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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Materials & Corrosion Control For additional information on this subject, contact

File Reference: COE10507 S.B. Jones on 874-1969 or S.P. Cox 874-2488

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Fabrication Methods

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Engineering Encyclopedia Materials & Corrosion Control

Fabrications Methods

Saudi Aramco DeskTop Standards

CONTENTS PAGES

THE CASTING PROCESS AND ITS APPLICATIONS........................................... 1

The Casting Process ........................................................................................ 6

Pattern Making..................................................................................... 6

Molding................................................................................................ 7

Melting Practices ............................................................................... 12

Defects in Castings ............................................................................ 19

Nondestructive Examination of Casting ............................................ 25

Repair of Castings.............................................................................. 26

Applications Of Cast Materials ..................................................................... 28

THE PLATE MANUFACTURING PROCESS AND ITS

APPLICATIONS ...................................................................................................... 31

The Plate Manufacturing Process .................................................................. 31

Hot And Cold Working...................................................................... 31

THE FORGING PROCESS AND ITS APPLICATIONS ........................................ 33

SEAMLESS AND WELDED PIPE MANUFACTURING AND THEIR

APPLICATIONS ...................................................................................................... 35

The Manufacture of Seamless Piping ............................................................ 35

Applications of Seamless Piping ................................................................... 35

The Manufacture of Welded Piping .............................................................. 36

Applications of Welded Piping ..................................................................... 36

THE CLADDING PROCESS AND ITS APPLICATIONS ..................................... 37

Co-Rolling ..................................................................................................... 37

Explosion Bonding ........................................................................................ 40

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TYPICAL WELDING PROCESSES, POSTWELD HEAT

TREATMENTS, AND THEIR APPLICATIONS .................................................... 42

Weld Quality ................................................................................................. 42

Carbon and Low-Alloy Steels............................................................ 43

Carbon Equivalent and Hardness ....................................................... 43

Preheating...................................................................................................... 43

Preheating vs Carbon Equivalent .................................................................. 44

Austenitic Stainless Steels ............................................................................. 44

Ferrite Control In Welds To Prevent Microfissuring..................................... 44

Sensitization .................................................................................................. 45

General Welding Practices ............................................................................ 45

Weld-End Preparations ...................................................................... 45

Joint Fit-Up and Cleanliness .............................................................. 46

Filler Metal Control (Low-Hydrogen Practices) ................................ 46

Weld Defects...................................................................................... 47

Welding Processes......................................................................................... 47

Arc Welding....................................................................................... 47

Shielded Metal Arc Welding (SMAW) ............................................. 48

Gas Tungsten Arc Welding (GTAW) ................................................ 49

Gas Metal Arc Welding (GMAW)..................................................... 50

Submerged Arc Welding (SAW)....................................................... 51

Weld Metal Overlays..................................................................................... 52

Corrosion-Resistant Weld Metal Overlays ........................................ 52

Defects in Weld Overlays .................................................................. 53

Postweld Heat Treatment (PWHT)................................................................ 57

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Stress Relief (SR)............................................................................... 57

Annealing........................................................................................... 58

Normalizing ....................................................................................... 58

Tempering .......................................................................................... 59

Quenching and Tempering................................................................. 59

Solution Annealing ............................................................................ 60

Stabilize Annealing ............................................................................ 60

Intermediate Stress Relief/Hydrogen Outgassing .............................. 61

PWHT — Fabrication Codes and Environmental

Requirements ..................................................................................... 61

REFERENCES........................... ............................................................................... 63

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Saudi Aramco DeskTop Standards 1

THE CASTING PROCESS AND ITS APPLICATIONS

Castings are components that are made in foundries. Castings are produced by allowing

molten metal to solidify in a mold. The casting process is very versatile with few restrictions

on the size or shape of a part that can be economically cast. In fact, there are certain

components, such as pump or reciprocating compressor casings, that cannot be economically

fabricated by any other method.

Cast alloys are available in a wide range of chemical compositions and mechanical properties.

Figure 1A lists the nominal chemical compositions of corrosion-resistant casting alloys, their

American Casting Institute (ACI) designations, and the American Iron and Steel Institute

(AISI) wrought equivalents.

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Composition of Common Casting AlloysCorrosion Resistant Castings

ACI* Nominal Chemical

AISI+ Wrought

Designation Composition (wt-%)

Equivalent

CA-15 0.15 1 max. 12 410

CA-6NM 0.06 4 12 0.7

CB-7Cu-1 0.07 4 16 Cb, 3Cu 17-4pH

CB-7Cu-2 0.07 5 15 Cb, 3Cu 15-5pH

CD-4MCu 0.04 5 26 2 3Cu Ferralium 255

CF-3 0.03 10 19 304L SS

CF-8 0.08 9 19 304 SS

CF-3M 0.03 11 19 2 316L SS

CF-8M 0.08 9 19 2 316 SS

CF-8C 0.08 9 19 Cb 347 SS

CG-8M 0.08 11 19 3 317 SS

CH-20 0.20 13 24 309 SS

CK-20 0.20 20 25 310 SS

CN-7M 0.07 29 20 2 3Cu Alloy 20Cb3

IN-862 0.07 24 21 5 Inconel 748

CW-12M 0.12 51 16 17 V, W, Fe Hastelloy C

CY-40 0.40 74 15 10Fe Inconel 600

Alloy 625 0.03 60 21.5 9 Cb Inconel 625

M-35 0.35 63 30Cu, Fe Monel 400

N-12M 0.12 62 28 V, Fe Hastelloy B

Notes:

• The materials specification for CB-7Cu-1 and -2 castings is ASTM A-747.

• The materials specifications for M-35 and CY-40 castings are ASTM A-494, A-743 and A-744.

• The materials specifications for CA-15 and CA-6NM castings are ASTM A-487 and A-743.

• The materials specifications for the balance of the above castings are either ASTM A-351, A-743 or

A-744. Note that many of the common casting alloys such as CF-3, CF-3M, CF-8, CF-8M, CF-8C and

CD-4MCu are included in all three materials specifications.

* American Casting Institute

+ American Iron and Steel Institute

C (max.) Ni Cr Mo Cu (Other)

Figure 1A

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Figure 1B shows the chemical composition of heat-resistant casting alloys, their American

Society for Testing and Materials (ASTM) specifications, and the AISI wrought equivalent

designations.

AISI+ Wrought

Equivalent

Nominal Chemical

Composition (wt-%) ASTM* Spec.

Designation

Composition of Common Casting Alloys

Heat Resistant Castings - Refinery and Petrochemical Service

A217GrC12 (HA) 0.20 max. 9 1 9Cr-1Mo

A297, A447, 0.2-0.5 13 25 0.2N max. 309 SS

and A-608 (HH) 2Mn, 2Si

A297, A351, 0.2-0.6 20 25 2Mn, 2Si 310 SS

and A-608 (HK) A-608 (HK-40) 0.35-0.45 20 25 2Mn, 2Si 310 SS(1)

HP 0.35-0.75 35 25 2Mn, 2Si None

HP-50 0.45-0.55 35 25 2Mn, 2Si None(2)

A-297 (HT) 0.35-0.75 35 17 2Mn, 2.5Si 330 SS

Proprietary Heat Resisting Centrifugal Casting Alloys

Paralloy H39W 0.45 35 25 1Cb None(3)

(HP modified)

Paralloy H20 0.40 20 25 310 SS(1)

(HK-40)

Manaurite 36X 0.35-0.45 33.5 25 1.5Mn, 1.5Si None(2)

Cb

Manaurite 36XS 0.35-0.45 33.5 25 2Mn, 2Si, Cb, None(3)

W

Manaurite 900 0.10-0.18 32.5 21 Cb Incoloy 800H(4)

Manaurite XA 0.50 max. 36 24 2Mn, 2Si, 2Cb None(3)

KHR-35CW 0.40-0.50 34.5 26 2Mn, 2Si, Cb, None(3)

(Kubota) W

(1) Tubes for reformer furnaces

(2) Tubes for both reformer and ethylene pyrolysis furnaces

(3) Tubes intended primarily for ethylene pyrolysis furnaces(4) Material for reformer furnace manifolds and transfer lines and ethylene pyrolysis furnace outlet lines.

* American Society for Testing and Materials

+ American Iron and Steel Institute

C Ni Cr Mo Other

Figure 1B

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Figure 1C lists the chemical composition of carbon and low-alloy Cr-Mo casting alloys and

their applicable ASTM and AISI wrought equivalent designations.

AISI+ Wrought

Equivalent

ASTM* Spec.

Designation

Nominal Chemical

Composition (wt-%)

Composition of Common Casting AlloysCarbon and Low-Alloy Cr-Mo Materials

A216GrWCA 0.25 0.7 max. 0.60 max. CS (1)

A216GrWCB 0.30 1.00 max. 0.60 max. CS (1)

A216GrWCC 0.25 1.20 max. 0.60 max. CS (1)

A352GrLCA (2) 0.25 0.7 max. 0.60 max. ITCS (3)

A352GrLCB (2) 0.30 1.00 max. 0.60 max. ITCS (3)

A217GrWC6 0.20 0.5-0.8 0.60 max. 1.0-1.5 0.45-0.65 1 1/4 Cr-1/2 Mo

A217GrWC9 0.18 0.4-0.7 0.60 max. 2.0-2.75 0.9-1.2 2 1/4 Cr-1Mo

A217GrC5 0.20 0.4-0.7 0.75 max. 4.0-6.5 0.45-0.65 5Cr- 1/2 Mo

A217GrC12 0.20 0.35-0.65 1.00 max. 8.0-10.0 0.9-1.2 9Cr-1Mo

(1) CS - Carbon Steel

(2) These castings are intended for low-temperature service and are impact tested.

(3) ITCS - Impact Tested Carbon Steel

Note: Specifications A216 and A217 restrict the concentration of individual residual elements, as well as

the total residual element content. However, these requirements are not shown in the Table. For specific information refer to the individual specifications.

* American Society for Testing and Materials+ American Iron and Steel Institute

C (max.) Mn Si Cr Mo

Figure 1C

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Figure 2 is a schematic flow diagram of the various steps involved in producing a steel

casting.

Inspection&

Shipping

Me l t ing

Coremaking

F i n a lHeat Treatment

Pouring

Shakeout

Initial

Heat Treatment

Cleaning &

Finishing

Riser Cutoff &

Gate Removal

Molding

M o l dClosing

Patternmaking

Figure 2. Flow Diagram for Producing a Steel Casting

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The Casting Process

An important first step in making a casting is the development of a pattern.

Pattern Making

The pattern is basically a full-size model of the component to be cast. Patterns may be made

from wood, cast aluminum, cast steel, cast iron, epoxy, reinforced epoxy, and polyurethane.

Metals are more widely used for multiunit production, since they have better abrasion

resistance and dimensional stability.

An important aspect of pattern design is the patternmaker’s shrinkage allowance. This must

be built into the pattern to account for the shrinkage of the molten metal during solidification.

The pattern is made slightly larger than the dimensions shown on the casting drawing.

Shrinkage of the casting will depend upon the material and the casting design. The same

pattern may require several different shrinkage allowances due to multiple wall thicknesses inthe casting. Shrinkage allowance also varies with the material; it is different for cast iron,

aluminum, and brass. This means that a different pattern is needed to produce the same

casting in each alloy. It is good practice to have the same foundry produce both the pattern

and castings, since shrinkage allowances vary with material, casting design, dimensions,

molding, and pouring practice.

Other important factors in the development of the pattern are gating and risering. Gating

provides flow paths for the liquid metal entering the mold. Risers are liquid metal reservoirs

that continuously feed the casting as shrinkage occurs during solidification.

Gating and risering requirements will vary with each metal.

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Molding

After the patterns are made as described above they are used to produce molds. A mold can

be thought of as a solid block of refractory with an internal cavity that has the shape and

dimension of the desired casting. It is produced by shaping refractory material around the

pattern.

Mold requirements include:

• It must be strong enough to support the weight of the molten and solidified

metal.

• It must permit gases formed during pouring to readily escape.

• The molding material must resist erosion by the hot metal during pouring and

must resist high-temperature degradation until the casting has solidified.

• It must have sufficient collapsibility so the metal can contract as necessary

during solidification.

• The material must strip cleanly away from the casting.

• The process must be economical.

Granular silica molding sand is a relatively inexpensive refractory material that meets most of

the requirements listed above. More expensive refractories are available, but they are used for

special applications and processes.

Moisture and binder contents of foundry sand must be closely controlled since they affect

warm and elevated temperature molding properties. Mulling machines are used to thoroughly

mix the sand, binding agent, and water.

With a clay bonded sand, the mixture is placed around the pattern and rammed to the required

hardness. With chemical binders the mold is given a light manual or machine compaction and

then chemically hardened. Molds are generally made in two halves to facilitate pattern

removal. The top half is termed the cope and the bottom half the drag.

Cores are component parts of the mold that form the internal passages and contours of the

casting that cannot be made by the pattern alone. A good example is the casting of a hollow

cylinder. A solid cylindrical pattern is needed to form a mold cavity that represents the OD

surface of the casting. A solid sand core is centered in the mold cavity by spacers, called

chaplets, to form the internal or ID surface. Cores are made from mixtures of sand and binder

and must have the same attributes as described above for molds. Cores may be made by hand

ramming or machine and may be dried or baked. For example, “green” cores are dried in

ovens at 177-230 °C (350-450 °F) long enough to develop the required strength and

collapsibility.

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Molding Processes - Static castings are produced by the green sand, dry sand, no-bake, floor

and pit, and shell molding processes. Precision static castings are made by vacuum, full mold,

magnetic, investment, and ceramic molding methods. Die casting uses special permanent

molds from high-strength, low-alloy steel or molybdenum. Centrifugal castings are made

using permanent molds manufactured from graphite, iron or steel, and are coated with a

refractory layer.

Highlights of some of the more important molding processes are summarized below:

• Green sand molding is the most common method used in foundries to produce

carbon, low-alloy, and stainless steel castings. Typically, the sand mixture

consists of 92 wt % silica sand, 6 wt % bentonite, and 2 wt % water. By

changing the proportions of the various additives the sand properties can be

changed, permitting its use on various types of mold making equipment. The

mold is hardened by pressure-baking, drying, or chemically-induced hardening

is not required. Green sand molds can be used within a few minutes or days of

closure.

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Figure 3 shows a green sand mold partially sectioned to show the principal

features.

Figure 3. Sand Mold Partially Sectioned to Show Detail

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• Dry sand molding is similar to green sand molding except that the moisture is

baked out of the mold in an oven. When petroleum-sand mixtures are used the

finished molds have very high strength and smooth surfaces, are durable, and

can be stored for a long period of time. However, this process has several

disadvantages: additional capital investment is required, and productivity

suffers due to extended mold drying times.

• No-Bake Molding - In the sodium silicate-CO2 and cold-box processes, CO2

gas is forced through the sand mixture to cure or harden the mold before the

pattern is removed. The mold cures by the chemical reaction between the CO2

and additives in the sand. These processes are used for cores and molds where

the castings require superior surface finish and dimensional stability.

• Shell molding is an automated process that is used to produce small to medium

size castings with close tolerances and a superior surface finish. The process

uses a hot ([200-260 °C] [390-500 °F]) metal pattern and a dry resin coated

sand mixture. Heat from the pattern melts the resin and bonds the sand particles

to form a mold approximately 9.5 mm (3/8 in) thick. Excess sand is

automatically removed, and heating is continued until the resin has cured and

hardened. The mold is then stripped from the pattern by the use of automatic

stripping pins. The shell molding cycle takes one to three minutes. Shell

molding can be used on all casting alloys.

• Investment molding is also called the “lost wax” or “precision casting” method.

In this process permanent metal dies are used to make disposable patterns that

are used to make ceramic molds (note that both the pattern and mold are

expendable).

The basic steps of the investment molding process are:

– Metal patterns are used to make disposable patterns of wax or plastic.

– The wax or plastic patterns are assembled onto a gating system.

– The assembled patterns are invested or coated with ceramic to form a

monolithic mold.

– The mold is heated to melt out the patterns and form the mold cavities.

– The mold is subjected to high-temperature firing to remove all traces of

pattern material and to develop ceramic bonding.

– The molten metal is cast into the mold.

– The solidified casting is shaken out of the mold, cleaned, and finished.

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The steps of the investment casting process are schematically shown in Figure

4.

Figure 4. The Basic Steps in Investment Casting Production

The advantages of the investment molding process are:

– Permits mass production of complicated shapes that are difficult or

impossible to produce by other casting methods.

– Permits precise reproduction of fine detail resulting in superior

dimensional accuracy and smoother surfaces than can be obtained with

other casting methods.

– Little or no finishing of the casting is required.

– Applicable to most alloys.

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The disadvantages of the process are:

– Not cost-effective for castings over 4. 5 Kg (10 lbs).

– High tooling costs.

– Low productivity.

• Ceramic Molding - The ceramic or ethyl silicate molding process is sometimes

referred to as the Shaw, Unicast, Osborn-Shaw, or Ceramicast Process. Graded

refractory fillers, hydrolyzed ethyl silicate, and a liquid catalyst are mixed and

blended in a slurry. The refractory filler material can be sillimanite, mullite,

zircon flour, silica flour, or calcined fire clay. The slurry is poured over the

pattern and sets within a few minutes, first to form a gel, and finally a rigid

mold. The mold is removed from the pattern during the gel state, placed in a

furnace, and fired at high temperature until ceramic bonding is complete.

Castings made by the ceramic molding process have excellent dimensional

accuracy and very smooth surfaces. Examples of products made by this process

are high-alloy jet engine manifolds and blades, and high-temperature gas

turbine blades and vanes.

Melting Practices

Both the electric-arc and induction melting methods are used by modern foundries. Highlights

of the equipment and important characteristics of each are reviewed in the following sections.

Electric-Arc Melting - An electric-arc furnace utilizes a bowl-shaped metal shell that is

refractory lined to form a melting chamber. A solid or molten steel charge conducts the

electric current between the carbon or graphite electrodes. Melting is initiated by the arcs

between the electrodes and the metal charge. The metal is melted by the heat from the arcs

and by radiation from the furnace walls and roof.

Most foundries use “basic practice” to refine the charge. Small amounts of lime are added

during melting and form a protective slag layer on top of the melt. Iron ore is added upon

completion of melting to drive the phosphorous from the melt into the slag. The phosphorous-

rich slag is removed and chemically basic compounds such as lime, fluorspar, and in some

cases silica plus coke and ferrosilicon, are added to the melt. These form the calcium carbideslag necessary for sulfur removal. This is called the “refining period.” An important

advantage of this process is the ability to produce high quality steel from low quality scrap.

“Basic practice” is required for the production of stainless and high alloy steels.

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Figure 5 is a cross section of an electric arc furnace illustrating both acid and basic refractory

linings. Since the acid lining process does not facilitate removal of phosphorous and sulfur

from the molten charge it is seldom used.

Figure 5. Cross Sectional Sketch of an Electric Furnace

Showing Typical Refractories for Acid and Basic Practices

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Induction Melting - An electric induction furnace consists of a carbon steel shell that has a

water-cooled copper induction coil installed on the ID of the vertical walls. The floor is lined

with firebrick. The furnace melting chamber is either a refractory crucible or the floor and

walls are lined with rammed and sintered refractory. The furnace also has a refractory-lined

removable steel top or lid, and a tapping spout.

The furnace is charged with scrap and a high frequency current is passed through the copper

coil. The current in the coil induces a much greater secondary current in the charge. Heat from

the induced current melts the charge and the magnetic fields associated with the currents

cause a stirring action that speeds up melting. However, the stirring action prevents the use of

a slag to protect or refine the melt. Scrap is continually added during melting to replace the

loss of easily oxidized elements. When melting is complete the heat is deoxidized and tapped-

usually within fifteen minutes. An advantage of an induction furnace is versatility, particularly

for the production of small lots of high alloy castings. A disadvantage is the need for costly,

high quality scrap, since the melt cannot be refined.

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Figure 6 is a cross-sectional sketch of an electric induction furnace.

Figure 6. Cross Sectional Sketch of a Typical Electrical Induction Furnace

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Some foundries have installed additional refining and processing equipment to improve

control of carbon content and to reduce sulfur content. This has been found to improve

mechanical properties, particularly the toughness of ferritic alloy castings. In AOD refining

(Argon-Oxygen Decarburization), the melt is lanced with argon and oxygen to control (in the

case of stainless steels to reduce) carbon content and to reduce sulfur levels.

Centrifugal Casting - Green sand, shell, investment, and ceramic molding processes are used

for static castings where the molten metal is poured into the mold and flows by gravity into

the mold cavity. However, in the centrifugal casting process (used to produce tubular or

cylindrical castings without using cores), the mold is rotated about its major axis, and

centrifugal force distributes the molten metal uniformly. Depending on the process and

product the rotation speeds will vary; centrifugal forces up to 150g may be used. A tube or

pipe with a particular wall thickness is made by pouring a calculated weight of molten metal.

The proper quantity of metal is based on the length of the mold, desired wall thickness, and

metal density. Castings of excellent quality are produced by this process. The external forcesacting on the metal usually preclude insufficient filling of the mold or the formation of shrink

cavities. HK-40 and HP-50 centrifugally cast tubes are used in the radiant sections of

Reforming and Ethylene Pyrolysis Unit furnaces.

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Figure 7 illustrates the production of horizontal centrifugal castings.

Figure 7. The Centrifugal Process

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One of the advantages of the centrifugal casting process is that castings can be made to

different lengths, thicknesses, and diameters in any castable alloy. Short lengths may be

obtained by sectioning a long casting into the required segments. It should be noted that

horizontal axis molds are available for the production of tubes and pipes up to 12m (40 feet)

long. The length and outside tube diameter are fixed by the mold cavity, but the tube

thickness will depend upon the weight of molten metal poured, as indicated above.

Short length cylindrical and intricately shaped castings are most often made in vertical axis

molds. Examples are flanges and rings. The inside surface of vertical axis castings is not

cylindrical but somewhat parabolic. The degree of parabolic taper will depend upon the speed

of rotation, the mold cavity OD, and the volume of metal poured. The faster a casting is spun

during pouring the less pronounced the internal taper. The lower limits of rotational velocity

are dictated by the amount of acceptable internal taper, while the upper limit is set by the need

to avoid excessive stresses in the solidifying outer skin of the casting.

The three principle types of molds for centrifugal casting are:

• A permanent mold made from steel, iron, or graphite. The mold is coated on

the inside surface with a thin layer of refractory to extend service life.

• A mold made from a steel flask lined with a rammed refractory mix and coated

with a refractory wash and baked until the coating is dry and hard.

• A refractory-lined spun or centrifugally-cast mold made from a steel flask. The

flask is rotated rapidly and refractory slurry is added until the required

refractory thickness is obtained. After spinning, the excess water is drained and

the coated mold is baked to cure the refractory.

In the centrifugal casting process, the molten metal is accelerated to mold speed by the

friction between the molten metal and the mold ID surface. The casting process is controlled

such that solidification begins after the entire mold surface is uniformly covered.

Solidification progresses from the mold/molten metal interface to the casting ID. The end

result is a cast tube that has a dense, sound structure throughout its wall with most of the

impurities concentrated adjacent to the ID. It is important to note that the ID is usually bored

out to remove these impurities prior to placing the tube in service.

As-cast microstructure is very important in the HK-40 and HP-50 furnace tubes mentioned

previously. To obtain good stress rupture properties, at least 50 % of the cross-sectional areashould consist of large columnar grains. The columnar grains nucleate on the OD and extend

to about mid-wall. In cross section the grains exhibit a length that is much greater than their

width. The size and extent of the columnar grain structure is related to the casting alloy, melt

cleanliness, degree of under cooling, and cooling rate. The balance of the structure, from

about mid-wall to the ID, consists of coarse, randomly oriented equiaxed grains. The dendritic

or equiaxed grains (same dimensions in all directions) nucleate in the melt and grow until they

meet the columnar grains.

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All grades of carbon, low-alloy, and high-alloy steels that can be statically cast can be

centrifugally cast. Examples of centrifugally cast products are: furnace tubes (discussed

above), pipe, retorts, furnace rolls, nuclear power piping, artillery gun tubes, and paper mill

rolls.

Although the available sizes of centrifugally cast tubes will vary among foundries, they are

being made up to 3.3 m (130 in) OD and up to 12 m (40 ft) long. Tube sections may be joined

by welding where longer lengths are required.

Defects in Castings

Because of the basic characteristics of the casting process, certain inherent flaws may occur in

castings. These flaws include:

• Sand and nonmetallic inclusions consist primarily of sand and slag particles

that become entrapped in the casting during solidification. The loose sand particles are generated during the installation of the cores in the mold. Small

particles of sand break free or are rubbed off mold and core surfaces. These

types of defects can occur anywhere in the casting. The inclusions appear as

dark images on negative radiographic film, since they are less dense than the

metal.

• Porosity is caused by gas bubbles that become entrapped during solidification.

The gases are generated when the molten metal vaporizes the moisture in a

green sand mold or when the binders vaporize. Usually these gases escape

through vents installed in the mold and cores. However, if the vents become

plugged, or if the mold is contaminated with oil or grease, the vents cannotaccommodate all of the gases, and a certain proportion remains trapped in the

metal. Large pores on the OD surface of the casting are termed blowholes.

Porosity appears as round or elongated dark spots on negative radiographic

film (light spots on a positive print). Pores can occur individually, in clusters,

or may be distributed randomly.

• Shrink cavities in a casting are the result of the solidification process. It must be

recognized that when molten metal solidifies, it shrinks, and that thin sections

of a casting solidify before thick sections. The shrinkage that occurs during

solidification results in a transfer of molten metal from the molten, thick

section, to the partially solidified thin section. If the riser attached to the thick section cannot deliver sufficient molten metal to accommodate this shrinkage, a

shrink cavity occurs in the thick section. Additional risering and gating, or an

increase in the wall thickness of thin sections, can minimize the potential for

shrink cavity formation. Shrink cavities appear as indistinctly outlined and

irregularly shaped dark areas on negative radiographic film.

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Figures 8 through 14 show the appearance of the most important defects.

Figure 8. Radiographic Appearance of Shrink in a Cast Aluminum Alloy

Figure 9. Radiographic Appearance of Inclusions in a Cast Aluminum Alloy

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Figure 10. Positive Print of Radiograph Showing Porosity (Light Spherical Areas)

Figure 11. Pinhole Porosity on Machined Face of a Casting

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Figure 12. Positive Print of Radiograph Showing a Hot Tear

Figure 13. Example of a Hot Tear at a Casting Cross Section

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Figure 14. Example of a Cold Shut

Defects such as porosity, sand inclusions, blowholes, cold-shuts, etc. , may occur on or

beneath the casting surface. Other defects such as shrink are subsurface. It is important to

recognize that just because casting surfaces appear to be free of defects there is no ensurance

that the interior of the casting is defect free.

An example of subsurface defects occurred a few years ago when several alloy CN7M

impeller and diffuser castings were ordered for vertical pit pumps in produced water service.

The as-cast surfaces appeared to be satisfactory and required little or no surface finishing.

However, leakage occurred around the stud bolts of the diffusers during hydrostatic testing.

Radiographic examination (not part of the original order) revealed large internal voids and

very poor casting quality. Since the same foundry also poured the impellers, these were

radiographed and found to contain similar large internal voids. All the castings were scrapped

and another foundry was given the order. This resulted in additional cost and a delay in pump

delivery.

Although this example emphasizes the importance of volumetric internal nondestructiveexamination (NDE), it should be noted that not all castings require radiography. The extent of

NDE and required examination techniques will depend upon the design temperature, pressure,

and the service requirements of the casting.

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Nondestructive Examination of Casting

The different types of nondestructive examinations of castings and a brief description of each

type are discussed in the following sections.

Radiographic Examination - Radiographic examination (RT) is used to determine the

internal soundness of a casting. It is primarily used to detect volumetric defects such as

porosity, sand and slag inclusions, and shrink cavities, but can also be used to detect linear

indications such as hot tears and cracks. It should be noted that RT is not as well suited for the

detection of linear defects as it is for volumetric defects. Linear defects parallel to the

radiation beam (perpendicular to the casting surface), or at a slight angle to the beam, can be

readily be detected. However, defects perpendicular to the radiation beam are not detectable.

If there is a question of interpretation, or if the presence of cracks or tears cannot be verified,

ultrasonic flaw detection can be used to inspect the areas in question.

Ultrasonic Examination - Ultrasonic examination (UT) for flaw detection is primarily used

on castings that have wall thicknesses in excess of 305 mm (12 in). At these thicknesses

radiographic examination is not practical due to poor radiographic image sensitivity and the

extended exposure times required to produce a film with sufficient density. UT examination

techniques are useful for the detection of linear discontinuities such as hot tears, cracks, cold

shuts, and so forth, because the ultrasonic waves can be directed at various angles to the

casting surface. UT techniques are often used to supplement radiographic examination,

especially when conflicts over defect interpretation occur.

For castings in critical service, or for castings poured from high-alloy austenitic materials,

calibration blocks must be made from the actual production steels. The blocks must be in the

same heat treatment condition and have the same surface finish and thickness as the casting

component to be examined. These requirements ensure the ultrasonic equipment is properly

calibrated and can detect unacceptable discontinuities in the casting.

Magnetic Particle and Liquid Penetrant Inspection - These inspection techniques are used

to detect surface discontinuities such as cracks, hot tears, cold shuts, inclusions, and porosity.

The liquid penetrant (PT) method can be used on all materials; the magnetic particle (MT)

method can only be used on carbon and low-alloy ferrous materials. In addition to surface

discontinuities, the DC MT technique can also detect subsurface discontinuities within 3 mm

(0.125 in) of the surface. It should be recognized that indications of subsurface defects are not

always reliable.

MT examination can be done with prod or yoke equipment. To avoid arc strikes, prods must

be firmly in contact with the material before the switches on the handles are used to activate

the magnetizing current. Arc strikes that do occur must be removed by grinding.

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Cleaning and developing solutions used for PT examination of austenitic stainless steel and

nickel-base alloys must have a combined total residual sulfur and halogen content of less than

one weight-percent. This restriction is necessary to prevent chloride-stress corrosion cracking

of austenitic stainless steels and to minimize the potential for sulfidation of nickel-base alloys.

Repair of Castings

Defects in castings can result in either major or minor repairs. Defects that result in major

repairs are:

• Those that cause leakage during hydrostatic testing.

• Those that result in repair cavities that exceed 20% of the wall thickness or 25

mm (1 in), whichever is less.

• Those that result in a repair area that exceeds 6500 mm2

(10 in2

).

Repairs required by other defects can be considered to be minor.

Casting defects, except those in cast iron, are usually repaired by welding. In some cases,

minor surface defects in castings with excess thickness can be removed by grinding. After

foundry personnel have removed the defective material, the cavity is inspected by MT or PT

to ensure that the defect has been completely removed. Welding is then done using ASME

B&PV Code Section IX qualified Welding Procedure Specifications (WPS’s) and qualified

welders. The requirements for preheat, interpass temperature control, postweld heat treatment

(PWHT), or normalizing and tempering heat treatments will depend on the particular casting

alloy and the depth of the repair and are defined in the WPS. Generally, the chemicalcomposition and strength of the deposited weld metal must closely match those of the casting.

Repair welding recommendations for carbon, low-alloy, and austenitic stainless steels are

summarized in the following material:

• Carbon Steel: For minor weld repairs the repair cavity and surrounding area

should be preheated to 100 °C (212 °F) prior to welding. PWHT or special heat

treatments are not normally required. For major repairs, in addition to the

preheat, a PWHT is recommended. PWHT should be performed at 593 °C

(1100 °F) minimum for one hour per inch of thickness (based on repair cavity

depth) with a minimum holding time of one hour.

PWHT after the completion of weld repairs is often recommended for castings

intended for certain critical services. Except for minor repairs on nonwetted

surfaces, PWHT is often recommended on castings in caustic, HF, amine, or

wet H2S service. After the completion of weld repairs and PWHT, the weld and

adjacent areas should be reinspected using the same NDE techniques and

acceptance criteria that were originally used to inspect the casting.

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• Low-Alloy Steel: Low-alloy steel castings are much more difficult to repair

than carbon steel, and all minor or major repair cavities must be preheated to

the following minimum temperatures prior to welding:

• 1Cr-Mo-150 °C (300 °F)

• 2Cr-lMo and 5Cr-Mo-200 °C (400 °F).

To prevent delayed cracking in these materials the preheat should be

maintained until the completion of welding. PWHT is required and should be

performed at the following minimum temperatures:

• 1Cr-Mo-593 °C (1100 °F)

• 2Cr-1Mo and 5Cr-Mo 677 °C 1250 °F).

Holding time is one hour per inch of thickness. Note that it may be necessary to

increase the PWHT temperature, especially if hardness limitations are required.

After the completion of weld repairs and PWHT, the weld and adjacent areas

should be reinspected using the same NDE techniques and acceptance criteria

that were originally used to inspect the casting. NDE techniques usually include

RT and MT for Cr-Mo castings.

• Austenitic Stainless Steels: The welding techniques used to repair austenitic

stainless steel castings are similar to those used to weld austenitic stainless steel

wrought material. In general, no preheat or PWHT is required. However, to

eliminate sensitization and restore optimum corrosion resistance, castingssubject to severe corrosive service are solution annealed [1040 °C (1900 °F)

minimum temperature] and water quenched after welding. If solution annealing

is not possible, alternative approaches are: using stabilized or low-carbon grade

castings and minimizing weld heat input. After welding, the weld and adjacent

areas should be reinspected using the same NDE techniques and acceptance

criteria that were originally used to inspect the casting.

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Applications Of Cast Materials

In the petroleum industry castings are widely used for valve bodies; pump casings and

impellers; compressor casings, blades, cylinders and rods; gas turbine blades and components;

power recovery turbine blades and components; steam turbine casings and blades; and

furnace tubes and pipe.

By far the largest number of applications are for castings made from carbon steel. Typical

applications include valve bodies, pump casings and impellers, compressor casings,

compressor cylinders and rods. Castings for these applications are made to ASTM A-216

grade WCB. For special applications, such as low-temperature service, castings are made to

ASTM A-352 to ensure sufficient notch toughness.

Cr-Mo castings are used in high-temperature service to provide resistance to sulfidation and

high-temperature hydrogen attack. For high-temperature sulfidation resistance in FCC Units,

5Cr-Mo castings are used for spent catalyst slide valves, pumps, valve bodies, and pipefittings. 5Cr-Mo and 9Cr-1Mo castings are also used as furnace components, pumps, valve

bodies, and pipe fittings in delayed coking units, crude units, and in the hydrogen-free, high-

temperature sulfur-bearing streams in CHD and hydrocracker units. 2Cr-1Mo and 9Cr-1Mo

castings are used for headers and return bends in PtR and CCR Unit fired heaters to resist the

effects of high-temperature hydrogen, to resist oxidation from the fireside of the heater, and to

provide adequate stress-rupture properties. In addition to heater components, 1Cr-Mo and

2Cr-1Mo castings are used in high-temperature piping systems within these units. The casings

for the high-temperature end of steam turbines are usually cast in 1Cr-Mo material because of

high-temperature strength and stress rupture considerations.

In corrosive environments such as refineries or chemical plants, austenitic stainless steel

castings, such as grades CF8, CF8M, CF3 and CF3M, are used in pumps, valves, and fittings.

In fact, grades CF8M and CF3M and their wrought counterparts (Type 316 and 316L SS) are

considered to be the “workhorse” materials in most chemical plants. The use of alloy

materials is usually based on the corrosivity of the process stream, but in some situations

product purity considerations are most important. In polystyrene plants the process is not

corrosive, but to prevent product contamination and ensure resin clarity the polymerization

reactors and overhead equipment are fabricated from Type 316 SS. In the production of

ethanol or isopropanol, reactors, separators, and piping are Type 316 SS because of stream

corrosivity. It should be recognized that austenitic stainless steel castings (like their wrought

counterparts) are susceptible to pitting and chloride-stress corrosion cracking and

consequently should not be used in aqueous chloride-bearing environments. For these

environments, alloys such as CN7M, CD4MCu, or IN862 should be used.

Steam turbine or compressor blades are usually made from martensitic stainless steel (12 wt-

% Cr) or age hardenable alloys such as Inconel 718 or 17-4 PH. These components may be

cast or made from wrought material. Proprietary alloys (in some cases derivatives of the

above alloys) are used by manufacturers like General Electric, Elliot Co., and Dresser-Rand.

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These alloys are used principally for their corrosion resistance and strength at high

temperatures.

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High-temperature gas turbine blades, vanes, and other components are made from cast nickel-

base alloys such as Udimet 500 or 700, Inconel 792 or 713, and Rene 80 or cobalt-base alloys

such as X-40 and the Stellites. Wrought materials are also used, and typical nickel-base alloys

include Inconel 718, X, 738 and 792, Nimonic 80 and 105, or Waspaloy. To resist sulfidation

and erosion, nickel-base alloys in high-temperature sulfur-bearing environments are flame

sprayed with a refractory ceramic coating. For erosion resistance cobalt-based alloys are

sprayed with similar coatings.

HK-40 (25 Cr-20 Ni) and modifications of the HP (25 Cr-35 Ni) classes of centrifugally cast

tubes are being used extensively in steam-methane reforming furnaces. HK 40 is considered

to be more cost-effective than HP up to 955 °C (1750 °F). At higher temperatures, tubes with

a higher nickel content should be considered to take advantage of improved mechanical

properties and carburization resistance. In addition to increasing the nickel content, some

manufacturers add small amounts of columbium and tungsten to further improve high-

temperature strength and carburization resistance. Examples of these materials include HPand the proprietary cast alloys Paralloy H39W, Kubota KHR-35CW, Abex TMA 6300,

Wiscalloy 25-35 Nb, Manaurite 36X, and Manaurite 36XS. At elevated temperatures these

alloys are superior to HK because they have better stress rupture properties and more

resistance to creep and oxidation. Subsequent to aging they have greater notch toughness at

low temperatures, which is important during heating or cooling cycles or when the furnace is

out of service for maintenance.

In Ethylene Pyrolysis Units, centrifugally cast furnace tubes usually contain 25-30 wt %

chromium, at least 35 wt % nickel, plus small amounts of columbium and tungsten. The

chromium ensures sufficient oxidation resistance; the nickel, columbium, and tungsten

provide the necessary carburization resistance to prevent metal dusting up to 1093 °C(2000 °F). Columbium and tungsten also improve high-temperature stress rupture properties.

Tubes may be HP, the proprietary casting alloys mentioned above, or 30Cr-30Ni Si, 28Cr-

48Ni W, and 28Cr-35Ni W Co. HK-40 tubes are not recommended above 955 °C (1750 °F)

in this service since the nickel content is not adequate to prevent carburization and metal

dusting.

Ethylene pyrolysis furnace tube internal bores should be honed after machining to improve

carburization resistance. A 125 RMS (root mean square) ID surface finish is usually

satisfactory.

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THE PLATE MANUFACTURING PROCESS AND ITS APPLICATIONS

The petroleum industry uses steel products in many different forms. To understand the

differences in steel products, their advantages and applications, the following section briefly

reviews how they are produced.

Highlights of page 58 in the Appendix, A Flowline of Steelmaking, are as follows: iron ore,

limestone, and coal are charged into a blast furnace. (Source: Reference No. 1, Steelmaking

Flowlines) The molten iron produced in the blast furnace is combined with scrap, and the

combined charge is refined in an electric furnace, open hearth furnace, or a Basic Oxygen

Furnace (BOF) to produce steel. The molten steel is charged to a continuous caster to produce

slabs or is cast into ingots.

The Plate Manufacturing Process

The next step in the steelmaking process is the hot working of the thick slabs from the

continuous caster into billets, blooms, and slabs of reduced cross section.

Hot And Cold Working

The billets that are formed may be subsequently hot rolled to form bar, rod, and wire

products. Tube rounds can also be formed that will subsequently be made into seamless pipe

and tubing. Blooms are hot rolled to form structural shapes or rails. The slabs are hot rolledinto:

• Plate

• Strip material that is pickled and oiled and subsequently cold rolled, annealed,

and cold temper-rolled into galvanized and other coated flat-rolled or tin mill

products.

• Skelp that is subsequently made into welded pipe or exchanger tubing.

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The diagram on page 59 of the Appendix illustrates the casting and subsequent hot working of

ingots into various steel products. (Source: Reference No. 1, Steelmaking Flowlines)

Molten steel from the electric, open-hearth, or BOF furnaces flows from ladles into ingot

molds or ingot railroad cars. When the ingot is solidified, a stripper crane lifts the mold away

while a plunger holds the ingot in position. After stripping, the ingots are placed in a soaking

pit and are held there until they reach the desired through-thickness temperature. They are

then moved to the roughing mill, where they are hot rolled into blooms, billets, or slabs. An

alternative method is to bypass the ingot step by positioning the ladle over the top of a strand

caster. Molten steel flows from the ladle into a reservoir called the tundish and then into the

molds of the caster. The copper molds are internally water cooled, which results in the

formation of a thin steel skin around the periphery of the molten steel. As the steel leaves the

mold and is cooled further, additional solidification occurs, and the thickness of the steel skin

increases. Complete solidification occurs as the partially solidified strands descend through a

water spray quench system. As can be seen in the diagram on page 59 of the Appendix, thecaster bends the slab from the vertical to the horizontal position before a torch cuts the slab to

the desired length.

Plate materials are used extensively for the pressure-retaining components of pressure vessels

(shell courses and heads), shell and tube heat exchangers (channels, channel covers, shells,

and shell covers), air coolers (header boxes), and tanks (shell courses, and bottom and roof

plates). They are also used for internals (pressure vessel trays, downcomers, and baffles) and

shell and tube exchanger cross baffles. New pressure vessels, heat exchangers, and tanks are

generally built to the appropriate ASME B&PV Code or API Code using ASME B&PV code

Section II A or B plate materials. In most situations, ASME materials are also used when

these types of equipment undergo alterations or are repaired. One of the most commonly usedcarbon steel plate materials is SA-516 Grade 70.

Examples of carbon steel, low-alloy, and high-alloy steel plate specifications are:

SA 202/202M 353/353M 542 737/737M

203/203M 387/387M 553/553M 738/738M

204/204M 412 562/562M 812/812M

225/225M 414/414M 612/612M 832/832M

240 442/442M 620/620M

263 455/455M 645/645M

264 480/480M 662/662M

265 516/516M 693

285/285M 517/517M 724/724M

299/299M 533/533M 736/736M

302/302M 537/537M

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THE FORGING PROCESS AND ITS APPLICATIONS

Forging is a process that involves the shaping of an ingot into a specific component or part by

hot working the metal under conditions of intense heat and pressure. Forging is performed by

hammering and/or pressing at metal temperatures around 1093 °C (2000 °F). It is important to

recognize that the ingot must be subjected to sufficient deformation in all three dimensions

during working to ensure that the as-cast ingot structure is completely eliminated, and that the

material undergoes complete recrystallization. Normally, the ingot is upset and the material

worked in such a manner that its cross section is reduced by a factor of 3 or 4. This amount of

working results in a forged component that has a homogeneous microstructure and exhibits

satisfactory mechanical properties in all three dimensions, including the through-thickness

direction. The forging process is used to make parts or components that must exhibit superior

mechanical properties. It is important to recognize that the mechanical properties of a forging

are superior to those of a casting.

The advantages of forging are:

• Forged materials often exhibit superior strength and toughness when compared

to both cast and plate materials. Consequently, wall thicknesses can be reduced,

making fabrication more economical.

• Machining costs are reduced, since the forged parts are close to the final

required size and shape.

• Scrap quantity is reduced.

Open die hot forging of a turbine shaft is illustrated on page 60 (A) of the Appendix. (Source:Reference No. 2, Steel Processing Flowlines)

An ingot of low-alloy steel is uniformly heated in a furnace and forged in a hydraulic press.

The piston driven flat-faced upper die is forced down onto the ingot which is moved and

turned on the bottom die as necessary to shape the component. It should be noted that

squeezing, rather than impact, characterizes open die forging. During the forging process the

partially shaped component is periodically returned to the furnace for reheating. When the

forging operation is complete, the turbine shaft will be finish machined, inspected, and

balanced.

Closed die forging of a conveyor roller is illustrated on page 60 (B) of the Appendix. (Source:Reference No. 2, Steel Processing Flowlines)

In this case, a billet from a rolling mill or strand caster is cut to the required length, placed

between two dies, and hammered in a stream hammer. The impact of hammering causes

plastic flow, and the hot metal fills both halves of the die. A finish machining operation

completes the fabrication of the roller.

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Forgings such as flanges, fittings, and valves are important components of piping systems.

Examples of carbon steel, low-alloy, and high-alloy steel forging specifications are:

SA 105/105M 350/350M (for low temperature service)

181/181M 522/522M

182/182M

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SEAMLESS AND WELDED PIPE MANUFACTURING AND THEIR

APPLICATIONS

The manufacture of seamless and welded pipe and the applications of each are presented in

the following sections.

The Manufacture of Seamless Piping

As shown on page 61 (A) of the Appendix, seamless pipe and tubing are made from tube

rounds using the piercing process. (Source: Reference No. 2, Steel Processing Flowlines)

Solid tube rounds are heated in a rotary hearth furnace and then transferred to a piercing mill

where the heated rounds are rapidly rotated and pierced under extremely high pressure. After

piercing, a mandrel of the desired size is inserted into the shell, and the partially formed pipe

is run through the mandrel mill to achieve the approximate wall thickness and diameter. After

reheating, the seamless tubes are rolled to precise sizes on a sizing or stretch-reducing mill.

Applications of Seamless Piping

Seamless pipe and tubing may be used in high pressure applications or where maximum

reliability is desired. Examples of ASME B&PV Code Section IIA specifications for carbon,

low-alloy, and high-alloy seamless pipe are:

SA 53* 376/376M

106 524

312*/312M 731/731M

333*/333M 790*/790M

335/335M

Examples of specifications for seamless tubing are:

SA 179/179M 210/210M 423*/423M

192/192M 213/213M 556/556M

199/199M 268*/268M 789*/789M

209/209M 334*/334M

*Specification also includes welded pipe or tubing.

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The Manufacture of Welded Piping

The diagram on page 61 (B) of the Appendix illustrates the production of longitudinally butt

welded pipe (butt welded tubing is similar). (Source: Reference No. 2, Steel Processing

Flowlines)

Skelp is uncoiled, levelled, and heated to the forming temperature in a furnace. The hot strip

is formed into pipe in the forming, welding, and reducing mill. Final pipe diameter and

surface finish are obtained by running the rough piping through the sizing mill. As a final

step, the finished piping is cut to the required length, inspected, and hydrostatically tested

prior to shipment.

Applications of Welded Piping

Welded piping and tubing are generally more economical than the corresponding seamless

products. However, it must be emphasized that for some critical applications, such ashydrogen or severe corrosive services, the longitudinally welded seam must be

radiographically examined to ensure structural integrity.

Examples of welded pipe specifications** are:

SA 106 587 813/813M

134 671 814/814M

358/358M 672

409/409M 691

Examples of welded tubing specifications** are:

SA 178/178M 249/249M

214/214M 250/250M

226/226M 557/557M

688/688M

**See lists of seamless pipe and tubing above for specifications that cover both seamless and

welded pipe and tubing.

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THE CLADDING PROCESS AND ITS APPLICATIONS

Co-Rolling

In the Lukens roll bonding process a cladding plate and a backer plate are joined by co-rollingunder intense heat and pressure and are integrally bonded over their entire surface. When

manufacturing clad materials, two backer plates are thoroughly cleaned and nickel plated. The

alloy cladding material is then placed on top of the nickel plating to form a “sandwich” with

the two sheets of alloy material in the center. The purpose of the nickel plating is to minimize

dilution of the alloy cladding material during bonding. To facilitate separation after rolling a

parting compound is inserted between the alloy sheets. The “sandwich” is then completely

seal welded around the periphery and rolled under intense heat and pressure to form a

metallurgical bond. Although roll-bonded plate can be produced in thicknesses up to 225 mm

(9 in), bonding can be a problem in thicknesses greater than 100 mm (4 in). Bond integrity is

usually checked by ultrasonic inspection. If areas of unbonded cladding are found they can

usually be weld repaired. The tables in Figures 15A, B, C, and D list the commonly used

cladding and backing alloys that are utilized in the manufacture of clad plate.

Composition and Specifications For Cladding Materals

Material

Chromium

SS Clad

ASTM

A-263

Chromium-

Nickel SS

Clad

ASTM

A-264

A240

A240 A240

A240

A240

A240

A240

A240

A240

A240

A240

A240

A240 A240

A240

410

410S

405

429

430

304

304L

309S

310S

316

316L

317

317L321

347

0.15

0.08

0.08

0.12

0.12

0.08

0.03

0.08

0.08

0.08

0.03

0.08

0.030.08

0.08

11.50/13.50

11.50/13.50

11.50/14.50

14.00/16.00

16.00/18.00

18.00/20.00

18.00/20.00

22.00/24.00

24.00/26.00

16.00/18.00

16.00/18.00

18.00/20.00

18.00/20.0017.00/19.00

17.00/19.00

0.75

0.60

0.60

0.75

0.75

8.00/10.50

8.00/12.00

12.00/15.00

19.00/22.00

10.00/14.00

10.00/14.00

11.00/15.00

11.00/15.009.00/12.00

9.00/12.00

A1 0.10/0.30

Mo 2.0/3.0

Mo 2.0/3.0

Mo 3.0/4.0

Mo 3.0/4.0Ti 5xC min. / 0.70 max

Cb 10xC min. / 1.10 max

ASTM

Spec.

Type of

Cladding Cr Ni Other C

Nominal Chemical Composition

)(

)(

Figure 15A

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Ni and

Ni-base

Alloy

Clad

ASTM

A-265

B162

B162

B127

B168

B409

B424

B463

0.15

0.02

0.30

0.15

0.10

0.05

0.07

-

-

-

14.0/17.0

19.0/23.0

19.5/23.5

19.0/21.0

99.00 min.

99.00 min.

63.0/70.0

72.0 min.

30.0/35.0

38.0/46.0

32.0/38.0

0.25

0.25

balance

0.50

0.75

1.5/3.0

3.0/4.0

0.40

0.40

2.5

6.0/10.0

balance

balance

balance

Al: 0.15/0.60

Ti: 0.15/0.60

Mo: 2.5/3.5

Ti: 0.60/1.2

Cb: 8xC min./1.0 max.

Mo: 2.0/3.0

Material ASTMSpec. Type ofCladding C Cr Ni Cu Fe Other


Ni 200

Ni 201

Monel 400

Inconel 600

Incoloy 800

Incoloy 825

Carp. 20

)(

Figure 15B


Cu and

Cu-base

Alloy

Clad

ASTM

B-432

B152

(No. 152)

B152

(No. 102)

B402

B402

Phos.

Deoxidized

Copper

Oxygen-free

Copper

Cu-Ni

90-10

Cu-Ni

70-30

9.0/11.0

29.0/33.0

99.90 min.

99.95 min.

86.5 min.

65.0 min.

1.0/1.8

.40/.70

P:.014/.040

No residual

deoxidants

Material

ASTM

Spec.

Type of

Cladding C Cr Ni Cu Fe Other


)(

Figure 15C

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Common Backing Steels

Structural-

Carton

Pressure

Vessel-Carbon

Pressure

Vessel-Alloy

A36

A113

A283

A285

A515

A516

A537

A203

A204

A302

A387

Gr A

Gr B

Gr C

Gr A

Gr B

Gr C

Gr D

Gr A

Gr B

Gr C

Gr 55

Gr 60

Gr 65

Gr 70

Gr 55

Gr 60

Gr 65

Gr 70

C1 1

C1 2*

Gr A

Gr BGr D

Gr E

Gr F*

Gr A

Gr B

Gr C

Gr A

Gr B

Gr C

Gr D

Gr 11*

Gr 12*

Gr 22*Gr 5*

Gr 9*

400(58)

415(60)

345(50)

332(48)

310(45)

345(50)

380(55)

415(60)

310(45)

345(50)

380(55)

380(55)

415(60)

450(65)

485(70)

380(55)

415(60)

450(65)

485(70)

485(70)

553(80)

450(65)

485(70)450(65)

485(70)

553(80)

450(65)

485(70)

515(75)

515(75)

553(80)

553(80)

553(80)

515(75)

450(65)

515(75)515(75)

515(75)

250(36)

228(33)

187(27)

180(26)

166(24)

187(27)

207(30)

228(33)

166(24)

187(27)

207(30)

207(30)

221(32)

242(35)

263(38)

207(30)

221(32)

242(35)

263(38)

345(50)

415(60)

256(37)

277(40)256(37)

277(40)

380(55)

256(37)

277(40)

295(43)

310(45)

345(50)

345(50)

345(50)

310(45)

277(40)

310(45)310(45)

310(45)

21

24

28

29

30

28

27

24

30

28

27

27

25

23

21

27

25

23

21

22

22

23

2123

21

20

23

21

20

19

18

20

20

22

22

1818

18

ASTM

Spec.

Tensile

Strength

MPa (ksi)

Yield

Strength

MPa (ksi)

Elongation

in 2in, min. %

* Normalized and Tempered Condition

Figure 15D

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Figure 16 shows the appearance of a co-rolled clad material.

Figure 16. Typical Appearance of Co-Rolled Clad Material

Explosion Bonding

The explosion bonding process by Explosion Fabricators Inc. or DuPont may be used to

integrally clad plate or piping materials. It may also be used to explosively join two dissimilar

metals that are not normally weldable to one another.

The figure on page 62 of the Appendix illustrates the arrangement of the backer plate,cladding plate, and the granular explosive within the frame and a detonator. (Source:

Reference No. 3, Explosive Fabricators, Inc.) The standoff, which is the distance between the

backer and cladding plates, is also shown.

The figure on page 63 of the Appendix illustrates a schematic of the explosion cladding

process. (Source: Reference No. 3, Explosive Fabricators, Inc.)

As the explosion occurs (in a progressive pattern), it forces the clad plate against the backer

plate. The high-velocity collision of the materials produces extensive shear deformation at the

bond surface. It also results in the formation of a high velocity liquid metallic jet that cleans

the metallic surfaces by sweeping away surface films and oxides. This results in optimummetal-to-metal contact which is needed to achieve bonding.

Page 64 of the Appendix includes photomicrographs of titanium clad carbon steel, copper-

nickel clad carbon steel and Type 304L SS clad carbon steel. (Source: Reference No. 3,

Explosive Fabricators, Inc.) Note the wave-like appearance of the bond. After straightening,

the clad-substrate bond is ultrasonically inspected and unbonded areas are weld repaired.

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Roll bonding and explosion bonding may be used to cost-effectively produce clad plates for

pressure vessel shells and heads, heat exchanger channels, shells and tube sheets, and clad

nozzles for pressure vessels and heat exchangers. In most situations, it is more economical to

use clad materials instead of solid alloy material. However, the transportation costs must be

added to the costs of roll or explosion bonding. In addition, lead time is subject to clad and

plate availability. In situations where pressure is low, or where equipment size is small, it may

be more economical to use solid alloy materials. However, when solid alloys are used, such as

the 300 series austenitic stainless steels, precautions must be taken to ensure that the

equipment is not vulnerable to chloride-stress corrosion cracking. Note that stress cracking

might result in the catastrophic failure of the vessel or exchanger.

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TYPICAL WELDING PROCESSES, POSTWELD HEAT TREATMENTS, AND

THEIR APPLICATIONS

Many metallurgical phenomena are involved in welding and each one contributes to the

weld’s final properties. Some of the most important of these include: melting, freezing, solid

state transformations, thermal strains, and shrinkage stresses. Different phases and

intermetallic compounds are formed when a weld solidifies and cools, and some of these

might exhibit poor strength and ductility.

During solidification, the crystals that freeze first will have a slightly different chemical

composition than those that freeze last. It should be emphasized that these small differences in

chemical composition can result in significant differences in mechanical properties. The purer

metals, which exhibit high strength and good toughness, have higher melting points and

freeze first; impure elements such as phosphorous, sulfur, etc., which promote the formation

of weak, brittle intermetallic compounds, have relatively low melting points and freeze last.

This difference in melting points causes intermetallic compounds and impurity elements to be

segregated at grain boundaries. This can result in hot cracking, due to the poor strength and

ductility of these grain boundary materials at elevated temperatures. It should be noted that

austenitic stainless steels are particularly susceptible hot cracking. Cracking in these materials

is minimized by restricting impurity element content, weld heat input and interpass

temperature.

Weld Quality

Weld cracking can also occur as a result of shrinkage stresses developed during solidification

and subsequent cooling. These stresses combined with brittle phases can lead to cracking.

Excessive quantities of dissolved gases in the molten weld metal usually result in porosity. As

the molten metal cools, the solubility of the dissolved gases decreases. During rapid

solidification, the dissolved gases become entrapped as distinct bubbles within the solidifying

material. Porosity in welds is similar to that in castings discussed earlier.

The weldability of metals is dependent on the following interrelated factors:

• Chemical composition

• Microstructure

• Thermal history

• Restraint

• Welding processes and techniques.

To ensure weld quality, these factors must be considered and specific controls and procedures

implemented.

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Carbon and Low-Alloy Steels

In plain carbon and low-alloy steels, the carbon content is usually restricted to 0. 30 and 0. 15

weight-percent respectively. When welding carbon steels the carbon content and cooling rate

are controlled to maximize the formation of ductile ferrite phase and minimize the formation

of less ductile pearlite and cementite. In low-alloy chromium-molybdenum steels, controlled

cooling is used to minimize the formation of hard, brittle martensite.

Chromium-molybdenum steels are given PWHT to temper any martensite that is formed

during cooling.

Carbon Equivalent and Hardness

In addition to carbon (the most important element affecting weldability), the effect of other

alloying elements can be estimated by equating them to an equivalent amount of carbon. An

empirical expression that is used to determine the effect of total alloy content on hardness iscarbon equivalent (CE).

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

Generally, steels with low CE values have excellent weldability. However, the susceptibility

to underbead cracking increases when the CE exceeds about 0. 40.

The graph on page 65 of the Appendix illustrates the relationship between carbon content and

maximum hardness for steels with 50 and 100 percent martensitic microstructures. (Source:

Reference No. 4, American Welding Society Welding Handbook, Volume 4, Seventh Edition)

Preheating

Most alloy steels and heavy wall carbon steels require preheat. The reasons for preheating are:

• To avoid cold cracking in the HAZ of hardenable steels.

• To increase weld joint toughness and improve brittle fracture resistance.

• To permit any hydrogen that enters the weld metal and HAZ to diffuse out of

the weld area.

• To reduce residual stresses from welding.

• To minimize shrinkage and distortion.

• To obtain the required mechanical and physical properties in the weld.

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Preheating vs Carbon Equivalent

Low carbon (0.15 wt-% max. carbon) or mild carbon (0.15-0.30 wt-% carbon) steels do not

normally require preheating prior to welding, as their carbon contents and CEs are relatively

low. However, when carbon content exceeds 0. 30 wt-% or CE exceeds 0.43 a 93 °C (200

°F), preheat is normally applied.

Carbon steels that contain small amounts of additional alloying elements generally require

higher preheat temperatures. Figure 17 lists recommended preheat temperature ranges for

various values of CE.

Table l

Preheat of Carbon Steel With Trace Alloys

CE < 0.43 Optional

CE 0.43 - 0.60 93-204ÞC (200-400ÞF)

CE > 0.60 204-371ÞC (400-700ÞF

Figure 17

Austenitic Stainless Steels

Austenitic stainless steels are readily weldable and do not require preheat or PWHT.

However, the major problems associated with the welding of these materials aremicrofissuring and hot cracking. The susceptibility to hot cracking can be controlled by

keeping the weld as cool as possible. This is done by limiting heat input during welding and

by limiting interpass temperature to a maximum of 177 °C (350 °F).

Ferrite Control In Welds To Prevent Microfissuring

Microfissuring can be prevented by ensuring that the deposited welds contain at least 5 %

ferrite. It is important that this requirement be given to the filler metal supplier so that the

required minor adjustments to the chemical composition can be made. A maximum ferrite

content of 15 % should be specified when the component is subjected to elevated temperature

service. This is necessary to minimize weld metal embrittlement because of the ferrite tosigma phase transformation at elevated temperature.

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Sensitization

Austenitic stainless steels are susceptible to sensitization when exposed to temperatures within

the range of 399-870 °C (750-1600 °F). Due to the nature of the welding process, a narrow

band of parent material adjacent to the weld nugget (the HAZ) is susceptible to sensitization

during welding. Sensitization occurs as a result of chromium carbide precipitation in the grain

boundaries of the material. Its effect is to reduce the corrosion resistance of the material.

The risk of sensitization during welding can be minimized by specifying the low-carbon “L”

grades (Type 304L SS, Type 316L SS, etc. ) or stabilized grades (Type 321 SS or Type 347

SS) of material. For conventional stainless steels (Type 304 SS or Type 316 SS) that have

become sensitized, the weldment may be given a solution annealing heat treatment. This heat

treatment eliminates the sensitization and restores the material’s corrosion resistance.

General Welding Practices

Weld-End Preparations

Weld bevels may be prepared by flame cutting, grinding, or machining. When flame cutting is

used, materials that require preheat for welding shall be preheated in the same manner prior to

cutting. Flame-cut surfaces shall be ground back sufficiently to remove all dross, heat-

affected material, and shall be reasonably smooth and true. The material must also be cleaned

of oil, grease, moisture, scale, rust, or foreign matter prior to welding. When welding

chromium-molybdenum or other air-hardenable materials, it is good engineering practice to

MT inspect the weld bevels for cracks or laminar defects prior to welding.

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Joint Fit-Up and Cleanliness

The Welding Procedure Specification (WPS) usually contains a sketch or drawing of the joint

with information on the angle(s) of bevel(s) and dimensions of the land and root opening. An

example of this is shown in Figure 18.

Figure 18

Prior to welding it is important to clean the surface as mentioned above, and to prepare and

fit-up the joint in accordance with the sketch or drawing.

Filler Metal Control (Low-Hydrogen Practices)

Filter metal control is an extremely important element of any fabrication process. It ensuresthat incorrect filler metals are not used during welding and that the completed joint has the

desired chemical composition and mechanical properties.

Low-hydrogen electrodes (an example is E-7018) are used on medium carbon and low-alloy

steels to prevent underbead and delayed cracking in the weld HAZ. To ensure that the

electrodes serve their intended purpose it is important that moisture pickup in the electrode

coating be prevented. Electrodes must be stored in heated ovens at temperatures

recommended by the manufacturer until they are used.

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Weld Defects

The ASME B&PV Code Section IX defines a defect as “a discontinuity or discontinuities

which by nature or accumulated effect (for example, total crack length) renders a part or

product unable to meet minimum applicable acceptance standards or specifications. This term

designates “rejectability.”

Defects can be linear or rounded. Cracks, lack of fusion (sidewall or interbead), and lack of

penetration are linear weld defects, while porosity, slag, or tungsten inclusions are considered

to be rounded defects. Section VIII, Div. 1 defines the acceptance criteria for the various

types of defects in terms of size, aggregate length, and percentage of thickness.

Appendix I - Appendix I, located on page 66 in the Participant Module Appendix, illustrates

the maximum acceptable rounded indications in typically clustered, assorted, and randomly

dispersed configurations. (Source: Reference No. 5, ASME Boiler and Pressure Valve Code)

Nondestructive examination (NDE) methods used to inspect welds for defects include visual,

dye penetrant, magnetic particle, radiography, and ultrasonic flaw detection.

Welding Processes

There are many welding processes that can be used to join metals. The American Welding

Society (AWS) has developed an overview of joining processes and their use on various

materials as shown in the table on page 67 of the Appendix. (Source: American Welding

Society Handbook, Volume 1, Eighth Edition) The legend below the Table explains the code

used to designate each welding process (for example, SMAW designates Shielded Metal Arc

Welding).

Arc Welding

Arc welding is the most important joining process. It is widely used in the petroleum, power

generation, and manufacturing industries. In this process an electric arc is used as the source

of heat to melt and join metals. The arc is struck between the workpiece and the tip of an

electrode. The electrode may be a consumable filler wire or rod, or it may be a

nonconsumable tungsten electrode. When a nonconsumable electrode is used, a separate

consumable filler wire is required.

The following sections contain a review some of the more commonly used welding processessuch as SMAW, GTAW, GMAW, AND SAW.

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Shielded Metal Arc Welding (SMAW)

Shielded Metal Arc Welding (SMAW) is a manual process that is also known as “stick”

welding. Heat from the arc between the electrode and workpiece melts the electrode and

parent metal to form a molten weld pool. The weld pool is protected from the effects of the

atmosphere by a liquid slag and shielding gas. The slag and the shielding gas are formed

when the flux coating on the electrode melts and vaporizes. The SMAW process is used

extensively for all manner of shop and field fabrication. Weld quality, while usually

acceptable, is inferior to that of welds made with other processes.

Figure 19 is an illustration of the SMAW process.

Figure 19. Shielded Metal Arc Welding Process

Note the electrode core wire, outer flux covering, weld pool, protective slag, shielding gas,

and the parent metal. SMAW is probably the most popular welding process. It is used

extensively for both shop and field welding. It exhibits maximum flexibility and can be used

to weld a variety of different metals over a wide range of thicknesses for minimal capital

investment. The SMAW process does have several limitations. These include relatively poor

productivity and the need for highly skilled welders. Poor productivity is primarily due to the

frequent starting and stopping associated with changing electrodes. In addition, the need to

deslag each pass and grind finished welds also reduces productivity.

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Gas Tungsten Arc Welding (GTAW)

The GTAW process is a manual process that is also known as TIG or heliarc. It is primarily

used to fabricate critical service equipment. It should be noted that welds in this type of

equipment must exhibit superior quality and are usually subjected to radiographic

examination. GTAW is used to deposit the root and first hot pass when joining alloy fired

heater tubes, and carbon and alloy steel piping materials. It is also used for depositing the root

and first hot pass of closure seams in vessels and heat exchangers when the joint is accessible

from only one side.

Figure 20 shows a schematic of the GTAW process.

Figure 20. Gas Tungsten Arc Welding

Heat is provided by the arc between the nonconsumable tungsten electrode and the parent

metal. The arc melts the parent metal to form the weld pool, and filler metal is added to the

weld pool to fill the joint. Shielding gas is required to protect the weld pool from the effects of

the atmosphere. The shielding gas can be argon, helium, or a mixture of both. Advantages of

the GTAW process include: superior quality welds, minimum spatter, and produces no slag;consequently, very little postweld cleaning is required. The major disadvantages are that the

welder must be highly skilled and productivity is relatively poor.

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Gas Metal Arc Welding (GMAW)

Gas Metal Arc Welding (GMAW) is a semiautomatic process that is also known as MIG. It is

used extensively for shop and field fabrication of carbon and alloy steel pressure vessels, heat

exchangers, tanks, and piping. Weld quality is adequate for most fabrications. However, it

should be noted that GMAW weld quality is superior to that of SMAW welds, but inferior to

that of GTAW welds.

As shown in Figure 21, the MIG process uses a solid wire consumable electrode. Heat is

generated by the arc between the consumable wire and the workpiece.

Figure 21. Gas Metal Arc Welding Process

The arc melts the workpiece and wire to form the molten weld pool. Argon, helium, carbon

dioxide, or mixtures of these gases shield the weld pool from the effects of the atmosphere.

Note that the shielding gas is coaxially fed with the consumable wire through the welding

torch. The high current “spray transfer” mode is used for high deposition rates (to maximize

productivity) in the flat or horizontal position. “Globular” and “short circuiting” transfer

modes are used for out-of-position welding (vertical and overhead) and for welding thin

materials. The “short circuiting” mode is susceptible to lack of fusion defects or “cold fold”

when used for full thickness welding on materials over 9. 5 mm (3/8 in) thick. GMAWadvantages are: good productivity due to high deposition rates, minimal weld finishing due to

the lack of flux or slag, and the ability to make out-of-position welds in steel and nonferrous

alloys. In addition, the process requires less welder skill than SMAW. Disadvantages include

the lack of control over filler metal addition, the inability to use the process in drafty

conditions because of shielding concerns, and the need to perform welding in close proximity

to the power source and wire feed system.

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Submerged Arc Welding (SAW)

The Submerged Arc Welding (SAW) process is a machine welding process that is monitored

by an operator. It is sometimes referred to as “sub arc”. It is used extensively for shop

fabrication of pressure vessels, heat exchangers, and longitudinally welded pipe. The process

is particularly cost-effective when used to fabricate large diameter, heavy wall equipment.

SAW can be used to join carbon, low-alloy, and high-alloy steels. Weld quality is generally

acceptable, but weld HAZ toughness can be low because of excessive grain growth during

welding due to high heat input. However, it is important to recognize that improved HAZ

toughness properties can be achieved by controlling heat input.

Figure 22 illustrates the working of the SAW process.

Figure 22. Submerged Arc Welding

Heat is generated by the arc between the consumable wire and workpiece. The arc is

submerged beneath a bed of granular flux and melts the parent metal, wire, and some of the

flux to form a weld pool protected by molten flux. The process is fully automated in that the

consumable welding wire (filler wire) and flux are continuously fed at constant, controlledrates. Advantages of the SAW process include: good productivity due to high deposition

rates, consistently satisfactory weld quality, and smooth and uniform weld finish. In addition,

since the process is readily automated it requires minimal operator skill. Limitations include:

the time and effort required to ensure adequate joint fit-up, the need to clean slag from the

weld metal, and the potential for low HAZ toughness as discussed above. In addition, the

process is limited to welding in the flat and horizontal positions.

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Weld Metal Overlays

By using appropriate welding procedures and filler metals that match the chemical

composition and mechanical properties of the parent metal, a weld metal overlay may be

applied to restore the required thickness of a pitted or corroded pressure vessel head, shell,

valve body, pump casing, etc. Weld overlay followed by grinding or machining may be used

to restore the corroded surface of a pipe or valve flange.

Corrosion-Resistant Weld Metal Overlays

An alternative to roll or explosion bonded cladding, corrosion protection can also be provided

by corrosion-resistant weld metal overlays. These are applied by depositing stringer beads of

corrosion-resistant weld metal onto the surface of a carbon or low-alloy steel backer plate.

The welding consumable may be a coated electrode, solid wire, strip, or flux-cored wire.

Shielding, using external shielding gases, fluxes, or both, are required to protect the weld

puddle from the atmosphere and ensure contaminant free sound deposits. The most frequentlyused overlay materials are: 300 series austenitic stainless steels, Inconel, Monel, nickel, and

copper-nickel. The typical microstructure of a corrosion-resistant weld metal is illustrated in

Figure 23. Note the difference in microstructure between the overlay and backer material.

Figure 23. Microstructure of Corrosion Resistant Weld Metal Overlay

The most important variables to consider when evaluating a weld overlay process are: dilution

of the alloy material, overlay thickness, and deposition rate. To increase productivity, the

objective is to deposit the required thickness of overlay in the shortest possible time with

minimum dilution of the weld deposit by the backing material. For example, when overlaying

stainless steel (SS) on carbon steel dilution of the SS, deposit by carbon and iron must be

minimized to maintain the intergranular and general corrosion resistance of the SS. Generally,

dilution should not exceed 15 %.

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In Figure 24, the effect of dilution on a SS weld deposit by carbon from the substrate is

illustrated by the heavy formation of carbides at the overlay substrate interface.

Figure 24. Effect of Dilution on a Stainless Steel Overlay

In actual practice, dilution is minimized by minimizing heat input and by making the first pass

with a filler metal that has a surplus of alloying elements. For example, the first pass of an

18Cr-8Ni SS overlay is made with a nominal 25Cr-12Ni filler metal such as E309.Subsequent passes are then made with an 18Cr-8Ni filler metal such as E308.

Overlays are usually made using the SMAW, GTAW, SAW, and GMAW processes with

special modifications and procedures to minimize dilution. SMAW and GTAW require

minimum capital investment, but exhibit relatively poor productivity, and consequently are

not cost-effective for overlaying large surfaces. However, both processes are very flexible and

can be used in either the shop or field, and are capable of producing sound deposits. SAW is

automated, can achieve high deposition rates, and is cost-effective for overlaying large

surfaces such as pressure vessels. Strip filler metal is used to further increase deposition rates.

SAW requires a high capital investment and is used primarily in the shop. GMAW can be

used in the shop or field, offers high deposition rates, and like SMAW and GTAW can beused for out-of-position overlay welding.

Defects in Weld Overlays

The most important defects in overlays are: microfissures, cracking due to brittle structure,

slag inclusions, porosity, and disbonding (separation of the overlay from the substrate).

Microfissuring and cracking at the overlay/substrate interface are illustrated in Figures 25 and

26.

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Figure 25. Overlay Microfissuring

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Figure 26. Cracking at Overlay-Substrate Interface

Microfissures may be found in austenitic stainless steel overlays, or when a fully austenitic

material has melted and resolidified. During the solidification of austenitic materials, sulfur, phosphorous, and tramp metallic and nonmetallic elements diffuse to the grain boundaries and

form low-melting point intergranular films, which remain molten below the solidus

temperature. As solidification continues and the material contracts, internal tensile stresses are

developed between the grains. Fissuring occurs when the partially solidified grain boundaries

can no longer accommodate the stresses.

To obtain fissure-free austenitic welds, filler metals should be selected to produce a weld

deposit with 3 to 12 percent ferrite. The ferrite phase acts as a “sponge” and absorbs the

impurity elements, minimizing the formation of low-melting point films. The Schaeffler or

other special diagrams can be used to select proper filler metals and to estimate the overlay

ferrite content as a function of chemical composition. (See Figure 27) Severn and Magnagages are used to check the ferrite content of production overlays. It should be noted that

these gages must be calibrated prior to use.

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21

20

19

18

17

16

15

14

13

12

11

0

16 2726252423222120191817

Auatensive

W R C F

e r r i t e

N u m b e r

S c h a e f f l e r A + M L i n e

0 % F e r r i t

e

2 % F e r r i t

e

4 % F e r r i t

e

5 % f e r r i t

e

6 % F e r r i t

e

7. 6 % F e

r r i t e

9. 2 % F e

r r i t e

1 0. 7 %

F e r r i t

e

1 2. 3 %

F e r r i t

e

1 3. 8 %

F e r r i t

e

0

2

4 6

8

1 0 1 2

1 4

1 6

1 8

Austenite + Ferrite

Chromium Equivalent = % Cr + % Mo + 1.5 x % Si + 0.5 x % Cb

N i c k e l E q u i v a l e n t = % N

i = 3 0 x % C

+ 3 0 x % N

+ 0 . 5 x % M

n

Figure 27. WRC Delta Ferrite Diagram

Cracking may occur when a brittle structure is formed in the overlay fusion zone. The brittle

structures are caused by:

• Carbon diffusion and the formation of carbide precipitates in the stainless steel.

• Formation of martensite as a result of excessive dilution of the stainless steel by

the carbon or low-alloy substrate.

• Formation of a brittle intermetallic phase in the stainless steel.

• Formation of brittle phases in copper alloy overlays, due to excessive iron

dilution from the carbon or low-alloy substrate.

Weld overlay/substrate disbonding may be due to lack of fusion and penetration during

welding. It may also be caused by shear stresses at the interface. The stresses are attributed to

differential thermal expansion during thermal cycling.

Overlay defects, such as slag inclusions and porosity, are due to poor shielding, improper

welding techniques, lack of proper interbead cleaning, and poor filler metal handling and

control techniques.

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Postweld Heat Treatment (PWHT)

Postweld heat treatment (PWHT) may be applied to an entire welded assembly, such as a

complete pressure vessel. It may also be applied locally, such that only the weld area is

heated. An example of local PWHT is the field installation of piping.

PWHT is performed for the following reasons: to relieve residual welding stresses, improve

toughness, improve corrosion resistance, reduce weld hardness, and reduce the susceptibility

to stress corrosion cracking. Some of the more common types of heat treatments are:

Stress Relief (SR)

In a stress relief heat treatment, the structure is uniformly heated to a suitable temperature

below the critical range of the base metal, held for a certain time period, then uniformly

cooled. As the name implies, the main purpose is to reduce residual welding stresses. In many

situations stress relief also reduces susceptibility to stress corrosion cracking.

Stress relief temperature ranges for a number of materials are given in Figure 28.

Typical Stress Relief Temperatures

Material Temperature

C F

Carbon Stee l565-648 1050-1200

Low Alloy Steel 620-788 1150-1450

Admiralty Brass 288-343 550-650

Copper Nickel Alloys 246-315 475-600Monel Alloys 538-648 1000-1200

Nickel Alloys 538-704 1000-1300

Aluminum 315-371 600-700

(Austenitic Stainless Steel) 871-899 1600-1650

Figure 28

Holding times at the stress relief temperatures for carbon and low-alloy steels are based on the

thickness of the structure. Usually one hour per inch of thickness is sufficient. For thin

materials a minimum time, usually 1/4 to 1/2 hour, is usually specified.

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Annealing

In annealing the structure is uniformly heated approximately 28-55 °C (50-100 °F) above the

critical temperature range to transform the microstructure to austenite. The material is then

uniformly slow cooled to produce the softest possible structure.

Annealing temperature ranges for various materials are listed in Figure 29.

Typical Annealing Temperatures


C F

Carbon Steel 871-927 1600-1700

Low Alloy Steel 843-927 1550-1700

Type 405 Stainless Steel 648-816 1200-1500Type 410 Stainless Steel 927-1010 1700-1850

Admiralty Brass 426-648 800-1200Copper Nickel Alloys 648-816 1200-1500

Monel 871-982 1600-1800

Nickel Alloys 927-1121 1700-2050

Cast Iron 788-899 1450-1650

Aluminum 77 5- 82 5 77 5- 82 5

Figure 29

Normalizing

In normalizing, ferritic steels and some cast irons are heated approximately 28-55 °C (50-100

°F) above the critical temperature range (similar to annealing) and slow cooled in still air. The

purpose of normalizing is to refine a coarse grain microstructure, homogenize the structure,

reduce stresses, and eliminate hard zones.

Normalizing temperature ranges for various materials are listed in Figure 30.

Typical Normalizing Temperatures


C F

Carbon Steel 871-927 1600-1700Low Alloy Steel 843-927 1550-1700

Cast Iron 885-927 1625-1700

Figure 30

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Tempering

In tempering, carbon steel and ferritic alloys are heated to a temperature below the critical

range to reduce the hardness of the martensite formed during the normalizing or quenching

heat treatment. By softening the microstructure (tempering the martensite), material toughness

is significantly improved.

Tempering temperature ranges for various steels are listed in Figure 31.

Typical Tempering Temperatures


C F

Carbon Steel 204-648 400-1200

Low Alloy Steel 315-677 600-1250

Cast Iron (Gray) 315-760 600-1400

Martensitic Stainless Steel (4XX) 232-371 450-700565-788 1050-1450

Figure 31

Quenching and Tempering

Quenching and tempering heat treatments of carbon and low-alloy steels are used to produce

the best possible mechanical properties and to optimize impact properties (toughness). Thesteel is heated to approximately 55 °C (100°F) above the critical range [e.g., 893-927 °C

(1550-1700 °F)], then quenched in water or oil to ambient temperature. The steel is then

tempered as described above.

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Solution Annealing

Solution annealing heat treatment is applied to austenitic stainless steel and nickel-base

austenitic alloys. Examples of such materials are Type 304 SS and Inconel 625. The purpose

of the heat treatment is to eliminate sensitization and restore corrosion resistance. The

material is heated to approximately 1065 °C (1950 °F) to dissolve grain boundary chromium-

carbides, held at temperature for about 1 hour per inch of thickness, and then water quenched.

For thinner sections, cooling in air may be sufficient. It should be noted that the solution

annealing temperature depends upon the specific alloy.

Figure 32 lists solution annealing temperature ranges for various materials.

Typical Solution Annealing Temperatures


C F

Austenitic Stainless Steel 1038-1093 1900-2000

(Type 304, 316, 321, 347)

Precipitation-Hardening Stainless 1024-1052 1875-1925

Steels (austenitic)

Monel K-500 982-1038 1800-1900

Nickel Alloys 1038-1149 1900-2100

Inconel X750 and Hastalloy Alloys 1149-1204 2100-2200

Figure 32

Stabilize Annealing

Stabilize annealing heat treatment is applied to the stabilized grades of austenitic stainless

steel (Types 321 and 347 SS). The purpose of the heat treatment is to maximize corrosion

resistance by preventing sensitization during welding. The steel is heated to approximately

871-899 °C (1600-1650 °F), held for 4-16 hours, and cooled. It should be noted that since the

material has been stabilized, the rate of cooling after heat treatment is not important.

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Intermediate Stress Relief/Hydrogen Outgassing

An intermediate stress relief or hydrogen outgassing heat treatment may be applied to carbon

and low-alloy steels that have picked up hydrogen during service or as a result of welding.

The purpose of the heat treatment is to remove or outgas the hydrogen from the parent

material and welds to prevent delayed cracking. In some situations this heat treatment is

necessary before attempting weld repairs or welded alterations.

Heat treatment temperature ranges and holding times are listed in Figure 33.

Typ ca Interme ate Stress Re e Hy rogen Outgass ng Treatment


Carbon Steel and 600°C (1100°F) for 15 minutes, slow cool. Low Alloy Steel and Low-Alloy Steel

or 325-400°C (620-750°F) for 2 hours minimum.

or 260-300°C (500-570°F) for 4 hours minimum.

Figure 33

PWHT — Fabrication Codes and Environmental Requirements

In the petroleum industry new equipment is fabricated and given PWHT in accordance with

the requirements of the following codes:

• ASME B&PV Code-Section I Power Boilers and Section VIII Div. 1.

Pressure Vessels (including heat exchangers)

• ASME/ANSI B31. 1 Steam Power Piping

ASME/ANSI B31. 3 Chemical Plant and Petroleum Refinery Piping

• API (American Petroleum Institute) Standard 620-Design and Constructionof Large, Welded, Low-Pressure Storage Tanks

• API Standard 650-Welded Steel Tanks For Oil Storage

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To reduce the susceptibility to stress corrosion cracking, the petroleum industry generally

considers PWHT of carbon steel equipment in the following services:

• Caustic

• Amine

• Wet H2S

• HF

• Deaerators

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REFERENCES

1. American Iron and Steel Institute, Steelmaking Flowlines, Washington, D. C. ,

USA

2. American Iron and Steel Institute, Steel Processing Flowlines, Washington, D.

C. , USA

3. Sharp, William F. , “Explosion Clad Metals”, Explosive Fabricators, Inc.

4. American Welding Society, American Welding Society Handbook, Miami,

Florida, USA, 1982.

5. ASME Boiler and Pressure Valve Code, Section IX.

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