guidelines for specification, welding and inspection of stainless alloy piping

7/27/2019 Guidelines for Specification, Welding and Inspection of Stainless Alloy Piping

1/19

GUIDELINES FOR SPECIFICATION, WELDING AND INSPECTION OF STAINLESS ALLOY PIPING

Craig Reid, P.Eng.

Bacon Donaldson Consulting Engineers

Richmond, B.C., Canada V7A 4V4

ABSTRACT

____________________________________________________________________________________________

The Corrosion & Materials Engineering Committee (C&ME) of the Tappi Engineering Division has developed two

technical information papers (TIPS) dealing with stainless alloy piping. The first, TIP 0402-24 "Guide to the use of

stainless steel pipe specifications" was issued in late 1998. The second, TIP 0402-26 "Guidelines for welding and

inspection of stainless alloy piping" was issued in early 1999. Both of these TIPS were sponsored by the Metals

Subcommittee of C&ME to provide guidance to mill engineers involved in ordering and installation of stainless

alloy (stainless steel and nickel base alloys) piping for a range of services including paper machine stock lines,

bleach plant, and pressure piping throughout the mill. They should prove useful in developing specifications and

evaluating proposals. This paper reviews the two TIPS.

____________________________________________________________________________________________

INTRODUCTION

In the Metals Subcommittee open forum at the 1991 Tappi Engineering Conference, a discussion was held on the

quality of field welded circumferential joints in paper machine stock line piping and TMP pressure piping, made

with flux protection of the weld roots instead of root purging. It was reported that fatigue cracking had occurred in

several circumferential butt welds after short service. Examination of cross sections showed the cracking had

initiated at severe lack of penetration (Figure 1). It was decided that a Technical Information Paper (TIP) on

welding and inspection of stainless steel piping would be useful to provide background for mill engineers involved

with the installation and maintenance of stainless steel pipe in pulp and paper mill services. The primary focus was to

be on manual circumferential butt welds made during shop spooling and field erection of stainless steel pipelines.

However, it became apparent that information on the specification of the pipe itself was also desirable and that nickelbase alloys as well as stainless steels should be included. Two task groups were initiated:

CA 910402.04 Specification of stainless alloy piping CA 920402.03 Fabrication and inspection of stainless alloy pipingThese Task Groups completed their assignments in 1998 and 1999 respectively with the issue of:

TIP 0402-24 Guide to the use of stainless steel pipe specification TIP 0402- 26 Guidelines for welding and inspection of stainless alloy pipingThe two new TIPs are not intended to serve as specifications, but to provide background information for those

preparing purchase inquiries and specifications for stainless alloy piping systems, and for evaluating proposals for

the installation of such systems. The TIPs could be considered "awareness" documents to help the mill engineer ask

the right questions. As such, they should be used in conjunction with "Recommended specifications for stainless steel

piping, fittings and accessories for the pulp and paper industry, 3rd

edition1, issued in 1986 by the M&ME

Committee


2/19

GENERAL CONSIDERATIONS

Codes and Standards

ANSI/ASME B31.3 Process Piping2

can be applied to pulp and paper industry stainless alloy piping. This code

sets minimum requirements for materials, design, fabrication, and inspection of process pressure piping in a range of

alloys and non metals.

B31.3 is intended to cover process piping including steam and process chemicals, where pressure is greater than 103

kPa (15 psig) regardless of temperature. It may be mandatory in some jurisdictions or it may be referred to in

specifications for stainless alloy piping. Simplistic specification statements like fabrication and inspection shall

conform to the ASME Code (ANSI/ASME B31.3) should be avoided, because confusion over specific requirementscan arise. For example, inspection requirements and acceptance criteria in B31.3 depend on the owners designation

of the fluid service category involved. These are discussed in a later section of this paper.

Alloy Selection

The stainless alloy pipe, fittings, and weld filler should be selected to provide an adequate margin of safety against

general and localized corrosion. Information on corrosion concerns and alloy selection is available in references 3 -5.

Depending on the stainless alloy involved, the corrosion resistance of a weld and the adjacent base metal can bereduced in relation to the unwelded base metal:6,7

the corrosion resistance of the weld filler metal may be reduced by microsegregation of molybdenum and other

alloying elements during solidification of the filler.8

the corrosion resistance of stainless steel weld filler metal to inhibited hydrochloric acid cleaning solutions can

be reduced by the formation of ferrite networks. The ferrite phase is corroded preferentially by hydrochloric

acid.9,10

the corrosion resistance of the heat affected zone in the base metal can be reduced by the precipitation of

undesirable phases or, when welding the 6% molybdenum and similar highly alloyed austenitic stainless steels,by the formation of an unmixed zone where the base metal is melted, but not mixed, with the weld filler.

8, 11

the corrosion resistance of the base metal may be reduced by formation of surface oxide scale as a result of

welding heat and inadequate protection of the weld root from oxidation (heat tint).12-14

Weld Filler Metal

To address weld metal microsegregation, a more highly alloyed (overmatching) filler metal may be used for some

services. Overmatching filler metal is most often required in acidic or near neutral oxidizing chloride environments,

in which the resistance to localized corrosion of the weld metal must at least match that of the wrought base metal.For example, nickel-chromium-molybdenum (nickel based) filler is commonly used for welding 6% molybdenum

austenitic stainless steels.8

Overmatching filler is generally not required in kraft liquor service or in paper machine

white water service.

Where an overmatching filler metal is used, it may be necessary to limit how much the filler metal is diluted by the

base metal to be sure the intended overmatching is achieved. With butted joints, or joints with narrow root gaps, so

li l fill b dd d h h l f hi i hi d F h 6% l bd i i i l


3/19

Table I. Weld filler metals for stainless alloys

Alloy Matching filler metal Overmatching filler metal

304L austenitic stainless steel AWS E or ER308L not generally used316L austenitic stainless steel AWS E or ER316L 317L is sometimes used

316L with 2.5% Mo 317L

317L austenitic stainless steel AWS E or ER 317L 904L is sometimes used

Alloy 20 AWS E or ER320

904L AWS E or ER385 AWS E or ERNiCrMo-3

6% molybdenum austenitic stainless

steels

matching filler not generally

recommended or available

AWS E or ERNiCrMo-3

high nitrogen, high molybdenum

superaustenitic stainless steels

matching filler not generally

recommended or available

appropriate fillers are available from alloy

suppliers2304 duplex stainless steel matching fillers may be available from

some alloy suppliers

AWS E or ER2209

2205 duplex stainless steel AWS E or ER2209 appropriate fillers are available - consult

alloy suppliers

2505 and 2507 type duplex stainless

steels

AWS E or ER2553 or

matching fillers available from alloy

suppliers

AWS E or ERNiCrMo-3 or as

recommended by alloy suppliers

nickel base alloys consult alloy supper for appropriate fillers

Weld Heat Affected Zone (HAZ)

The formation of phases or structures that can reduce the corrosion resistance of the base metal heat affected zone

depends on alloy chemistry and welding procedure . For austenitic stainless steels and nickel base alloys, heat input

should be minimized. For duplex stainless steels, the heat input should be controlled within an optimum range .15,16

Surface Oxides

The negative effects of weld-related oxide scales and heat tint are best eliminated by preventing their formationthrough adequate inert gas purging of the weld root. Where oxide scales and heat tint form they can be removed by

pickling with pastes or fluids containing nitric and hydrofluoric acids, or by other surface cleaning methods including

abrasive blasting and sanding. Acid pickling usually provides the best results but is seldom practical for field welds

because of access, safety, and environmental considerations. Hand wire brushing does not remove heat tint. Power

wire brushing appears to, but in fact only smears it.

Requirements for prevention and/or removal of oxide scale and heat tint should be based on an assessment of the

costs and benefits in a given application. For example, heat tint is not a concern in alkaline liquor service.

Weld Quality

Welds in stainless alloy piping should, in general, meet the quality requirements of ANSI/ASME B31.3. Special or

supplementary requirements may be warranted for some service conditions depending on:

corrosivity loading conditions - e.g. vibration, hammer, stock stick-slip phenomena, etc. ANSI/ASME B31.3 provides


4/19

site for fiber/deposit build up which could cause crevice corrosion or release inclumps causing process problems (e.g. headbox approach piping)

lack of fusion (embedded) stress raiser for crack initiation porosity, slag (if aligned may be interpreted as a linear defect by some codes) if embedded are stress raisers if exposed on the process side are stress raisers and sites possible sites for

corrosion

poor root profile (excessive reinforcement, steep merging angle to base metal) stress raiser for crack initiation site for fiber hang up irregular oxidized root stress raiser for crack initiation site for fiber/deposit build up site for corrosion in some servicesGUIDE TO THE USE OF STAINLESS STEEL PIPE SPECIFICATIONS

This TIP summarizes ASTM/ASME specifications commonly used for stainless steel pipe. Nickel base alloys werenot included. The TIP discusses general considerations in the specification of stainless steel pipe and presents three

summary tables based on the method of manufacture of the pipe, i.e. centrifugally cast, seamless, and welded.

The tables in the TIP compare specification requirements which govern the:

corrosion resistance of the pipe - filler metal, heat treatment dimensional tolerances of the pipe nondestructive testing performed on the pipe - hydrotest (HT) and optional or supplementary tests that may be

specified, including corrosion tests.

The corrosion resistance can be affected by the filler metal (if any) used for the longitudinal weld in the pipe, and by

the heat treatment (if any) and cleaning given the pipe after welding. Surface finish can influence both corrosion

resistance, and also the performance of the pipe in special applications like paper machine head box approach piping.

Surface finishes are generally better with cold worked products.

Dimensional tolerances can influence the field fit up required during shop and field welding. ASTM A530 Standard

specification for general requirements for specialized carbon and alloy steel pipe sets basic dimensional

requirements but the specific standard should also be checked for more restrictive requirements on wall thickness,

outside diameter and ovality. A530 governs straightness through two paragraphs:

14.1 the finished pipe shall be reasonably straight

14.2 for metal arc welded pipe the maximum deviation from a 3 m (10 foot) straightedge placed so thatboth ends are in contact with the pipe shall be 3.2 mm (1/8 inch). This is pro-rated for lengths shorter

than 3m.


5/19

Significant cost for routine maintenance - this condition could indicate a problem such as poor alloy choice forthe actual service, poor pipe manufacture with insufficient inspection, or improper installation. Consideration of

alternative manufacturing methods of inspections may be warranted.

Tolerable cost for maintenance with low risk - pipe purchased to one of the basic specifications will likely beadequate.

The welded pipe table in the TIP has been adapted as Table II of this paper to list the current ASTM specifications

for welded austenitic stainless steel pipe (which is commonly used in pulping, bleaching, and paper machine

applications). The welded pipe table in the TIP also includes duplex , ferritic, and martensitic stainless steel pipe.

Inspection of the tables in the TIP should be the first step in identifying and specifying the requirements judged

necessary for stainless steel pipe to offer the expected service performance. Once the appropriate specification(s)

have been identified, the current issue(s) should be obtained and reviewed before they are referenced in contractdocuments or purchase orders. The titles of ASTM specifications are keyword searchable on the ASTM website at

www.astm.org. This is a good way to determine the current issue of a specification, and to search for other relevant

specifications. Individual ASTM specifications can be purchased on the website and many are available for

downloading or transmission by facsimile.

The ASTM specifications for stainless steel pipe each include an "Ordering Information" section, which is intended

to guide the user through the choices embedded in these specifications. The ordering information can also be

supplemented by additional requirements identified by the purchaser in the purchase order. Ordering pipe by

referring simply to the ASTM specification and the alloy type leaves options open to the pipe supplier, which can

affect the suitability of the ordered pipe for the intended service.

Example Use of the TIP

Suppose thin wall (gauge), large diameter, 6% molybdenum or equivalent austenitic stainless steel pipe is required

for a chlorine dioxide bleaching stage. The circumferential butt joints will be made with the nickel base filler, AWS

ENiCrMo3. An overmatching filler metal is required to ensure the welded joints match the corrosion resistance of

the pipe base metal (see later section). Some percentage of the circumferential butt welds will be inspected by

radiography according to the provisions of ANSI/ASME B31.3 "Process piping ".

2

Seamless thin wall pipe is generally not available and review of Table II indicates the basic purchase options are:

pipe welded without filler (autogenous welding) followed by a solution annealing heat treatment - A312, A813,A409

pipe welded with filler, without subsequent solution annealing (as-welded) - A358, A409, A778To ensure competitive pricing, it is planned to ask for quotations in both solution annealed and as-welded pipe.

However, pipe supplied as-welded should be made with AWS ENiCrMo-3 filler for the longitudinal welds so the

specification used must allow for this. It is also desired that longitudinal welds in both annealed and as-welded pipebe capable of passing radiographic inspection, since portions of the longitudinal welds in pipe or fillings will likely

be radiographed when the circumferential butt joints are radiographed. In fact, ANSI/ASME B31.3 paragraph

341.4.1(b) requires that radiographs of circumferential joints maximize coverage of the intersections with

longitudinal joints - at least 38 mm (1 ) of the longitudinal joint is to be examined. Moreover, the acceptance

criteria for longitudinal welds in ANSI/ASME B31.3 are more stringent than those for circumferential welds for

example incomplete penetration, undercutting, and surface porosity or exposed slag are not acceptable while for


6/19

Review of A778 indicates the 6% molybdenum austenitic stainless steels are not included so this specification is

dropped from consideration.

For autogenously welded pipe A312, A409, and A 813 appear to be equivalent from Table II except for a differencein OD tolerance. All three specifications require solution annealing and allow radiographic testing (RT) as a

supplementary requirement. Review of the specifications is needed to determine the requirements for radiography,

which are generally left to agreement between the pipe manufacture and the purchaser.

A409 allows filler metal to be used in welding, but this would be unlikely as matching filler is not generally

available for the 6% molybdenum grades, and there is no point in using ENiCrMo-3 filler if the pipe is to be solution

annealed.

For as welded pipe, A358 and A409 both require ENiCrMo-3 filler in 6% molybdenum pipe. As-welded delivery is

optional for A358 so should be stated explicitly in the inquiry and purchase order to avoid confusion. Both the use offiller and as-welded delivery are optional for A409 pipe. Thus the inquiry and purchase order should state explicitly

that AWS ENiCrMo-3 filler metal be used and the pipe be supplied as welded. It would also be prudent to require

positive materials identification (PMI) of the pipe and weld filler with an appropriate minimum molybdenum

requirement for the weld filler. Longitudinal welds in pipe are generally made by automatic electric arc processes

and so little weld filler can be used that the welds approach the composition of welds made without filler. Such

near autogenous welds would be prone to pitting corrosion in chlorine dioxide service. This chemistry requirement

need not necessarily increase the cost of as-welded pipe if it is clearly stated at the inquiry stage.

A358 allows for radiography but reference to Table II show the class must be specified - class 2 does not require

radiography, class 5 requires spot radiography, and classes 1,3, and 4 require 100% radiography of the weld. A409allows radiography as a supplementary requirement but, as noted previously, leaves the requirements up to

agreement between the manufacturer and purchaser.

A358 has tighter tolerances than A409 for OD, wall thickness, and ovality so may be more expensive. However, it

will also be easier to fit up for welding, and lower welding and fit up costs might offset a higher first cost for the

pipe itself. Straightness is governed by ASTM 530

When as-welded pipe is ordered, it is also important to address the surface finish. A409 states the pipe is to be free

of scale and contaminating iron particles, but does not state the means of surface finishing. Pickling, or blasting are

mentioned as possibilities. In the case of chlorine dioxide service, 6% molybdenum stainless steel pipe wouldusually be ordered pickled and passivated. The effects of surface finish are discussed in more detail in a later

section. There may be little gain in requiring pickling and passivation of the pipe if the circumferential weld seams

are made to a lower quality standard.

The specification of 304L, 316L, or 317L austenitic stainless steel pipe involves similar considerations. Generally

as-welded pipe possesses adequate corrosion resistance if the base alloy appropriate for the service has been

selected. Overmatching filler is generally not needed.

FITTINGS

TIP 0402-24 does not address fittings and review of the ASTM specifications shows they are different enough to

warrant separate consideration. Table III lists ASTM specifications for fittings.

Table III. ASTM standard specifications for stainless steel fittings

A182-96 Forged or rolled alloy-steel pipe flanges, forged fittings, and valves and parts for high-temperature


7/19

For example, all A403 fittings are heat treated and are supplied to two basic classes: CR which does not require

nondestructive testing and WR which does. WR fittings are further subdivided as:

WP-W 100% radiography of all welds made with the addition of filler metal, but ultrasonic testing may besubstituted for welds made by the fitting manufacturer

WP-WX 100% radiography of all welds WP-WU 100% ultrasonic testing of all welds

A778 requires only visual examination. Hydrostatic testing and radiography are not required and are not included

as supplementary requirements. It is stated that welds shall be full penetration with or without the addition of filler

metal.

ANSI/ASME B31.3 does not include as-welded pipe and fittings which would thus be considered as unlisted

components per paragraph 302.2.3, which states that unlisted components which conform to a published

specification or standard may be used provided the designer shall be satisfied that composition, mechanical

properties, method of manufacture, and quality control are comparable to the corresponding characteristics of listed

components. All longitudinal welds in unlisted components must be 100% visually inspected and it is also

necessary to apply the radiography requirement for the appropriate service of B31.3. B31.3 service categories and

their radiography requirements are summarized in a later section of this paper. It is apparent that A774 fittings mightrequire radiography if used in a piping system covered by ASNI/ASME B31.3. It should be noted that ANSI/ASME

B31.3 does not allow ultrasonic testing to be substituted for radiography to attain weld joint quality factors for

longitudinal welds in fitting or pipe (B31.3 Interpretations 11.05 and 11.20 respectively). In view of the potential

complications, it might be wiser to purchase A403 fittings to avoid the possibility of rejectable fittings being

discovered during field radiography of circumferential butt joints.

GUIDELINES FOR WELDING AND INSPECTION OF STAINLESS ALLOY PIPING

This TIP outlines considerations for fitup, welding, and inspection of circumferential butt welds made during shop

spooling and field installation of stainless alloy pipe and fittings; both thin wall (gauge) and thick wall (schedule).

The stainless alloys included are austenitic stainless steels, duplex stainless steels, and nickel base alloys. It is

assumed that:

the weld root will not normally be accessible for direct visual inspection or repair

all circumferential butt welds will be made using filler metal a solution heat treatment will not be performed after welding.The goal is to provide guidance in obtaining welds appropriate for the intended service. Figures 3- 5 illustrate welds

in thin wall stainless steel piping that were not fit for the intended purpose due to incomplete root penetration. The

welds were leaking due to corrosion and cracking due to fatigue.


8/19

Cutting and grinding. Machining, sawing, grinding, or plasma cutting are commonly used to cut stainless alloy

pipe. If plasma cutting is used, care should be taken to protect the process side of the pipe from spatter. It is good

practice to grind or sand plasma cut edges to remove the hard glassy residue from the cutting process.

Overheating during grinding should be avoided, as it can reduce the corrosion resistance in some cases. Only

grinding or sanding discs designed for use with stainless alloys should be used. Grinding or sanding discs

previously used on carbon steel should not be used on stainless alloys .

Cleaning. The weld joint should be free of burrs, lubricants, grease, paint, filings, and cuttings. Solvent cleaning

should be done with non-chlorinated solvents. Tools or materials used to clean joints should not contaminate the

stainless alloy with iron, carbon or other residues. For example wire brushes or steel wool used to clean joints

should be made from austenitic stainless steel (which is nonmagnetic) and be marked as stainless steel.

Joint Preparation. Joint preparation can influence penetration and weld metal dilution by base metal. An

inadequate root gap may result in incomplete penetration and incomplete mixing of the filler metal with the base

metal. The joint preparation and root gap must be included in the welding procedure specification (WPS).

Fit up. Good fit up of the joint is crucial for high quality welding. Poor fit up can make it difficult for the welder to

fuse both side of the weld root. An unfused root can be a stress raiser for fatigue cracking and/or a site for deposit

formation and corrosion.

Large diameter thin wall pipe and fittings are often out of round and require adjustment during fit up. Jointalignment can be done with mechanical devices and should be free of depressions and bumps. Heat should not be

used in the alignment of joints where it has been determined that the corrosion resistance of the alloy may be

reduced.

Misalignment tolerances should be specified in consideration of the wall thicknesses involved and the service

requirements. ANSI/ASME B31.3 requires that the inside surfaces of pipe be aligned within the dimensional

tolerances of the welding procedure specification (WPS,) but provides no further guidance.

A commonly used guideline for allowable misalignment is 1.6 mm (1/16). For thin wall pipe ( e.g. 10 gauge pipewith a 3.5 mm wall) this could produce misalignment up to 50% of the pipe wall thickness and present the welder

with a challenge, depending on the weld acceptance criteria.

Welding

Welding processes. Gas tungsten arc welding (GTAW), sometimes referred to as TIG welding, generally gives the

best quality and is preferable for the root pass and one or more of the subsequent passes. Subsequent passes can be

made by:

GTAW gas metal arc welding (GMAW), sometimes referred to as MIG welding flux cored arc welding (FCAW) shielded metal arc welding (SMAW)GMAW and FCAW are sometimes referred to generically as wire feed welding, however, the specific terms above

are preferable to avoid confusion.


9/19

Welding Procedures. Written welding procedures are essential. Section IX "Welding and brazing qualifications"17

of the ASME Boiler and Pressure Vessel Code lists the welding variables to be included in a welding procedure

specification (WPS) and also defines how the WPS must be qualified. The welding procedure qualification (WPQ) is

documented and is said to support the WPS. The tests involved in qualifying a WPS are given in Section IX which

is referred to by ANSI/ASME B31.3. Thus, if B31.3 is mandatory in a given jurisdiction, so are the relevant parts

of Section IX.

It should be realized that the WPS and WPQ requirements of ASME Section IX represent minimum mechanical

requirements for pressurized service, and it may be necessary to supplement them with additional requirements

relevant to the intended service.- e.g. requirements for corrosion resistance or special surface finish.

It is good practice to post the welding procedures (and inspection standards) at the job site, for ready access by

welders, inspectors, and owners personnel.

Welder Qualification and Certification. Welders should generally be qualified per ASME Section IX which

gives minimum requirements for welder qualification (WQ) in the WPSs which will be used. A welder may qualify

to weld a range of pipe sizes and wall thicknesses in different positions, by passing an appropriate test on a particular

pipe in a particular position. If the welder has not welded to a given WPS in a certain time, he is required to

requalify in that procedure. For non- pressurized, non-critical services (e.g. drains) the owner may elect to specify

alternative qualification requirements.

To simplify WPS requirements, some jurisdictions and associations have devised prequalified welding procedureswhich may be used by contractors. Several jurisdictions and associations have also devised welder qualification

schemes in which each welder carries a log book which lists the procedures in which he is currently qualified ( or

certified). Certification does not necessarily guarantee that welders will be capable of meeting the requirements of a

given job. For difficult welding positions, or for severe service welds, it may be desirable for welders to be skill

tested using the welding equipment, welding positions and pipe sizes that will be used in the job. Welder

qualification or skill testing on production welds is generally not recommended.

Tack welding. Tack welds that will become part of the final weld should be performed after preweld purging has

been completed. At least 4 tack welds should be made, spaced 90 apart around the pipe. For pipe 250 mm (10)diameter and larger, tacks should be made at least every 150 mm (6) and should be long enough to resist weld

shrinkage forces which will try to pull the root closed. The welder should check tack welds for cracking and any

cracked tack welds should be ground out. Before making the root pass, tack welds should be free of oxides and both

ends should be ground and tapered to promote complete fusion into the root pass.

Weld Root Shielding. It is good practice to protect the weld root from oxidation. Severe oxidation of a weld root

can lead to lack of penetration, irregular root profile, and reduction of corrosion resistance and mechanical

properties, including fatigue resistance. Weld heat tint may be acceptable in some services, but this depends on the

stainless alloy used and the severity of the corrosion involved.

The best way to prevent weld root oxidation is to shield the weld root with an inert gas such as argon. The WPS

should include the minimum purge gas flow rate, as well as the purging time for each pipe diameter and purge dam

spacing to be employed. It is not necessary to purge the entire run of pipe if purge dams are used. Water soluble

purge dams are available for closure welds in a pipe run. AWS D10.1118

may be used as a guideline for argon

purging.


10/19

Each welder can inspect the weld root visually for oxidation before he closes the root pass.

Portable oxygen monitors are also available to check the concentration of oxygen remaining in a purged volume.

British Standard BS 747519

states :

less than 1% oxygen is often specified for stainless alloy piping systems less than 0.5% oxygen is required to prevent root discoloration ( heat tint) in pipe up to 50 mm ( 2) diameter oxygen levels approaching zero are needed to prevent heat tint in pipe greater than 200 mm ( 8) diameter.Reference color charts have been developed which show the heat tint on stainless steel weld roots as a function of

purging gas type and oxygen concentration at the weld root20,21 Where it has been established that control of heat tint

is important for corrosion performance, a reference weld or color chart can be used as an acceptance standard.

Fluxes are available which are intended to protect the weld root from oxygen. They are typically supplied as a dry

powder, which is mixed with alcohol to make a paste and then applied on the inside surface of the pipe adjacent to

the weld root and on the faces of the weld preparation. During welding the flux melts and flows over the molten root

and adjacent parent metal. Unfortunately, flux can dry out and fall off the inside surface of the pipe if the pipe is

handled roughly during fit up.

When mixed and applied according to the manufacturers instructions, fluxes can be effective in limiting oxidation

of the molten weld root and allowing full penetration welds with reasonable profiles. They are less effective in

preventing heat tint, and they leave a flux residue on the root which may be undesirable in some services. Figure 6shows a cross section of a GTAW weld in type 316L stainless steel thin wall pipe. The weld was made with

voltage and current specified in the WPS and with flux applied in accordance with the manufacturers

recommendations. The weld profile is good, but at higher magnifications, grain boundary contamination is apparent

at the toes of the root pass. This may be a site for corrosion in some services. Figure 7 shows a weld made with flux

shielding by the same welder, but with 70 A current instead of the specified 60A. The weld cap is good, but the root

side is rejectable.

Because of the sensitivity of flux shielding to procedural variations, it is recommended that written instructions for

the use of flux be developed and included in the (WPS). Flux should be used in welder skill testing. It is furtherrecommended that the acceptability of flux shielding be evaluated in consideration of the service requirements.

Reference 22 reports the results of an investigation where both purged and flux protected welds in 317L stainless

steel pipe were tested in ferric chloride solution, (which is a "not unreasonable" simulation of a bleach crevice

environment), and in a paper machine simulated white water. In ferric chloride solution, nitrogen and argon purged

welds did not pit, while flux protected roots experienced severe pitting. In the simulated white water none of the

weld roots pitted, but the flux protected roots rusted. The simulated white water used for the test was, however, not

particularly corrosive due to a high ratio of sulfate ion to chloride ion.

Flux cored and flux coated filler rods. Proprietary filler rods with flux coating or flux cores are also available asan alternative to inert gas purging for root pass welding. The effectiveness of these fillers depends on welder skill

and adherence to the manufacturer's instructions. For example, the pipe wall thickness and the weld root gap can

influence performance..

If flux coated or flux cored filler rods are to be used, the instructions for their use should be included in the WPS and

they should be used in welder skill testing.


11/19

Identification of welds. It is good practice to mark each weld with the name or number of the welder.

Identification of welds is required by ANSI/ASME B31.3.

Post Weld Cleaning

As outlined in the "General considerations" section of this TIP, the nature and extent of post weld cleaning of weld

roots should be based on consideration of the effect on service performance and the cost of cleaning. Complete

removal of oxide and heat tint may require a combination of mechanical and chemical cleaning. This may be difficult

for long runs of pipe where worker access to the inside is not possible.

The most effective chemical cleaning agents for stainless alloys are pastes, gels, or solutions containing nitric andhydrofluoric acids. These agents are harmful to human tissue, and proper safety precautions should be followed in

their use. In addition, most jurisdictions will have environmental regulations governing proper disposal. Safety,

application, and disposal instructions are available from the manufacturers of these products.

Inspection

The requirements for weld inspection methods and scope, hold points, and weld acceptance criteria should be clearly

understood. A pre-job quality meeting between the owner (or owners representative), the welding contractorand welders, and inspection personnel can be useful in identifying and resolving differences in understanding before

the work begins.

Some means of dispute resolution should be agreed upon before work begins. The dispute resolution agreement

should address options, responsibilities, and costs for obtaining a second opinion on rejected welds including cut

out of welds for verification of volumetric inspection results (radiographic testing or ultrasonic testing).

Visual (VT) and radiographic testing (RT) are commonly used for circumferential welds in stainless alloy piping.

Ultrasonic testing (UT) is generally limited to thicker wall piping, e.g. for wall thickness greater than about 5mm(0.2).

Visual examination. VT should be performed to ensure the fit up meets alignment and gap requirements. The root

side of all circumferential butt welds should be visually examined by the welder before the root is closed. This

requires a flashlight if the root is not readily visible from one end of the pipe. Larger diameter pipe may be inspected

from the inside, but this usually requires a confined space procedure.

Each weld pass can be visually examined from the outside for freedom from cracking, slag, porosity, and undercut.

In addition, the final weld pass should be examined for a smooth transition to base metal. Visual examination shouldalways precede volumetric examination (RT, UT).

Visual examination can be performed for weld root oxidation and heat tint, if this is a specification requirement, and

if some weld roots are visually accessible. A videoprobe can be used for visual examination of weld roots for

incomplete penetration, excessive reinforcement, and oxidation or heat tint.


12/19

Extent of inspection. Each welders first two production welds should be inspected using the inspection techniques

that have been specified. If these welds are acceptable, subsequent welds should be inspected at random so that

some percentage of each welders work or total weld length is inspected - 5 to 10% is not uncommon. ANSI/ASME

B31.3 paragraph 341.3.4 provides for increasing the extent of examination when defects are revealed.

ANSI/ASME B31.3 is a useful guide to inspection requirements and may be mandatory in some jurisdictions.

However, it is not adequate simply to reference B31.3, as the extent of inspection depends on the fluid service

designation determined by the owner. The owner is free to increase the extent of inspection as part of the

engineering specification, but cannot decrease it if this code is mandatory. Table IV summarizes the approach taken

in B31.3. Reference should be made to B31.3 for further details.

Table IV. Inspection requirements in ANSI/ASME B31.3

Fluid Service Definition Examinationcategory D non-flammable

non-toxic

not damaging to human tissues

design pressure 1 MPa (150 psig)

-28C design T 186C

(-20F design T 366 F)

VT only required

extent is that necessary to satisfy the examiner

see paragraph 341.4.2 of B31.3

normal fluid

service

fluid service not classified as category D,M,

severe cyclic, or high pressure.

VT plus either RT or UT required

extent is a minimum of 5% of fabrication

covering the work of each welder or welding

operator


category M potential for personnel exposure judged to

be significant

single exposure to a small quantity of the

fluid can cause serious irreversible harm

VT plus either RT or UT required

extent of VT is 100%

extent of RT or UT is a minimum of 20%

see paragraph M341.4 of B31.3

severe cyclic the displacement stress range, Se, due to

bending and torsional stresses exceeds 0.8 of

the allowable displacement stress range, SA

and the number of full displacement cycles

during the expected life of the system

exceeds 7,000

VT plus RT

extent of VT and RT is 100%

UT may be substituted for RT if specified in

the engineering design


high pressure service for which the owner specifies the use

of B31.3 chapter IX

high pressure is defined as greater than that

allowed by ASME B16.5 "Pipe flanges and

fittings" Class 2000 rating for the specified

design pressure and temperature

VT plus RT

extent of both is 100%

UT cannot be substituted for RTsee paragraph K341.4 of B31.3


13/19

misalignment less than or equal to 50% of the pipe wall thickness, providing both sides of the joint are fused and therequirements for incomplete penetration are met

incomplete penetration less than 20 mm (3/4) length of continuous incomplete penetration in 150 mm ( 6) of weld or a total of 20 mm of separated occurrences of incomplete penetration in 6 of weld. underfill underfill to a maximum of 20% of the pipe wall thickness provided both sides of the joint have been fused and

the underfill has rounded contour. root protrusion

root protrusion less than or equal to 2.4 mm (3/32) provided the root profile is smooth so as

not to act as a stress raiser

ANSI/ASME does not address weld root oxidation or heat tint. If control of weld root oxidation or heat tint has been

specified, then an appropriate reference weld or color chart can be used as a comparator for inspection.

REFERENCES

1. "Recommended specifications for stainless steel piping, fittings and accessories for the pulp and paper industry,3

rdedition". Atlanta, Tappi Press, 1986.

2. "ASME Code for pressure piping, B31.3:Process piping". ANSI/ASME B31.3. New York, The AmericanSociety of Mechanical Engineers.

3. "Corrosion". D.F. Bowers. Chapter 10 (pp. 349-407) in Mill control & control systems: quality & testing,environmental, corrosion, electrical. Volume 9 of Pulp and Paper Manufacture,3

rdedition. Atlanta, TAPPI,

1992.4. "Corrosion in the pulp and paper industry". edited by Andrew Garner. Pages 1187-1220 in Corrosion. Volume

13 of the Metals Handbook, 9th

edition. Metals Park, Ohio, ASM International, 1987.5. "Stainless steels for pulp and paper manufacturing". Washington, D.C., Committee of Iron and Steel Producers

of the American Iron and Steel Institute, 1982. Distributed by the Nickel Development Institute. Currently

being revised.6. "Guidelines for the welded fabrication of nickel-containing stainless steels for corrosion-resistant services".

Richard E. Avery and A.H. Tuthill. Nickel Development Institute Publication 11 007. Toronto, 1992.7. "Corrosion behavior of welded stainless steel". T.G. Gooch. Welding Journal :Welding Research Supplement,

May 1996, pp. 135s-154s8. "Corrosion behavior of stainless steel, nickel base alloy and titanium weldments in chlorination and chlorine

di id bl hi h hill i h d d d th i l i


14/19

11. "GTAW root pass welding of 6% molybdenum austenitic stainless steel pipe - open root joint with hand fedfiller". Tappi TIP 0402-20. Atlanta, Tappi Press, 1994.

12. "Fabrication and post fabrication cleanup of stainless steels". Arthur H. Tuthill , Nickel Development InstitutePublication 10 004, Toronto, 1989.

13. "Effect of some surface treatments on corrosion of stainless steel". G.E. Coates. CORROSION '90 Paper #539.Houston, NACE International, 1990.

14. "Effect of post weld cleaning on corrosion resistance of austenitic and duplex stainless steel weldments in bleachplant service". D.W. Christie. 7

thInternational Symposium on Corrosion in the Pulp and Paper Industry,

Orlando, 1992, pp.87-95. Atlanta, TAPPI, 1992.

15. "Welding of duplex stainless steel". Tappi TIP 0402-23. Atlanta, Tappi Press, 1998.16. "Qualification of duplex stainless steel welds". Tappi TIP in preparation 1999.17. "ASME Boiler and pressure vessel code section IX: qualification standard for welding and brazing procedures,

welders, brazers, and welding operators". New York, American Society of Mechanical Engineers.18. "Recommended practices for root pass welding and gas purging". ANSI/AWS Standard D10.11. Miami,

American Welding Society.19. Specification for fusion welding of austenitic stainless steels. British Standard BS 7475. London, British

Standards Institution.20. "Root surface quality requirements - high efficiency purging or pickling? ". J. Vagn Hansen, T.S. Nielsen, and

P. Aastrup. Paper 46 in volume 2 ofDuplex 94: 4th

International conference on duplex stainless steels,

Glasgow, 1994. Cambridge, Abington Publishing, 1995.21. "Reference colour charts - for purging gas in stainless steel tubes". J. Vagn Hansen. FORCE Institute report94.34. Copenhagen, FORCE Institute, 1994.22. "Corrosion evaluation of stainless steel root weld shielding". Margaret Gorog and Linda Sawyer. CORROSION

99 paper no. 277. NACE, Houston, 1999, 6pp.


15/19

Figure 1. Fatigue crack at root of incompletely penetrated circumferential butt joint in 316L thin gauge pipe. Weld

was made with poor fitup and flux for root protection (16X, electrolytic etch in 10% oxalic acid).


16/19

Figure 3. Inside surface of a circumferential butt weld in a 316L stainless steel stock line in corrosive paper machine

white water service. The butt weld was made with flux for root protection and was not fully penetrated. The crevice

in the root, combined with flux residue and weld heat tint, resulted in severe corrosion and leaking.


17/19

Figure 5. External repairs made to fatigue cracking which initiated at an incompletely penetrated root in a paper

machine stock line weld. Such external repairs are temporary, as fatigue cracking will persist if root imperfectionsand cyclic loading remain. Note that the cracking has diverged into the base metal.


18/19

Figure 7. Cross section of a circumferential butt weld made by the same welder as Figure 6 with properly appliedflux for root protection, but with 10 amps greater welding current - i.e. outside the specified parameters. The weld

cap is good, but the weld root is not acceptable (16X electrolytically etched in 10% oxalic acid).


19/19

Table II. ASTM specifications for austenitic stainless steel pipe

ASTM Title Filler

metal

Heat treat Cold

work

Size range1

Tolerances Tests

OD wall oval HT ET UT PT RT Corr

A312-

98

Seamless and welded

austenitic stainless steel pipes

N annealed opt 1/8 - 30 NPS

SCH 5-80

broad -12.5% 1.5% Y opt opt S5 S5 S7

A358-

98

Electric fusion welded

austenitic Cr-Ni alloy steel

pipe

Y annealed

as welded optusually > 3

NPS

+0.5% -0.01 1% Y S3 Y2 S6

A409-

95ae1

Welded large diameter

austenitic steel pipe

opt annealed

as welded opt

14-30 NPS

SCH 5, 10

broad -12.5% 1.5% Y opt opt S2 S6

A778-

98

Welded unannealed austenitic

stainless steel tubular

products

opt not annealed 3 - 48 OD

.062/0.50

wall

broad +12.5% 1.5% S4 S2

A813-

95

Single or double welded

austenitic stainless steel pipe

N annealed 1/8 - 30 NPS

SCH 5 - 80

moder

ate

+12.5% 1.5% Y S5 S6 S7 S8

A814-

96

Cold worked welded

austenitic stainless steel pipe

N annealed Y 1/8 - 4 NPS

SCH 5 - 80

tight < + 10% OD

tols

Y S5 S6 S7

General notes

Unless otherwise noted, the standard broad tolerances for dimensions listed in ASTM A530 Standard specification

for general requirements for specialized carbon and alloy steel pipe apply.

These specifications require mechanical testing to meet specified minimum values. The actual test results need not

be reported unless specified in the purchase order.

If hydrotesting (HT), eddy current testing (ET), ultrasonic testing (UT), liquid penetrant testing (PT), or radiographic

testing (RT), is required it is it is noted by a Y in the appropriate column. If these tests are available as

supplementary requirements the supplementary requirement number is noted in the appropriate column. If there is

no entry the test is not addressed by the specification.

Note 1: The specifications generally allow for provisions of other diameters and wall thicknesses than those listed

providing the pipe complies with all other requirements of the specification.

Note 2: Class 2 does not require radiographic inspection. Classes 1, 3, and 4 require 100% RT. Class 5 requires

spot radiography at a minimum of 1 foot every 50 feet.

guidelines for specification, welding and inspection of stainless alloy piping

Documents