Classification of Carbon and Low-Alloy Steels

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Abstract: The American Iron and Steel Institute (AISI) defines carbon steel as follows:Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Steels can be classified by a variety of different systems depending on:

The composition, such as carbon, low-alloy or stainless steel. The manufacturing methods, such as open hearth, basic oxygen process, or

electric furnace methods. The finishing method, such as hot rolling or cold rolling The product form, such as bar plate, sheet, strip, tubing or structural shape The deoxidation practice, such as killed, semi-killed, capped or rimmed steel The microstructure, such as ferritic, pearlitic and martensitic The required strength level, as specified in ASTM standards The heat treatment, such as annealing, quenching and tempering, and

thermomechanical processing Quality descriptors, such as forging quality and commercial quality.

Carbon Steels

The American Iron and Steel Institute (AISI) defines carbon steel as follows:

Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Carbon steel can be classified, according to various deoxidation practices, as rimmed, capped, semi-killed, or killed steel. Deoxidation practice and the steelmaking process will have an effect on the properties of the steel. However, variations in carbon have the greatest effect on mechanical properties, with increasing carbon content leading to increased hardness and strength. As such, carbon steels are generally categorized according to their carbon content. Generally speaking, carbon steels contain up to 2%

total alloying elements and can be subdivided into low-carbon steels, medium-carbon steels, high-carbon steels, and ultrahigh-carbon steels; each of these designations is discussed below.

As a group, carbon steels are by far the most frequently used steels. More than 85% of the steel produced and shipped in the United States is carbon steel.

Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels, tin plate, and wire products.

For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher manganese content up to 1.5%. These materials may be used for stampings, forgings, seamless tubes, and boiler plate.

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon-manganese steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.

High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. High-carbon steels are used for spring materials and high-strength wires.

Ultrahigh-carbon steels are experimental alloys containing 1.25 to 2.0% C. These steels are thermomechanically processed to produce microstructures that consist of ultrafine, equiaxed grains of spherical, discontinuous proeutectoid carbide particles.

High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition.

The HSLA steels have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

HSLA Classification:

Weathering steels, designated to exhibit superior atmospheric corrosion resistance

Control-rolled steels, hot rolled according to a predetermined rolling schedule, designed to develop a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure on cooling

Pearlite-reduced steels, strengthened by very fine-grain ferrite and precipitation hardening but with low carbon content and therefore little or no pearlite in the microstructure

Microalloyed steels, with very small additions of such elements as niobium, vanadium, and/or titanium for refinement of grain size and/or precipitation hardening

Acicular ferrite steel, very low carbon steels with sufficient hardenability to transform on cooling to a very fine high-strength acicular ferrite structure rather than the usual polygonal ferrite structure

Dual-phase steels, processed to a micro-structure of ferrite containing small uniformly distributed regions of high-carbon martensite, resulting in a product with low yield strength and a high rate of work hardening, thus providing a high-strength steel of superior formability.

The various types of HSLA steels may also have small additions of calcium, rare earth elements, or zirconium for sulfide inclusion shape control.

Low-alloy Steels

Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.

For many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.

As with steels in general, low-alloy steels can be classified according to:

Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels

Heat treatment, such as quenched and tempered, normalized and tempered, annealed.

Because of the wide variety of chemical compositions possible and the fact that some steels are used in more than one heat-treated, condition, some overlap exists among the alloy steel classifications. In this article, four major groups of alloy steels are addressed: (1) low-carbon quenched and tempered (QT) steels, (2) medium-carbon ultrahigh-strength steels, (3) bearing steels, and (4) heat-resistant chromium-molybdenum steels.

Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications. However, a few steels, such as HY-80 and HY-100, are covered by military specifications. The steels listed are used primarily as plate. Some of these steels, as well as other, similar steels, are produced as forgings or castings.

Medium-carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels are covered by SAE/AISI designations or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and welding wire.

Bearing steels used for ball and roller bearing applications are comprised of low carbon (0.10 to 0.20% C) case-hardened steels and high carbon (-1.0% C) through-hardened steels. Many of these steels are covered by SAE/AISI designations.

Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.2%. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalized and tempered, quenched and tempered or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants.

  The Metallurgy Of Carbon Steel

The best way to understand the metallurgy of carbon steel is to study the ‘Iron Carbon Diagram’.  The diagram shown below is based on the transformation that occurs as a result of slow heating.  Slow cooling will reduce the transformation temperatures; for example: the A1 point would be reduced from 723°C to 690 °C.  However the fast heating and cooling rates encountered in welding will have a significant influence on these temperatures, making the accurate prediction of weld metallurgy using this diagram difficult.

Austenite    This phase is only possible in carbon steel at high temperature.  It has a Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbon in solution.

Ferrite   This phase has a Body Centre Cubic structure (B.C.C) which can hold very little carbon; typically 0.0001% at room temperature.  It can exist as either: alpha or delta ferrite. 

Carbon   A very small interstitial atom that tends to fit into clusters of iron atoms.  It strengthens steel and gives it the ability to harden by heat treatment.  It also causes major problems for

welding , particularly if it exceeds 0.25% as it creates a hard microstructure that is susceptible to hydrogen cracking.  Carbon forms compounds with other elements called carbides.  Iron Carbide, Chrome Carbide etc.

Cementite   Unlike ferrite and austenite, cementite is a very hard intermetallic compound consisting of 6.7% carbon and the remainder iron, its chemical symbol is Fe3C.  Cementite is very hard, but when mixed with soft ferrite layers its average hardness is reduced considerably. Slow cooling gives course perlite; soft easy to machine but poor toughness.  Faster cooling gives very fine layers of ferrite and cementite; harder and tougher

Pearlite   A mixture of alternate strips of ferrite and cementite in a single grain.  The distance between the plates and their thickness is dependant on the cooling rate of the material;  fast cooling creates thin plates that are close together and slow cooling creates a much coarser structure possessing less toughness.  The name for this structure is derived from its mother of pearl appearance under a microscope.  A fully pearlitic structure occurs at 0.8% Carbon.  Further increases in carbon will create cementite at the grain boundaries, which will start to weaken the steel.

Cooling of a steel below 0.8% carbon     When a steel solidifies it forms austenite.  When the temperature falls below the A3 point, grains of ferrite start to form.  As more grains of ferrite start to form the remaining austenite becomes richer in carbon.  At about 723°C the remaining austenite, which now contains 0.8% carbon, changes to pearlite.  The resulting structure is a mixture consisting of white grains of ferrite mixed with darker grains of pearlite.  Heating is basically the same thing in reverse.

  Martensite   If steel is cooled rapidly from austenite, the F.C.C

structure rapidly changes to B.C.C leaving insufficient time for the carbon to form pearlite.  This results in a distorted structure that has the appearance of fine needles. There is no partial transformation associated with martensite, it either forms or it doesn’t.  However, only the parts of a section that cool fast enough will form martensite; in a thick section it will only form to a certain depth, and if the shape is complex  it may only form in small pockets.  The hardness of martensite is solely dependant on carbon content, it is normally very high, unless the carbon content is exceptionally low.

Tempering   The carbon trapped in the martensite transformation can be released by heating the steel below the A1 transformation temperature.  This release of carbon from nucleated areas allows the structure to deform plastically and relive some of its internal stresses. This reduces hardness and increases toughness, but it also tends to reduce tensile strength.  The degree of tempering is dependant on temperature and time; temperature having the greatest influence. 

Annealing   This term is often used to define a heat treatment process that produces some softening of the structure.  True annealing involves heating the steel to austenite and holding for some time to create a stable structure.  The steel is then cooled very slowly to room temperature.  This produces a very soft structure, but also creates very large grains, which are seldom desirable because of poor toughness.

Normalising   Returns the structure back to normal.  The steel is heated until it just starts to form austenite; it is then cooled in air. This moderately rapid transformation creates relatively fine grains with uniform pearlite.  

Welding   If the temperature profile for a typical weld is plotted against the carbon equilibrium diagram, a wide variety of transformation and heat treatments will be observed. 

Note, the carbon equilibrium diagram shown above is only for illustration, in reality it will be heavily distorted because of the rapid

heating and cooling rates involved in the welding process. 

a)  

b) 

   

c)   

d) 

Mixture of ferrite and pearlite grains; temperature below A1, therefore microstructure not significantly affected.

Pearlite transformed to Austenite, but not sufficient temperature available to exceed the A3 line, therefore not all ferrite grains transform to Austenite.  On cooling, only the transformed grains will be normalised. 

Temperature just exceeds A3 line, full Austenite transformation.  On cooling all grains will be normalised

Temperature significantly exceeds A3 line permitting grains to grow.  On cooling, ferrite will form at the grain boundaries, and a course pearlite will form inside the grains.  A course grain structure is more readily hardened than a finer one, therefore if the cooling rate between 800°C to 500°C is rapid, a hard microstructure will be formed.  This is why a brittle fracture is most likely to propagate in this region.  

Welds  The metallurgy of a weld is very different from the parent material.  Welding filler metals are designed to create strong and tough welds, they contain fine oxide particles that permit the nucleation of fine grains.  When a weld solidifies, its grains grow from the course HAZ grain structure, further refinement takes place within these course grains creating the typical acicular ferrite formation shown opposite. 

 

  

Duplex stainless steels

Typically twice the yield of austenitic stainless steels.  Minimum Specified UTS typically 680 to 750N/mm2 (98.6 to 108ksi).  Elongation typically > 25%.

Superior corrosion resistance than a 316.  Good Resistance to stress corrosion cracking in a chloride environment. 

Duplex materials have improved over the last decade; further additions of Nitrogen have been made improving weldability. 

Because of the complex nature of this material it is important that it is sourced from good quality steel mills and is properly solution annealed.  Castings and possibly thick sections may not cool fast when annealed causing sigma and other deleterious phases to form. 

The material work hardens if cold formed; even the strain produced from welding can work harden the material particularly in multi pass welding.  Therefore a full solution anneal is advantageous, particularly if low service temperatures are foreseen.

The high strength of this material can make joint fit up difficult.

Usable temperature range restricted to, -50 to 280°C

Used in Oil & Natural Gas production, chemical plants etc.

Standard Duplex S31803 22Cr 5Ni 2.8Mo 0.15N PREn = 32-33

Super Duplex:   Stronger and more corrosion resistant than standard duplex. S32760(Zeron 100) 25Cr 7.5Ni 3.5Mo 0.23N  PREn = 40    

Micro Of Standard Duplex

Dark Areas:- Ferrite

Light Areas:- Austenite

Duplex solidifies initially as ferrite, then transforms on further cooling to a matrix of ferrite and austenite.  In modern raw material the balance should be 50/50 for optimum corrosion resistance, particularly resistance to stress corrosion cracking.  However the materials strength is not significantly effected by the ferrite / austenite phase balance.   

The main problem with Duplex is that it very easily forms brittle intermetalic phases, such as Sigma, Chi and Alpha Prime.  These phases can form rapidly, typically 100 seconds at 900°C.  However shorter exposure has been known to cause a drop in toughness, this has been attribute to the formation of sigma on a microscopic scale.  Prolonged heating in the range 350 to 550°C can cause 475°C temper embitterment.  For this reason the maximum recommended service temperature for duplex is about 280°C.

Sigma (55Fe 45Cr) can be a major problem when welding thin walled small bore pipe made of super duplex, although it can occur in thicker sections.  It tends to be found in the bulk of the material rather than at the surface, therefore it probably has more effect on toughness than corrosion resistance.  Sigma can also occur in thick sections, such as castings that have not been properly solution annealed (Not cooled fast enough).

However most standards accept that deleterious phases, such as sigma, chi and laves, may be tolerated if the strength and corrosion resistance are satisfactory.

Nitrogen is a strong austenite former and largely responsible for the balance between ferrite and austenite phases and the materials superior corrosion resistance.  Nitrogen can’t be added to filler metal, as it does not transfer across the arc. It can also be lost from molten parent metal during welding.  Its loss can lead to high ferrite and reduced corrosion resistance.  Nitrogen can be added to the shielding gas and backing gas, Up to about 10%; however this makes welding difficult as it can cause porosity and contamination of the Tungsten electrode unless the correct welding technique is used.  Too much Nitrogen will form a layer of Austenite on the weld surface.  In my experience most duplex and super duplex are TIG welded using pure argon.

Backing / purge gas should contain less than 25ppm Oxygen for optimum corrosion resistance.

Fast cooling from molten will promote the formation of ferrite, slow cooling will promote austenite. During welding fast cooling is most likely, therefore welding consumables usually contain up to 2 - 4% extra Nickel to promote austenite formation in the weld.  Duplex should never be welded without filler metal, as this will promote excessive ferrite, unless the welded component is solution annealed. Acceptable phase balance is usually 30 – 70% Ferrite

Duplex welding consumables are suitable for joining duplex to austenitic stainless steel or carbon steel; they can also be used for corrosion resistant overlays.  Nickel based welding consumables can be used but the weld strength will not be as good as the parent metal, particularly on super duplex. 

Low levels of austenite: - Poor toughness and general

corrosion resistance. 

High levels of austenite: - Some Reduction in strength and reduced resistance to stress corrosion cracking. 

Good impact test results are a good indication that the material has been successfully welded.  The parent metal usually exceeds  200J.  The ductile to brittle transition temperature is about –50°C. The transition is not as steep as that of carbon steel and depends on the welding process used. Flux protected processes, such as MMA; tend to have a steeper transition curve and lower toughness.  Multi run welds tend to promote austenite and thus exhibit higher toughness

Tight controls and the use of arc monitors are recommended during welding and automatic or mechanised welding is preferred.  Repair welding can seriously affect corrosion resistance and toughness; therefore any repairs should follow specially developed procedures.  See BS4515 Part 2 for details. 

Production control test plates are recommended for all critical poduction welds.

Welding procedures should be supplemented by additional tests, depending on the application and the requirements of any application code:-

A ferrite count using a Ferro scope is probably the most popular.  For best accuracy the ferrite count should be performed manually and include a check for deleterious phases.

Good impact test results are also a good indication of a successful welding procedure and are mandatory in BS4515 Part 2.

A corrosion test, such as the G48 test, is highly recommended. The test may not model the exact service corrosion environment, but gives a good qualative assessment of the welds general corrosion resistance; this gives a good indication that the welding method is satisfactory. G48 test temperature for standard duplex is typically 22°C, for super duplex 35°C

 

Typical Welding Procedure For Zeron 100 (Super Duplex)

Pipe 60mm Od x 4mm Thick     Position 6G

Maximum Interpass 100°C     Temperature at the end of welding < 250°C

1.6mm Filler Wire          85 amps  2 weld runs (Root and Cap)

Arc energy  1 to 1,5 KJ/mm         Travel speed  0.75 to 1 mm/sec

Recommended Testing

1. Ferric Chloride Pitting Test To ASTM G48 : Method A 2. Chemical analysis of root 3. Ferrite count