The Metallurgy Of Carbon Steel

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

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

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

Carbon A very small interstitial atom that tends to fit into clusters of iron atoms. Itstrengthens steel and gives it the ability to harden by heat treatment. It also causesmajor problems for welding , particularly if it exceeds 0.25% as it creates a hardmicrostructure that is susceptible to hydrogen cracking. Carbon forms compoundswith other elements called carbides. Iron Carbide, Chrome Carbide etc.

Cementite Unlike ferrite and austenite, cementite is a very hard intermetallic

Cementite Unlike ferrite and austenite, cementite is a very hard intermetalliccompound consisting of 6.7% carbon and the remainder iron, its chemical symbol isFe3C. Cementite is very hard, but when mixed with soft ferrite layers its averidgehardness is reduced considerably. Slow cooling gives course perlite; soft easy tomachine but poor toughness. Faster cooling gives very fine layers of ferrite andcementite; harder and tougher

Pearlite A mixture of alternate strips of ferriteand cementite in a single grain. The distancebetween the plates and their thickness isdependant on the cooling rate of the material; fastcooling creates thin plates that are close togetherand slow cooling creates a much coarser structurepossessing less toughness. The name for thisstructure is derived from its mother of pearlappearance under a microscope. A fully pearliticstructure occurs at 0.8% Carbon. Furtherincreases in carbon will create cementite at thegrain boundaries, which will start to weaken thesteel.

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. Asmore grains of ferrite start to form the remaining austenite becomes richer incarbon. At about 723C the remaining austenite, which now contains 0.8% carbon,changes to pearlite. The resulting structure is a mixture consisting of white grainsof ferrite mixed with darker grains of pearlite. Heating is basically the same thingin reverse.

Martensite If steel is cooled rapidly from austenite, the F.C.C structure rapidlychanges to B.C.C leaving insufficient time for the carbon to form pearlite. Thisresults in a distorted structure that has the appearance of fine needles. There is nopartial transformation associated with martensite, it either forms or it doesnt.However, only the parts of a section that cool fast enough will form martensite; in athick section it will only form to a certain depth, and if the shape is complex it mayonly form in small pockets. The hardness of martensite is solely dependant oncarbon content, it is normally very high, unless the carbon content is exceptionallylow.

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

Annealing This term is often used to define a heat treatment process that producessome softening of the structure. True annealing involves heating the steel toaustenite and holding for some time to create a stable structure. The steel is thencooled very slowly to room temperature. This produces a very soft structure, butalso creates very large grains, which are seldom desirable because of poortoughness.

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

Welding If the temperature profile for a typical weld is plotted against the carbonequilibrium diagram, a wide variety of transformation and heat treatments will beobserved.

Note, the carbon equilibrium diagram shown above is only for illustration, in reality it willbe heavily distorted because of the rapid heating and cooling rates involved in the weldingprocess.

a)

b)

c)

d)

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

Pearlite transformed to Austenite, but not sufficient temperature available toexceed 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 coolingall grains will be normalised

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

Welds The metallurgy of a weld is very different fromthe parent material. Welding filler metals are designedto create strong and tough welds, they contain fineoxide particles that permit the nucleation of fine grains.When a weld solidifies, its grains grow from the courseHAZ grain structure, further refinement takes placewithin these course grains creating the typical acicularferrite formation shown opposite.

Recommended Reading

Metals and How To Weld Them :- Lincoln Arc FoundationA very cheap hard backed book covering all the basic essentials of weldingmetallurgy.

Welding Metallurgy Training Modules:- (Devised by The Welding Institute ofCanada) Published in the UK by Abington Publishing. Not cheap but theinformation is clearly represented in a very readable format.

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Page last updated 08 May 2002

Residual Stress

Magnitude Of Stresses- A Simple Analogy

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Page last updated 08 May 2002

Strain Age Embrittlement

This phenomenon applies to carbon and low alloy steel. It involves ferriteforming a compound with nitrogen; iron-nitride (Fe4N). Temperatures around250C, will cause a fine precipitation of this compound to occur. It will tend topin any dislocations in the structure that have been created by cold work orplastic deformation.

Strain ageing increases tensile strength but significantly reduces ductility andtoughness.

Modern steels tend to have low nitrogen content, but this is not necessarily truefor welds. Sufficient Nitrogen, approximately 1 to 2 ppm, can be easily pickedup from the atmosphere during welding.

Weld root runs are particularly at risk because of high contraction stressescausing plastic deformation. This is why impact test specimens taken from theroot or first pass of a weld can give poor results.

Additions of Aluminium can tie up the Nitrogen as Aluminium Nitride, butweld-cooling rates are too fast for this compound to form successfully. Stressrelief at around 650 degrees C will resolve the problem.

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Page last updated 08 May 2002

HOW TO AVOID PWHT

The above picture is of a new pressure vessel that failed during itshydraulic test. The vessel had been stress relieved, but some parts ofit did not reach the required temperature and consequently did notexperience adequate tempering. This coupled with a small hydrogencrack, was sufficient to cause catastrophic failure under testconditions. It is therefore important when considering PWHT or itsavoidance, to ensure that all possible failure modes and theirconsequences are carefully considered before any action is taken.

The post weld heat treatment of welded steel fabrications is normallycarried out to reduce the risk of brittle fracture by: -

Reducing residual Stresses. These stresses are created whena weld cools and its contraction is restricted by the bulk ofthe material surrounding it. Weld distortion occurs whenthese stresses exceed the yield point. Finite elementmodelling of residual stresses is now possible, so that thecomplete welding sequence of a joint or repair can bemodelled to predict and minimise these stresses.

Tempering the weld and HAZ microstructure. Themicrostructure, particularly in the HAZ, can be hardened byrapid cooling of the weld. This is a major problem for low

rapid cooling of the weld. This is a major problem for lowand medium alloy steels containing chrome and any otherconstituent that slow the austenite/ferrite transformationdown, as this will result in hardening of the micro structure,even at slow cooling rates.

The risk of brittle fracture can be assessed by fracture mechanics.Assuming worst-case scenarios for all the relevant variables. It isthen possible to predict if PWHT is required to make the fabricationsafe. However, the analysis requires accurate measurement of HAZtoughness, which is not easy because of the HAZs small size andvarying properties. Some approximation is possible from impacttests, providing the notch is taken from the point of lowesttoughness.

If PWHT is to be avoided, stress concentration effects such as: -backing bars, partial penetration welds, and internal defects in theweld and poor surface profile, should be avoided. Good surface andvolumetric NDT is essential. Preheat may still be required to avoidhydrogen cracking and a post weld hydrogen release may also bebeneficial in this respect (holding the fabrication at a temperature ofaround 250C for at least 2 hours, immediately after welding).

Nickel based consumables can often reduce or remove the need forpreheat, but their effect on the parent metal HAZ will be no differentfrom that created by any other consumable, except that the HAZmay be slightly narrower. However, nickel based welds, like mostaustenitic steels, can make ultrasonic inspection very difficult.

Further reduction in the risk of brittle fracture can be achieved byrefining the HAZ microstructure using special temper bead weldingtechniques.

Further Information On: -

Temper Bead Welding Technique

Fracture Mechanics (Link temporarily Disabled)

Residual Stresses Metallurgy of Steel

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Page last updated 10 June 2002

Alloying Elements

ManganeseIncreases strength and hardness; forms a carbide; increases hardenability;lowers the transformation temperature range. When in sufficient quantityproduces an austenitic steel; always present in a steel to some extent because itis used as a deoxidiser

SiliconStrengthens ferrite and raises the transformation temperature temperatures; hasa strong graphitising tendency. Always present to some extent, because it isused with manganese as a deoxidiser

ChromiumIncreases strength and hardness; forms hard and stable carbides. It raises thetransformation temperature significantly when its content exceeds 12%.Increases hardenability; amounts in excess of 12%, render steel stainless. Goodcreep strength at high temperature.

NickelStrengthens steel; lowers its transformation temperature range; increaseshardenability, and improves resistance to fatigue. Strong graphite formingtendency; stabilizes austenite when in sufficient quantity. Creates fine grainsand gives good toughness.

Nickel And ChromiumUsed together for austenitic stainless steels; each element counteractsdisadvantages of the other.

TungstenForms hard and stable carbides; raises the transformation temperature range,and tempering temperatures. Hardened tungsten steels resist tempering up to6000C

MolybdenumStrong carbide forming element, and also improves high temperature creepresistance; reduces temper-brittleness in Ni-Cr steels. Improves corrosionresistance and temper brittleness.

VanadiumStrong carbide forming element; has a scavenging action and produces clean,

inclusion free steels. Can cause re-heat cracking when added to chrome mollysteels.

TitaniumStrong carbide forming element. Not used on its own, but added as a carbidestabiliser to some austenitic stainless steels.

PhosphorusIncreases strength and hardnability, reduces ductility and toughness. Increasesmachineability and corrosion resistance

SulphurReduces toughness and strength and also weldabilty.Sulphur inclusions, which are normally present, are taken into solution near thefusion temperature of the weld. On cooling sulphides and remaining sulphurprecipitate out and tend to segregate to the grain boundaries as liquid films, thusweakening them considerably. Such steel is referred to as burned. Manganesebreaks up these films into globules of maganese sulphide; maganese to sulphurratio > 20:1, higher carbon and/or high heat input during welding > 30:1, toreduce extent of burning.

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Page last updated 02 June 2002

Pre-heat Calculator

Pre-Heat Calculator to EN1011 Part 2 - Non Alloyed And Low AlloySteels.

Information on how to use this page

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Manual Metal Arc Welding

Heat InputKJ/mm = Note, this box is not for data input.

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HydrogenScale

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A {MMA Rutile & Cellulosic Electrodes, Worst Case} >15ml/100g

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Note Thickness must be 2 x T for a butt weld

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Page last updated 13 April 2002