Metallurgy of Carbon Steel

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: theA1 point would be reduced from 723°C to 690 °C. However the fast heating and coolingrates encountered in welding will have a significant influence on these temperatures, makingthe accurate prediction of weld metallurgy using this diagram difficult.

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.

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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 averagehardness 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 ferrite andcementite in a single grain. The distance betweenthe plates and their thickness is dependant on thecooling rate of the material; fast cooling creates thinplates that are close together and slow coolingcreates a much coarser structure possessing lesstoughness. The name for this structure is derivedfrom its mother of pearl appearance under amicroscope. A fully pearlitic structure occurs at0.8% Carbon. Further increases in carbon will createcementite at the grain boundaries, which will start toweaken 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. Asmore 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 topearlite. The resulting structure is a mixture consisting of white grains of ferritemixed 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 rapidlychanges to B.C.C leaving insufficient time for the carbon to form pearlite. This resultsin a distorted structure that has the appearance of fine needles. There is no partialtransformation associated with martensite, it either forms or it doesn’t. However, onlythe parts of a section that cool fast enough will form martensite; in a thick section itwill only form to a certain depth, and if the shape is complex it may only form insmall pockets. The hardness of martensite is solely dependant on carbon content, it isnormally very high, unless the carbon content is exceptionally low.

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 to





reduce 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 to austeniteand holding for some time to create a stable structure. The steel is then cooled veryslowly to room temperature. This produces a very soft structure, but also creates verylarge grains, which are seldom desirable because of poor toughness.

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. Oncooling, 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 hardened thana finer one, therefore if the cooling rate between 800°C to 500°C is rapid, ahard microstructure will be formed. This is why a brittle fracture is mostlikely to propagate in this region.

Welds The metallurgy of a weld is very different from theparent material. Welding filler metals are designed tocreate strong and tough welds, they contain fine oxideparticles that permit the nucleation of fine grains. When aweld solidifies, its grains grow from the course HAZ grainstructure, further refinement takes place within thesecourse grains creating the typical acicular ferrite formationshown opposite.

Recommended Reading

Metals and How To Weld Them :- Lincoln Arc Foundation

A 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 the informationis clearly represented in a very readable format.

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Page last updated 08 May 2002





Metallurgy of Carbon Steel

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