The Metallurgy Of Carbon SteelThe 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) Mixture of ferrite and pearlite grains; temperature below A1, therefore microstructure not significantly affected.

b) 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. 

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

d) 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. 

Residual Stress

Magnitude Of Stresses- A Simple Analogy

Strain Age EmbrittlementThis phenomenon applies to carbon and low alloy steel.  It involves ferrite forming a compound with nitrogen; iron-nitride (Fe4N).  Temperatures around 250°C, will cause a fine precipitation of this compound to occur.  It will tend to pin any dislocations in the structure that have been created by cold work or plastic deformation.

Strain ageing increases tensile strength but significantly reduces ductility and toughness.

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

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

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

HOW TO AVOID PWHT

The above picture is of a new pressure vessel that failed during its hydraulic test.  The vessel had been stress relieved, but some parts of it did not reach the required temperature and consequently did not experience adequate tempering.  This coupled with a small hydrogen crack, was sufficient to cause catastrophic failure under test conditions.  It is therefore important when considering PWHT or its avoidance, to ensure that all possible failure modes and their consequences are carefully considered before any action is taken.

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

Reducing residual Stresses.  These stresses are created when a weld cools and its contraction is restricted by the bulk of the material surrounding it.  Weld distortion occurs when these stresses exceed the yield point.  Finite element modelling of residual stresses is now possible, so that the complete welding sequence of a joint or repair can be modelled to predict and minimise these stresses.

Tempering the weld and HAZ microstructure.  The microstructure, particularly in the HAZ, can be hardened by rapid cooling of the weld.  This is a major problem for low and medium alloy steels containing chrome and any other constituent that slow the austenite/ferrite transformation down, 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 is then possible to predict if PWHT is required to make the fabrication safe.  However, the analysis requires accurate measurement of HAZ toughness, which is not easy because of the HAZ’s

small size and varying properties.  Some approximation is possible from impact tests, providing the notch is taken from the point of lowest toughness. 

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

Nickel based consumables can often reduce or remove the need for preheat, but their effect on the parent metal HAZ will be no different from that created by any other consumable, except that the HAZ may be slightly narrower.  However, nickel based welds, like most austenitic steels, can make ultrasonic inspection very difficult.

Further reduction in the risk of brittle fracture can be achieved by refining the HAZ microstructure using special temper bead welding techniques.

Alloying ElementsManganese Increases strength and hardness; forms a carbide; increases hardenability; lowers the transformation temperature range.  When in sufficient quantity produces an austenitic steel; always present in a steel to some extent because it is used as a deoxidiser

Silicon Strengthens ferrite and raises the transformation temperature temperatures; has a strong graphitising tendency.  Always present to some extent, because it is used with manganese as a deoxidiser

Chromium Increases strength and hardness; forms hard and stable carbides.  It raises the transformation temperature significantly when its content exceeds 12%. Increases hardenability; amounts in excess of 12%, render steel stainless.  Good creep strength at high temperature.

Nickel Strengthens steel; lowers its transformation temperature range; increases hardenability, and improves resistance to fatigue. Strong graphite forming tendency; stabilizes austenite when in sufficient quantity.  Creates fine grains and gives good toughness.

Nickel And Chromium Used together for austenitic stainless steels; each element counteracts disadvantages of the other.

Tungsten Forms hard and stable carbides; raises the transformation temperature range, and tempering temperatures.  Hardened tungsten steels resist tempering up to 6000C

Molybdenum Strong carbide forming element, and also improves high temperature creep resistance; reduces temper-brittleness in Ni-Cr steels.  Improves corrosion resistance and temper brittleness.

Vanadium Strong carbide forming element; has a scavenging action and produces clean, inclusion free steels. Can cause re-heat cracking when added to chrome molly steels.

Titanium Strong carbide forming element. Not used on its own, but added as a carbide stabiliser to some austenitic stainless steels.

Phosphorus Increases strength and hardnability, reduces ductility and toughness.  Increases machineability and corrosion resistance

Sulphur Reduces toughness and strength and also weldabilty.   Sulphur inclusions, which are normally present, are taken into solution near the fusion temperature of the weld.  On cooling sulphides and remaining sulphur precipitate out and tend to segregate to the grain boundaries as liquid films, thus weakening them considerably.  Such steel is referred to as burned.  Manganese breaks up these films into globules of maganese sulphide; maganese to sulphur ratio > 20:1,  higher carbon and/or high heat input during welding > 30:1, to reduce extent of burning.