WELD METAL ZONE:
Weld metal zone is attained due to the fusion of a mixture comprising parent metal and electrode (or filler metal) and solidification from the molten state. The nature of the micro structure of the weld metal gives an indication of the rate of cooling applied on the weld. Depending upon the chemical composition, a martensite structure in the weld indicates a very fast cooling rate; fine pearlite, and coarse pearlite showing comparatively slower rates of cooling respectively . In this report, development of microstructure in the weld metal and the heat affected zones (HAZ) in mild steel has been emphasized. The reactions that take place in an arc welding process depend on three important factors namely oxidising effect of the arc atmosphere, concentration and nature of de-oxidants and temperature of the liquid metal . The various subzones formed in the microstructure due to non-uniform heat distribution have also been discussed. Finally the influence of carbon equivalent on micro-structural changes in the weld metal is also accessed.
2.1 WELD METAL ZONE:
The metallurgical properties of a weld metal are very different from that of the parent material. The fine oxide particles present in the weld filler metals induces nucleation of fine grains. When a weld solidifies through gradual cooling in a carbon steel, grain growth takes place from the course grain structure in the heat affected zone (HAZ) and the structure usually obtained after further refinement is an acicular ferrite( 2). Other transformation products such as pearlite, bainite,martensite and sometimes even retained austenite (ζ) may be found between the ferrite grains as illustrated in 3.
Mild steel and low carbon alloy steels transform in to austenitic phase when subjected to heat during the welding process. When subjected to slow cooling, ferrite starts to form as a result of ejection from austenite as the temperature falls below the upper critical temperature boundary ‘A3'. Ferrite (α) comprises of a solid solution of carbon in body centred cubic crystal structure. With further drop in the temperature, ferrite is precipitated; austenite grains reduce in size and carbon content increases. As the temperature falls to lower critical temperature A1 (723oC) the remaining austenite will change to a lamellar structure of alternate plates of ferrite and cementite as shown in 5, called pearlite .
Phase changes in the microstructure of mild steel have been depicted in the 4 with the help of iron and iron carbide diagram and can be summarised as follows:
a. When temperature is lower than lower critical temperature line A1, mixture of ferrite (α) and pearlite (P) grains constitute the steel microstructure. Microstructure is not significantly affected.
b. Pearlite (P) transformed to Austenite, but not sufficient temperature available to exceed the upper critical temperature line A3, therefore not all ferrite grains will transform to Austenite.
c. Once the welding temperature has reached ‘A3', the remaining ferrite grains transforms to austenite.
d. Austenite grain formation and growth takes place on heating to a temperature above the upper critical line ‘A3'. The microstructure consists of course pearlite with formation of ferrite at the grain boundaries. Rapid cooling especially between the temperatures ranging from 800°C to 500°C can result in a hard microstructure that is susceptible to brittle fractures .
Upon rapid cooling of steel from austenite phase, there is insufficient time for the carbon to form pearlite. The time over which carbon atoms can diffuse is considerably reduced as a result of which carbides precipitate from the austenite and forms bainite characterized by plate like structure. At even more rapid cooling rates the Face-centred cubic (FCC) structure of the austenitic phase consequently transforms to a distorted version of Body-centred cubic structure retaining the same composition as that of the austenite. This distorted structure is associated with the formation of martensite (m), characterised by the formation of fine needles as shown in 6.Martensite is formed only in those parts of a section that has been cooled sufficiently fast and may form in small pockets in thick sections owing to the difference in cooling rates .
Though the microstructure of both martensite and bainite appear quite similar bainite is darker. The hardness of martensite is dependent on carbon content . The hardness of martensite increases with the carbon content. Bainite is further classified into lower and upper bainite on the basis of different carbon diffusion rates at the temperature of its formation. Lower bainite forms at relatively lower temperature than upper bainite and closer to region of martensite formation as shown in the TTT diagram (9). As a consequence of welding, different sub zones appear in the microstructures of weld and HAZ regions. Rapid cooling results in martensite formation in the entire weld metal region and HAZ. There is variation in the physical characteristics and arrangement of laths of the martensite in different regions of the weld metal and HAZ owing to non-uniform cooling rates in different parts .
This type of martensite long is featured by long plates and commonly found in plain carbon and low alloy steels ranging to 0.5 wt% carbon. These occur in packets with each lath separated from another by angle shaped boundaries . Martensitic laths (as shown in 9) occur in a columnar arrangement in the regions of weld metal close to heat affected zone (HAZ). This is due to more effective and rapid cooling. While martensite grains are found in equi-axed packets in the HAZ region adjacent to the zone of columnar grains. As the central zone of the weld pool is subjected to very high temperatures and inefficient cooling, the microstructure of this region contains equi-axed packets of martensitic laths. The martensitic grain sizes in the HAZ are coarser than the sub-zones of the weld metal. The microstructure of HAZ region close to the base metal is found to contain matrix of unaffected ferrite and pearlite surrounding the homogenous martensitic phase .
2.2 HEAT AFFECTED ZONE
Heat affected zone is referred to as the portion of the base metal which has been subjected to a complex thermal cycle during the welding process involving rapid cooling. As a result the mechanical properties and microstructure of this region have been altered by the thermal cycle. Since the heat affected zone is subjected to a wide range of temperatures from the melting range of the steel down to comparatively much lower temperatures it consists of a series of graded structures. The HAZ in plain carbon steels usually contains a variety of microstructures, from narrow regions of hard martensite to coarse pearlite . The HAZ is therefore the weakest area in welds and is most susceptible to welding failure. In fusion welds the width of this region is only few millimetres. Extensive and rapid rate of formation of austenite grains occurs once mild steel is heated to temperature above the upper critical temperature (A3) during the welding process. Grain coarsening takes place beyond a specific point. Additives like aluminium, niobium etc could be introduced in to the steel to retard the austenite grain growth and result in finer grains . The heat-affected zone in an arc weld may be characterised by three zones namely: supercritical, inter-critical and subcritical. Variations in the microstructure and width HAZ can be observed with each weld run in the case of multi pass welds .
The supercritical zone is further classified into three regions which are discussed and illustrated below :
(a) The grain growth zone:
Grain growth is observed when the temperature during the welding process is greater than the grain coarsening temperature, adjacent to the weld metal zone (fusion boundary). The grain growth or coarsening of the structure occurs on subjecting the parent metal to a temperature above the upper critical (A3) temperature in the weld thermal cycle . The microstructure in this region depends on the austenite grain size and transformed structure. The maximum grain size and the extent of this grain growth region increases with the heat input rate and longer time of subjecting to temperatures higher than the grain coarsening. Therefore the largest grain size is generated in the case of electro-slag welds .
In mild and low carbon steel, pro-eutectoid ferrite is formed initially at the prior austenite grain boundaries and either a ferrite-pearlite, ferrite-bainite structure or a mixture of both develops inside the grain. Austenite transforms in to upper bainite, lower bainite, martensite or mixture of these owing to increased cooling rates and higher carbon content. With increased amount of coarse grains in the microstructure of the austenite phase, transformation would result in harder microstructures like bainite or martensite instead of ferrite-pearlite structure .
(b) The grain refined zone:
The grain refined zone is located adjacent to the grain growth region where the parent metal is subjected to lower temperatures and slower rate of cooling compared to grain growth zone. The parent metal has been heated to just above A3 temperature where grain refinement is completed and the finest grain structure exists. The microstructures of ferrite and pearlite areas are considerably fine indicating complete recrystallization .
(c)The Transition Zone:
In the transition zone, the microstructure is similar to the refined zone with the exception that pearlite regions have been made much finer. Partial allotropic recrystallization takes place but with no alteration in the ferrite grains. The metal is heated to temperatures ranging between the Al and A3 critical temperatures . Pearlite is transformed into austenite by heating in this range and by subsequent cooling reforms the pearlite.
The inter-critical region in low carbon and mild steel would comprise a microstructure of the martensite or bainite obtained from cooling austenite. As a result of the welding process, only the pearlite region in the initial ferrite-pearlite structure undergoes transformation in to austenite. Here the hard martensite grains are surrounded by the unaltered ferrite matrix .
Usually no major micro-structural changes occur in this region. However spheroidization is possible in localised regions. Also strain ageing could be experienced around any pre-existing crack in the parent metal .
Outside the heat affected zone, the microstructure of a portion of the parent metal is left unaltered owing to insufficient heating from welding process.
2.3 INFLUENCE OF CARBON EQUIVALENT ON WELD MICROSTRUCTURE
Since hardness is an indicator of the tensile strength and the extent of embrittlement, hardenability and prior austenite grain size must be considered. Hardenability is the ability to form hard metallurgical microstructure such as martensites or any other hard phases is dependent on the carbon equivalent and the cooling rate of the steel involved in cooling from the transformation temperature. Carbon equivalent gives a measure of hardenabilty of the steel. Carbon equivalent relates to the combined effects of different alloying elements employed in carbon steels. The tendency to form hard, brittle phases such as bainite and martensite during cooling increases with the carbon equivalent value and cooling rate. 
The metallurgical characteristics of steels are mainly determined by its chemical composition. The influence of various alloying elements can be better understood from the 14.
Even minor changes in the chemical composition of the base and filler metals could considerably increase cracking. ‘The risk of cracking also increases with increasing hardness of the Heat Affected Zone (HAZ) in welding for a particular hydrogen level and joint restraint.' The diagram shows the influence of certain alloying elements on the hardness of the weld microstructure. Carbon equivalent is significant in determining the occurrence of embrittlement and cracking during welding. Carbon equivalent has the limitation of accounting only for the chemical composition of the steel. 
The variation in the microstructure of the weld metal can be attributed to the non-uniform heating, difference in cooling rates and its chemical composition. Also with each succeeding pass in a multi-pass weld, the microstructure of the heat affected zones is affected. With rapid cooling rates, hard phases like martensite form the microstructure and make the steel brittle. The size of austenite grains produced during the arc welding plays a significant role in determining the size of the acicular ferrite grains. The nature of the grains, coarse or fine depends on the aspect of heat input which varies with each welding technique. Carbon and other alloying elements which are added to increase the strength of the steel, is responsible for brittle and hard phases. Carbon equivalent accounts for hardenability of the weld metal and is quite useful in when one has to assess the possibility of cracking as a consequence of welding thermal cycle.
1) Metallurgy of welding by J.F. Lancaster,1980
2) R.Krishnan1, R.K.SinghRaman, K.Varatharajan and A.K.Tyagi, “Microstructure and oxidation resistance of different regions in the welding of mild steel”, Journal of Materials Science Letter.
3) Kishor P. Kolhe, C.K. Datta, “Prediction of microstructure and mechanical properties of multipass SAW”, Journal of materials processing technology 197 ( 2008 ), pg:241–249
9) Interpretation of the Microstructure of Steels by H. K. D. H. Bhadeshia (http://www.msm.cam.ac.uk/phase trans/2008/Steel_Microstructure/SM.html)
12) Physical Metallurgy for engineering by Donald .S. Clark, Wilbur R. Vainey
13) Alfonso Rafael Fernández Fuentes, Nelson Guedes de Alcântara, Sergio Haro Rodríguez, Alejandro López Ibarra; “Effect of in service weld repair on the performance of CrMo steel steam pipelines.” Materials Research, Mat.Res.vol.9no.2São Carlos(2006)