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The microstructure of a material is significant when it comes to the properties and characteristics of a particular material. It would be perfect if the properties and characteristics, which are related to the microstructure, of the parent metal, heat affected zone and the weld metal is the same. However the probability of occurrence of such a situation is very less since the parent metals are used in the wrought form and the weld metals are used in the cast form. Wrought materials got superior strength, ductility and toughness when it is weighed against the materials in the cast form. Even then the weld metal properties draws near the properties of the wrought material, since it is a minuscule casting which is rapidly cooled. This situation is particularly related with the ferrous materials, which includes mild steel also (Houldcroft and John, 1988). The report holds the information regarding the development of microstructure during the arc welding of mild steel plate, changes which occur in the heat affected zone and the change in the structure of the steel when the carbon equivalent of the steel was increased.
Steel with a low carbon content of 0.25% is known as mild steel. Mild steel is easy to weld and fabricate because of its low carbon content since it would not get harden by heat treatment. This leads to the lack of hardened zones in the heat affected zones and welds, even though there is quick cooling. As the carbon content increases, the ease in welding reduces because of the quenching action (Davies, 1993).
Welding is primarily classified into two welding methods and they are (1) Plastic welding and (2) Fusion welding. It can be further fragmented into eight divisions on the basis of its specific processes and they are (1) Cold welding, (2) Thermit Welding, (3) Gas welding, (4) Resistance welding, (5) Arc welding, (6) Braze welding, (7) Forge welding, (8) Induction welding. The welding processes such as Cold welding, Pressure welding, Resistance welding and Forge welding comes under the Plastic welding division whereas the welding processes such as Gas welding, Thermit welding, Induction welding and Arc welding belongs to the Fusion welding processes (Clark, 1962).
The electrode material and shielding technique are the basis of classification of Arc welding processes. In mass production, the automatic welding technique is very important and the Arc welding technique is well adapted to it. Added on to this, Arc welding technique imposes a lot of flexibility to the joining of both thin and heavy sections of a material. Another trait of Arc welding process is that the heat application in this particular welding process is highly concentrated when compared to other welding processes (Clark, 1962).
Microstructure of weld metal
The microstructure of the weld metal is primarily dependent upon the alloy content of the carbon steel. Whereas in carbon, carbon manganese and micro-alloyed steel, the weld metal microstructure is mainly affected by the welding procedure and composition of the weld.
According to Lancaster, 1999, the microstructure of Carbon-Manganese alloy steel is affected by the aspects such as cooling rate, composition, plastic strain and the presence of non-metallic nuclei. Figure 2.1 shows the effect of cooling rate and composition on structures produced in the weld.
The above details show that the steel containing less than 0.30% C will have similar microstructures after the welding process. During the Arc welding of mild steel a number of discrete structural zones, such as unaffected, transition, refined, coarsened, fusion and deposited metal zones are formed. These zones are shown in the diagram and it is compared with the relevant section of the iron-iron carbide diagram. Many of these zones will not be having discrete line of demarcation and they appear to be merged together (Clark, 1962).
In the unaffected zone, the parent mild steel is not heated to an adequate amount to reach the critical range. Therefore, the structure is unchanged and the unaffected zone represents the archetypal grain structure of the parent mild steel. The figure shows the microstructure of the unaffected zone of mild steel. It consists of a typical combination of ferrite and pearlite (Clark, 1962).
Next to the unaffected zone, there exists a region where there is a temperature range, between the A1 and A3 transformation temperatures, in which a limited allotropic recrystallization takes place and this particular zone is known as the transitional zone. The transition zone has a microstructure of both ferrite and pearlite. But the size of the pearlite region will be different from that in the unaffected zone. The pearlite region will be much finer which is due to the heating of the mild steel to the critical range and due to the cooling after the heating process. During the heating process, the pearlite will be transformed into austenite and then transformed into finer pearlite grains on cooling (Clark, 1962).
After the transition zone, comes the refined zone. In this zone, the temperature is heated just above the A3 temperature and the finest grain structure exists in this region as a result of the extensive grain refinement. The figure shows the microstructure of the refined zone of the mild steel. The microstructure consists of much finer structures of pearlite and ferrite. These structures are formed from the austenite which existed at a temperature just above the upper critical temperature (Clark, 1962).
The region next to the refined region is known as the coarsened zone. In this zone, the temperature is higher than the A3 temperature and the grain structure will be coarsened. When it comes to the coarsened zone, the microstructure will be dominated by pearlite grains and ferrite will be of smaller grain. Due to the prevailed rate of cooling, the pearlite will show a higher rate of finer grains than that existed in the original pearlite areas, when it is magnified (Clark, 1962).
The actual melting of the parent metal takes place when the temperature is higher than the solidus and the zone in which this takes place is known as the fusion zone. In the fusion zone, the microstructure will be of a very coarse structure. This type of structure is common in mild steel where the particular structure is formed from the large austenite grains when the cooling rate is of a medium pace. The following figure shows the microstructure in the fusion zone (Clark, 1962).
Deposited metal zone
The deposited metal zone is a zone along with the fusion zone where there is a coarse grain structure and it happens when a filler metal is added to the weld. The structure of deposited metal zone is shown in the figure. As you can see in the figure, the microstructure consists of columnar structure of ferrite and pearlite (Clark, 1962).
Heat affected zone
The possibility of performing a welding process without building up a thermal gradient in the parent metal is almost negligible. The temperature and the speed of the welding process is very influential in deciding the spread of heat into the parent metal. The thermal gradient will get compressed by the high power welding at high speed (Houldcroft and John, 1988).
The schematic sketch of a weld, heat affected zone and relevant portion of the iron-carbide phase diagram is shown in the figure 3.1. The base metal is heated up to a peak temperature and it varies along with the distance from the fusion line. If the lower critical temperature, A1, was surpassed by the peak temperature, then there will be a transformation from ferrite to austenite. This transformation will be complete and an austenitic microstructure is formed when the temperature goes beyond the upper critical temperature, A3. The ferrite structure is stable at room temperature and has bcc crystal structure whereas the austenite structure is stable at high temperature and has fcc crystal structure (Raj et al, 2006).
The heat affected zone of an arc weld in steel is classified into three regions, such as supercritical, intercritical and subcritical regions, from a metallurgical perspective (Lancaster, 1999).
The supercritical zone
The supercritical zone can be classified into the grain growth region and the grain refined region. Coarse grain heat affected zone (CGHAZ) is the term which is used to refer to the region of heat affected zone where extensive growth of austenite grains takes place when the temperature goes beyond the temperature of 1300 degree Celsius. The region next to the CGHAZ, which is at a temperature range of 900 to 1200 degree Celsius, is known as the Fine grained heat affected zone (FGHAZ). In this region of the steel, the austenite grain size remains small (Raj et al, 2006).
The intercritical zone
The intercritical region is narrow when compared to other zones and partial transformation takes place in this zone. The region of HAZ, which is having a temperature range in between the critical temperatures A1 and A3 is referred as Inter critical heat affected zone (ICHAZ) (Raj et al, 2006).
The subcritical zone
In the subcritical zone, not much observable alteration in the microstructure will be there except the occurrence of a small region of spheroidization, which is difficult to detect. The tempered zone and unaffected base material comes under this zone (Raj et al, 2006).
The microstructures such as ferrite and other metastable phases are formed during the cooling cycle of a welding process, from an austenite microstructure which was formed at high temperatures. For welds produced with adequate pre-heat or for high heat input welding, the cooling rate will be less and this leads to the formation of a mixture of ferrite and carbides whereas in a high cooling rate scenario, microstructures such as bainite or martensite are formed from austenite. The formation of bainite and martensite is also affected by the amount of carbon content and alloying elements. This particular trait of steel to form a hard microstructure such as bainite or martensite from austenite phase when cooled at high rate is generally referred to as hardenability and this increases with the austenite grain size and alloy content of the steel. Therefore in the case of mild steel, the microstructure of the heat affected zone (HAZ) is of carbide and ferrite after performing an arc welding even if it is performed without any preheating (Raj et al, 2006).
The effect in the increase of carbon-equivalent of steel
The carbon equivalent plays an important role in deciding the microstructure of the steel. Along with this, the cooling rate during the welding process too plays a decisive role in this regard. The probability of formation of martensite or bainite in high carbon equivalent steels is high and in order to avoid that situation, use of distinctive techniques, such as preheating and post-heating are required (Clark, 1962).
Carbon equivalent calculation
In order to discuss about the effect of carbon equivalent in deciding the microstructure of mild steel during the arc welding process, first we have to discuss the formula which is used to calculate the carbon equivalent of steel. The carbon equivalent can be calculated by the formula.
CE= C% + (Mn%)/6 + (Cr%+Mo%+V%)/5 + (Ni%+Cu%)/15 (Davies, 1993).
This formula is relevant to the plain carbon and carbon manganese steel but it is not applicable to micro-alloyed high strength low-alloy steel or low alloy Cr-Mo type.
Due to Ito and Bessyo, the formula used by Japanese Welding Engineering Society is Pcm= C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B (Lancaster, 1999).
As mentioned in the previous sections, the formation of hard microstructures such as bainite and martensite is dependent upon the cooling rate as well as the carbon equivalent in the steel. During the welding process of mild steel, the heat will be absorbed faster by the steel and creates a sudden fall of temperature (Raj et al, 2006).
Here, steels with three different carbon contents are compared with the help of an Iron-Iron Carbide Equilibrium diagram. The steel with less than 0.83 percent carbon content is known as hypo-eutectoid steels, steel with 0.83 percent carbon content is known as eutectoid steel and steel with more than 0.83 percent carbon content is known as hypereutectoid steel (Clark, 1962).
Steel with 0.1% carbon content
This type of steel belongs to the hypo-eutectoid steel. As you can see from the Iron-Iron carbide diagram, when a 0.1% C steel is cooled at an appropriate rate from 2800 F to room temperature, a mixture of austenite and delta solid solution is formed from the delta solid solution and liquid. On further cooling, grains of austenite are formed from the former followed by formation of ferrite and austenite. By the time the cooling is done till the room temperature, a microstructure of ferrite and pearlite will be formed (Clark, 1962).
Steel with 0.8% carbon content
This form of steel has a composition which is very near to the composition of eutectoid steel. During the process of cooling of this steel from 2800F, the transformation starts from the molten state into a liquid and austenite form. Then on further cooling, formation of austenite followed by the eutectoid called pearlite will occur (Clark, 1962).
Steel with 1.2% carbon content
This form of steel belongs to the hypereutectoid steel. During the cooling process of this steel from 2800F, the transformation starts from the molten state of steel into a liquid and austenite form. Then on further cooling, there will be formation of austenite, combination of austenite and cementite, and ends with ferrite and cementite at room temperature (Clark, 1962).
Microstructure of steel is a very important deciding factor when it comes to its properties and behaviour. It is obvious from this report that the cooling rate during the welding process, composition of weld metal and the type of welding process plays a vital role in the formation of the different form of microstructures in the weld metal.
The weldability and hardenability of the steel depends a lot on the carbon content of the steel to be welded. As the carbon content of steel increases, the weldability of that particular steel decreases and its hardenability increases. This proves that the composition of the weld metal plays an imperative role in the characteristics of a welded material.
This report illustrates that the weldability of mild steel is quite good and the role of composition of mild steel in achieving so. It also gives you an idea about the various changes that occur to the microstructure of the mild steel during the arc welding process.
- Clark, D. and Varney, W. (1962) Physical metallurgy for Engineers. 2nd edition New York: D Van Nostrand Company.
- Davies, A.C. (1993) The science and practice of welding, vol 2, The practice of welding. 10th edition Cambridge: Cambridge University Press.
- Houldcroft, P. and John, R. (1988) Welding and cutting. 1st edition Cambridge: Woodhead-Faulkner Limited.
- Raj, B., Shankar, V. and Bhaduri, A. (2006) Welding Technology for Engineers. 1st edition Oxford: Alpha Science International Limited.
- Lancaster, J.F (1999) Metallurgy of Welding. 6th edition UK: Abington Publishing.