Cooling rate increases with welding speed and for a given welding speed the cooling rate in­creases with decreasing weld-pool size. The thermal cycle at any point in the medium is gov­erned by its distance from the moving heat source. As the distance from the heat source in­creases the peak temperature reached decreases and the temperature further lags behind the source. Fig. 5.6 (a) shows the variation of temperature with time at different distances from the heat source. Weld microstructures will depend upon the cooling rates [Fig. 5.6 (b) and (c)].

Fig. 5.6 Variation of temperature with time at different distances from the heat source (b) fusion boundary (c) outer boundary of HAZ

5.2.1 Weld-Metal and Solidification

Welded joints contain a melted zone, which on solidification comparises the weld-metal. It is composed of varying mixtures of filler metal and base metal melted in the process. Its chemi­cal composition can be tailored by the composition of the filler metal used but its micro-struc­ture and the attendent mechanical properties are a direct result of the sequence of events that occur just before and during the period of solidification. These events include gas metal reac­tions in the vicinity of the weld, reactions with non-metallic liquid phases (slag or flux) during welding and solid-state reactions occuring in the weld after solidification. Let us first consider solidification.

Solidification. In arc-welding the molten weld pool is contained in a surrounding solid metal. Thus a liquid-solid interface, present at the fusion boundary provides an ideal nuclea - tion site (heterogeneous nucleation). There is no homogeneous nucleation and thus the super­cooling is negligible. Since the heat flow in welding is highly directional towards the cold metal, hence the weld acquires a columnar structure having long grains parallel to the direc­tion of heat flow (Fig. 5.7).

In the case of pear-shaped growth shown on the right, the columnar grains growing from apposite sides meet at the middle of the weld. This midplane solidifies last and often contains impurities and porosity. It is prone to fracture at low strains. This defect is called ingotism and can be corrected by adjusting the joint gap configuration and weld procedure.

There is a unique dependence by the dendrite arm spacing on energy input. The more rapid the solidification, the more closely spaced are the dendrites.

Fig. 5.7 Columnar structure of welds Left: Shallow weld;

Right: Deep pear-shaped weld.

When solidification is extremely rapid, dendrites do not develop fully, under these con­ditions a much shorter projection of the freezing interface into the liquid weldpool occurs which is called a cell structure. Spacing between cells are normally smaller than those between dendrites and the segregation of solutes is not so extensive. Examples of dendrites and cells are shown in Fig. 5.8.