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Cooling rate increases with welding speed and for a given welding speed the cooling rate in-creases with decreasing weld-pool size. The thermal cycle at any point in the medium is gov-erned by its distance from the moving heat source. As the distance from the heat source in-creases the peak temperature reached dein-creases 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)].

Distance from heat source

Temperature

Time

Fig. 5.6 (a) Temperature variation with time at various distances from heat source

Heat-affected zones

Weld

Heat Heat HeatHeat

Heat Heat

Melting point

°C

Heating

Cooling

°C

Heating Cooling

Time Time

Lowest temperature for metallurgical change

(b) Fusion boundary (c) Outer boundary of heat-affected zone

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 chemi-cal composition can be tailored by the composition of the filler metal used but its micro-struc-ture 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 reac-tions in the vicinity of the weld, reacreac-tions 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 nucleanuclea-tion). There is no homogeneous nucleanuclea-tion and thus the super-cooling 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 direc-tion 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 con-ditions 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.

Growthdirection Growthdirection

X X Y Y

Cell Cell Cell Cell Cell Cell

Liquid

Fig. 5.8 Schematic of solute distribution for cellular and dendritic growth patterns.

5.2.2 Gas-Metal reaction

The absorption of gas from the arc or flame into the weld-pool causes gas-metal reaction (since both the metal and the gas are at higher temperatures). There are two types of such reactions.

In the first type the gas may be just dissolved in the liquid metal. In the second type, the gas and liquid metal may chemically react to form stable chemical compounds. In case this chemi-cal compound is soluble it may cause embrittlement of the welded joint.

An insoluble reaction product may produce surface scale or slags and thus physically interferes with the formation of the weld pool. In this case the excess gas is either prevented or a flux is used to dissolve or disperse the reaction product.

When the gas is dissolved in the liquid weld pool, the gas evolves during cooling as its solubility decreases with fall of temperature. Gas bubles are formed. If these bubles are trapped, the weld becomes porous and of low quality. This defect is common in metals whose oxides are easily reducible by hydrogen, and can be avoided by the addition of a suitable deoxidant in the filler metal.

Another important gas-metal reaction is the diffusion of the gas into the parent metal from the weld pool. When the temperature of the thermal cycle is high, this diffusion process may be quite fast. The diffusion of hydrogen into the HAZ may again cause an embrittlement of the welded joint.

5.2.3 Liquid-Metal Reactions

During welding, non-metallic liquid phases are produced that interact with the weld metal.

These may be slag layers formed by the melting of flux in SMAW, SAW, etc. They may also be produced as a result of reactions occuring in the molten weld-pool and remain in or on top of the weld metal after welding.

The flux layers used in SMAW or SAW etc. processes are designed to absorb deoxidation products produced in the arc and molten metal. They usually float to the surface of the weldpool forming part of the slag, but some may remain in the metal as inclusions.

Another important effect of liquid solid interaction is hot cracking, which occurs during solidification. The interdendritic liquid, the last region to freeze, has a substantially lower freezing temperature than the bulk dendrite. The shrinkage stresses produced during solidification act upon this small liquid region and produce interdendritic cracks. These cracks occur at temperatures close to bulk solidification temperature, therefore, they are called hot cracks.

5.2.4 Solid State Reactions

Among the solid state reactions, the most important phenomenon is the formation of cold cracks or delayed cracks. This type of cracking is confined to steels that can be hardened.

These steel contain a hard phase called martensite.

The cracks occur after the weld completely cools down, sometimes hours after or even weeks after welding. This is always associated with the presence of hydrogen in the weld metal.

At high temperature the steel is F.C.C. austenite, a form in which hydrogen is quite soluble. On cooling the austenite changes to pearlite or martensite, and there is drastic reduc-tion of hydrogen solubility. In plain carbon steels this transformareduc-tion takes place at a rela-tively high temperature (about 700°C), even if cooling is rapid, there is sufficient mobility so that much of the rejected hydrogen diffuses out of the metal. Moreover the transformation product (ferrite plus carbide) formed in the HAZ are relatively ductile and crack resistant.

A rapidly cooled hardenable steel transfoms at a much lower temperature (generally below 400°C) and often room temperature, so the hydrogen is locked into the structure which may also be hard and brittle. It is this combination that induces cracking. This has led to the development of low hydrogen electrodes. These electrodes have to be protected from moisture.

5.2.5 Macro and Microstructure of Weld, Heat–Affected Zone (HAZ) and Parent Metal The metallurgical changes that takes place in weld and HAZ significantly affect the weld quality. The wide variety of changes that may take place depend on various factors, e.g.,

(a) the nature of the material (i.e. single-phase, two-phase) (b) the nature of the prior heat-treatment

(c) the nature of the prior cold working

We now consider typical examples of these changes.

Let us consider the fusion welding of two pieces of a single-phase material, which have been cold worked to yield a desired orientation. These cold worked grains result in a high strength and low ductility. However, on fusion welding, a random grain growth again takes place within the melt boundary, which, in turn, results in a low strength. Within the heat affected zone, the grains become coarse due to heat input (annealing), and a partial recrystallization also occurs. In either case, the strength falls much below that of the parent material. With increasing distance from the melt boundary, the grains become finer until the heat unaffected zone with elongated grains is reached. All these changes are shown in Fig. 5.9.

Original workpiece edge

Melt boundary Coarse

Fine Recrystallized grains

Original cold worked metal

Liquid

Solid qm

Heat affected zone

Ductility Strength

Fig. 5.9 Characteristics of welded joints in pure metals.

Let us now consider a two-phase material which derives its strength mostly from pre-cipitation hardening. In this case, the strength within the melt boundary is again too low. But, in the immediately adjacent heat affected zone, the thermal cycle results in heating and quench-ing followed by further agquench-ing. This agquench-ing process recovers some of the strength. The material beyond this zone is only overaged due to the heat of welding and becomes harder with the loss of strength. Hence, the strength and ductility variation near the joint are as shown in Fig. 5.10.

Precipitation hardened Overaged

Original precipitation hardened metal

Liquid

Strength

Ductility Heat affected

zone

Fig. 5.10 Characteristics of welded joints in precipitation hardened alloy

The two examples we have considered clearly demonstrate that various types of metal-lurgical changes are possible during welding, particularly for complex alloys. These changes are governed by the non-equilibrium metallurgy of such alloys, and must be clearly under-stood to yield a satisfactory fusion weld. Also, a decision on the postwelding heat treatment to be given, must be taken to restore the desirable characteristics of the joint.