The fabrication of welded-structure and their use in service involves most of the principal phases included in the metallurgy of ferrous and non-ferrous alloys.
The processes which are common in the welding are as follows:
(i) Fusion,
(ii) Casting,
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(iii) Hot and cold forming,
(iv) Heat-Treatment,
(v) Metallurgical structure.
Regardless of the process employed, a thermal cycle is introduced during heating in the welding process. In it, the metal is heated over a range of temperature (up to fusion) and followed by cooling to ambient temperatures.
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In other words a differential heating will take place, thereby the metal remote from the weld will be simply warmed out, but as the weld area is approached, progressively higher temperatures are obtained, resulting in a corresponding complex mixture of microstructure particularly in steel.
The heating and cooling also results in setting up internal stresses and plastic strain in the vicinity of the weld. Also at higher temperatures, certain chemical changes are liable to take place. Thus, whatever the process it may be, in a thermal cycle, physical, chemical and metallurgical properties are liable to change.
Fig. 9.12 shows the thermal gradient during a welding process.
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Depending upon the welding process used and the metal being welded, there exist wide variations in the maximum liquid temperature and in the slope of temperature gradient curve for the unused metal.
The slope of thermal gradient depends upon the way in which heat is being supplied per unit volume of metal per unit time and the thermal conductivity of base metal parts. The heat affected zone is wide with gas welding than arc welding because heat is concentrated for longer time in gas welding.
As a rule, the flow of heat in the weld zone is highly directional towards the adjacent cold metal, there by producing what is called columnar grains at right angles to the fusion line. Columnar-structure is a characteristic of the metal of single-pass welds.
Thus the original structure consisting of ferrite and pearlite in steels is changed to another micro-structure. The composition of the first crystals which from in a molten alloy may be quite different from the composition of liquid but as the freezing proceeds, the crystals readjust their composition to that of the initial liquid alloy in order to satisfy the condition of equilibrium.
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The weld metal, when it is in the molten state has a good capacity of dissolving gases which come into contact with it, such as oxygen, nitrogen and hydrogen. As the metal starts cooling, capacity of dissolving gases goes on decreasing, thus causing additional volume of gases to be evolved at the time when the metal is becoming mushy and, therefore, incapable of permitting the gases to escape freely. The entrapment of gases causes gas pockets and porosity in the final weld.
Fig. 9.13 shows the mechanism of fusion or liquid state welding process. The material around the joint is melted in both the parts to be joined and if necessary, a filler material is added. Three distinct zones, viz. fusion zone, heat affected un-melted zone, and unaffected original base metal can be observed. Heat for fusion welding is produced at a high temperature and at a high rate in a sharply defined, isolated zone.
Fig. 9.14 shows the structure of a weld section. It may be noted that columnar (long elongated) crystals are formed near fusion faces due to directional cooling of weld towards the centre. Since the inner part of weld cools more uniformly it results in an enlarged but regular crystal structure. Surface of weld being in contact with air cools very fast and small and slightly chilled crystal structure can be noted there. The parent metal in heat affected zone experiences grain growth.
In case of thick plate, several passes of weld would be required and the structure of previous weld would be refined by the heat in the subsequent welding. For single pass welding, post weld heat treatment is desirable to refine the weld metal structure.
The most of ferrous and non-ferrous commercial metals can be welded, provided appropriate welding conditions are employed.
The following common metals and non-metals are used during heating in order to improve the weldability of metal:
(i) Carbon,
(ii) Manganese,
(iii) Phosphorus,
(iv) Sulphur,
(v) Silicon,
(vi) Nickel,
(vii) Copper,
(viii) Chromium,
(ix) Molybdenum,
(x) Vanadium,
(xi) Aluminium,
(xii) Titanium.
The above mentioned elements exert their influence on the micro-structures and physical properties across the welded joint:
(i) By strengthening of ferrite in the solid solution.
(ii) By formation of carbides.
(iii) By formation of inter-metallic compounds.
(iv) By oxidation and de-oxidation in the weld.
(v) By increasing or decreasing the hardenability of the heat-affected zone.
(vi) By controlling the grain size.
(vii) By raising and lowering the transition temperature (i.e., the temperature at which the material ceases to display ductile properties and becomes brittle).
One of them or all of them plays a vital role in changing the chemical composition of metal and the thermal effect during welding operation.