Structure of the Weld and the Weld-Affected Zone

During the welding process, the metal adjacent to the weld is also heated. The temperatures existing in a mild steel plate during weld­ing are given in Fig, 11-16. Since the temperatures adjacent to the weld exceed the critical temperatures, the grain size of the metal in this zone is affected.

The effect of the welding heat on the metal adjacent to the weld is shown in Fig. 11-17. In this case the metal is iron and two welds are shown. The upper weld is made on iron that has not been cold - worked, while the lower weld is on previously cold-worked iron. Observe the difference in the shape of the original grains in these two metals.

In both cases, the metal in the weld has columnar grains. Adja­cent to the weld the temperature exceeded the grain-coarsening temperature and, as a result, this region always is characterized by its coarse grains. Further away from this region there is an area where the maximum temperature exceeded the critical temperatures but did not exceed the grain-coarsening temperature. This results in a region of very fine grains that is always present in the case of iron and steel.

Still further away there is a region where the maximum tempera­ture was above 950F but did not exceed the lower critical tempera-

Structure of the Weld and the Weld-Affected Zone

Courtesy of George E. tinner t, “Welding Metallurgy" (New York; American Welding Society, 1965}

Fig. П -16. The distribution of the temperature in a mild steel plate at an instant during welding. W-Liquid weld metal (puddle). The shaded area is the metal that is in the mushy stage.

ture. The temperature range did not affect the grains in the upper piece (Fig. 11-17), in which the original grains are not cold-worked. However, in the lower piece, which was cold-worked, the grains were refined and the residual stresses relieved. In the lower view the second region of refined grains can be seen. It is interesting to note the coarser-grained region between the two fine-grained regions. This is the result of heating to just below the lower critical tempera­ture. To avoid this coarse-grain structure, the maximum temperature for stress-relief annealing should not exceed 1200F.

In summary, the region adjacent to the weld is always character­ized by coarse grains which is followed by a region where the grains are highly refined. Cold-worked steels have a second region where the grain structure has been refined, which does not exist in a steel that has not been cold-worked. The region in which the large grains exist is less ductile than the fine-grain region and the other regions

Courtesy of George E. Linnert, "Welding Metallurgy” (.Ww York: American Welding Society, 1965)

Structure of the Weld and the Weld-Affected Zone

Structure of the Weld and the Weld-Affected Zone

950° F

1670° F

1670° F 950° F

50 76 (00 125 «50


Fig. 11*17. Single pass weld in iron. (Top) Iron annealed before welding. Note grain refinement in the vicinity of the zone that has reached 1670F during welding. Chart at the right shows that welding did not affect hardness. (Bottom) iron cold-worked (cold-rolled) before welding. Note grain refinement in vicinity of the zones that reached 950F (approximate temperature of recrystallization) and 1670F. Chart at right shows that welding has softened the iron in the zones that were heated above 950F.

where smaller grains exist, unless they are very severely cold - worked.

An advantage of multiple-pass welding is that the following pass refines the grain in the previous pass. The second pass of a two-pass weld in mild steel will, for example, refine the grains in the first bead.

The weld will consist of a fine-grained lower bead and a coarse­grained upper bead; therefore, it will have a better ductility than a single-pass weld.

In the case of mild steel with a.2 percent carbon content, the weld metal will have a Widmanstatten structure at room temperature. All of the metal adjacent to the weld that has been heated above 2000F will also have this structure. The metal that did not reach 2000F, but was heated above 1500F, has a structure consisting of small grains of ferrite among which small grains of pearlite are distributed. This structure is more ductile than the Widmanstatten structure.

The next region is that which has not been heated above 1560F but above the lower critical (1333F) temperature. This region will have some rather large ferrite grains and clusters of finer ferrite and pearlite grains.

The region that did not reach the lower critical temperature remains essentially unchanged although there is some tendency for spheroidite to form. Usually the speed of welding is so high that spheroidization of the cementite in the pearlite rarely occurs. This region will consist of the original ferrite and pearlite grains.

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