Cold-working imparts stresses in metals that remain after the cold-working operation is finished. These stresses are called residual stresses. When the cold-working is severe the grains are distorted as a result of this operation (see Fig. 11-17). Machining operations also impart residual stresses in metals as a result of the cold-working effect, the severity depending upon the size of the cut, the sharpness of the cutting tool, and the type of machining operation. There are other causes of residual stresses in metals such as grinding stresses caused by the grinding operation and residual stresses in castings resulting from a solidification of the cast metal in the mold. Welding will also impart residual stresses in a metal.

The grains of a bar of cold-drawn or cold-worked steel are distorted in a direction perpendicular to the cold-working operation and this bar will contain severe residual stresses. If this bar is heated
to below the lower critical temperature, usually 1000 to 1200F, the distorted grains will recrystallize and the residual stresses will be relieved, A new, and finer, grain structure will result from this operation, and the hardness of the bar will be lower.

This operation is called “stress relieving” or “stress-relief ’ anneal­ing. When welding a piece of cold-drawn or cold-rolled steel, the metal adjacent to the weld that has been heated to a temperature above 950 to 1000F, but not exceeding 1333F, will also be stress relieved as described and the grain structure will be refined.

Structure of the Weld

When making a butt weld (see Fig. 11-15) to join two mild steel plates, the liquid in the puddle is mostly above the melting point, but

Courtesy of George £. Linnert, “Welding Metallurgy" (New York: American Welding Society, 1965>

Fig. 11-15. Three views of crystals in a single-pass weld. (Top) Single-pass butt weld on a plate. (A) Three views of crystals or grains from top, side, and end. m the single-pass weld shown above. (B) A backing strip may exert a chilling action causing the crystals to grow vertically upward at the root of the bead.

at the boundary of the parent metal it is just at the melting point. The parent metal adjacent to the weld is in a mushy condition.

As the source of heat (the arc) is moved on, the mushy region solidifies and the molten metal in the puddle starts to freeze. The molten metal adjacent to the metal at the sides of the weld and, if present, at the solid weld bead starts to solidify first. Grains start to grow out from these surfaces. These grains have a columnar shape, as shown in Fig. 11*15.

As the metal in the weld continues to cool it solidifies completely. Upon cooling from below the solidification temperature to room temperature it undergoes phase changes (see diagram in Fig. 11*12). The temperatures at which these changes occur are lower than those shown in the iron-carbon diagram because of the relatively fast cooling rate of the weld.

The phase changes would occur exactly as described on pages 165 and 166 in a.2 percent carbon steel if the weld cooled slowly. However, the actual weld puddle cools very rapidly. For this reason a somewhat different microstructure will form. The microstructure in the weld will consist partly of a Widmanstatten structure and partly as minute plates of pearlite.