Thermal Stresses Developed In High-strength Steels Subjected To Thermal Cycles Simulating Weld Heat-affected Zone

Residual stress less than tensile yield stress of the material is obtained for usual groove welds of high-strength steels (See Fig. 1). In the present research attention was focused on the difference in thermal stress cycle between mild steel and high-strength steel welds. Experiments were made in such manner that a round bar specimen (Fig. 3) set in a rigid frame (Fig.2) was subjected to a thermal cycle simulating weld heat-affected zone. The thermal cycle was given by high frequency induction heating and air-cooling or controled-cooling. Thermal stress developed in the specimen was measured by a load-cell (capacity 2 tons) connected to the specimen. A mild steel and HY-80 steel were used for the experiments (See Table 1.)Thermal cycles measured at several points on the specimen are shown in Figs. 4 and 5. Free expan-sions as shown in Figs. 6 and 7 were obtained between end chucks of the specimen during thermal cycles. Uniform temperature distribution along parallel part of the specimen was not expected because of high frequency induction heating. Peculiar shape of the free expansion curves is obtained as the result of nonuniform temperature distribution (See Fig. 8.).Figs. 9 thru 11 show thermal stress cycles plotted for surface temperature at the center of the specimen. It was found that factors by which residual stress is lowered are the decrease in Ar transformation temperature range over which yield stress of the material is rather small and the increase in expansion during the transformation. Therefore, rapid cooling from higher temperature than Ac1 point lowers residual stress level in high yield-strength steel welds (See Fig. 12).Degree of restraint for the rigid frame used was investigated by an analogical system shown in Fig. 13. The restraint was about one-third as large as a bar with uniform temperature fixed at the ends.