溶接熱サイクル過程において塑性予歪をうけた鋼の室温における延性

Abstract

This report discusses mechanical properties, in particular ductility, at room temperature of steels subjected to prestrain during weld thermal cycles. Three kinds of steels, a plain carbon steel (SM-41A), 50 kg/mm2 high strength steel (SM 50) and a heat-treated high strength steel (HY 80) were used for experiments. (See Table 1.) Thermal cycle simulating weld heat-affected zone was applied to a round bar specimen shown in Fig. 1 and plastic strain was given at a temperature during cooling. Fig. 2 shows schematic explanation of the test procedure. High frequency induction heating was used to obtain a synthetic weld thermal cycle. Plastic strain was given for about 4 seconds at a temperature during cooling through the tensile load produced by compressed air. Tension test was done at room temperature on the specimensubjected to the thermal cycle and prestraining. Attention was paid to general elongation or strain at the maximum load in addition to usual mechanical properties such as reduction in area, yield stress, ultimate tensile strength and fracture stress.The mechanical properties at room temperature change with cooling conditions, prestraining temperature and magnitude of prestrain. Prestrainig at 400°C to room temperature, in particular in the temperature range of blue brittleness, increases flow stress and decreasees ductility at room temperature (See Figs. 10 and 11.) Serious effect was the decrease in general elongation at room temperature. (See Figs. 12 and 15.) General elongation of SM41A steel and HY80 steel decreased respectivity from 24% and 15% as received to 15% and 9.5% after thermal cycle and to only 8% and 1% after thermal cycle combined with 5% prestrain at 200-300°C during cooling. The general elongation decreased with the increase in ratio of yield stress to ultimate tensile strength. (See Fig. 24).