Fig. 8 shows the relationship between the temperature, the heating time and the hardness of the stainless steel specimen, which was measured after heating in the furnace. The hardness of the stainless steel increases according to rather low heating temperature and short time due to the aging effect of stainless steel.The hardness of the stainless steel specimen increases slightly as temperature increases to 950 C, decreases in the range between 950 and 1000 C, and then increases when the temperature exceeds 1200 C [5].
In the absence of ultrasonic vibration and static pressure, the increase in hardness of stainless steel specimens could be attributed to such high temperature rise of over 1200 C at the weldment. However the hardness increase effect should actually ascribed mainly to the vibration as no visible light emission due to the high temperature rise at the welded part was observed. In fact, the maximum temperature rise directly mea-sured from the thermo-electromotive force among alu-minum, copper and steel was only 441 C [4].