maximum increment of hardness measured is over 20% in the stainless steel specimen. Fig. 7 shows the hardness distribution of 6-mm-thick stainless steel specimens. The decrease and increase in hardness are also noted adjacent to the welded surface of the stainless steel specimen.
The increase in hardness is observed for the first time, which is contrary to the decreases in hardness being noted in various high-power ultrasonic applications.