Abstract
Laser pulses with about 0.2J energy and 8ns duration from a frequency-doubled Nd : YAG laser were focused to 1mm and irradiated to the water-immersed sample of high tensile strength steel (JIS SHY685) without coating. Various laser scanning patterns i.e., single spot, line and area, and irradiating pulse density were examined to understand the generation process of a compressive field in area scanning. The detailed distributions of the surface residual stress on the specimens were measured using synchrotron radiation. A tensile residual stress was observed in the center region of the single spot, whereas a compressive field appeared around the spot. In the line scanning, a tensile component was observed in the final spot of the line, however it decreased and changed into compression by the increase in the number of the irradiating pulses. It can be concluded from these results that the compressive residual stresses in the line and area scanning were generated by off-center overlapping of successive laser pulses.
