2003 年 21 巻 2 号 p. 267-273
HAZ characteristics of laser welded ultra-fine grained steels were investigated using a 20 kW CO2 laser facility. A narrow softening region was clearly observed in HAZ welded at low welding speed of less than 0.017 m/s. The softening was caused by coarsening of ferrite grains which were heated above Ac1. On the other hand, HAZ softening was suppressed at higher welding speeds of more than 0.033 m/s due to the formation of tiny dispersed MA constituents and reducing grain coarsening. The prior austenite grain size was also examined at various welding speeds. The result indicate that the prior austenite grain size decreased with increasing the welding speed and with decreasing the initial ferrite grain size.