2022 年 108 巻 9 号 p. 631-641
Friction stir welding (FSW) was performed under the two welding conditions (rotation speed - traveling speed) of 150 rpm - 100 mm/min and 200 rpm - 400 mm/min using 24%Ni - 0.1%C (mass%) steel. The rapid cooling utilizing liquid CO2 was exploited to strictly evaluate the microstructural evolution of austenite in stir zone after FSW. In addition, the effect of the microstructural characteristics of austenite on martensitic transformation behavior during cooling were compared with that of 6%Ni - 0.63%C steel. Irrespective of the welding conditions, fine equiaxial grains with simple shear texture of {111}<110> orientation formed. At 200 rpm - 400 mm/min, finer grain formed without grain growth during cooling. On the other hand, at 150 rpm - 100 mm/min, recovery and grain growth occurred during relatively slow cooling after FSW. The grain growth preferentially proceeded presumably due to the strain-induced grain boundary migration of grain with lower dislocation density having {110}<110> orientation. Larger amount of retained austenite remained in the stir zone of 6%Ni - 0.63%C steel FSWed at 200 rpm - 400 mm/min due to both finer austenite grain size and higher dislocation density. Moreover, the influence of austenite grain orientation on the thermally induced martensitic transformation during cooling was revealed to be negligible, which is different from that of deformation induced martensitic transformation.
