Abstract
Microstructures of KA-SCMV28, KA-STPA29 and KA-SUS410J3 weld joints were investigated mainly by electron backscatter diffraction pattern (EBSP) in order to clarify the differences of fine grained HAZ formation mechanism among these steels. The welded specimens were heat treated at 740-760°C for several hours as the post weld heat treatment (PWHT). In the base metal of KA-SCMV28 and KA-STPA29, most of the observed boundaries were block and packet boundaries and the ratio of the boundaries with rotation angle of 60° was high. In the fine grained HAZ region of these steels, boundaries with rotation angles of 22 to 46°, which were different from K-S relationship, were detected and the ratio of block boundaries with rotation angle of 60° decreased. In order to investigate the formation mechanism of fine grains, as-welded specimen of KA-SCMV28 without PWHT was observed. Boundaries (22-46°) which were different from K-S relationship have already formed in the as-welded condition but the ratio of these boundaries was lower than in the specimen after PWHT. It was considered that fine grains were formed by the transformation of α’ to γ to α’ or α’ to α during welding cycle. On the other hand, in KA-SUS410J3 most of boundaries were block and packet boundaries and distinct fine grain region was not observed. In addition, distribution of the rotation angle was not much different between the base metal and HAZ region.
