Experimental assessment of temperature distribution in heat affected zone (HAZ) in dissimilar joint between 8Cr-2W steel and SUS316L fabricated by 4 kW fiber laser welding

High-Cr steel (Fe-8Cr-2W-0.4Mn-0.2V-0.06Ta, so-called F82H steel) is one of the candidate structure materials for fusion blanket system, and its dissimilar welding technology to stainless steel (SUS316L) is an inevitable issue. Fiber laser welding is a promising technique to joint F82H and SUS materials, because of its non-mandatory vacuum environment during welding, and the reduced distortion and residual stress. Purpose of the present study is to identify the microstructure characteristics of dissimilar joint between F82H and SUS316L fabricated by fiber laser welding. M₂₃C₆-type precipitates were evaluated using an electron back-scatter diffraction device equipped with a scanning electron microscope, an electron probe micro analyzer, and a transmission electron microscope. The microstructure of as-welded specimens was composed of heat affected zone (coarse grain (CGHAZ) and fine grain (FGHAZ)) in F82H side, and weld metal (WM). Distributions of size, density and chemical composition of precipitates in HAZ were examined as a function of the distance from WM/HAZ interface. Besides, isochronal annealing experiments on F82H performed to obtain relationship between annealing temperature and Cr (or C) concentration in M₂₃C₆ precipitates. Cr-rich M₂₃C₆ comparing with original F82H (tempered F82H) one was observed at temperature ranging from 1103 to 1173 K. By comparison of their microstructure with that of HAZ, temperature distribution in HAZ upon welding was evaluated. It is suggested that the HAZ/F82H interface and the FGHAZ/CGHAZ boundary were heated up to 1150 K and 1400 K, respectively. Further applications of this method to clarify the temperature history and its distribution in HAZ, as well as increase in diagnostic accuracy, are greatly expected. (author)