Chiang Mai Journal of Science

Microstructural characterization of lean duplex stainless steel UNS S32101 welded joints using scanning electron microscopy (SEM) and backscatter diffraction (EBSD) is discussed in the present paper. The influence of isothermal aging heat treatment at 650°C on the microstructures of weld, heat affected zone (HAZ) and parent metal was investigated. The welded joint in the as-welded condition shows Cr2N nitride precipitation in the HAZ, while isothermal aging at 650°C for 15 min causes further precipitation of nitrides, both in the parent metal, as well as in the HAZ and the weld area. Increasing the aging time at this temperature to 90 min causes the formation of carbides at the grain boundaries and nitride precipitation inside ferritic grains in each zone of the welded joint.

                The EBSD analysis of the welded joint after aging at 650°C for 90 min revealed that ferrite content decreased from 45% in parent metal to 40% in HAZ and 39% in the weld. The higher austenite content in the weld, closer to equilibrium, optimized the mechanical properties of welded joints. Grain size of austenite increased gradually from the parent metal over the HAZ to the weld where it was the highest. The ferrite shows smallest grain size in the weld, in the parent metal and the weld it was similar. The orientation relationship between adjacent grains of austenite and ferrite observed from EBSD in the weld metal and the HAZ follows both (K-S) and (N-W) orientation relationship but favors the (N-W) relationship. The presence of such relationship between austenite and ferrite in the weld metal together with the decrease of high-angle grain boundaries in both phases in the welded joint can possibly positively influence the reduction of residual stresses and decrease the susceptibility for crack initiation in the weld metal.