Effects of Aging at 550 °C on α′-Martensitic Transformation of High Si-Bearing Metastable Austenitic Stainless Steel Weld Metal

The high Si-bearing 15Cr–9Ni–Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment (SHT) at 850 °C for 3 h. The effects of 550 °C aging on the α′-martensitic transformation of the as-welded and the SHT weld metals were investigated. The results showed that the weld metal had poor thermal stability of austenite. The precipitation of NbC during the 850 °C SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α′-martensite. The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α′-martensite at the early stage of 550 °C aging. The homogenized diffusion of C and Cr in the matrix during 550 °C aging led to the restoration of austenitic thermal stability and the decrease of α′-martensite content. The C-depleted α′-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix, while the Cr-depleted α′-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.