Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si-Bearing 15Cr–9Ni–Nb Austenitic Stainless Steel Weld Metal

Two 15Cr–9Ni–Nb austenitic stainless steel weld metals with 2.5% Si and 3.5% Si (namely 2.5Si and 3.5Si samples, respectively) were designed and prepared through tungsten inert gas (TIG) welding and then hold at 800 °C or 900 °C for 3 h for stabilization. The microstructure and mechanical properties were investigated both for the as-welded and after-stabilization heat treatment (SHT) weld metals. There are 3.0–4.0% martensite and 2.5–3.5% δ ferrite in the 2.5Si as-welded weld metal and 6.0–7.0% δ ferrite in the 3.5Si as-welded weld metal. After SHT, a large amount of martensite formed in the 2.5Si weld metal, and δ → γ transition occurred during the SHT process both for the 2.5Si and 3.5Si weld metals. There were a large amount of coarse NbC and few nanoscale NbC in the as-welded weld metal. During the SHT, a large amount of nanoscale NbC formed in the matrix, while a large number of G phases formed at the austenite grain boundaries and the δ/γ interfaces. The decrease in solid solution C and δ ferrite content led to the decline of the yield strength of the weld metal after SHT. The martensite formed in 2.5Si weld metal after SHT had less effect on strength because of its low carbon content. The G phases formed during the SHT reduced the impact energy of the weld metal because it promoted the intergranular fracture, while the δ → γ transition reduced the amount of the δ/γ interfaces and avoided the intergranular fracture, which was beneficial for the impact toughness of the weld metals.