Influence of carbon content and ferrite morphology on the sensitization of duplex stainless steel

Abstract

The influence of ferrite morphology and carbon content on the intergranular corrosion behavior of 308 stainless steel was investigated using four wrought alloys and six weld deposited alloys. The four wrought alloys were heat treated at four different annealing temperatures to introduce four different amounts of ferrite. The annealed samples along with the weld deposited alloys were aged at temperatures ranging from 480 to 700°C for times varying between 15 min and 1000 h and then tested for intergranular corrosion susceptibility in acidified copper-copper sulfate solution. For a given carbon content there exists a critical amount and distribution of α-γ boundary area above which the alloy is immune and below which it is susceptible to intergranular corrosion. For amounts and distributions of α-γ boundary area less than the critical value two types of sensitization behavior are possible. First, there may be a sufficient amount and distribution of α-γ boundary area to insure rapid healing of the sensitized microstructure. Second, there may be an inadequate amount or distribution of α-γ boundary area to produce either immunity or rapid healing and the alloy behaves as a fully austenitic alloy regardless of the amount of ferrite present. A model is presented which describes as a function of carbon content the critical amounts and distributions of α-γ boundary area required for rapid healing and immunity to sensitization.