Comparison Between Hot Rolled and PM/HIP Processed Duplex Stainless Steel UNS S31803

Abstract

Duplex stainless steels (DSS) are corrosion resistant alloys (CRA) which became largely employed due to its excellent combination of high strength, pitting corrosion resistance and toughness. The steel with basic composition 21.0–23.0%Cr, 4.5–6.5%Ni, 2.5–3.5%Mo, 0.08–0.20%N was developed in the 1980s and became the most popular DSS used in the oil and gas industry. The mechanical properties and corrosion resistance are optimized with a microstructure containing equal parts of ferrite (δ) and austenite (γ). This balanced microstructure is obtained by the chemical composition control and heat treatment. A solution treatment at 1050–1100 °C followed by water cooling is recommended. Although other heat treatments are not common in DSS, a significant hardening effect can be obtained by short duration exposition to 475 °C. Long thermal aging in the 350–550 °C interval has been extensively studied by many authors, and was proved to cause embrittlement and decrease of corrosion resistance, due to spinodal decomposition of ferrite into Cr-rich (α′) and Cr-depleted (α″) regions. However, a heat treatment at 475 °C for 4 h or 8 h may increase the yield and ultimate tensile strength without significant decrease of toughness and corrosion resistance, as observed previously. The goal of this work is to compare the microstructure, mechanical properties and corrosion resistance of two duplex stainless steels with similar composition, but produced by different methods: hot rolling and powder metallurgy PM/HIP. These two fabrication processes may be concurrent for some applications, such as manifolds in the oil and gas industry. The response of both materials to short duration aging at 475 °C was compared.