Investigation of the effect of low temperature aging on the mechanical properties and susceptibility to sulfide stress corrosion cracking of 22%Cr duplex stainless steel

Abstract

Duplex stainless steels (DSS) are corrosion resistant alloys largely used in the oil and gas production industry. Optimum properties are usually obtained with equal fractions of ferrite (δ) and austenite (γ) in the microstructure, which can be obtained by a proper solution heat treatment. However, work hardening can be used for applications where mechanical resistance must be increased, such as in casing and tubulars for oil and gas production. These pipes operate under high pressure in a harsh environment, composed by oil and high chloride water, low pH and high H2S and CO2 partial pressures. Additional strengthening can also be provided by low temperature aging which promotes the fine precipitation of a Cr-rich α’ phase by spinodal decomposition of ferrite. In this work, a DSS with 22%Cr (UNS S31803) received with an un-balanced microstructure (64% of ferrite), was aged at 475 °C for 8 h. For comparison, the material was correctly solution treated to obtain a balanced microstructure, and aged in the same way of the as received one. The effects on tensile mechanical properties, hardness and impact toughness were determined. The sulfide stress corrosion (SSC) susceptibility was evaluated by slow strain rate tensile (SSRT) tests in low pH and high pH2S chloride solution.