Magnetic and Thermoelectric Detection of Sigma Phase in 2507 Duplex Stainless Steel

Abstract

Duplex stainless steel has significantly broadened the range of applications of stainless steel. They have a dual-phase microstructure containing ferrite and austenite at approximately a 50–50% phase ratio. Their corrosion resistance is much better compared to the traditional austenitic stainless steel, especially in surroundings containing chloride ion. Moreover, the large stress yield of duplex steels offers significant advantages in structural applications. The ferrite phase in some duplex stainless steels is metastable due to its composition. Consequently, the ferrite can decompose to a secondary austenite and sigma phase due to heat input. The sigma phase is a hard and brittle intermetallic compound phase that significantly deteriorates the mechanical and corrosion-resistant properties of duplex stainless steel. The embrittlement can cause a safety risk in industrial applications. This paper is a preliminary study to investigate what physical properties can be used to obtain information on sigma-phase-induced embrittlement. In this work, the effect of plastic deformation and heat treatment was studied in the appearance of the sigma phase in 2507 duplex stainless steel. Magnetic saturation polarization and thermoelectric power measurements were used to monitor the microstructural changes due to cold rolling and heat treatment. It was found that the magnetic saturation polarization and thermoelectric power measurements can be effective tools for monitoring the sigma-phase formation in duplex stainless steels due to heat input. Their application helps to prevent the embrittlement problems caused by the sigma-phase formation in duplex stainless steel structures.