Influence of aging heat treatment at 850 °C in the microstructure, mechanical and magnetic properties of Duplex steel UNS S3180

Duplex stainless steels are those that have a microstructure constituted by two phases, ferromagnetic ferrite (alpha) and paramagnetic austenite (gamma), which are present in approximately equal volumetric fraction. In this work, the microstructure of duplex steel UNS S31803 was modified by a thermal solubility treatment at temperature of 1050 degrees C for 30 minutes, followed by water cooling. Next, specimens were aged for 5, 15, 30, 60 and 180 minutes at temperature of 850 degrees C. This treatment promoted the formation of a deleterious phase, the sigma phase, which is formed by the decomposition of ferrite, rich in the elements Cr and Mo. The formation of this phase changes the mechanical and chemical properties of the duplex and super duplex steels. The magnetic and mechanical properties were evaluated in each of the conditions mentioned and a correlation with the microstructure was made. The magnetic properties of the specimens were obtained in a Vibrating Sample Magnetometer (VSM), where the material hysteresis curves were obtained. The mechanical properties of the samples were characterized by Vickers micro-hardness measurements performed with a load of 0.2 kgf. Microstructural characterization was carried out by using an optical microscope. VSM tests results showed that as the aging time increased, the volume fraction of sigma phase also increased, causing a diminution of the steel duplex magnetic properties. The diminution of the magnetic properties of the duplex steel with the aging time is attributed to the lower fraction of ferrite as more sigma phase is formed by the decomposition of ferrite. The hardness results showed an increasing of the hardness of the material as the aging temperature increased, which confirms the higher brittleness of the steel due to the higher amount of sigma phase.