Study of the phases transformation in high temperatures in the uns S31803 duplex stainless steel.

Abstract

Duplex Stainless Steel, with ferritic-austenitic microstructure, have excellent mechanical propieties and corrosion resistence. However, when duplex stainless steel are exposed to temperatures between 300°C and 1000°C, some phase transformations can ocurr such as chromium nitrides precipitation of a Cr-rich phase (?'), sigma phase and the chromium carbides precipitation. The formation of such compounds leads to loss in both corrosion resistence and fracture toughness. The negative effects of the formation of chromium nitrides, carbides, ?' precipitation and sigma phase are due to chromium depletion in the matrix. The aim of this work is to identify and characterize the phase transformations, wich occur when aging heat tratments are carried out at temperatures at which the kinetics is the fastest for the reactions mentioned. The aging heat treatments were then carried out at 450°C, 600°C and 850°C for 10h, 24h and 100h. Microstructural characterization was done by using SEM. The identification of the phases by its morphology and then colour acquired after the matallographic etching can be made in a conclusive way because many of the phase transformations ocurred duringaging treatment are well know. The age hardening was measured by Brinnel hardness tests and the corrosion resistance by DL-EPR eletrochemical test.