Abstract
Duplex stainless steels, with ferritic-austenitic microstructure, have excellent mechanical properties and corrosion resistance. However, when duplex stainless steels are exposed to temperatures between 600 and 1000 °C, some phase transformations can occur such as chromium nitrides precipitation, chromium carbides precipitation and the sigma phase formation. The formation of such compounds leads to loss in both corrosion resistance and fracture toughness. The negative effects of the formation of chromium nitrides, carbides and the sigma phase are due to the chromium depletion in the matrix. The phase transformations cited above occur initially at ferritic-austenitic interfaces and at the grain boundaries. The aim of this work is to identify and characterize the phase transformations, which occur when aging heat treatments are carried out at temperatures at which the kinetics is the fastest for the reactions mentioned. At first, the samples were annealed at 1100 °C for 40 min. The aging heat treatments were then carried out at 850 °C for 6, 40 e 600 min. Microstructural characterization was done by using optical microscopy with different etchings, in order to identify each phase formed in the duplex stainless steel during aging heat treatments. The toughness was also evaluated by using Charpy impact test. Impact tests show that loss of toughness was related to phase transformations.
Highlights
The duplex stainless steels, with ferritic-austenitic microstructure, have better corrosion resistance than the austenitic stainless steel and better mechanical properties than the ferritic stainless steel[1]
Different etchings were used to characterize the microstructure of the samples after annealing and aging heat treatments
After annealing the samples were etched with different etchings
Summary
The duplex stainless steels, with ferritic-austenitic microstructure, have better corrosion resistance than the austenitic stainless steel and better mechanical properties than the ferritic stainless steel[1] When these steels are exposed to temperatures ranging from 300 to 500 °C the α’ phase formation can occur. At temperatures ranging from 500 to 1000 °C (Fig. 1), phase transformations such as the sigma phase formation, chromium nitrides and carbides precipitation are favored[2]. The occurrence of these phases can often cause loss of toughness and a decrease in corrosion resistance.
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