Analysis of Super Duplex Stainless Steel Properties as an Austenite-Ferrite Composite

Abstract

Super duplex stainless steel (SDSS) is considered as a composite formed from a microstructure of an approximately equal mixture of two primary constituents (γ-austenite and α-ferrite phases) and the secondary precipitates (sigma, chi, alpha-prime, etc.). While the formation of these phases affects the properties of SDSS, however there are no rules that govern the relationship. In this work, the relationship between toughness as well as corrosion behavior of SDSS (UNS 32760) and the microstructure constituents has been experimentally investigated, and analyzed in view of the composite principles. Another two stainless steels namely; fully austenitic SASS (UNS N08367) and fully ferritic FSS (UNS S42900) are considered to simulate the constituent's primary components in the composite which are austenite γ and ferrite α phases respectively. Samples of the composite and constituent's steels are first subjected to solution annealing, where the composite steel has a microstructure of γ austenite and α ferrite grains. They were then subjected to similar different isothermal heat treatment cycles, for the formation of secondary phase precipitations within the transformation temperature ranges of each of γ and α primary grains. Impact toughness and corrosion (specific weight loss) tests were conducted on the annealed and isothermally treated samples. The composite rule of the mixtures (ROM) is used to analyze the relationship between the toughness and corrosion properties in the composite SDSS and the SASS and FSS constituent's steels. The analysis indicates that in case of toughness, ROM applies well on the composite and constituents' steels in the solution annealed and in isothermal treatment conditions, where better matching between experimental and calculated results is observed. When applying ROM for corrosion weight loss, a great difference is found between the experimental and calculated results, which is much reduced for solution treated samples ferritic and austenitic temperature ranges of 480˚C-500˚C and 700˚C-750˚C as for ferrite and austenite respectively.