Simulation of the precipitation of sigma phase in duplex stainless steels

Abstract

The precipitation of sigma phase within the ferrite component of a duplex stainless steel has been simulated using a two-dimensional computer model which takes into account the partitioning of alloy elements between ferrite and austenite. The model is based on a cellular automaton and, despite having a rather simple set of transition rules, is able to simulate changes in the volume fractions of the austenite, ferrite, and sigma phases. The microstructures produced are similar in appearance to those in the real system. Comparison of the model and the real system may assist in the assessment of the various phenomena occurring. Use is made of the model to examine many of the factors that might conceivably be harnessed to retard precipitation of the sigma phase in duplex stainless steels.

An Experimental and Numerical Approach for the Welding Effects on the Duplex Stainless Steel Microstructure

“…However, DSS are frequently submitted to welding operations and, depending on the thermal history undergone by the material during this procedure, the precipitation of brittle intermetallic phases such as sigma (σ) and chi(χ) [3][4][5][6][7][8][9] may occur besides of impairing the adequate balance between the fractions of the ferrite and austenite phases at the heat affected zone (HAZ). In terms of secondary phases, the sigma phase is the most important intermetallic compound that could precipitate, since it has a deleterious influence on the mechanical properties and corrosion resistance of the DSS and higher volumetric fraction observed among all them [10][11][12][13][14][15][16] . In fact, some works [11][12][13][17][18][19][20][21] have shown that the impact toughness and corrosion resistance of the DSS drastically decrease even in the presence of very low amounts of sigma phase.…”

“…In fact, some works [11][12][13][17][18][19][20][21] have shown that the impact toughness and corrosion resistance of the DSS drastically decrease even in the presence of very low amounts of sigma phase. Sigma phase is a hard, brittle and non-magnetic intermetallic phase with a tetragonal crystalline structure being formed basically by elements Fe, Cr and Mo 3,10,14 and usually forms after long holding times at temperatures between 650 and 950 °C and after cooling from high temperatures, as those occurring at the DSS HAZ during welding 22 . Higher heat input implies lower cooling rate favoring austenite reformation at the HAZ, yields a more satisfactory balance between the fractions of ferrite and austenite, but makes also possible the precipitation of brittle intermetallic phases such as sigma phase and grain growth depending on the holding time and temperature reached by the weldment.…”

Efforts to Save Nickel in Austenitic Stainless Steels

“…[34][35][36][37] In particular, work strain accelerates precipitation of the s phase. 38,39) Therefore the duplex steels are prone to cracking during hot rolling, which reduces product yield.…”

Microstructural Transformation in a Root Pass of Superduplex Stainless Steel Multipass Welding

“…Superduplex stainless steels have been used in the chemical and petrochemical industries since they can be formed and welded with standard equipment and techniques 1 . However, they are susceptible to the formation of secondary phases like sigma(σ), chi(χ), γ 2 , M 23 C 6 and Cr 2 N 2,3 when the material is exposed to the temperature range of 300°C -1000°C 4 .…”