Abstract
This work determines the kinetics of sigma phase formation in UNS S31803 Duplex Stainless Steel (DSS), describing the phase transformations that occur in isothermal aging between 700 and 900 ºC for time periods up to 1032 hours, allowing the determination of the Time-Temperature-Precipitation (TTP) diagram for sigma phase and proposing a model to predict the kinetics of sigma phase formation using a Johnson-Mehl-Avrami (JMA) type expression. The higher kinetics of sigma phase formation occurs at 850 ºC. However, isothermal aging between 700 and 900 ºC for time periods up to 1032 hours are not sufficient to the establishment of thermodynamic equilibrium. Activation energy for both nucleation and growth of sigma phase is determined (185 kJ.mol-1) and its value is equivalent to the activation energy for Cr diffusion in ferrite, indicating that diffusion of Cr is probably the major thermally activated process involved in sigma phase formation. The determined JMA type expression presents good fit with experimental data between 700 and 850 ºC.
Highlights
Duplex Stainless Steels (DSS) are widely used in applications that demand high mechanical resistance, toughness and corrosion resistance[1,2,3,4,5]
The same occurred in the other studied temperatures, suggesting that equilibrium is not reached at the studied temperatures even considering the long-term aging applied
The calculated equilibrium volume fraction of nitride at 850 °C is higher than the volume fraction found after 1008 hours aging at this temperature (0.4%), another indication that equilibrium is not reached at the studied temperatures
Summary
Duplex Stainless Steels (DSS) are widely used in applications that demand high mechanical resistance, toughness and corrosion resistance[1,2,3,4,5]. During heating between 700 and 900 °C precipitation of a hard and brittle Cr-rich phase, called sigma, can occur, resulting in brittleness and reduction in corrosion resistance[6,7,8,9,10,11,12,13,14]. At 850 °C, sigma phase can be formed by three distinct mechanisms: nucleation and growth from original ferrite, eutectoid decomposition of ferrite ( forming secondary austenite) and growth from austenite after total consumption of original ferrite[6,7]. These three mechanisms lead to Cr depletion at the metallic matrix surrounding sigma phase, resulting in lower corrosion resistance
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