Transformation of austenite during isothermal annealing at 600–900 °C for heat-resistant stainless steel

Abstract

In this work, the results of the transformation kinetics of austenite heat-resistant stainless steel (AISI 310S) with 2.27wt.% silicon were presented. The results of microstructural and fractography analysis, as well as the results of hardness after the isothermal heat treatment of the steel in the temperature range from 600 to 900°C at different annealing times (from 1 to 956h) are shown. It was found that the microstructure of steel after isothermal annealing consisted of austenite, carbide (M23C6) and σ-phase. The number of σ-phase precipitates increases with higher annealing times. In longer annealed steel samples the significant coarsening of precipitates was observed, as well as a large amount of σ-phase particles in the form of a chain and a network of plates. Precipitates were both observed at grain boundaries and within austenite grains. σ-phase precipitation involves two mechanisms: transformation γ→Cr23C6→σ and transformation γ→α′→σ. The hardness began to increase after 48h of annealing at a temperature of 600°C, while in the temperature range of 700–900°C the hardness increased with annealing times higher than 8h. The slope of hardness curve is caused by microstructural changes. The fracture surface mode was intercrystalline brittle. Relatively large dimples can be related to large particles of precipitates which occurred after long annealing times.