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

• We examine transformation kinetics of austenite during isothermal annealing of heat-resistant stainless steel.
• Long-time exposure to high temperatures 600–900 °C leads to microstructural changes.
• Sigma phase particles occurred in the form of network of plates.
• After fracture occurred intercrystalline brittle mode of fracture surfaces.
• The increase in hardness was associated with annealing duration and content of sigma phase.

In this work, the results of the transformation kinetics of austenite heat-resistant stainless steel (AISI 310S) with 2.27 wt.% 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 956 h) 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 48 h 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 8 h. 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.