Effects of chemical composition and thermomechanical treatment on austenite to ferrite transformation in 12 wt-%Cr steels

Abstract

The kinetics of austenite to ferrite transformation was studied in 11–12 wt-%Cr steels having an essentially austenitic microstructure at hot rolling temperatures (750–1050°C). The effects of chemical composition, high temperature γ/δ phase balance, and deformation before the transformation were assessed. The phase transformation was monitored using dilatometry, metallography, and hardness measurements. Small variations in chemical composition, particularly in the nickel and manganese content, resulted in significant differences in the kinetics of the transformation. These are a result of changes in the Ac₁ temperature, pre-existing δ ferrite content at high temperature, and probably the solute drag effect. Deformation at low temperatures of 850–750°C accelerated the transformation. The magnitude of this effect wasfound to depend on the degree of deformation and the cooling rate above the transformation temperature. Using a reduction of 30%, the cooling rate that resulted in a specific fraction of ferrite in the final structure was increased threefold. The results suggest that if the steel composition, particularly the nickel and manganese content, can be adjusted within narrow limits, controlled rolling together with controlled, retarded cooling can be applied to produce 11–12 wt-%Cr steels with adequate mechanical properties and excellent weldability, without the need for tempering.