Formation and stabilization of reverted austenite in supermartensitic stainless steel

The formation and stabilization of reverted austenite upon inter-critical annealing was investigated in a X4CrNiMo16-5-1 (EN 1.4418) supermartensitic stainless steel by means of scanning electron microscopy, electron backscatter-diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy and dilatometry. The results were supported by thermodynamics and kinetics models, and hardness measurements. Isothermal annealing for 2 h in the temperature range of 475 to 650 degrees C led to gradual softening of the material which was related to tempering of martensite and the steady increase of the reverted austenite phase fraction. Annealing at higher temperatures led to a gradual increase in hardness which was caused by formation of fresh martensite from reverted austenite. It was demonstrated that stabilization of reverted austenite is primarily based on chemical stabilization by partitioning, consistent with modeling results.