Nitriding of Lightweight High Manganese and Aluminum Steels

Abstract

Austenitic high manganese and aluminum steels have exceptional combinations of high strength and toughness with excellent wear resistance. Adding aluminum in levels from 6 to 8.8 wt% reduces the density by 10–15 % compared with quenched and tempered Cr and Mo steels but also decreases strain hardening and abrasive wear resistance. Wear resistance may be improved by a low-cost heat treatment in a nitrogen atmosphere to produce a hard layer of surface AlN. In the current study, the effect of aluminum and silicon content on the kinetics of nitriding process was evaluated for a Fe–30 %Mn–(6–9 %) Al–(1–1.6 %)Si–0.9 %C steel in the temperature range of 900–1100 °C (1652–2012 °F). Results show that up to a 550-μm-thick surface layer of AlN plates can be produced, depending on the time and temperature. Increasing the amount of silicon from 1.1 to 1.6 % Si had no statistical effect on the diffusion of nitrogen in the temperature range of 900–1100 °C (1652–2012 °F). Increasing the amount of aluminum from 6 to 8.8 % Al decreased the diffusivity of nitrogen and increased the calculated activation energy from 64 to 79 kJ/mol. The lower than expected values of the activation energy for the diffusion of nitrogen in austenite is suggested to be the result of the development of high-diffusivity pathways at the interface between the AlN and the austenite matrix.