Steel, as a fundamental material for many industries, will continue to hold its position in the forging applications, where achieving a balance of combination of strength and ductility is required. Presently, a significant portion of steel forgings is derived from quench and tempering (Q + T) steels. As efforts and goals towards carbon neutrality intensify, there is a growing need to develop materials and processes with equivalent properties but that will reduce carbon emissions, as the Q+T heat treatment is cost and energy intensive. A recent development introduces a novel class of martensitic steels that attain their final properties through air cooling directly from the forging heat [1].

These air hardening ductile forgings steels are alloyed with approximately 4 wt.% manganese and a attained a uniform martensitic microstructure through direct air-hardening from the forging heat. By eliminating the energy-intensive quench and tempering treatment, substantial reductions in CO2 emissions are achieved. As the first commercial melts of this steel grades are no on the market, there is curiosity about its performance in various applications [2]. Therefore, within the scope of this study, efforts have been made to understand the performance of AHD steel in plasma nitriding.

In this study, the influence of the aluminum composition in AHD steel and, in addition, the impact of process parameters on plasma nitriding performance were investigated. AHD steels exhibit a surface hardness of approximately 1000 HV after plasma nitriding, competing the hardness of commercial steels. The samples were exposed to nitriding processes at different temperatures (from 400-550 °C) for varying durations. Subsequently, mechanical and chemical properties were characterized. Electron Probe Microanalysis (EPMA) was employed for chemical analysis, and X-ray Diffraction (XRD) was used to identify the phases formed in the nitride layers. The high surface hardness is attributed to the formation of aluminum nitrides, with a decreasing trend for lower aluminum concentrations. Furthermore, a significant decarburization of the nitrided layer is observed was observed. The research reveals that, in specific conditions, carbon and nitrogen jointly diffusion and stabilize austenite, leading to a multiphase microstructure.

References
[1] A. Gramlich; W. Hagedorn; K. Greiff; U. Krupp Advanced Engineering Materials, 2023, 25, 2201931.
[2] A. Gramlich; M. Auger; S. Richter HTM Journal of Heat Treatment and Materials, 2022, 77, 298-315