Aluminium Nitride-containing Steels for High-Temperature Low-Pressure Carburizing Processes

Abstract: By increasing the temperature at which low-pressure carburizing is performed, the process time for production of case-hardened steels decrease but run a risk of abnormal grain growth occurring during carburizing. Particles present in the steel pin the grain boundaries and can prevent this growth depending on their size and volume fraction. In this report, the grain growth and particle stability in specifically aluminium nitride containing steels are investigated through thermodynamic calculations, precipitation simulations, and mock carburizing in atmospheric- and low pressures. The results show that no or minimal grain growth occurs at lower temperatures and for steels with a higher pinning force from the particles, and that abnormal grain growth happens otherwise. This is consistent with contemporary theory and there is a promising possibility of carburizing at 1000 °C with only small changes in alloy design from current commercial alloys. A theoretical reduction of equilibrium fraction of particles was calculated at low pressures, but the experimental results were not enough to confirm nor deny its existence in practice. For the purposes of alloy design, high combined nitrogen and aluminium content is recommended for sufficient high temperature stability and an optimal ratio of aluminium to nitrogen seems to be just above 2:1. High aluminium does cause a higher coarsening rate of the particles, which needs to be considered should attempts to fully optimize design be made.