Heat treatment experiments of an air-hardening medium-manganese steel show that an increase in the austenitising temperature, despite increasing the grain size, leads to better toughness values after quenching. Furthermore, the influence of the austenitising temperature on toughness is reduced at lower cooling rates, and this behaviour cannot simply be explained by differences in the microstructure. Notably, in addition to high amounts of manganese and silicon, the steel contains small amounts of phosphorus. The results show that phosphorus not only contributes to temper embrittlement but also segregates to austenite grain boundaries at lower austenitising temperatures or during slow cooling from higher austenitising temperatures. This reduces grain boundary cohesion, which is detrimental to impact toughness. The phenomenon was validated by Auger electron spectroscopy measurements on the fracture surfaces. Therefore, such air-hardening steels can reach a high hardness even at low cooling rates, but toughness is sensitive to the cooling rate and austenitising temperature owing to phosphorus segregation in austenite.
David et al. (2026) studied this question.