Phosphorus embritlement in austenite during heat treatment of an air hardening steel

Heat treatment experiments of an air hardening steel show surprisingly that an increase of the austenitizing temperature leads, despite the higher grain size, to better toughness values after quenching. Interestingly there is only a small influence of the austenitizing temperature on toughness at lower cooling rates, whereby this behavior cannot simply be explained by differences in the microstructure. However, the steel contains, beside higher amounts of manganese and silicon, also small amounts of phosphorous. The results show that phosphorus is not only a dominant factor for temper embrittlement but also tends to segregate to austenite grain boundaries at lower austenitizing temperatures or during slow cooling from higher austenitizing temperatures promoted by higher manganese contents. This reduces grain boundary cohesion, which is very 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, however toughness is very sensitive to the cooling rate and austenitizing temperature due to phosphorus segregation in austenite.