Phosphorus embrittlement in austenite during heat treatment of an air hardening forging steel

David Daniel - University of Applied Sciences Upper Austria (Austria)

Due to the increasing demand to save CO2, air-hardening forging steels, which achieve their mechanical properties during cooling from the forging heat without further heat treatment, are interesting alternatives to conventional quenching and tempering steels. For the design of such steels a precise knowledge of the continuous cooling transformation behavior and factors influencing toughness are crucial. Therefore, a new manganese alloyed steel was produced by a semi-industrial vertical continuous casting process following hot rolling. Subsequently, the effect of the heat treatment parameters on strength, toughness and microstructure was investigated. In addition, the content of segregated elements at the austenite grain boundary was measured using auger electron spectroscopy (AES). From the heat treatment experiments can be concluded that surprisingly an increase of the austenitisation temperature leads, despite the higher grain size, to better toughness values after fast cooling. Decreasing the cooling rate has no significant influence on strength and hardness but reduces toughness. Interestingly there is only a small influence of the austenitisation temperature on toughness at lower cooling rates. As the microstructure at the lower cooling rate is still predominantly martensitic, this behavior cannot simply be explained by a different microstructure. However, the steel contains, beside rather high amounts of manganese and silicon, also small amounts of phosphorous. Phosphorus is not only a dominant factor for temper embrittlement but also tends to segregate to austenite grain boundaries at lower austenitisation temperatures or during slow cooling from higher austenitisation temperatures at higher manganese contents. This reduces grain boundary cohesion, which is very detrimental to the toughness. This phenomenon was validated by AES measurements on the fracture surfaces. Therefore, such new air hardening forging steels reach high strengths even at low cooling rates, however toughness is very sensitive to cooling rate and austenitisation temperature due to phosphorus segregation in austenite.