A re-examination of the B0 and BS temperatures of steel

Abstract A new explanation of the physical significance of the B 0 (the theoretical upper temperature limit) and the B S (the practical upper temperature limit) temperatures in steel is proposed. It is found that the B 0 temperature of the bainite reaction in steel should be defined by the point of intersection between the thermodynamic equilibrium curve of austenite→ferrite transformation by coherent growth (curve Z γ→ α → ) and the thermodynamic equilibrium curve of austenite→cementite transformation (curve ES in the Fe–C phase diagram). The B S temperature for the bainite reaction is approximately 50∼55°C lower than the B 0 temperature. Using this method, the B S and B 0 temperatures for plain carbon steel were found to be 630°C and 680°C, respectively. The effects of steel composition on the B S temperature are discussed. The formula B S (°C)=630−45 Mn−40 V−35 Si−30 Cr−25 Mo−20 Ni−15 W can be used to predict the B S temperature of a wide range of commercial steels.