A new empirical formula for the bainite upper temperature limit of steel

The definition of the practical upper temperature limit of the bainite reac tion in steels is discussed. Because the theoretical upper temperature limi t of bainite reaction, B-0, can neither be obtained directly from experimen tal measurements, nor from calculations, then, different models related to the practical upper temperature limit of bainite reaction, B-S, are reviewe d and analyzed first in order to define the B-0 temperature. A new physical significance of the B-S and B-0 temperatures in steels is proposed and ana lyzed. It is found that the B-0 temperature of the bainite reaction in stee ls can be defined by the point of intersection between the thermodynamic eq uilibrium curve for the austenite --> ferrite transformation by coherent gr owth (curve Z(gamma --><(<alpha>)over right arrow>)) and the extrapolated t hermodynamic equilibrium curve for the austenite --> cementite transformati on (curve ES in the Fe-C phase diagram). The B-S temperature for the bainit e reaction is about 50-55 degreesC lower than the B-0 temperature in steels . Using this method, the B-0 and B-S temperatures for plain carbon steels w ere found to be 680 degreesC and 630 degreesC, respectively. The bainite re action can only be observed below 500 degreesC because it is obscured by th e pearlite reaction which occurs prior to the bainite reaction in plain car bon steels. A new formula, B-S(degreesC) =,630-45Mn-40V-35Si-30Cr -25Mo-20N i-15W, is proposed to predict the B-S temperature of steel. The effect of s teel composition on the B-S temperature is discussed. It is shown that B-S is mainly affected by alloying elements other than carbon, which had been f ound in previous investigations. The new formula gives a better agreement w ith experimental results than for 3 other empirical formulae when data from 82 low alloy steels from were examined. For more than 70% of these low all oy steels, the B-S temperatures can be predicted by this new formula within +/- 25 degreesC. It is believed that the new equation will have more exten sive applicability than existing equations since it is based on data for a wide range of steel compositions (7 alloying elements). (C) 2001 Kluwer Aca demic Publishers.