Bainite formation: Influence of strain energyWednesday (28.09.2016) 11:15 - 11:30 Part of:
Bainite microstructure forms the crux of several modern structural steels. With the ever growing significance of bainite in the field of structural steels, a generalized and predictive model describing the bainite formation kinetics is extremely important in order to select the right alloy and heat treatment during production. However, the fundamental mechanism of bainite formation is still unclear. This undermines the predictive capabilities of any of the existing theories of bainite formation.
The displacive theory of bainite formation suggests that the bainite formation can occur only under certain thermodynamic limits. One of the limits is derived assuming the diffusionless nature of the bainitic growth. According to this limit, bainite formation is governed by the T0’ limit theory, where bainite formation can occur only below T0’ temperature. T0’ temperature is temperature where the driving force for diffusionless austenite to bainite transformation for a given composition of steel is zero. T0’ temperature also accounts for the strain energy of bainite. However, several researchers question the validity of T0’ limit theory since the experimental Bs temperature does not correspond with the T0’ temperature.
According to the displacive theory of bainite formation, strain energy associated with the bainite formation is assumed to be approximately 400 J/mol irrespective of the temperature of bainite formation. This strain energy influences the driving force or the undercooling required for bainite formation.
In this work, it is proposed that the strain energy associated with the bainite formation is a function of temperature. The strain energy associated with bainite formation is a function of aspect ratio of the bainitic subunits, the shear modulus of the austenite and the Poisson’s ratio of the austenite. These parameters are influenced by temperature. The temperature dependence of strain energy can heavily influence the T0’ limit theory since the required undercooling varies as a function of strain energy. Based on the temperature dependence of the strain energy, the T0’ limit theory is revisited and its validity is evaluated by comparing with experimental Bs temperatures.