Shear Transformation of Austenite in Steels Considering Stresses’ Effects

The currently known main mechanisms of martensitic transformation of austenite in steel during cooling, physical models, and schemes of shear rearrangement of the iron crystal lattice during γ→α transformation under the action of internal stresses are considered. An analysis of the available kinetic model of the shear transformation of austenite in steel is carried out considering the influence of stresses. As shown, for the transformation of retained austenite into martensite, two main conditions have to be fulfilled. The first one is thermodynamic condition due to decreasing the temperature of the steel down to the temperature of the end of the transformation of retained austenite. The second condition is kinetic one due to increasing the level of internal stresses in austenite, the cooling rate at the final treatment stage or mechanical impact. To calculate the effect of stresses on the transformation of retained austenite in steel, a new equation is proposed that takes into account the minimum stress required for a crystallogeometrically ordered displacement of iron atoms during the formation of martensite. As shown, the effect of alloying elements and cooling rate on the temperature of the beginning and end of the austenite transformation can be calculated using the relations given in the article. To calculate the quantity of martensite formed depending on the transformation temperature, an improved Koistinen–Marburger equation is suggested. This equation takes into account not only the temperature of the beginning, but also the temperature of the end of the austenite transformation, i.e., increases the accuracy of the obtained result.