Effect of alloying elements in steels on the kinetics of the austenite to bainite transformation

The effects of different alloying elements on the bainite reaction kinetics in steel were investigated. Dilatometric studies were performed on four Fe--C--X ternary systems in particular (X = Mo, Ni, Cr, Mn). The investigation was extended to include higher combination systems of these same alloying elements such as Fe--C--Mo--Ni, Fe--C--Ni--Cr, and Fe--C--Ni--Cr--Mo in order to determine what their interaction effects are in the bainite transformation. All the alloying elements studied accelerated the austenite decomposition at isothermal temperature treatments just above the martensite transformation temperature (Ms) giving for the reaction start curve an S-like shape. Of the alloying elements studied Mn, Ni, Cr (in order of effectiveness) shifted the bainite reaction to longer times (increasing the hardenability). Mo, instead, accelerated the bainite reaction. All the alloying elements lowered the temperature range of bainite reaction, further additions, even for mixed addition such as Cr--Ni, Mo--Cr Mo--Ni suggested an additive effect in lowering the reaction temperature. However combined additions gave a multiplicative effect on the reaction start times for the isothermal decompositions of austenite, suggesting strong interaction effects. The combined addition of Cr--Ni was more effective than the addition of Ni and Cr alone. Similarly the time for Mo--Ni alloys, and for Mo--Cr was more effective with respect to bainite hardenability