Investigation of bainite transformation in medium carbon low alloy steels

The effects of small additions of common alloying elements (C, Ni, Cr, Mn, Mo and Si) on the kinetics of the bainite reaction in 4340 steel were studied. The 4340 steel was selected as a base because it had tne necessary pearlitic hardenability and this enabled studies of the bainite reaction to be made without interference from the proeutectoid or pearllte reactions. A rapid magnetic permeability method was developed to facilitate this investigation. It consisted of quenching the sample from an austenitlzing temperature to a subcritical temperature in an isothermal bath which was held within the field of an inductor coil. The increase in permeability accompanying austenite decomposition increased the inductance of the coil, and this changed the resonant period of oscillation of the circuit. An automatic continuous recording of the period provided a convenient and accurate method for following the austenite decomposition. Computer aided methods were used to analyze the isothermal reaction data and to plot the time-temperature-transformation (TTT) diagrams.