Characterization of microstructural phases of steels by sound velocity measurement

For investigating the effect of phases on sound velocity, thin disk shaped samples were prepared from AISI 4140 and AISI 5140 steels. An initial heat treatment was applied to all specimens for obtaining a uniform reference microstructure. To eliminate the effect of grain size all specimens were treated identically through austenitization at 850 °C/30 min. Samples, each consisting of only one phase, namely, martensite, bainite, fine pearlite-ferrite, and coarse pearlite-ferrite were obtained by quenching, and several isothermal heat treatments. The microstructures were characterized by metallographic examinations and hardness measurements. By measuring sound velocities for both longitudinal and transversal waves, the reference values were obtained for each individual phase. Martensite has the lowest sound velocity while coarse pearlite-ferrite has the highest. The differences in sound velocity arise primarily from differences in the elastic moduli of the phases which are affected by the degree of lattice distortion and misorientation.