Nondestructive investigation of the effect of quenching and tempering on medium-carbon low alloy steels

The aim of the study is to investigate the effect of quenching and tempering on sound velocity of steels, and to contribute to the nondestructive control and optimisation of the quenching/tempering systems. Disk-shaped specimens were prepared from AISI/SAE 4140 and AISI/SAE 5140 steel bars. All specimens were austenised and quenched identically to eliminate the effect of grain size on sound velocity, and a constant tempering time was chosen. Microstructures of the samples were characterised by metallographic examinations and hardness measurements. The reference values were obtained for as-quenched and tempered structures by measuring sound velocities for both longitudinal and transversal waves. The sound velocity is lowest in the as-quenched martensite, and however, increases after tempering as a function of tempering temperature. The changes in sound velocity can be explained primarily by differences in the elastic modulus that is affected by the degree of lattice distortion and misorientation in the microstructure