Microstructural investigation of SAE 1040 steel specimens by ultrasonic measurements

The aim of this study is to characterise the microstructures of the as-quenched and tempered SAE 1040 steel specimens by sound velocity and attenuation measurements. An initial heat treatment was applied to the specimens to obtain a uniform starting microstructure. Following 850C/0.5 h austenitisation and water-quenching of all specimens, a tempering process was applied at 200C and 600C for 2 h. Microstructures of the specimens were characterised first by metallography and hardness measurements. Then, the velocities of both longitudinal and transversal waves were measured. Backwall echo patterns were obtained to compare the attenuation behaviour. By using the specimens having the same thickness, and by eliminating the effects of grain size and texture, only the influence of microstructural phases on sound velocity and attenuation was investigated. Sound velocity increased as a function of tempering temperature, however no meaningful variation in sound attenuation was observed.