Abstract

The aim of this study is to evaluate the microstructural changes, in particular the morphological changes of cementite, in an AISI 1045 steel treated at 700°C by means of optical and scanning electron microscopy and ultrasonic techniques. The ultrasonic evaluation was carried out using the pulse-echo contact technique and 5, 10, and 22 MHz transducers to determine the acoustic parameters such as longitudinal ultrasonic wave velocity and attenuation coefficient. The non-conventional ultrasonic evaluation also included the analysis of signals in frequency. The results obtained from the microstructural evaluation reveal the fracture of the cementite lamellae of the pearlite and an increase in the degree of spheroidization with the increase of the treatment time at subcritical temperature. Ultrasonic evaluation exhibited a decrease in the longitudinal wave velocity in the transient state of change of pearlite morphology. Also, an increase in longitudinal velocity with increasing degree of spheroidization was observed. In contrast, the attenuation coefficient measurements show a tendency to increase with heat treatment time and with the microstructural changes of the material caused by the heat treatment. On the other hand, the spectral analysis suggests higher energy loss with longer treatment times due to the phenomena of ultrasonic wave attenuation by scattering, as a result of the morphological change that occurs in the pearlitic colonies.