Ultrasonic velocity measurements during phase transformations in steels using laser ultrasonics

This article presents accurate laser-ultrasonic measurements of longitudina l velocity in the 500-1000 degreesC temperature range in carbon steel sampl es for different conditions known to affect phase transformations such as c ooling rate, carbon concentration, and rolling. Measurements were performed during continuous heating and cooling at rates varying between 0.1 and 20 degreesC/s. Carbon concentrations ranged from 0.0% to 0.72%. Hot-rolled and cold-rolled samples were measured. For the hot-rolled samples, a reproduci ble hysteresis was observed in the dependence of the ultrasonic velocity ve rsus temperature of samples having a significant carbon concentration. This hysteresis is attributed to the combined effects of the phase transformati on and of the ferromagnetic-paramagnetic transition. In particular, the rat e of change of velocity with temperature during heating suddenly diminishes at the Curie temperature, and the velocity behavior during cooling shows c learly the start and end of phase transformations, even allowing discrimina tion between ferrite and pearlite nucleations. For the cold-rolled samples, significant drops in ultrasonic velocity were observed at the transformati on temperatures when the samples were heated for the first time. However, t he magnitude of these drops decreased for subsequent heating. These drops a re attributed to irreversible crystallographic orientation changes caused b y phase transformations. This effect was modeled using the Kurdjumov-Sach t ransformation relationship. The model calculation agrees well with the expe rimental data. (C) 2001 American Institute of Physics.