Acoustic Emission During Phase Transformation in Steel

Abstract

Acoustic emission was monitored during phase transformations that occur during cooling in a wide variety of steels. Acoustic emission was generated during the formation of martensite but not during the formation of ferrite, bainite, or pearlite. This observation is consistent with the rapid, diffusionless, shear-like nature of martensite formation and the slow, diffusion-controlled growth of ferrite, bainite, or pearlite. The martensite start temperatures, and the temperature range of martensite formation determined by acoustic emission were in good agreement with those determined by metallographic or dilatometric methods. The intensity of acoustic emission generated during martensite formation decreased markedly as the carbon content of the steel decreased, becoming nearly undetectable in a maraging steel. This decrease in intensity correlates with a morphological change from large plate-shaped martensite units to smaller lath-shaped martensite units as the carbon content of the steel is decreased.

Application of acoustic emission to monitor bainitic and martensitic transformation

“…To calculate the volume fraction of martensite f m as a function of temperature, the K-M equation (2) was applied [17,18]:…”

“…In the calculations of the volume fraction of martensite as a function of the transformation temperature (Fig. 5), a constant value α m = 0.013 was assumed in the K-M equation (2). In practice, the α m coefficient value depends on the carbon content, which changes during the transformation.…”

Theory of Acoustic Emission From Phase Transformations

“…Despite the reporting by several workers [8,10] of intense acoustic emission during some martensitic transformations, effects of micro mechanism (transformation velocity, volume, etc.) upon acoustic emission have not been studied.…”

“…Thus, diffusion-controlled phase changes, while often resulting in significant stress changes, cause mainly quasi-static surface displacement and no direct acoustic emission (as is usually the case with bainite and pearlite formation during cooling of low alloy steels [8]). In these cases acoustic emission is not a viable candidate for microstructure control during processing.…”

“…This implies that the formation of a typical 30 JLm length of martensite in some alloys is formed in as little as 30 ns. In this time, elastic waves only propagate --0.1 mm and transient sample displacements are observed [8,10] as the sample returns to mechanical equilibrium.…”