Characterization of deformation-induced metallurgical change in austenite stainless steel sheet with acoustic microscopy

Abstract

This article reports on the non-destructive evaluation (NDE) of metallurgical changes induced by the deformation processes. In particular, the martensite and reversed austenite states of a meta-stable austenitic stainless steel sheet were evaluated. The sheet was elongated up to 40% at room temperature, and was then brought below room temperature to produce martensite. This was then followed by annealing for reversion. First, martensite content was measured with a Feritscope. Second, the surface acoustic wave (SAW) velocity was measured with a high numerical aperture spherically focused ultrasonic beam with a frequency equal to 600 MHz. The measured SAW velocity values were found to be dependent on the elongation, the ambient temperature during elongation, and the annealing temperature. A useful trend was found in the correlation between the measured SAW velocity and the martensite content as measured with the Feritscope. Second, using a high frequency (i.e., 800 MHz) acoustic imaging technique, the deformed and annealed grain structures were mapped. In comparing the acoustic images with the optical images, the deformed grains shown in the acoustic images were found to be significantly clearer than those shown in the optical images. These studies reveal the importance of NDE, in connection with the forming of stainless steel structures, to map the deformation induced metallurgical changes. Here, unique NDE techniques are seen to be effective in mapping the changes with the use of hypersonic SAW velocity measurements and imaging.