Ultrasonic Techniques for Assessment of Microstructures and Residual Stresses in Austenitic Stainless Steels

In recent years, ultrasonic measurements are found to offer unique and nondestructive methods of determining the microstructures and quantifying residual stresses. Ultrasonic techniques developed based on velocity measurements are found to be more reliable than the existing techniques for characterizing the annealing behaviuor of cold worked austenitic stainless steel. The variation in ultrasonic velocity with annealing time/temperature exhibited three stages. Based on the microstructural investigations using metallography, the three stages have been identified to be recovery, progress of recrystallization and completion of recrystallization. Ultrasonic based residual stress measurements are nondestructive and offer quantitative estimation of surface/sub-surface and bulk residual stresses. In the present work, critically refracted longitudinal L_CR waves have been employed to measure residual stresses in A-TIG weld joints of type 316 LN stainless steel of various thickness. There was significant reduction in the magnitude of tensile residual stresses in A-TIG weld joints with increasing thickness of the plates joined. This residual stress measurement technique has several advantages including high spatial resolution for longitudinal residual stresses across the weld joints, easy and simple to use on fabricated components, cost effective and non-destructive.