Nonlinear ultrasonic measurement on cold rolled and sensitized 304 austenitic stainless steel
Geibel, Timm Dennis
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Austenitic stainless steels have a wide range of applications in the energy industry due to their high temperature performance and high corrosion resistance. Type 304 SS, is a widely used stainless steel, but is susceptible to sensitization, the formation of chromium carbide precipitates along the grain boundaries, causing chromium depletion when exposed to a certain temperature, ultimately leading to intergranular stress corrosion cracking (IGSCC). Sensitization preferably tends to occur in the heat affected zone (HAZ) of welds where heat is induced for a certain amount of time during welding. IGSCC is a main cause of the cracking in boiling water reactor piping systems and therefore is of great concern. Nonlinear ultrasound is a nondestructive evaluation method measuring the change of harmonic frequencies over propagation distance quantified by the nonlinearity parameter, β. The research evaluates the changes of microstructure caused by cold work (cold rolling) using two nonlinear ultrasound methods, Rayleigh and longitudinal waves. Moreover it shows the sensitivity of nonlinear ultrasound to sensitization and combines sensitization and cold rolling, then is compared with previous work    that investigated just the isolated effect of sensitization, or cold work. The results show that (1) cold work causes significant variations in the nonlinearity parameter which can be attributed to the formation of dislocations and twins as a consequence of the plastic deformation and phase transformation and that (2) cold work has large influences on the sensitization behavior of 304 SS.