Measurements of acoustic nonlinearity in cold-rolled and heat-treated 304 austenitic stainless steel using nonleaner ultrasound
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In energy industry, austenitic stainless steels have been used in a number of different applications. Their excellent mechanical properties, good formability, and high corrosion resistance make them a popular candidate material. Among other, SS 304 is the most common stainless steel. However, it has one disadvantage that the material is susceptible to sensitization. The sensitization occurs when the material is exposed to a certain temperature even for a short duration, where chromium and carbon forms chromium carbide precipitates along the grain boundaries. The formation of the precipitates causes chromium depletion and ultimately leads to intergranular stress corrosion cracking (IGSCC). The heat affected zone (HAZ) of weld in 304 SS structures is usually the most probable locations for sensitization. The IGSCC is of great interest because most cracking in high temperature pipes, such as water reactor pipes, initiates in HAZ due to the sensitization. Nondestructive evaluation methods based on nonlinear ultrasound such as the second harmonic generation technique are highly sensitive to material damage at microstructure level. The second harmonic generation technique measures the change of the second harmonic frequency in the initially monochromatic wave, and relates property changes in the tested materials to the measured nonlinearity parameter, β. This paper evaluates microstructures of cold rolled and heat treated stainless steel 304 by using nonlinearity parameter, β. Using longitudinal waves, the change of microstructural properties of cold rolled 304 SS is first evaluated. Next, combined effect of cold rolled and heat-treated stainless steel is evaluatedusing the same technique. The results are then compared with those from previous works      , which have examined each effect individually. The result shows that the cold work causes drastic increases in nonlinearity of 304 SS due to the plastic deformation and formation of dislocations. Also, the cold rolling affects the sensitization in different ways depending on the percent cold rolled of 304 SS. The heat treatment at 675 °C induces two different effects in this material: sensitization and recrystallization. With different percent of cold rolling, the combined effect of cold rolling and heat treatment result differently on the nonlinearity of 304 SS, depending on domination of either sensitization or recrystallization.