Ultrasonic Nondestructive Evaluation Of Austenitic Stainless Steel

Austenitic stainless steel is the solid solution that carbon is in theγphase, chromium and nickel as the main alloying elements. With good corrosion resistance and low temperature toughness, strong high-temperature creep and without brittle transition temperature, etc, Austenitic stainless steel has been widely used to make equipment and parts that are required good overall performance. However, the grain will continue to grow in high-temperature environment, resulting in creep damage and reducing strength .Additionally, corrosive media will lead to corrosion of parts and reduce its'strength in corrosive environment. Therefore, in order to ensure the safety of equipment, we have to measure the grain size and thickness. Although metallographic method is the conventional method of measuring grain size, it is not suitable for detection of in-service equipment because of destructive, many processes and long period. On the basis of the feature that the austenitic stainless steel is a single austenite phase after solution treatment and any change in the speed of sound can be attributed to change in grain size of this feature, we use the propagation speed and attenuation to study the relationship between average grain size and ultrasonic velocity and attenuation, which provides a quick, comprehensive and non-destructive monitoring method for in-service austenitic stainless steel parts.Firstly, we select the specimen material and design specimen size in view of materials science and ultrasonics. Then we carry out metallographic analysis on 0Cr18Ni9 stainless steel specimens after solution (air cooling and water quenching) with using 500mm gauge network lattice cut-off point method to measure the average grain size. Finally, we study ultrasonic evaluation of the average grain size in three ways (ultrasonic velocity, the relative attenuation and spectrum analysis) and establish the ultrasound evaluation model of austenitic stainless steel combined with the metallographic test. It shows that: (1)If increasing solution temperature or extending holding time in the same solution temperature, grain size will increase: the grain under water quenching grows up faster than that under air-cooled. (2) Ultrasonic velocity is reducing as the grain growth: the relationship between average grain size and ultrasonic speed under water quenching shows an exponential decay trend, however that under air cooling shows a linear trend. (3) For austenite stainless steel with different grain sizes , when the frequency is constant , the more coarse the grains are, and the more uneven the organization is, the more heavier the acoustic attenuation is . The relationship between average grain size and attenuation coefficient under water quenching shows a cubic polynomial curve growth trend. (4) The ultrasonic scattering is also different, as the acoustic impedance in the adjacent grains is different. The larger the grains are, the more serious the scattering is. Echo frequency spectrum decreases and structure noise frequency spectrum increases as the grain grows up. The EDS point analysis shows that if increasing solution temperature or extending holding time in the same solution temperature, solid solubility will increase. Because the solute atoms dissolve to the solvent to change the lattice constant of the sovent, which results in the lattice distortion, the binding force between atoms and elastic modulus E will reduce. The damping effect from dislocation, twin crystal or other crystal defects also reduces ultrasonic speed.