Nonlinear Ultrasonic Characterization For Intergranular Corrosion Susceptibility Of 304 Stainless Steel

Stainless steel is used in all walks of life due to its excellent performance, but stainless steel after sensitization is very prone to occur intergranular corrosion and causes to failure accident. Thus it is significant to achieve fast and non-destructive testing of intergranular corrosion susceptibility. According to study the relationship between sensitization degree and nonlinear ultrasonic parameter of stainless steel, a new method is proposed in this paper.Firstly, the influence of the system parameters on the nonlinear ultrasonic testing is discussed and the scheme of parameter selection is proposed. The precision of the experiment can be effectively improved by Hanning window for signal modulation and more pulse for inspiring. The nonlinear coefficient is not affected by the attenuation and the output voltage level, But it will be seriously affected by high gain regulation.304 stainless steel of different sensitization conditions is tested by nonlinear ultrasonic equipment in this paper, then the feasibility of the new method is discussed supplemented by EPR test. And the mechanism is analyzed through metallographic observation and X-ray diffraction, The results are as follows:1.There is no obvious difference between the sound velocity and the attenuation coefficient of the samples.2.The nonlinear parameters of the samples varied significantly with the increase of the sensitization temperature and sensitization time.3.Pearson correlation coefficient calculation results show a good correlation between the nonlinear parameters and sensitive degree, the relationship curves also shows the nonlinear parameters increases with sensitive degree, so it is feasible to characterize the degree of sensitivity by nonlinear ultrasonic technique.4.Metallographic microstructure observation and X-ray diffraction results show that: chromium carbide (Cr23C6) is precipitated in the grain boundary and it increases with sensitization temperature and time. So analysts believe that:the mismatch between grain boundary carbide (Cr23C6) and the austenitic matrix caused local strain fields which interfered the propagation of ultrasonic; with prolonging of sensitization time, increasing of precipitation further exacerbated the distortion of the ultrasonic.