Ultrasonic Nondestructive Evaluation Of Hydrogen Damage And The Sensitization Degree In304Stainless Steel

304stainless steels (0Cr18Ni9) with good mechanical properties, strength at low temperature, heat resistance and corrosion resistance are widely used in metallurgical machinery, aviation and navigation, petroleum chemical industry and instrument fields, etc. But when they work long time in high temperature, high pressure and the hydrogen conditions,304stainless steels will also generate hydrogen damage. In addition, when they expose to the corrosive environment or experience improper temperature range (400～850℃) heating, they maybe emerge sensitization phenomenon, which make materials’intergranular corrosion tendency increase obviously. That would seriously affect the service life of industrial parts or even lead to catastrophic accidents. Therefore, it’s urgent to study a nondestructive testing method to evaluate hydrogen damage and the sensitization degree in304stainless steel, in order to achieve an aim:which not only doesn’t affect the normal operation of industrial components but also can realize real-time monitoring, preventing serious accidents.Based on the interaction between ultrasonic signals and materials’organization structure, this study measured transverse wave velocities, longitudinal wave velocities and the attenuation coefficient of304stainless steel and used spectrum analysis method analyse echo signal. Combined with metallographic analysis, electronic probe analysis of the microstructure and hardness test results, this paper discussed the feasibility and ultrasonic nondestructive characterization parameters for hydrogen damage degree, solid solution processing product grain size and sensitization degree in304austenitic stainless steels. The results were as follows:Hydrogen damage will influence the service life of austenitic stainless steel parts seriously and even cause accidents. During our experiment, we used electrolysis hydrogen charging method to prepare304stainless steel hydrogen damage samples, and discussed the influences of ultrasonic attenuation,sound velocity, and frequency domain properties due to micro structure change in hydrogen charging process. The results showed that:with hydrogen charging time prolonged, the ultrasonic velocity of304stainless steels appeared the trend of "increase-reduce-increase", while ultrasonic attenuation appeared "reduce-increase-reduce" trend. The above results were due to that hydrogen charging will change materials’elastic modulus, dislocation motion resistance and stress state. The grain size is a critical parameter for materials because it affects materials’ mechanical properties. Using ultrasonic testing detect304stainless steels to study ultrasonic nondestructive characterization parameters for average grain size. It showed that:with solid solution time prolonging,304stainless steel grain size increased, transverse wave velocity, longitudinal wave velocity were decreasing, and attenuation coefficients are growing continuously. The above studies are mainly attributed to the effects of the solution treatment to the grain size and the effects of the grain size to the propagation of ultrasonic wave through materials. Based on the least square method, the linear relationships between the transverse wave velocities, the longitudinal wave velocity and the average grain size were obtained: vt=-0.49254d+3188.40138,vt=-0.07632d+5772.88964; meanwhile, the relevance of the attenuation coefficient to the average grain size was also described in the studied specimens: α=1.2605×10-15d6.5012+0.0995Austenitic stainless steels which have emerged sensitization will generate intergranular corrosion more likely. So we used ultrasonic detecting method to study ultrasonic nondestructive characterization parameters for the sensitization degree in304stainless steel. The results were as follows:ultrasonic velocities did not provide significant changes when ultrasound propagated in sensitized specimens, but the attenuation increased apparently with increasing sensitization treatment time. The attenuation increase was mainly attributed to the continuous distribution and possible coalescence of the carbides along the grain boundaries and the increased scattering effects of precipitated carbides to the propagation of ultrasonic wave through materials. The spectral analysis exhibited a shifting of the peak frequency to high frequency direction and an obvious increase of the peak amplitude in the amplitude spectrum with prolonging sensitization treatment time. The increase of the peak frequency and the peak amplitude was due to that the carbides along the grain boundaries became intensive and tiny with increasing sensitization treatment time, which increased the interference of the higher frequency wave.