20G low carbon steel and 15CrMo steel was chosen for the experimental material and exposed in hydrogen gas at 18MPa at 450 ℃ and 500 ℃ for 240, 480 and 720 hours respectively in order to obtain different degrees of hydrogen attack. Ultrasonic attenuation measurements of hydrogen attack on HA samples were taken. The data obtained were analysed statistically. The results showed that the experimental values of ultrasonic attenuation coefficient at different placed on one sample obeyed the normal probability distribution. Both the mean value μ and the variance σ in the normal distribution increases with the exposure time. Therefor the values of μ could be used for prediting the degree of hydrogen attack. Flow echo ratio N also can be used to measure the degree of hydrogen attack. Hydrogen attack on 304 austenitic stainless steel in the service for 20000 h at 450 ℃ under 5MPa was detected to result from the occurrence of Cr_(23)C_6. The methane bubbles were formed preferentially along grain boundaries and twin boundaries, and some of them in grains. It is pointed ont that diffusion process is not the controlling factor of bubbles growth based on the observed results by SIMS and TEM. The mechanism of bubbles growth is belong to dislocation creep model. Acoustic emission (AE) behaviour during tensile in air has been studied for a low-carbon steel 20G which had been hydrogen attacked after exposed to hydrogen. The experimental results show that AE activity caused by hydrogen attack is decreased, but the counts of high amplitude AE signals is increased. The source of AE signals is a intergrainular fracture caused by coalescence of the micro-bubbles or microcracks on the grain boundaries.
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