The propagation behavior of disbonding and surface crack near the interface between overlay and base metal is one of the most important problems occurring in service pressure vessels. In order to evaluate the possibility of disbonding and surface crack penetrating into the base metal, the following research work has been carried out. The microstructure, distribution of chemical elements and hardness have been studied using metallograph and EMPA. The diffusion of elements, formation of plate-like martensite and carbide are discussed. The hydrogen distribution curves along cross-section were obtained using SIMS with high pressure hydrogen charging overlaid specimen for the first time. The experimental result showed that the hydrogen distribution in the interface corresponded to its microstructure. Measurement of hydrogen distribution indicated that there were a peak of hydrogen accumulation at the interface and two hydrogen valleys near the interface, one valley at the overlay side and the other at the base metal side. During discharging, the peak of hydrogen accumulation at the interface decreased, the valley at the overlay side vanished slowly and the valley at the base metal side vanished quickly. The relationship between the microstructure and diffusion accumulation of hydrogen were discussed. A new model of hydrogen diffusion accumulation that the peak and valleys of hydrogen are caused by the cooperation of Cr_(23)C_6 trapping clusters and residual stress is proposed, and the mechanism of hydrogen transportation during discharging is suggested. A new quantitative method of disbonding defect was presented using ultrasonic test of specimen with hydrogen induced disbonding. According to the characteristic of the microstructure, the factors which affected the UT were discussed. The dynamic characteristic of disbonding propagation and environment factors which affected the sensitivity of disbonding were studied by monitoring the course of disbonding, the factors which affected the incubation period were discussed. The relationships among the microstructure, the hydrogen diffusion accumulation and hydrogen induced cracking behavior of the interface were investigated. The experimental results show: the course of disbonding was a multiple points cracking; the paths where hydrogen disbonding cracks nucleated and propagated were classified into four types; the disbonding cracks would not penetrate into the base metal. According to the theory of molecular hydrogen pressure, the critical hydrogen content was calculated. The propagating course of disbonding and surface crack was studied by constant strain rate test. The distribution of hydrogen concentration near notch tip of weld overlay interface zone under mode I loading has been studied using SIMS for the first time, and the trend of the crack propagation is predicated in theory. The experimental results of microzone hydrogen analysis and constant strain rate test showed that the disbonding and surface crack wouldn't penetrate the base metal. According to the results of microzone hydrogen analysis, a new model of hydrogen diffusion accumulation that the hydrogen peak is caused by the cooperation of Cr_(23)C_6 trapping clusters, stress distribution and dislocation configuration is proposed. On the basis of the experimential results, the author proves in theory and in experiment that the disbonding and surface crack wouldn't penetrate the base metal.
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