Study On Hydrogen Induced Crack Sensitivity Of X80Pipeline Steel

X80pipeline steel is the main gas transmission pipeline used in the Natural Oil and Gas Transmission Construction Project. Security problems such as pipeline rupture, caused by material failure as a result of hydrogen damage and in most cases, may arise especially when it’s serving in long distance under high pressure. In order to ensure the using safety and reliability of pipeline steel, it is necessary to inspect if there exist HIC by the method of ultrasonic flaw detection. But due to the insufficient systematic researching of the law and influence factors of HIC of pipeline steel, especially the lack of concrete researches on the factors that influence the hydrogen diffusion and HIC during the process of stacking slow cooling, some inspection qualified rate fluctuations phenomenon occurs frequently in the practical production. Therefore, this paper takes the X80pipeline steel as the research object, HIC sensitiveness, HIC mechanism and their influence factors are studied by electrochemical penetration, microstructure analysis, finite element software simulation, etc. It analyzes the law of hydrogen diffusion and HIC during the practical stacking slow cooling, optimizes the X80pipeline steel stacking slow cooling technology and improves the slab fault detection qualification rate. The main research content and conclusions are as follows:(1)X80HIC sensitiveness is studied by the electrochemical method. The results show that the content of diffusible hydrogen increases with the prolonging of hydrogen charging time first and then inclines to stable value, and it increases with the increase of ampere density.(2)Hydrogen induced crack characteristics of X80pipeline steel are studied. The results show that HIC can be divided into two types:One is hydrogen bubbling which generates on the surface; the other is the internal HIC which exists mainly in the form of stepped cracks. These provide theoretical basis for HIC mechanism.(3)The influence factors of X80pipeline steel HIC are studied. The results show that banded structure is the gathering place for the initiation and expansion of HIC; different types of inclusions have different influence on HIC, proving that inclusions have a close relationship to the sensitiveness of HIC.(4)The effective hydrogen diffusion coefficient under different temperatures is tested with the method of electrochemical penetration, and the linear relation equation between the effective hydrogen diffusion coefficient of X80pipeline steel and temperature is obtained.(5)The sensitiveness of temperature and HIC to the diffusion of Hydrogen in X80 pipeline steel during the stacking slow cooling is simulated. The high temperature interval is500-400℃and hydrogen density in the slab center is far higher than that in the lower temperature interval350-100℃, the sensitiveness in high temperature area is much larger than that in low temperature. According to the result of research, the X80pipeline steel stacking slow cooling technology is improved, meanwhile off-line testing temperature is improved too. The optimization process is stacking starting temperature of about450℃, stacking time greater than24h and the offline testing temperature of85℃.Based on the research on the hydrogen diffusion law and HIC sensitiveness during the process of stacking slow cooling, we optimize the stacking slow cooling technology, which has great guiding significance to the practical production. The method is promoted and has been applied in work field, which effectively improves fault detection qualification rate of X80pipeline steel slab, solves the check missed problems caused by HIC.