高强度系泊链钢的氢脆与海水腐蚀疲劳机理研究

Mooring chain steel is one of the most important materials in marine engineering equipment construction. To meet the demand of building deep ocean oil drill platform, the R5 steel is now being developed for higher strength. However, the higher strength the steel is, the more serious the sensitivity of hydrogen brittleness will be. Therefore, it is necessary for this high strength mooring chain steel to investigate the sensitivity of hydrogen embrittlement and mechanism of marine corrosion fatigue. This project aims at studying the following respects. First, through controlling the heat-treatment, the changes of carbide size, distribution and type, the amount change of retained austenite are systematically researched. In addition, some subtle changes in elements (such as Cr, P) distribution throughout carbides, retained austenite, base α-Fe phase and their interfaces, in hydrogen trap sites distribution are also observed from nanoscale. Hence, the microstructure is understood better. Second, a relationship between microstructure and hydrogen embrittlement is established in theory through the investigation of changes with microstructure in hydrogen behavior such as diffusible hydrogen, trapping hydrogen and hydrogen diffusivity, in hydrogen-induced embrittlement under different hydrogen charge conditions. Third, after researching the effect of hydrogen on crack initiation and crack propagation, the relationship of hydrogen embrittlement and fatigue properties is elucidated. Fourthly, the changes of mobile hydrogen and trapping hydrogen are investigated in these different heat-treatment steels after marine corrosion fatigue. Thus the effect of microstructure on hydrogen formation is set forth. Finally, through dynamically hydrogen charging to simulate the procedure of taking in hydrogen, the fatigue behavior is investigated in order to explain the mechanism of marine corrosion fatigue in this high strength mooring chain steel. In a word, the final goal is to provide a theory law for developing high strength steels used in ocean engineering and for its security application.

系泊链钢是海洋工程装备建设的关键材料，针对深海石油钻井平台的要求而开发的R5系泊链钢，强度大幅提高，氢脆敏感性会更加严重。因此，研究高强度系泊链钢的氢脆与海水腐蚀疲劳性能很有必要。本项目通过高强度系泊链钢的热处理控制，系统研究碳化物的大小、分布、类型上的变化、残余奥氏体数量上的变化，并在纳米尺度上研究Cr、P等合金元素在相中及其界面的分布、氢陷阱的分布等微细观变化；结合该钢在不同电解充氢条件下可扩散氢、陷阱氢、氢扩散系数等氢行为变化以及氢脆敏感性变化，在理论上建立组织与氢脆之间的关系；结合氢对裂纹萌生、扩展的影响，阐明氢脆与疲劳性能之间的关系；并结合该钢在海水中腐蚀疲劳后可扩散氢、陷阱氢的变化，阐明组织对海水腐蚀疲劳中氢产生方面的作用；最后通过电解渗氢模拟氢原子进入高强度系泊链钢的疲劳行为研究，以揭示高强度系泊链钢在海水中的腐蚀疲劳机理，最终为高强度海洋工程用钢开发、安全应用提供理论依据。

系泊链钢是海洋工程装备建设的关键材料，针对深海石油钻井平台的要求而开发的R5系泊链钢，强度大幅提高，氢脆敏感性会更加严重。因此，研究R5高强度系泊链钢中的氢行为、氢脆、海水中的腐蚀行为及海水腐蚀疲劳性能都是非常重要的。本项目通过对已经研发的R5高强度系泊链钢进行热处理控制，即920°C+890°C、920°C、940°C淬火，560-640°C不同温度的回火，系统研究碳化物的大小、分布、类型上的变化，晶界结构的变化以及位错组态的变化，在纳米尺度上研究C、Cr、Nb、V等合金元素在相中及其界面的分布、氢陷阱的分布等微细观变化；结合该钢在不同组织、不同电解充氢条件下可扩散氢、陷阱氢、氢扩散系数等氢行为变化以及氢脆敏感性变化，在理论上建立组织与氢脆之间的关系；结合该钢在不同组织、不同pH值海水中的极化曲线、阻抗谱及失重等腐蚀行为，阐明组织与腐蚀性能之间的关系；电解充氢条件下结合氢对裂纹萌生、扩展的影响，阐明氢脆与疲劳性能之间的关系；最后通过电解渗氢模拟氢原子进入高强度系泊链钢的疲劳行为研究，以揭示高强度系泊链钢在海水中的腐蚀疲劳机理。得到重要的结果是：H在系泊链钢的残余奥氏体中明显聚集，而Cr的碳化物中很少聚集，Nb的碳化物中聚集氢的能力大大超过Cr的碳化物；不同碳化物位置（内部/边界位置）捕获氢的能力也有区别。回火温度比淬火温度对这种系泊链钢力学性能、氢脆敏感性的影响要大。造成回火温度对预充氢的氢脆敏感性不同的主要因素是位错组态，即可逆氢陷阱，它也直接影响氢的扩散系数及相同充氢条件下的氢浓度。随着回火温度的升高，耐海水腐蚀性能有所提高，海水中慢拉伸的应力腐蚀性能也有所提高，与预充氢的氢脆敏感性规律一致；但回火温度对海水中的疲劳寿命影响不大，这可能与动态充氢的氢脆敏感性有类似的规律，有待进一步的证实。通过本项目的研究，建立组织、腐蚀性能、氢脆敏感性及海水中腐蚀疲劳性能之间的关系，可以为高强度海洋工程用钢开发、安全应用提供理论依据。