Developmental Research on Hydrogen Induced Fracture in High Tension Bolts

F13T高强螺栓自引入钢结构和桥梁领域以来，其抗拉强度超过1300 Mpa的延迟性失效已有报道。这些失效大多被认为是由扩散氢引起的，因为这些失效通常发生在海边或工业区域等污染环境中。虽然许多关于富氢焊缝金属的氢致裂纹(HIC)的研究已经报道，但由于缺乏关于空气中氢的吸收、金属中的扩散过程、局部高应力场的积累和裂纹萌生的冶金知识或机理，高压螺栓中的扩散氢的行为尚未得到很好的解决。因此，为了求解应力作用下氢在金属中的扩散过程，不仅需要考虑氢的浓度梯度，还需要考虑应力分布。进行了包括应力场贡献在内的数学推导，将结果应用到有限元方法中，并用数值方法计算了扩散氢在螺纹底周围积累所需的孕育时间。F13T螺栓试验材料为热处理用含硼高强度钢。每个试件在HCl或H_2SO_4电解液中阴极充氢数小时后，在其设计应力下施加恒定载荷，通过测量声发射来观察裂纹的萌生和扩展，并将有限元分析得到的氢在螺纹底部周围的扩散过程与实验结果进行了比较，从孕育到断裂的时间来看，两者吻合得很好。

The delayed failures of the F13T high tension bolts whose tensile strength were over 1300 MPa have been reported since they were introduced into the field of steel structures and bridges. Most of these failures have been thought to be induced by the diffusible hydrogen because these failures often occurred under polluted circumstances like seaside or industrial areas.Although many studies about hydrogen induced crack (HIC) have been reported in a hydrogen rich weldmetal, behavior of diffusible hydrogen in high tension bolt has not yet been solved satisfactorily due to the lack of metallurgical knowledge or mechanism about absorption of hydrogen from the air, diffusible process in metal, accumulation in the locally higher stressed field and initiation of crack. Therefore, there is the need to take into account not only the hydrogen concentration gradient but also the stress distribution in order to solve the diffusional process of hydrogen in metals under stress.Mathematical development including the contribution of the stress field was done, results were applied into the finite element method (FEM), and a numerical method was used to calculate the incubation time required for diffusible hydrogen to accumulate around the bottom of a thread.Delayed fracture tests with pre-charged specimens were also performed. The testing material used for F13T bolt was high strength steel contained Boron for heat treatment. After each specimens was cathodically charged with hydrogen for several hours in HCl or H_2SO_4 electrolyte, they were stressed with a constant load under its design stress, and observed initiation and propagation of crack by measuring acoustic emission till they were fractured.The diffusible process of hydrogen around the bottom of thread obtained by FEM analysis were compared with these experimental results, and showed a good agreement with that in view of incubation time till fracture.