水素侵入による980MPa級溶接金属の低温割れ限界条件

Abstract

Prevention of weld cracking is primely important for ensuring the integrity of steel construction. Especially, for high strength steel weld, which is susceptible to hydrogen cold cracking, it is important to clarify the critical condition for cracking in order for expanding use of high strength steel applications. In this study, critical conditions of hydrogen content and residual stress in the high strength steel weld with tensile strength level of over 980MPa were investigated. Cold cracking test was conducted using Y-grooved constraint weld joint by intentionally introducing hydrogen gas, and critical hydrogen content in the weld metal to cold cracking was determined. Cold cracking occurred in the weld metal from the root, bottom portion in the weld metal, when hydrogen content exceeds about 2ppm, and fracture surface was typical grain boundary fracture caused by hydrogen embrittlement. Then, residual stress distribution in the weld meal was measured by neutron diffraction technique. Same Y-grooved weld joint with no cold cracking was used for stress measurement. Neutron beam was filtered by 2mm×2mm slit to measure the residual stress in the small region. Reference sample without residual stress was prepared by introducing multiple saw cuts with small intervals in the same weld joint, and lattice spacing of stress free condition was first measured. Then, lattice spacing in the three different directions along the centerline of the weld metal were measured to calculate residual stress in the three directions. All portions in the weld metal center showed tensile residual stress. The root portion showed highest tensile stress of over 1110MPa in the transverse direction, which is the same as the crack opening direction. Therefore, it is considered that high level of tensile residual stress is the key factor for cold cracking. Hydrogen concentration to the root region by pressure stress was discussed in terms of local hydrogen content condition for cracking.