大入熱ＳＡＷ Ｂｏｘ柱のＨＡＺ割れ防止対策の検討

Abstract

One-pass submerged arc welding (SAW) with high heat-input is widely employed for the corner seam of box-shaped columns, however, an internal crack similar to lamellar tearing occasionally occurs along the center-thickness line of the flange plate. In this paper, microstructures, hydrogen concentrations and residual stresses at the cracking position have been investigated to clarify the mechanism of the cracking. In addition, appropriate countermeasures against the cracking in terms of the quality of plates and the conditions of welding have been studied, by application of the one-pass SAW experiment in the laboratory and the numerical analysis of hydrogen diffusion.The generation and the propagation of the cracking were closely related to the transit of hydrogen concentration at the cracking position, while the effect of residual stresses on the cracking was small. The dominant material factors of this cracking were elongated manganese sulfides (MnS) in the center segregation band, and martensite-austenite constituents (M-A) around MnS formed by intercritical heat affection of one-pass SAW.Moisture contained in the flux was the greatest source of hydrogen, so that drying of the flux was the most effective countermeasure in the welding conditions. As a groove for the one-pass SAW corner joint, a bevel groove was preferable to a V groove to prevent cracking. PWHT at 300°C for longer than 2 hours was also effective. As countermeasures in the production of steel plates, alleviating center segregation by soft reduction in continuous casting, shape control of sulfides by addition of Ca, and lowering Ceq by application of thermo-mechanical-control-process were effective.