Prevention of HAZ cracking in corner joints of high heat input SAW box columns

Abstract

Summary One-pass submerged-arc welding (SAW) with high heat input is extensively used for the corner welds of box columns, though internal cracking of lamellar tearing type in some cases occurs along the centreline of the flange plate thickness. This paper describes an investigation of weld microstructures, hydrogen concentrations, and residual stresses at the cracking position to clarify the cracking mechanism. Appropriate measures to prevent cracking in terms of material- and fabrication-related factors are also studied on the basis of laboratory-scale one-pass SAW experiments and numerical analysis of hydrogen diffusion. The generation and propagation of cracking are closely related to the transition of the hydrogen concentration at the cracking position, whereas the effect of the residual stress on cracking is small. The material-related factors controlling this cracking include elongated manganese sulphide (MnS) in the centre segregation band and martensite-austenite constituent (M-A) around the MnS formed by the intercritical heat effect of one-pass SAW. Moisture contained in the flux is the greatest source of hydrogen, so that flux drying is the most effective preventive measure affecting the welding conditions. To prevent cracking, a bevel groove for a one-pass corner joint SAW is preferable to a V groove. PWHT at 300°C for more than 2 hours is also effective in this respect. Effective fabrication-related preventive measures include alleviation of centre segregation by hardenability reduction in continuous casting, shape control of sulphides by the addition of Ca, and Ceq reduction through application of thermomechanical control.