Thermal Elasto-plastic Analysis on Hot-cracking Formation in Initial-bead of Butt-welded Joints

Abstract

It is well-known fact that solidification cracking frequently occurs in the initial bead of butt-welded joints. Such a solidification cracking is a troublesome problem because the one-side welding with backing materials is becoming widely used in recent years even in a relatively short welded joint. The weld metal solidification cracking occurs by rupture of liquid films of low freezing temperature segregate present between dendrites or cells during the final stages of solidification. The main object of this work is to seek for the origin of driving force and elucidate the growth mechanism for this solidification cracking. Numerical calculations are performed for four analytical models simulated to the initial pass of butt-welded joints using the finite element method for temperature analysis and the boundary element method for thermal elasto-plastic analysis. Calculated results of maximum strain rates in the middle cross section of the weld metals within the solidification brittleness temperature range are comparable to the experimental values obtained from the Trans-Varestraint test, however the amount of abrupt change of the tensile plastic strain is small compared with the value predicted from the experiments. The cooling rate at the toe of the weld metal immediately after commencement of solidification is so fast that a considerable amount of tensile plastic strain is generated suddenly in this part and its strain rate amounts to the order of 10-8/sec. It may be suggested that an effective way to prevent such a solidification cracking is to reduce the cooling rate at the toe of the weld metal during the final stages of solidification.