Finite element analysis is commonly used to investigate the thermal-mechanical phenomena during welding. To improve the computing efficiency of finite element analysis for welding thermal conduction, a novel Newton–Raphson method (NRM) without the computation of inverse matrix and a hybrid method combing the NRM and conventional implicit method (IMP) were developed. Comparison of computing time between the hybrid method implemented in an in-house software JWRIAN and the IMP used in a commercial software ABAQUS indicated that the computing speed of the former was about 4.5 times faster than that of the latter. Additionally, compared to the conventional IMP, the NRM exhibited higher computing efficiency in the analysis of transient thermal conduction during the welding heating process. Meanwhile, a combined hybrid method of the NRM and IMP was verified to be more efficient in analyzing the welding thermal conduction throughout the heating and cooling processes. Moreover, the thermal cycles computed by the hybrid method were consistent with those from experimental measurement, indicating the high accuracy of the hybrid method. Furthermore, the hybrid method was used to predict the temperature field of the corner boxing fillet joint welded by a low transformation temperature weld metal for generation of compressive residual stress.

We investigated whether low transformation temperature (LTT) welding materials are beneficial to the generation of compressive residual stress around a weld zone, thus enhancing the fatigue performance of the welded joint. An experimental and numerical study were conducted in order to analyze the residual stress in multi-pass T-welded joints using LTT welding wire. It was found that, compared to the conventional welded joint, greater tensile residual stress was induced in the flange plate of the LTT welded joints. This was attributed to the reheat temperature of the LTT weld pass during the multi-pass welding. The formerly-formed LTT weld pass with a reheat temperature lower than the austenite finish temperature converted the compressive residual stress into tensile stress. The compressive residual stress was generated in the regions with a reheat temperature higher than the austenite finish temperature, indicating that LTT welding materials are more suitable for single-pass welding.

The as-welded low-transformation-temperature (LTT) weld metal with martensite/retained austenite (RA) dual phase exhibits high toughness and ductility, but the yield strength (YS) is very low. After low-temperature postweld heat treatment at 300°C, the YS, toughness and ductility of dual-phase LTT weld metal increase dramatically, while there is a slight effect on mechanical properties of full martensite LTT weld metal. During the low-temperature postweld heat treatment, carbon atoms diffuse from martensite into RA, which increases the stability of RA. The improvements of mechanical properties for dual-phase LTT weld metal after low-temperature postweld heat treatment are attributed to the increased stability of RA and enhanced transformation-induced plasticity effect.

Two types of low-transformation-temperature weld metals were devised, one associated with primary austenite solidification, the other primary ferrite solidification. The martensite start temperature of both low-transformation-temperature weld metals was about 125°C. Experimental results showed that low-transformation-temperature weld microstructure associated with primary austenite solidification was martensite with 8.0% retained austenite, whereas that one related to primary ferrite solidification primarily consisted of martensite and δ-ferrite. Accordingly, both welded joints had little distinction between distortion and residual stress, indicating that weld metal associated with primary ferrite solidification played the same function as primary austenite solidification on residual stress reduction. Moreover, the low-transformation-temperature weld metal associated with primary ferrite solidification had higher tensile strength and hardness than that based on primary austenite solidification.