Mechanical Behavior Study On Solid-state Phase Transformation Of High-strength Low-alloy Steel During Welding

The high-strength low-alloy steels are widely used in shipbuilding, automobile manufacturing and nuclear power engineering, et al. for the excellent mechanical strength, good ductility and welding performance. Because of the influences of alloying elements, the mechanical behavior of solid-state phase transformation of different low-alloy steels varies significantly, which brings a certain degree of uncertainty to the prediction of welding residual stresses and deformation. As a result, studies on modeling approaches of solid-state transformation and mechanical properties of the high-strength low-alloy steels during welding has become a research focus in recent years, as well as the coupled relationship among phase transformation, microstructures and stress-strain under different cooling rate.Several different types of solid-state phase transformation would take place simultaneously during welding for high-strength low-alloy steels because of the multiple alloy elements, which brings great difficulties to the establishing of phase transformation model and finite element simulation. On the one hand, the accuracy of traditional transformation model on prediction of multi-phase transformation and the multi-phase mechanical properties is relatively poor. It is also difficult to describe the effects of alloying elements on the transformation rate and achieve unified modeling for different types of steels. On the other hand, the relationship among temperature, microstructures and joints mechanical properties cannot be described accurately using the traditional model, as well as the evolution of the welding deformation and residual stress. Therefore, the establishment of multi-field coupled mode to effectively describe the phase transformation behavior of the high-strength low-alloy steels and the studies on the relationship among temperature, microstructures, joints mechanical properties, welding deformation and residual stresses are the chief research content of this paper.In this work, the solid-state transformation behavior of several high-strength low-alloy steels has been studied using the thermo-mechanical simulator, Gleeble-3500. The characteristic of the transformation and its mechanical behavior under different cooling conditions have also been studied using the optical microscopy and scanning electronic microscopy, respectively. On the basis of experiment and FEM simulation, a optimized transformation model has been proposed by error analysis to describe the martensite transformation of low-alloy steels. The optimized transformation model was compared with Koistinen-Marburger equation, and was proved more accurate in predicting the martensite volume fraction of low-alloy steels. In order to further verify the accuracy of the optimized model, a cyclic uniaxial test was conducted numerically and experimentally. The effects of the transformation kinetics on the uniaxial stress evolution were discussed based on the simulated results and experiment data.In order to further study the effects of complicated transformation kinetics, hardening model and annealing temperature under multiaxial complex stress state, the relationship among hardening model, annealing temperature and mechanical properties were discussed, and a modified annealing model was proposed. On the basis of the discussion above, the stress-strain evolutions of 304 L stainless steel, 16MND5 and Weldox960 during welding were studied experimentally and numerically. The results indicated that there is a big difference as whether or not considering recrystalization during simulating the residual stress. However, it seems that the stress was slightly affected by the annealing temperature. The effects of thermal cycle, solid-state transformation and the annealing temperature on the residual stress were also discussed. By analyzing the stress-strain state in different locations during welding, the stress-strain conceptual graph and the transient stress-strain distribution map were presented, in which the state of stress and the strain under quasi-steady temperature fields are described.By the discussion of the relationship among transformation, temperature distribution, microstructure properties and stress, the thermo-mechanical-metallurgical coupled model was constructed more perfect, and was applied to study the welding deformation and residual stress of large-scale complex structure. In order to consider the real situation during the welding process, a thermo-mechanical interface element was developed. The significant character in the interface element model is that the distortion could be estimated well by considering the effect of the fusion course, effective penetration and fillet weld size on the strength of welded joint. Meanwhile, a mixed heat source model was proposed to describe the heat flow distribution of laser welding based on the orthogonal testing and inverse calculation. Welding deformations and residual stresses were discussed, and the multi-field coupled model with interface element model was proved accurate and applicative in welding simulation of large-scale complex structure. Meanwhile, a thermal-metallurgical-displacement interface element model was proposed to analyse the microstructure deformations and fracture behavior.On the basis of the discussion, the 3D multi-fields coupled model and the interface element model were applied to the deformation and residual stress prediction of large scale structure welding. By the comparison of experimental results and simulation results,the deformation and residual stress distribution rule was discussed. The results also further verify the accuracy and applicability of the 3D multi-fields coupled model and the interface element model.