Thermomechanical finite element simulation of welding, and elevated-temperature mechanical behaviour of high and ultra-high strength steels

Abstract

The development of high and ultra-high strength steels (HSSs/UHSSs) emerged as an engineering approach to enable lightweight structural applications and an environmental solution to reduce the steel production-related carbon footprint. There is increasing use of HSSs/UHSSs in a diverse range of industrial applications due to their excellent strength-to-weight ratio, weldability, and toughness. Fusion welding techniques and inbetween, arc welding processes are indispensable as efficient methods to make permanent joints between components and structural members made from HSSs/UHSSs. Ensuring the safe application of assemblies made from these materials and exploiting their maximum load-bearing capacities require extensive research on the behaviour of such materials due to welding heat and elevated temperatures that cause welding residual stresses, different welding deformations, and strength degradation.

Consequently, the development of welding residual stresses and distortions in different HSSs/UHSSs was investigated by means of finite element (FE) simulation of welding. Sequentially coupled thermomechanical formulation was adopted in all the cases considered, and mathematical modelling of the heat source was based on a double ellipsoidal heat source model. In this regard, welding distortions due to bead-on-plate welding of a specimen made of S960MC were simulated incorporating solid-state phase transformation models. Comparison of the simulated results and experimental measurements showed that a more accurate prediction of angular and out-of-plane bending distortions is possible when the effect of phase transformation is considered.

Development of welding residual stresses and distortions in T-joints made of S700MC under different external restraints and welding sequences for continuous and short noncontinuous fillet welds were numerically and experimentally investigated in separate studies. Thermo-elastic–plastic FE models were developed considering material and geometrical nonlinearities. The results showed that for both continuous and short fillet welds, the effects of mechanical boundary conditions on sequential and cumulative welding distortions and welding residual stresses are more significant than the welding sequence. While angular distortion and transverse residual stresses were highly affected by the configuration of external constraints, longitudinal residual stresses were found to be less affected. Comparing the results of the two studies showed that localized transverse stress fields with higher peak magnitudes resulted from short fillet welds.

The strength degradation of two grades of UHSSs, namely, S960MC and S1100, due to elevated temperatures in as-received and as-welded conditions was investigated experimentally. Standard specimens were manufactured, and steady-state hot tensile tests from room temperature to 900 °C were carried out. The reduction factors of the constitutive mechanical properties of the two steels were determined and compared with several leading standard predictive models and datasets from the literature. Finally, predictive equations were proposed to estimate those reduction factors for the tested UHSSs at elevated temperatures.