Welding Integrity of HSLA Welds

Abstract

The novel approaches in modelling and experimental investigations on welding integrity for single and multi-pass high strength low alloy steel (HSLA) welds is presented here. High level of welding stresses (mainly tensile mode) are generated during the construction of pipelines which are due to non-uniform temperature distribution and cooling rates as well as using clamps during welding and mechanical handling loads which occur during lifting the front end of pipeline to place it on supports. Existing of such high tensile stresses combined with hydrogen rich cellulosic electrodes used by Australian pipeline industry has increased the risk of Hydrogen assisted cold cracking (HACC) in the weld metal, in particular root pass of girth welding, which clearly has detrimental effect on weld integrity and its performance in service. The prediction and measurement of this welding stresses in pipeline construction was limited to a few studies. The current project was therefore started with emphasis on the effect of welding stresses on HACC susceptibility of the root pass of pipeline girth welding using Welding Institute of Canada (WIC) weldability test. 3D finite element models are developed and validated against neutron diffraction measurements to investigate the effects of various welding process parameters and regimes on residual stresses for the root pass of the WIC test. Chapter 4 presents the experimentally validated numerical simulation results of the root pass of the WIC test. However after initial modelling and experimental measurements the project was extended toward measurement of residual stresses (using neutron diffraction) for multi-pass welding in HSLA welds coupled with microstructural characterization and mechanical property studies with the view of full applicability for pipeline/ pressure vessel applications. It was found that welding parameters have significant effects on the residual stress-microstructural-mechanical property interrelationships of multipass HSLA welds. Therefore a set of welded specimens was prepared to investigate such interrelationships for the following welding parameters: Heat input effects (variable travel speed); The alterations in welding direction (welding sequence); The effects of welding process (combination of modified short arc and fluxed core arc welding versus shielded metal arc welding) It was found that increasing the heat input (reduction in welding speed) is beneficial in reduction of residual stresses. This could be correlated with the formation of microstructural constituent of mainly polygonal ferrite in the weld metal and HAZ of high heat input welds. Furthermore, there was no significant effect on the magnitude of the residual stresses, microstructural characteristics and mechanical properties when the weld deposition direction was changed. The findings also indicated using SMAW lead to significant reduction in residual stresses in comparison with the combination of MSAW+FCAW processes. The results of such investigations are presented in chapters 5-7. After identifying such interrelationship effects, this research study develops further to investigate the effects of mitigation techniques on relaxation of residual stresses/strains in weldments. In-situ neutron diffraction studies was conducted, coupled with in-depth microstructural/mechanical property studies, in which the rate of relaxation, holding time effects and the underlying mechanism behind stress relaxation for HSLA steel welds was investigated. One of the core finding of this investigation is the insignificant effects of holding time on stress/strain relaxations of HSLA welds. It was also found that strain relaxation in the initial stage of heating (temperatures up to about 360-370 ºC) is due to reduced yield strength with increasing temperature, while for the higher temperatures (370-600 ºC) strain relaxation is due to development of creep strain. Microstructural studies also indicate existing of sub-grains in the PWHT specimen, which is due to dislocation climb, pointing out to creep (creep strain development) as the driving mechanism behind stress relaxation during PWHT. The results are presented in chapter 8 of the thesis. Such findings will also be fully elaborated in the up-coming publications. Future work will be investigating the effects of plate thickness and the type of material on strain relaxation behaviour for various weld joints to establish a “time-temperature-rate of relaxation-thickness-holding time-material type” envelope. Such findings could offer the prospect of more economic heat treatment standards that combine cost savings with optimised mechanical properties and residual stress states.