Welding residual stress is an important variable in the assessment of structural integrity as it would increase the risk of brittle fracture. Post-weld heat treatment (PWHT) of welds is normally employed to mitigate the welding residual stresses in welded joints, as well as temper microstructures to improve mechanical properties of the weld and heat affected zones (HAZs) (eg fracture toughness). The normal practice of PWHT is to place a component in an enclosed furnace for uniform heating of the entire component so that no additional thermal stresses and residual stresses would be generated in the component due to the PWHT process. However, it is not always possible to place the entire component in a furnace for heat treatment (eg circumferential weldes in a pipeline), then a local PWHT may be the only option in which the welded joints are locally heat treated. Compared to the fully controlled (ie furnace) PWHT of welded joints, there is little work on local PWHT with regard to its effectiveness in stress relief. Hence, it is of great practical importance to understand the effects of various parameters in local PWHT, such as heating rate, cooling rate, soak temperature, hold time, heated band width, soak band width and insulation conditions) on stress relief.
This project combines experimental techniques (including characterisation of material properties and microstructures, and measurements of residual stress) and finite element analysis (FEA) approach (including thermal, elastic-plastic and creep analyses) to investigate systematically the effects of the various parameters involved in a local PWHT process, with a view to provide a more concrete guidance on stress relief through a local PWHT.
In the experimental work, the main focus is on quantifying the extent of stress relief in ferritic steel pipe girth welds due to different combinations of local PWHT parameters (eg heated band width, soak band width, soak temperature and hold time). Internal residual stresses were measured non-destructively before and after local PWHT using neutron diffraction technique. Microstructures in the weld and HAZs will also be investigated, as well as the effects on fracture toughness due to local PWHT.
In the analytical aspects, FEA will be used to simulate the local PWHT process for understanding the effects of the various parameters on stress relief. The temperature dependence of material mechanical properties (mainly the yield strength and modulus of elasticity), plastic deformation and creep behaviour occurring in the local PWHT will be taken into account. The analytical work will be a complement to the experimental work in providing a more fundamental understanding of the local PWHT process and the significance of the local PWHT parameters in reliving welding residual stresses.