Effects of Thermal Insulation on Residual Stresses of Welded Joints

Abstract

Residual stresses are generally retained in a component, such as a section of a welded pipe, assembled coupling, or angle joint for the entire service life of the component. Residual stresses contribute to tensile stresses which are partly responsible for failure of welded structures through processes such as fatigue failure, stress corrosion cracking, or corrosion fatigue. Some of the chief factors responsible for setting up residual stresses in welds include; rate of cooling, stresses already present in the parent material, type of structure and neighboring joints, heat present during welding depending on, current, electrode size, and speed in arc welding. From this research, the following conclusions were drawn. Lagging the welded pipe coupons decreased tensile residual stresses. For instance, axial tensile residual stresses on the inner surface of the pipe were lowered by 31.4 percent when 25 mm thick insulation was used. Insulating the weld caused an increase in compressive residual stresses. Axial compressive residual stresses on the outer surface of the pipe increased by 176.9 percent when 37.5 mm thick insulation was used. The aim of the research was to study ways of minimizing residual stresses in order to improve the quality of welded structures, focusing on oil pipelines. The method studied in this research was lowering the rate of cooling of welded pipe coupons by insulating them. Residual stresses in the pipe coupon were calculated using finite element simulation. ABAQUS was used to model the welding process of API 5L X65M pipe coupons. Trend-lines of stresses obtained at different cooling rates were drawn for easy comparison and conclusions made.