Thermal and residual stress analysis of welded joints in cladded pipelines

Abstract

The approach employed in this research in resolving the challenge of welding induced stresses on the dissimilar material joints of cladded pipes and the aftermath of weld is unique in approach compared to other researchers in that the transient heat recorded with high temperature thermocouples (positioned at strategic points) and Pico logger uncovered the trend and rate of heat transmitted throughout the dissimilar material welded joint during welding. This further revealed that the rate depended on the nature of the material, the distance from the weld line, weld axis, weld start, the thermal conductivity of material, phase change and not just the assumed proximity to the heat source. Hence the thermocouple closest to the heat source was not necessarily the first to receive the heat neither received the highest amount of heat compared to the rest of the thermocouples, most especially those placed farthest. This was further validated with the aid of finite element analysis of the welded joint of dissimilar materials and was confirmed for different clad thicknesses. Butt welding of two dissimilar materials: stainless steel of grade AISI 316 and mild steel of grade CR4 was carried out with the aid of the tungsten arc weld at a voltage of 240v using metal filler elements of A15 copper wire and 304/316 SS filler metals for the carbon steel and stainless steel sections of the weld, at the Brunel University laboratory. In this thesis different thicknesses (2mm and 12mm) of stainless steel clad in the dissimilar material clad joints has been investigated using scanning electron microscope (SEM), EBSD, XRD and EBSD to examine the dissimilar interface region and carbides in adjacent clads to generate the diffusion interface occurring in the dissimilar welded joint in order to guarantee the structural integrity of the structures for improved product quality and reliability. This resulted in generation of diffusion representation of the microstructural occurrences by reason of the results obtained from the microscopic and macrostructural analysis which stands out from all other authors. The approach of resolving the thermally induced weld stresses using the Gaussian theorem also differs from the approaches of other researchers and proved effective in the FEA of modelling and validation of both thermal and stress models and with respect to the weld direction. It was discovered from the weld induces direction, the radial and axial shrinkage effects radial shrinkage increase with increasing angle of inclination whereas the axial shrinkage at lower increments differs from those at higher increments of the axial length – cause of creep effect experienced at higher shrinkages.