Effect of Martensitic Transformation on Stress Evolution in Multi-Pass Butt-Welded 9% Cr Heat-Resistant Steel Pipes

It has been recognized that low temperature martensitic transformation can reduce harmful tensile stress and generate beneficial compressive stress in weld zone of single pass welded joints. The influence of martensitic transformation is even greater in 9% Cr steel because of its high hardenability and low transformation temperature (about 100 similar to 400 degrees C). However, compressive stress was confined in certain parts of weld zone in multi-pass butt-welded 9% Cr steel pipes. In this work, stress evolution in a multi-pass butt-welded 9% Cr steel pipe was predicted using Abaqus software, and the effect of martensitic transformation was further investigated. The simulated results show that the overall pattern for the axial and hoop stresses appears to be similar, despite the lower magnitudes for axial stress. The maximum compressive stress was found in the final weld pass, and the maximum tensile stress was formed in the weld pass adjacent to the final weld pass. Stress in weld passes adjacent to weld root is relatively low. Tensile stress due to thermal contraction in the final weld pass was relieved by martensitic transformation and clear compressive stress was formed. However, little effect of martensitic transformation was found on the significant tensile residual stress in weld passes adjacent to the final weld pass. The final weld pass has the primary effect on the formation of residual stress. Compressive stress was indeed generated by martensitic transformation in former weld pass, however it was relieved by weld thermal cycle of latter weld pass. As a result, the effect of martensitic transformation appears to be confined to the final weld pass. The influence of martensitic transformation is greater around outer surface than that around inner surface.