Effect of Multipass Welding Using a Low Transformation Temperature Filler Metal on Residual Stress and Toughness

Abstract

Residual stresses in welded components are consequence of stress and/or thermal gradients and influenced by factors such as joint geometry, variation in strength of the material, preheat temperature, heat input, post-weld heat treatment and phase transformation strains. During the 70’s, it was observed that the level of residual stress accumulated in a constrained sample during cooling from austenite could be reduced after transformation to martensite or bainite. Some works have evaluated effect of welding using a low transformation temperature martensitic filler metal on the level of residual stress in single pass joints. According to these studies, martensite start temperature in the range 200–250°C can be extremely effective for mitigation of tensile residual stresses. The outcome of most of these works was on one hand increase of fatigue life due to the mitigation of tensile residual stresses via transformation strains, on the other a significant reduction of the fracture toughness.In the present study, sections of API 5L class B steel tubes were multipass welded using a 12Cr-5Ni low transformation temperature filler metal in addition to a conventional filler metal. Residual stresses in the inner and outer surfaces were measured by X-ray diffraction. Aspects related to the improvement of toughness in the weld metal due to the tempering of one pass by the subsequent were also discussed.