Stress Relaxation Cracking and the Curious Case of the Uncracked Welds SRC Mechanisms Part II

Abstract This paper investigates the phenomenon of stress relaxation cracking (SRC), a damage mechanism affecting process equipment and piping operating at elevated temperatures. It is characterized by intergranular cracking with exceptionally low ductility in welds and cold-formed components at elevated temperatures. The “soft denuded zone model,” frequently cited as an explanation for this mechanism, was refuted in a companion paper [1] and an alternative model is proposed. The paper describes the key characteristics of SRC, highlighting that most welds do not experience cracking. The highest susceptibility occurs at the low end of the creep temperature range, where secondary weld residual stress alone can induce cracking due to extremely low creep ductility. The proposed model attributes SRC to creep cavitation under high stress, primarily residual stress resulting from welding or forming. While most welds can relieve residual stress through relaxation, certain welds develop triaxial residual stress due to constraint and have very low stress relaxation rates, making them susceptible to SRC. Triaxial stress state is shown to reduce ductility by limiting dislocation mobility. Factors that impede dislocation movement, including extensive cold forming, precipitation, and the presence of interstitial solutes, further increase susceptibility. Creep cavitation is enhanced by prior cold work, causing nucleation at secondary particles located on grain boundaries. Susceptibility to SRC is influenced by grain size, orientation of grain boundaries to the stress, and lattice orientation between adjacent grains. Elemental segregation at grain boundaries may also contribute to SRC, although this is generally a minor factor. Mitigation involves stress relaxation through post-weld or post-fabrication heat treatment at elevated temperatures. However, some damage accumulation is anticipated during stress relief, potentially reducing the component’s lifespan.