Effects of Ce and La on microstructure and sulfide stress cracking susceptibility of API-5CT-C110 casing steel

This study investigated the effects of Ce and La addition on microstructure and sulfide stress cracking (SSC) behavior of API-5CT-C110 casing steel. The role of rare earth (RE) elements on the SSC resistance was assessed via uniaxial tensile testing and double cantilever beam testing (NACE TM0177 A and D methods). The experimental results show that the appropriate addition of RE is beneficial to the improvement of SSC resistance without compromising strength. RE addition is found to refine prior austenite grains and carbides, and modify the coarse inclusions into spherical RE oxysulfides. The electron backscattered diffraction analysis indicates that the steel with RE has lower high angle grain boundary and kernel average misorientation, which could effectively hinder crack propagation and thus reduce the SSC susceptibility. Furthermore, the proportion of deformed grains is reduced significantly due to RE addition, and substructure grains with low storage energy are increased, which is also a vital factor enhancing the SSC resistance.