Effect of alloying elements on the susceptibility to sulfide stress cracking of line pipe steels

pipe steels on the susceptibility to sutfide stress cracking (SSC) was studied with respect to hydrogen permeation, crack nucleation, and crack propagation. The SSC susceptibility was evaluated using a constant elongation rate test (CERT) method in a NACE Standard TM0177 solution. The corrosion properties were evaluated with potentiodynamic, potentiostatic, and linear polarization methods in the same solution used in CERT The hydrogen diffusion rate through the steel matrix was determined electrochemically by measuring the hydrogen oxidation current density using Devanathan-Stachurski cells. Crack nucleation and propagation behavior were investigated using optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The cracks nucleated predominantly at nonmetallic inclusions and propagated through the steel matrix in a quasi-cleavage manner regardless of the test materials' compositions. The number of inclusions, which acted as crack nucleation sites, was not influenced by the addition of the alloying elements. Addition of Cu-Ni (Steel A) and Mo (Steel B) increased SSC susceptibility by increasing the hardness value of the steel matrix due to the formation of banitic components in the microstructure, and by increasing hydrogen permeation flux through the steel matrix. Addition of Mo (Steel B) increased hydrogen permeation flux by both inducing pitting in sutfide film, which formed on the steel surface, and by decreasing the number of precipitates by forming banitic matrix. Addition of Ti (Steel D) decreased SSC susceptibility by reducing hydrogen permeation flux due to the formation of Ti-Nb(C, N) precipitates, which acted as strong hydrogen trapping sites. The SSC susceptibility index estimated from the hydrogen permeation flux (J), area fraction of inclusion (R-i), and the hardness value (Hv) of steel adequately explained the SSC data obtained in this study Optimization between the mechanical properties and SSC resistance by addition of various alloying elements can be made by controlling those three factors.