STRESS ORIENTED HYDROGEN INDUCED CRACKING BEHAVIOR OF HEAT AFFECTED ZONE OF L360MCS STEEL IN WET H<SUB>2</SUB>S ENVIRONMENT

Abstract

Stress oriented hydrogen induced cracking (SOHIC) behavior of heat affected zone (HAZ) of L360MCS steel in wet H2S environment was studied by separating crack, SEM and EDS. The cracks are formed by four-point bending test in wet H2S environment. The microstructure of HAZ is obviously different from the base metal, and the hardness of HAZ is lower than those of the base metal and welding joint. The fracture surface of the cracks was separated and the hy- drogen blisters were torn off. The cross section and fracture surface of the cracks are analyzed by SEM and EDS. Results show that the cracks of SOHIC appeared as axial crack and vertical crack, and the former is preferential. The appearance of cracks is associated with the low hardness value of HAZ. The dimension of hydrogen blasters in tension stress zone of bended specimen is bigger than that in unstressed specimen, which is due to hydrogen concentration in bended specimen is higher. Both axial and vertical cracks nucleate independently and propagate in the quasi-cleavage manner. Tension stress not only increases the supersaturated hydrogen concentration in tension stress zone but directly promotes vertical crack propagation. Axial crack nucleates at MnS, CaS and precipitated phase, and propagates under internal hydrogen pressure. Vertical crack mainly nu- cleates on precipitated phase and propagates under both internal hydrogen pressure and tensile stress.