The role of heat treating on the sour gas resistance of an X-80 steel for oil and gas transport

In this work, the role of the microstructure in the stress sulfide cracking (SSC) resistance of an API X-80 steel was investigated by exposure of as-received and heat-treated specimens to a H₂S-saturated aqueous National Association of Corrosion Engineers (NACE) solution. It was found that for similar corrosive environments and applied stress intensity factors of 30 to 46 MPa√m, crack growth in LEFM (linear elastic fracture mechanics) compact specimens is strongly influenced by heat treating. In the as-received alloy, crack growth in the direction normal to rolling was controlled by metal dissolution of the crack tip region in contact with the corrosive environment, with crack growth rates of the order of 1/W(da/dt)∼8.3×10⁻⁴ h⁻¹. Alternatively, crack growth in the direction parallel to the rolling direction did not show metal dissolution, but instead hydrogen embrittlement along segregation bands. In this case, crack growth rates of the order of 1.2×10⁻³ h⁻¹ were exhibited. In the martensitic condition, the rate of crack propagation was relatively fast (1/W(da/dt)∼4.5×10⁻² h⁻¹), indicating severe hydrogen embrittlement. Crack arrest events were found to occur in water-sprayed and quenched and tempered specimens, with threshold stress intensity values (K _ISSC) of 26 and 32 MPa√m, respectively. Apparently, in the water-sprayed condition, numerous microcracks developed in the crack tip plastic zone. Crack growth occurred by linking of microcracks, which were able to reach the main crack tip. In particular, preferential microcrack growth occurred across carbide regions, but their growth was severely limited in the ferritic matrix. Quenching and tempering (Q&T) resulted in a tempered martensite microstructure characterized by fine distribution carbides, most of which were cementite. In this case, the crack path continually shifted to follow the ferrite interlath boundaries, which contained mostly fine cementite precipitates. As a result, the crack was tortuous with numerous bifurcations along ferrite grain boundaries. Most of the tests were carried out in NaCl-free NACE solutions; the only exception was the as-received condition where 5 wt pct NaCl was added to the sour environment. In this case, crack growth did not occur after exposing the specimen to the salt-free NACE solution for 30 days, but addition of 5 pct NaCl promoted crack propagation.