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-r eceived and heat-treated specimens to a H2S-saturated aqueous National Asso ciation of Corrosion Engineers (NACE) solution. It was found that for simil ar corrosive environments and applied stress intensity factors of 30 to 46 MPa root m, crack growth in LEFM (linear elastic fracture mechanics) compac t specimens is strongly influenced by heat heating. In the as-received allo y, crack growth in the direction normal to rolling was controlled by metal dissolution of the crack tip region in contact with the corrosive environme nt, with crack growth rates of the order of 1/W(da/dt) similar to 8.3 X 10( -4) h(-1). Alternatively, crack growth in the direction parallel to the rol ling direction did not show metal dissolution but instead hydrogen embrittl ement along segregation bands. In this case, crack growth rates of the orde r of 1.2 x 10(-3) h(-1) were exhibited. In the martensitic condition, the r ate of crack propagation was relatively fast (1/W(da/dt) similar to 4.5 X 1 0(-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 root m, re spectively. Apparently, in the water-sprayed condition, numerous microcrack s 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 temperin g (Q&T) resulted in a tempered martensite microstructure characterized by f ine 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. Mos t of the tests were carried out in NaCl-free NACE solutions; the only excep tion was the as-received condition where 5 wt pet NaCl was added to the sou r 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.