Microstructure has a very strong influence on the sulphide stress cracking (SSC) resistance of high strength steels. In this work a commercial heat of a low alloy carbon steel was used. Six different heat treatments were performed using two austenitization temperatures (890º and 1110ºC) and three tempering temperatures (500º, 600 º and 700°C). The microstructure was analysed using optical and electron microcopy (SEM and TEM). The SSC behaviour was examined using slow strain rate testing (SSRT) of smooth samples. After quenching, packets of fine autotempered lath martensite were the main component, although retained austenite and plates of ferrite with very small carbides inside were also detected. After tempering at 500º and 600ºC, the matrix did not change its morphology very much. The retained austenite was transformed into a thin film of carbides located between laths and in prior austenite grain boundaries (PAGB). The specimens tempered at 700ºC showed recovery (more in the material austenitized at 890ºC) although most of the microstructure retained the laths morphology. In SSRT, materials tempered al 500 º and 600 º C broke during the elastic elongation and a mixture of Intergranular and transgranular fracture was observed. Materials tempered at 700 º C broke in the plastic range and transgranular fracture was observed (although some inter1ath fracture parallel to the direction of deformation was also detected). Large carbides, together with inclusions are sites of crack nucleation and they may also help the propagation. High-angle boundaries (PAGB and boundaries of lath martensite packets) are sites where cracks are arrested.
Sulphide stress cracking (SSC) of high strength, low alloy steels is a phenomenon of particular concern to the oil and gas industry. Frequently, different components of oil wells and refineries are in contact with acid brines containing hydrogen sulphide. If stress is applied to such components, catastrophic failures can occur. It has been established that SSC is a particular case of hydrogen embrittlement (1, 2). strength and microstructure have a strong Influence on the susceptibility to SSC. The higher the strength of a material, the higher Is its susceptibility to SSC (35). The influence of the microstructure Is less clear, probably because a full characterization of all the components has not always been carried out. Even though optical microscopy and hardness are important tools, they are not precise enough to detect all the constituents present (retained austenite, twins, carbides, etc). Electron microscopy provides the resolution and analytic capacity to obtain a deeper knowledge of the microstructure. An important step towards obtaining materials with better strength and resistance to SSC Is to understand the real influence of the different constituents of the microstructure (grain boundaries, segregated impurities, carbides etc) on the process of initiation and propagation of cracks.
The heat treatment used in pipes for the oil industry is mainly quench and temper. It has been stated that materials produced by this process have a better combination of strength and resistance to SSC than those produced by normalising and tempering.
