The needs of oil and gas operators are challenging the industry to push the limits in sour service materials always further by finding the best compromise between high strength steels and good resistance to Sulphide Stress Cracking (SSC). It is in this context that materials have been recently developed, so as to respond to increasingly severe service with high level of Specified Minimum Yield Strength (SMYS). This communication presents the combination of computational modeling methods, advanced metallurgical characterization techniques, scale-up methods (from laboratory to mass production) for designing sour service steel grades with improved SSC resistance. Basics, pros and cons of several tools are discussed; namely X-Ray Diffraction from synchrotron beam, Transmission Electronic Microscopy, Electron Back Scatter Diffraction, Thermo-Calc™. Their efficiency to optimize design has been validated on a selection of lab casted prototypes and applied to the development of a new 125 ksi SMYS material with enhanced SSC resistance, able to sustain 0.1 bar H2S at ambient temperature. Some results demonstrating the SSC resistance as evaluated with NACE TM0177-2016 Method A of the newly developed steel sampled from pipes already produced at industrial scale will be shown.
During the last decades, low alloyed steels with improved resistance to Sulfide Stress Cracking (SSC) have been developed for covering specific applications as heavy wall casings1 or expandable tubings2 or for reaching higher mechanical properties, such as 125 ksi Specified Minimum Yield Strength (SMYS) materials.3-6 For the latter, relevant sour environments for developed grades are mild, meaning that all sour applications cannot be covered while a strong interest exists for O&G operators to use high strength materials when designing wells. Consequently, there is an incentive to push the limits of use of high strength sour service steels by enhancing their resistance to SSC. Several recommendations were already published when designing high strength sour service grades: hardness level shall be limited as much as possible and be preferentially below 22 HRC7, microstructure shall present a minimum required amount of martensite8 which is well known to be ideal for combining high mechanical properties and high resistance to hydrogen. Besides, many authors highlighted some other influencing parameters related to the material or the process; below a short and incomplete review:
- The benefit to increasing the tempering temperature9 as much as possible for reducing dislocations densities10 and for increasing the proportion of transgranular cracking11;
- The benefit to reducing prior austenite grain size12,14. To do so, it has been recommended to apply multiple quenching steps9;
- The benefit to reducing the amount of inclusions that affect the plastic elongation11 and specifically to avoid oversized oxide particles9. Also, the presence of residual elements such as S affect dramatically the Sour Service performance13;
- Upper bainite should be avoided9,14 and the addition of boron is known to enhance hardenability of low alloyed steels9;
- The beneficial impact of Mo for Sour Service is well established15 with an optimum content at 1.2% has been proposed16. The benefit would be due to fine carbide precipitation and to a decrease of the corrosion rate17;
- The impact of niobium content on SSC resistance has also been investigated. This element would help to decrease hydrogen permeation flux18. Niobium prevents prior austenite grain size growth during austenitization and allows higher tempering temperature thanks to niobium carbide precipitation19. It has been observed a reduction of SSC growth rate inside DCB specimens with increasing Nb content20;
- While the maximum content in Ni at 1% given in NACE† MR0175/ISO 15156 document7 has been agreed as a good compromise21, Kappes and his co-authors highlighted that this maximum content in Ni is dubious because based on outdated test practices (NACE TM0177 Method B) and claimed that the most important is to have the appropriate tempered martensite microstructure22.