In searching for plausible replacements of 17-4PH (UNS S17400) HH1150 for selected oilfield applications where the presence of reservoir fluid is a possibility, a series of NACE TM0177 Tensile Test Method A tests were conducted on various commercially available chromium-manganese austenitic stainless steels as well as 17-4PH in the 110ksi-to- 130ksi yield strength range (34 – 35 HRC max). Complementarily, microstructure analyses and electrochemical tests, among other standard tests, were applied to all alloys for comparison purposes and were used to explain the sulfide stress cracking performance of each alloy. The test results reveal that chromium-manganese austenitic stainless steels with higher PREN (higher than 25) and clean grain boundaries (e.g. no semi or continuous networks of precipitates) can have similar or better sulfide stress cracking performance than 17-4PH in slightly more aggressive sour environments than recommended by NACE MR0175/ISO15156 for 17-4PH. Other chromium-manganese austenitic stainless steels (PREN less than 20) exhibited major signs of embrittlement and are not suitable for future expanded investigations. As inferred by PREN, the electrochemical testing also demonstrates an improved performance of several austenitic alloys in artificial seawater compared to 17-4PH.
Chromium - Manganese (Cr-Mn) austenitic stainless steels with 110ksi [758MPa] minimum yield strength are well-established in the oilfields for directional drilling applications where their non-magnetic property is paramount to the surveying of the well trajectory. Today, such steels are commonly available both as solid and hollow bar stocks from a variety of major domestic and foreign mills. The performance of such steels towards pitting, crevice, stress-corrosion cracking (SCC), and corrosion fatigue have been known to vary greatly, with limited data readily available in the technical literature. Due to high chromium (>13wt.%) and manganese (>15wt.%) content, Cr-Mn stainless steels possess a stable austenite phase with a high nitrogen interstitial solubility. An elevated nitrogen (N) content improves both strength and enhances corrosion resistance while retaining room temperature toughness1. At a microstructure level, it further augments the austenite stability over that of strain-induced martensite; nitrogen indeed increases the shear strength of the face-centered cubic (fcc) austenite lattice, requiring more cold-work to build up the energy for formation of strain-induced martensite2. Nitrogen is thus not only an effective interstitial solid-solution strengthener but also extends the work-hardening ability, increasing responsiveness towards cold and warm forging. It follows that Cr-Mn austenitic stainless steels are not heat-treatable, meaning their strength does not increase after thermal treatments. Due to their stable austenite, Cr-Mn stainless steels with high nitrogen content exhibit near-invariant magnetic permeability, as required by directional drilling applications. The effects of heat-treatments tend to be negative, often resulting in sensitization and embrittlement due to the formation of nitrides and chromium depleted grain-boundaries. Nitrogen has positive effects on corrosion resistance in austenitic stainless steels; it increases resistance to pitting and crevice corrosion and resistance to stress-corrosion cracking in hot chloride solutions3. Since practically free of nickel (Ni), Cr-Mn austenitic stainless steels are cheaper than Nickel - Chromium - Molybdenum austenitic alloys such as Alloys 925 or 718; the latter alloy also being used extensively both in Drilling and Production under slightly different specifications (e.g., AMS 5662N vs. API 6ACRA)4,5. Cr-Mn steels are today not known to be used in oilfield production, particularly in applications that require withstanding major tensile stresses. In benign oilfield conditions (slightly sour), Cr-Mn stainless steels have the potential to provide satisfactory cracking resistance whereas in severe downhole conditions the risk of using these alloys is higher6.