UNS S20910, also known as XM-19 according to ASTM A276, is an austenitic stainless steel used in several industrial applications including oil and gas, chemical, nuclear and cryogenic industry due to its combination of high strength and excellent corrosion resistance. In solution-annealed condition, UNS S20910 shows strength levels significantly higher than conventional austenitic stainless steels. Strength can be further increased by strain hardening without deteriorating other material properties. UNS S20910 exhibits remarkable corrosion resistance in most industrial environments, however, high chloride contents in combination with elevated temperatures may challenge the pitting corrosion resistance of this grade. To enhance pitting corrosion resistance, the microscopic cleanliness can be improved by electro-slag remelting (ESR). In this study, the influence of ESR and strain hardening processes on the pitting susceptibility of UNS S20910 has been investigated. Electrochemical tests were conducted at elevated temperatures in different alkaline chloride-containing solutions ranging from 80 to 140 g/l. Additionally, exposure tests according to ASTM G48 Method A were performed on material in strain-hardened condition. Experimental results confirmed differences in the pitting resistance of UNS S20910 due to the refinement its microstructural cleanliness reached by ESR. It was also confirmed that strain hardening has no detrimental influence on the corrosion behavior of UNS S20910.
During drilling operations, the components in the drill string including the bottom hole assembly (BHA) remains in permanent contact with the drilling fluid. Therefore, besides non-magnetic properties and high strength the corrosion resistance of the materials utilized for the BHA plays a decisive role specially in applications involving harsh environments. In fact, strain-hardened CrMn-austenitic steels commonly used in directional drilling technology show a high susceptibility to pitting corrosion and environmentally assisted cracking in drilling fluids with a high chloride (Cl−) content at elevated temperatures.1 Due to its chemical composition including high quantities of chromium (Cr), molybdenum (Mo) and nitrogen (N), UNS S20910 represents a very attractive alternative for these challenging environments. UNS S20910 is a Mn-containing austenitic stainless steel with excellent corrosion resistance even under very aggressive environmental conditions.2 As shown in Table 1; besides Mn, Cr, Mo and N, UNS S20910 also contains a high amount of nickel (Ni) and small additions of niobium (Nb) and vanadium (V). Nb is added to increase the strength through Z-Phase precipitation. Z-Phase is a complex nitride containing also small amounts of iron (Fe), V, Mo and Ni.2-4 Furthermore, Nb can also be present in the microstructure as a primary carbonitride (Nb(C,N))4 and is expected to have a negative effect on the corrosion properties. Electro-slag remelting (ESR) is a commonly used secondary metallurgical process step to refine the microstructure and reduce the amount of unwanted inclusions such as Mn-Sulfides and Oxides in stainless steels and Ni-alloys. Since the cleanliness of the microstructure can be significantly improved, it is assumed that ESR also improves the corrosion resistance of remelted material.
