Retardation of intermetallic phase formation in experimental superferritic stainless steels
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INTRODUCTION
SUPERFERRITIC stainless steels developed around a 29 pct chromium 4 pct molybdenum composition have excellent corrosion resistance in hot chloride-containing solutions and many other environments.tl~] Curiously, although commercial development of these alloys began most noticeably in the 1970s, ferritic stainless steels with from 20 to 40 pct chromium and up to 6 pct molybdenum were the subject of patents and articles as early as 1911.I1.5.61 The early alloys were, however, brittle and did not achieve much application. Binder and Spendelow[7] and Hochmann et al.tS] published the solution to the problem of brittleness in ferritic stainless steels in the early 1950s; poor toughness was not due to any embrittling effect of chromium p e r se but rather to the effect of the content of carbon and nitrogen, which is exacerbated at higher levels of chromium and molybdenum. Since the normal steelmaking techniques of the time were not able to reduce carbon and nitrogen levels in the steel to the low levels required to ensure that these alloys were ductile, the matter rested. However, the development in the late 1960s of vacuum-oxygen decarburization and argon-oxygen decarburization has permitted the manufacture of ferritic stainless steels with total contents of carbon and nitrogen of below 0.02 pct. This is generally sufficiently low to ensure good mechanical properties. The relatively high chromium and molybdenum contents of these more recent superferritic alloys confer resistance to both general and localized corrosion. The alloys are also highly resistant to stress corrosion cracking (SCC). If desired, the further addition of 1 to 2 pct nickel provides resistance to reducing acids (such as dilute H2SO4) and generally improves the impact toughness, although it lowers the resistance to SCC. [2,4,9] Maintenance of the content of carbon plus nitrogen at less than 0.02 pct in total provides
S. NANA, Senior Technician, and M.B. CORTIE, Assistant Director, are with the Physical Metallurgy Division, Mintek, Randburg 2125, Republic of South Africa. Manuscript submitted December 22, 1994. 2436---VOLUME 27A, SEPTEMBER 1996
resistance to intergranular corrosion and good toughness. Overall, alloys of this type can match or exceed the corrosion resistance of austenitic stainless steels.12,4J However, a disadvantage of these materials is the possible formation of very small, undesirable precipitates when they are held for prolonged periods at temperatures between 300 ~ and 500 ~ and intermetallic phases when they are exposed for protracted times between 650 ~ and 900 ~176 The former heat treatments cause the precipitation of chromium carbonitride followed by Cr-rich t~",[1~ whereas the latter may result in the nucleation and growth of the sigma and chi phases. (The notation a" will be used for the chromium-based bcc phase instead of a', following the lead of Pickering.t~5] This is because the designation a' is more often used nowadays for the low-carbon martensite formed in modem austenitic and dual-phase stainless
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