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ABSTRACT
UNS S41429 is a martensitic stainless steel with a unique alloy chemistry suitable for carburizing or nitriding to create a deep, hard, and stainless surface layer while maintaining a strong, tough and ductile core. UNS S41429 can achieve surface hardness greater than 60 HRC via carburizing while maintaining its stainless properties and fine grain size. It can also be nitrided to a surface hardness of 70 HRC with improved corrosion resistance compared to a similarly-nitrided common martensitic stainless steel (UNS S42000). Salt fog testing per ASTM B117 was done to compare the corrosion resistance of carburized UNS S41429 with a nominal surface hardness of 61 HRC to through-hardening grades UNS S44004 (59 HRC) and UNS S42700 (61 HRC), and carburizing stainless grade UNS S42670 (63 HRC). Salt fog corrosion testing was also performed on salt-bath nitrided UNS S41429 samples with a surface hardness of 70 HRC. Results indicated that carburized UNS S41429 possesses superior corrosion resistance in salt fog testing with a surface hardness of 61 HRC versus all other stainless steels investigated.
Key words: Carburizing, nitriding, QPQ, stainless steel, bearings, gears, surface hardness
INTRODUCTION
Surface-hardened stainless steels are attractive for applications requiring a combination of high surface hardness, excellent core toughness and good corrosion resistance, like bearings and gears that are exposed to marine environments. Unfortunately, the carburizing and nitriding processes can reduce the corrosion resistance of conventional carburizing and nitriding stainless steels. Similarly, the corrosion resistance of through-hardening stainless steels can be compromised after standard heat treatments. Both carburizing and through-hardening stainless steels form undesirable chromium-rich carbides like M23C6 and M7C3 in the carbon-rich surface layer, thereby degrading the corrosion resistance of the hardened surface.1' ? 3 Nitrided stainless steels can exhibit similarly poor corrosion resistance due to the formation of chromium-rich nitrides that precipitate in the nitrided layer. Hard surface layers in both carburized and nitrided stainless steels are achievable' but corrosion resistance is sometimes no better than in carburized or nitrided alloy steels. Carburizing or nitriding at temperatures above the solvus for M23C6 and M7C3 carbides limits the formation of deleterious Cr-rich phases, but will generally cause significant grain growth and result in poor toughness.4 Low-temperature carburizing techniques have been developed for austenitic stainless steels like UNS S31603 that...