Surface Hardened, Corrosion Resistant Fasteners for DoD Applications

Case hardening by carburization has long been recognized to produce wear resistant surfaces in steels.  Historically, when case hardening was applied to Cr containing alloys such as stainless steel, corrosion performance was degraded because of chromium carbide formation.  Recently, it was shown that a “Low Temperature Carbon Super Saturation” (LTCSS) surface modification treatment of 316L stainless steel (SS) enhanced surface hardness and fatigue resistance without compromising ductility (1,2). Carbon penetrates into the alloy at a depth of up to 25µm from all surfaces accessible to the treatment atmosphere. The industrial implementation of LTCSS surface modification process is extremely simple, can be used in non-line of sight configurations, maintains sample dimensions, and is relatively inexpensive. A growing body of corrosion work performed at NRL on LTCSS-treated austenitic stainless steel alloys show that LTCSS improves the pitting and crevice corrosion resistance of these alloys. The ambient temperature seawater pitting potential of 316SS is increased from +200mV for untreated specimens to +950mV for LTCSS-treated 316SS – a change that suggests LTCSS-treated 316SS is essentially immune to pitting in ambient temperature seawater (3).

Fastener corrosion and failure is a considerable maintenance issue incurring significant replacement costs and occasionally costly unplanned system downtime.  A SERDP workshop in 2006 identified fasteners as a common issue for all Services. This paper will report on corrosion resistance of LTCSS treated 316 stainless steel fasteners and fastener materials. References
1. U.S. patent 6,093,303 (July 25, 2000), Williams P et al. Swagelok Company.
2. G. M. Michal. F. Ernst, H. Kahn, Y. Cao, N. Agarwal, F. Oba, and A.H. Heuer, Acta

Materialia 54, 1597 (2006).3. . F. J. Martin, E. J. Lemieux, T. M.Newbauer ,  R. A.Bayles,  P. M. Natishan,  H.

Kahn, G.M.Michal, F. Ernst, and A.H. Heuer, “Localized Corrosion Resistance of LTCSS-Carburized Materials to Seawater Immersion”, ECS Trans. 3, (31) 613 (2007)