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CHARACTERIZATION OF STRESS CORROSION CRACKING ( SCC ) IN ULTRA-HIGH STRENGTH STAINLESS STEEL

Ultra-high strength precipitation hardened martensitic stainless steels (UHSSS) are increasingly used in aerospace applications as a replacement for ultra-high strength low alloy steels since stainless character eliminates the need to plate high strength components with toxic cadmium for corrosion resistance. However, modern UHSSS alloys remain susceptible to severe intergranular stress corrosion cracking (IGSCC) and transgranular stress corrosion cracking (TGSCC) when stressed in chloride containing environments. The primary goal of this research is to characterize the stress corrosion crack growth kinetics and fracture morphology for hydrogen environment assisted cracking (HEAC) in UHSSS. Subcritical crack growth experiments have been optimized to provide high resolution measurements of SCC crack growth in Custom ® 465 and Ferrium ® PH48STM at constant applied potential in aqueous NaCl solution. Results establish that a combination of composition and heat treatment changes applied to Ferrium ® PH48STM have increased the SCC resistance and changed the dominant fracture mechanism from intergranular to transgranular, compared to Custom ® 465. Metallurgical variables are hypothesized to control the concurrent increase in SCC resistance and change in fracture mechanism and Ferrium ® PH48STM. Introduction Ultra-high strength stainless steels (UHSSS) have become increasingly common replacements for ultra-high strength steels (UHSS) in high strength military and aerospace applications [1]. Conventional UHSS is susceptible to corrosion damage and requires the use of toxic cadmium plating or similar protection schemes to improve corrosion resistance in field environments [2,3]. Precipitation hardened UHSSS offer an outstanding combination of mechanical properties and general corrosion resistance without the need for cadmium plating [3]. Stress corrosion cracking (SCC) is a well know phenomena in conventional UHSSS such as PH 17-4 and PH 18-8 [4–6]. Both intergranular (IG) and transgranular (TG) SCC in UHSSS has been attributed to hydrogen environment assisted cracking (HEAC) [4,6,7]. Critically, high strength required for aerospace or military applications exasperates HEAC driven SCC in UHSSS. Control of metallurgical variables is crucial to maximize the combination of alloy strength, toughness, and HEAC resistance. Modern UHSSS alloys, Ferrium ® PH48STM 1 and Custom ® 465 2 , offer significant mechanical property improvements over previous generations of UHSSS with improved SCC resistance [3,8–10]. The microstructure of 1 Ferrium ® PH48STM is a registered trademark of QuesTek® Innovations LLC. 2 Custom 465 ® is a registered trademark of CRC Holdings Inc., a subsidiary of Carpenter Technology Corporation.