New high-strength, corrosion-resistant alloys for marine corrosion, fastener and boat shaft applications

Abstract

High strength and corrosion resistant nickel alloys Hastelloy C-86 ® (UNS N06686) and Hastelloy C-22HS ® (UNS N07022) are currently being evaluated and/ or used by the Navy for current and future bolting applications where both high strength, toughness, ductility along with excellent resistance to corrosion and hydrogen embrittlement are required. This includes present and new ship design. The 686 alloy is currently into final stages of data gathering and mil spec preparation. Alloy 686 is a solid solution nickel base alloy obtaining it’s strength through cold work. The C-22HS alloy was recently introduced for applications requiring both corrosion resistance and high strength. Being a C-type alloy (Ni-Cr-Mo), C- 22HS has excellent corrosion resistance in both reducing and oxidizing environments. The alloy is capable of being age-hardened to effectively double the yield strength over other Ctype alloys. The C-22HS alloy has generated interest in the oil & gas industry due to its resistance to sour brine environments. C-22HS is being included in NACE MR0175/ISO 15156 at NACE Level VII (highest severity), exhibiting resistance to severe sour brine environments. Properties of the new alloy relevant to marine corrosion and Navy fastener applications are presented. This material has also been shown to be resistant to hydrogen embrittlement in marine and other severe environments, exhibiting potential for corrosion immunity. Efforts in developing alternate processing routes for C-22HS alloy for very high strength applications are detailed. The high strength of C-22HS alloy is imparted through the formation of Long Range Ordering (LRO) domains of Ni2(Cr, Mo) which develop during a two-step age-hardening treatment. In standard C-22HS alloy, the formation of the LRO domains nearly doubling of the room temperature yield strength up to values around 100 ksi. The yield strengths of the as-cold worked bars ranged from 191 to 205 ksi (1317 to 1413 MPa), and the cold-worked + age-hardened bars ranged from 199 to 206 ksi (1372 to 1420 MPa). In these material conditions, C-22HS also exhibits excellent ductility and fracture toughness of _150 ft-lbs (_203 J) from -300ºF to +550ºF (-149 to 288°C). The C - 22HS alloy is presently being tested for Navy Subsafe high strength fastener approval. An important feature of the C-22HS alloy in the age-hardened condition is that the alloy retains excellent corrosion resistance. Both alloys C-86 and C-22HS have excellent potential for all marine and military applications where excellent strength, toughness and corrosion resistance are required. In addition to the nickel alloy just discussed, NITRONIC® 50HS (UNS S20910) Stainless Steel provides a combination of corrosion resistance and strength not found in any other commercial material available in its price range. As a result, the use of this alloy for boat shafting is significantly increasing for commercial and naval shipping. Superior corrosion resistance, high yield strength, exceptionally low magnetic permeability and outstanding cryogenic properties define this alloy usefulness for boat shafting.