W. D. Cao, ATI Allvac, Monroe, NC

Summary:

Ultra-high strength precipitation hardening stainless steels have gained much attention in recent years, mainly due to their excellent mechanical properties and corrosion / stress corrosion cracking resistance. The majority of these types of steels are strengthened with precipitation hardening elements Al, Ti and Cu, either individually or in combination.

There have been a number of different approaches to designing alloys of this type. Some alloy developers preferred to use Al alone or in combination with Ti or Cu while others have favored Ti as the major strengthening hardener. A thorough study of the effects of Al, Ti and Cu on phase transformation, microstructure, and mechanical and chemical properties would be a useful foundation for developing new PH stainless steels. A research program was initiated at Allvac to pursue such a study in a 12Cr-9 Ni-base, martensitic precipitation hardening stainless steel. More than 40 pilot plant heats weighing 50 to 300-lbs were made. The Al content in the test steels varied from 0 to 2%, Ti from 0 to 2% and Cu from 0 to 3%. The effect of individual elements and their combination on martensitic transformation, aging kinetics, strength, toughness, SCC and corrosion resistance were studied.

It was found that Al had a minor influence on Ms temperature, but both Ti and Cu noticeably suppressed this temperature. The three elements have quite different effects on aging kinetics, leading to peak aging occurring at different temperatures. Ti resulted in the greatest strengthening and the least impact on fracture toughness. Al had a lesser strengthening effect, and the least effect on notch toughness. Pitting/crevice corrosion was adversely effected by Ti. All results will be introduced and a new alloy developed from this study, Allvac® S240™, will be discussed.