Stress Corrosion Cracking Of Hardenable Stainless Steels

Abstract

Abstract Hardenable stainless steels, like other high-strength steels, are susceptible to cracking under the combined influence of tensile stress and corrosion, depending on the severity of the stress, the nature of the media and to a large extent, on their hardness. The most severe conditions are acid solutions containing sulfides, which promote entry of hydrogen into the steel and where chlorides are present to promote pitting. In such media cracking may occur in the standard grades under stress at hardnesses of Rockwell C24, in severe marine atmospheres at C40 and in industrial atmospheres at C45. Type 422, a modified 12 percent chromium alloy, tempered at 1200 F to Rockwell C30 and the precipitation-hardening alloys, 17-4 PH and 17-7 PH, overaged to the same hardness level were the most resistant materials tested. Types 410, 416 and 431 performed as well as or somewhat better than low alloy steels when heat treated to equivalent hardness levels. Tempering them at 1100 F or higher temperatures provided good resistance to cracking in the severest media. Internal stresses generated during quenching can promote stress corrosion cracking without imposing additional external stresses. Interrupted quenching (martempering) is helpful in minimizing stresses from this source and is recommended as a heat treating practice for valve trim and other parts used fully hardened in service likely to cause corrosion cracking.