Aircraft parts are often made from Type 431 stainless steel, where both corrosion resistance and hardenability are required. The alloy has good ductility, and has performed well in atmospheric marine environments. Type 431 stainless steel contains 15 to 16.5 percent chromium, 2 to 3 percent nickel, and small amounts of molybdenum, manganese, silicon, and carbon. The alloy is heat treatable to an ultimate tensile strength of 200,000 psi. Final machining of parts is usually done before heat treatment, due to the high cost of machining hardened steels. The heat treat and quench operations require very close control in order to obtain the desired metallurgical properties. Slow or delayed quenching after heat treatment will result in the precipitation of M23 C6 carbides, which are mostly chromium carbides, at the grain boundaries. In addition, the quenched material must be given a minus 100 F cold-soak treatment to assure complete transformation of austenite to martensite. The presence of any of these unwanted phases will detract from the properties of the pure martensitic structure. When chromium carbides have been precipitated, stress corrosion cracking occurs as a result of corrosion of the chromium depleted zones adjacent to the grain boundaries. The chromium depleted zone is considerably anodic to the grain centers and to the chromium carbide precipitates. Also, the corrosion rate will be accelerated considerably by the unfavorable ratio of anode to cathode area.
