The Physical Metallurgy of Precipitation-Hardenable Stainless Steels

The precipitation-hardenable stainless steels have been developed during the past two decades to meet the needs of the defense program. They offer corrosion resistance, fabricability, and strength at room and moderately elevated temperatures. In comparison with titanium and its alloys, they are stronger and can be used at higher temperatures. In many applications the corrosion resistance and fabricability of the precipitation-hardenable stainless steels allow them to compete with the stronger hot-work die steels. The precipitation-hardenable stainless steels are classified as martensitic e.g., Stainless W and 17-4 PH, semiaustenitic e.g., 17-7 PH, PH 15-7 Mo, AM 350, and AM 355, or austenitic A-286 and HNM. At normal annealing temperatures, these alloys are predominantly austenitic but, depending on composition and heat treatment, the austenite may transform to martensite on cooling or during mechanical deformation. Classification, therefore, is on the basis of austenite stability. Factors to be considered in the fabrication and heat treatment of these alloys include the following 1 Mechanical properties are critically dependent on proper heat treatment. 2 Mechanical deformation may have a bearing on response to certain thermal treatments. 3 Dimensional changes occur during heat treatment. 4 After certain heat treatments these alloys are subject to intergranular attack in acid solutions.