PH stainless steels (precipitation-hardening) contain around 17% chromium and 4% nickel. These steels can develop very high strength through additions of aluminum, titanium, niobium, vanadium, and/or nitrogen, which form coherent intermetallic precipitates during a heat treatment process referred to as heat aging. As the coherent precipitates form throughout the microstructure, they strain the crystalline lattice and impede the movement of dislocations, or defects in a crystal’s lattice. Since dislocations are often the dominant carriers of plasticity, this serves to harden the material. Precipitation-hardening stainless steels have high toughness, strength, and corrosion resistance. Precipitation-hardening stainless steels have been increasingly used for a variety of applications in marine construction, aircraft and gas turbines, chemical industries and nuclear power plants.
17-4PH Stainless Steel
For example, precipitation-hardened stainless steel 17-4 PH (AISI 630) have an initial microstructure of austenite or martensite. Austenitic grades are converted to martensitic grades through heat treatment (e.g. throung heat treatment at about 1040 °C followed by quenching) before precipitation hardening can be done. Subsequent ageing treatment at about 475 °C precipitates Nb and Cu-rich phases that increase the strength up to above 1000 MPa yield strength. In all heat treatments performed the predominant microstructure is lath martensite. Unlike austenitic alloys, however, heat treatment strengthens PH steels to levels higher than martensitic alloys. Precipitation-hardening stainless steels are designated by the AISI 600-series. Of all of the available stainless grades, they generally offer the greatest combination of high strength coupled with excellent toughness and corrosion resistance. They are as corrosion resistant as austenitic grades. Common uses are in the aerospace and some other high-technology industries.
In nuclear industry, 17-4PH steel may be used in controle rods. Control rods usually constitute cluster control rod assemblies (PWR) and are inserted into guide thimbles within a nuclear fuel assembly. The absorbing material (e.g. pellets of boron carbide) is protected by the cladding usually made of stainless steel. The control element has some components that need to have high resistance to impacts when the safety function is activated, so the material of this component must have high mechanical strength and toughness. One of the materials in which can be specified for this application is PH stainless steel 17-4PH (UNS 17400).
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