bridges, coastal works) and in the water industry. Precipitation hardening Precipitation hardening stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than other martensitic grades. There are three types of precipitation hardening stainless steels: Martensitic 17-4 PH (AISI 630 EN 1.4542) contains about 17% Cr, 4% Ni, 4% Cu, and 0.3% Nb. Solution treatment at about followed by quenching results in a relatively ductile martensitic structure. Subsequent aging treatment at precipitates Nb and Cu-rich phases that increase the strength up to above 1000 MPa yield strength. This outstanding strength level is used in high-tech applications such as aerospace (usually after remelting to eliminate non-metallic inclusions, which increases fatigue life). Another major advantage of this steel is that aging, unlike tempering treatments, is carried out at a temperature that can be applied to (nearly) finished parts without distortion and discoloration. Semi-austenitic 17-7 PH (AISI 631 EN 1.4568) contains about 17% Cr, 7.2% Ni, and 1.2% Al. Typical heat treatment involves solution treatment and quenching. At this point, the structure remains austenitic. Martensitic transformation is then obtained either by a cryogenic treatment at or by severe cold work (over 70% deformation, usually by cold rolling or wire drawing). Aging at — which precipitates the Ni3Al intermetallic phase—is carried out as above on nearly finished parts. Yield stress levels above 1400MPa are then reached. Austenitic A286(ASTM 660 EN 1.4980) contains about Cr 15%, Ni 25%, Ti 2.1%, Mo 1.2%, V 1.3%, and B 0.005%. The structure remains austenitic at all temperatures. Typical heat treatment involves solution treatment and quenching, followed by aging at . Aging forms Ni3Ti precipitates and increases the yield strength to about at room temperature. Unlike the above grades, the mechanical properties and creep resistance of this steel remain very good at temperatures up to . As a result, A286 is classified as an Fe-based superalloy, used in jet engines, gas turbines, and turbo parts. Grades There are over 150 grades of stainless steel, of which 15 are most commonly used. There are several systems for grading stainless and other steels, including US SAE steel grades. The Unified Numbering System for Metals and Alloys (UNS) was developed by the ASTM in 1970. The Europeans have developed EN 10088 for the same purpose. Terminology In its early history, stainless steel was sometimes called rustless steel. Both adjectives, stainless and rustless, are duly