Question

Explore how the phenomenon of peak aging and over aging occurs, and why the size of the precipitates has a great impact on the strength and other mechanical properties of precipitation-hardened martensitic stainless steel.

Answer 1

Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.Precipitation hardening stainless steels are iron-nickel-chromium alloys containing precipitation hardening elements such as aluminum, titanium, copper, niobium and molybdenum. Precipitation hardening (PH) stainless steels derive strength due to the precipitation of fine coherent precipitates . The major advantage when compared to martensitic stainless steels is that the PH steels can be fabricated in annealed condition and subsequently precipitation hardened at lower temperatures. Such treatment eliminates the need for fast cooling and attendant problems like warpage and cracking due to martensitic transformation. The strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix.This is accomplished by appropriate heat treatments.The process is called precipitation hardening because the small particles of the new phase are termed "precipitates”.

“Age hardening" is also used to designate this procedure because the strength develops with time, Alloys that are hardened by precipitation treatments include Al-Cu, Cu-Be, Cu-Sn, and Mg-Al; and some ferrous alloys.The composition of the precipitation hardening steels is dictated by three factors viz., the electron-atom ratio, the atomic size and the compressibility. PH steels can be made to austenitic, semi austenitic and martensitic types. PH stainless steels have high strength, good toughness and corrosion resistance both at ambient and elevated temperatures. PH stainless steels find extensive use in aircraft structures, landing gears, armament and other engineering applications. The steels achieve high strength by the precipitation of Ni3Al, NiAl or copper in a martensitic or austenitic matrix.Martensitic precipitation hardening stainless steels have a predominantly austenitic structure at annealing temperatures of around 1040 to 1065°C. Upon cooling to room temperature, they undergo a transformation that changes the austenite to martensite. In FeCr-Ni martensite, many substitutional elements produce precipitation hardening. The effect of these elements on precipitation hardening varies significantly. While Ti and Cr be have strong influence on Al, Nb, Mo and V have moderate effect and Cu, Co and Zr have less effect. Many high strength precipitation hardening martensitic steels are so alloyed that the strengthening is derived primarily from the precipitation of coherent ordered precipitates such as NiAl as in 13-8 grade or BCC copper rich precipitates or FCC copper precipitates as in 15-5 and 17-4 grades. Depending on the ageing temperature and time, the size and sometimes the shape of the precipitates vary.