Precipitation hardening heat treatments

Sm-Co anisotropic powders have been developed to prepare magnets meeting high operating temperature requirements [135]. These powders are produced by pulverization of bulk ingots which have been submitted to the precipitation hardening heat treatment. 

PH (Precipitation hardening). These are iron-chromium-nickel alloys containing precipitating element, such as Al. Strength is achieved by precipitation hardening heat treatment. One example is (17-7 PH)... 

The most common heat-treatable copper alloys are the beryllium coppers. They possess a remarkable combination of properties tensile strengths as high as 1400 MPa (200,000 psi), excellent electrical and corrosion properties, and wear resistance when properly lubricated they may be cast, hot worked, or cold worked. High strengths are attained by precipitation-hardening heat treatments (Section 11.9). These alloys are costly because of the beryllium additions, which range between 1.0... 

Solid solution alloys are readily fusion welded, normally in the annealed condition. Some noteworthy examples of solid solution alloys are Ni 200, the Monel 400 series, the Inconel 600 series, the Incoloy 800 series, Hastelloys and some Nimonic alloys such as 75, and PE 13. Because the HAZ does not harden, heat treatment is not usually required after welding. Precipitation hardened alloys may be susceptible to postweld heat-treatment (PWHT) cracking. Some of these alloys are the Monel 500 series. Inconel 700 series, Incoloy 900 series, and most of the Nimonic alloys. 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

In Ni—P electroless deposits, there can be as much as 10% by weight of phosphoms. The amount depends on the added complexing agents and the pH. The Ni—P deposits are fine-grained supersaturated soHd solutions, which may be precipitation hardened by heat treatment to form dispersed Ni P particles in a nickel matrix. 

The Zn—A1 system permits manipulation of the mechanical properties by suitable heat treatment. The aluminum-rich alpha phase is especially suitable for solution hardening since it can be supersaturated by as much as 30 wt % zinc. Furthermore, both alpha and beta phases can be strengthened by precipitation because of decreasing solute solubiUty with decreasing temperature. 

Most wrought alloys are provided in conditions that have been strengthened by various amounts of cold work or heat treatment. Cold worked tempers are the result of cold rolling or drawing by prescribed amounts of plastic deformation from the annealed condition. Alloys that respond to strengthening by heat treatment are referred to as precipitation or age hardenable. Cold worked conditions can also be thermally treated at relatively low temperatures to affect a slight decrease in strength (stress rehef annealed) to benefit other properties, such as corrosion resistance and formabiUty. 

Austenitic stainless steels are the most corrosion-resistant of the three groups. These steels contain 16 to 26 percent chromium and 6 to 22 percent nickel. Carbon is kept low (0.08 percent maximum) to minimize carbide precipitation. These alloys can be work-hardened, but heat treatment will not cause hardening. Tensile strength in the annealed condition is about 585 MPa (85,000 Ibf/in"), but workhardening can increase this to 2,000 MPa (300,000 Ibf/in"). Austenitic stainless steels are tough and ducdile. 

Fig. I.I2 Curves showing the relationship between strength, stress-corrosion susceptibility and heat treatment for a high-strength precipitation-hardening aluminium alloy...

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