Generally speaking, heat treating is done in order to alter the mechanical properties, the metallurgical structure, or the residual stress state of a metal product.
As it pertains to aluminum alloys, the term “heat-treatable” is used in order to differentiate those which can be strengthened by heating and cooling versus those which cannot.
“Non-heat-treatable” alloys depend mostly on cold work to increase strength while other alloys utilize heating to decrease strength and ductility, called annealing.
Heat treating to increase the strength of aluminum alloys encompasses a three-step process:
1. Solution Heat Treatment: heating an alloy to a suitable temperature, which it’s held at until one or more constituents enter into a solid solution, upon where it is then cooled rapidly to hold these constituents in solution
2. Quenching: development of supersaturation
3. Age Hardening: spontaneous hardening of a metal that takes places after it is quenched and then stored at ambient temperature or treated with mild heat
The following aluminium alloys are commonly used in aircraft and other aerospace structures:
7068, 7075, 6061, 6063, 2024, 5052, and 7050 aluminum.
When it comes to stainless steel, Custom 455 can be hardened by cold working and with a heat treatment. However, Custom 455 comes in 4 different specs, AMS 5860, AMS 5672, AMS 5617, and AMS 5578, with all yielding different properties depending on how they are heat treated.
For example, AMS 5617 wire is cold drawn or shaved after solution heat treatment, which varies according to the spec and alloy shape.
Depending on chemical composition, fabrication requirements and intended service, nickel and cobalt alloys may be subjected to one or more of five principal types of heat treatment.
These methods include stress relieving, annealing, stress equalizing, solution treating, and age hardening.
1. Annealing: generally carried out at temperatures between 705 and 1205°C in order to produce a recrystallized grain structure and softening in work-hardened alloys.
2. Stress Relieving: involves temperatures for nickel and nickel alloys ranging from 425 to 870°C to remove or reduce stresses
3. Stress Equalizing: a low-temperature heat treatment utilized to balance stresses in cold worked material without a significant decrease in the mechanical strength produced by cold working
4. Solution Treating: deposits age-hardening constituents and carbides into solid solution byway of a high-temperature treatment
5. Age Hardening: conducted at intermediate temperatures (425 to 870°C) to develop maximum strength by precipitation of a dispersed phase throughout the matrix
Popular nickel alloys include Incoloy 909, Inconel 718, Inconel 625, and Inconel 601, while MP35N, L-605, and Hayne 188 represent the most prevalent cobalt alloys.
Nickel and cobalt alloys are known for having great strength and wear-resistant properties at high temperatures. As a result, these alloys are commonly used to replace steel in aircraft jet engines, with the most popular use for the high-temperature cobalt alloy being in gas turbine (turbojet) aircraft engines.
Other common applications for nickel and cobalt alloys include engine plumbing, pumps, valves, piping systems, processing equipment, turbines, assemblies, tools, chemical processing, oil and gas well piping.