Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (i.e. grain size and composition) is one of the most important factors in determining the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure.
Complex heat treating schedules are often devised by metallurgists to optimize an alloy's mechanical properties. These alloys may undergo five or more different heat treating operations to develop the desired properties.
A technique used to recover cold work and relax stresses within a metal. Annealing typically results in a soft, ductile metal. When an annealed part is allowed to cool in the furnace, it is called a "full anneal" heat treatment. When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment. During annealing, small grains re-crystallize to form larger grains. Typical annealing processes include, "normalizing", "stress relief" annealing to recover cold work, and full annealing.
To quench, a metal must be heated into the austenitic crystal phase and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other gas (such as nitrogen), oil, polymer dissolved in water, or brine. The quenched hardness of a metal depends upon its chemical composition and quenching method. Cooling speeds, from fastest to slowest, go from polymer (i.e. silicon), brine, fresh water, oil, and forced air.
Some metals are classified as precipitation hardening metals. When a precipitation hardening alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form inter-metallic particles. These inter-metallic particles will nucleate and fall out of solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age "naturally" meaning that the precipitates form at room temperature, or they may age "artificially" when precipitates only form at elevated temperatures. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations - assembly of rivets, for example, may be easier with a softer part.
Examples of precipitation hardening alloys include 2000 series, 6000 series, and 7000 series aluminum alloy, as well as some superalloys and some stainless steels.
The Gayston Corporation currently deals with the heat treatment of many precipitation hardening alloys due to our product mix, and is performs all product heat treatment in house in its oven department.
