Heat treating is a general term used to describe processes in which metals are heated and subsequently cooled to induce a number of desired modifications. These processes are used to improve various properties of metal, including strength, abrasion resistance, corrosion resistance, durability, and hardness, among others. Some of the most popular heat-treating techniques include carburizing, nitriding, hardening and tempering, annealing and stress relief, and cryogenic hardening.
The outcome of any heat treating method is essentially based on three variables: the temperature to which the metal is heated, the amount of time it is held at that temperature, and the method and rate of cooling. These variables are adjusted according to the type of material being treated and the desired effect of the treatment. Cooling can be achieved with gas, in salt, in air, or in oil.
ThermoFusion is a metalworking company specializing in heat treating and brazing. We can achieve heating temperatures of up to 2400° F and cryogenic temperatures ranging from -100° F to -327° F. The work we do supports various industries, from automobile and aerospace to computers and heavy equipment.
Carburizing is the process of adding additional carbon atoms to the surface and sub-surface of a steel, to increase surface hardness. This technique reinforces the metal part’s surface and improves its microstructure and mechanical properties by enabling carbon to diffuse into it. The depth at which carbon is able to diffuse depends on the type of material used, the level of carbon in the atmosphere, the temperature used, and the metal’s length of exposure to that temperature. The hardening happens only after the part is quenched (cooled), which locks the carbon into place.
In addition to improving surface hardness, carburizing enhances wear resistance and fatigue strength. It is best suited for low-carbon steels with carbon contents ranging from 0.05% to 0.3% and can be performed on parts of varying complexity. Carburizing is the treatment of choice for metals that require improved durability, wear resistance, and fatigue strength for their intended application.
Like carburizing, nitriding is a thermochemical case-hardening process used to improve the hardness, wear resistance, and fatigue life of metal parts. With nitriding, however, nitrogen is diffused into the metal’s surface to achieve the hardening effect. The nitriding process involves heat treating a ferrous material, then exposing it to active nitrogen at tightly controlled sub-critical temperatures. The temperature used during active nitrogen exposure is typically set below the final tempering temperature to ensure that the metal’s mechanical properties are not affected.
Nitriding is most effective when performed on alloy steel materials containing nitride-forming elements (Cr, Al, Mo, V, W, Ti) as this enables the nitrogen to easily form alloy nitride precipitates with the metal. Examples of steels that are compatible with the nitriding process include 4130, 4140, 4150, 4340, 8640, 17-4, 15-5, 4xx stainless,and Nitralloy 135. ThermoFusion offers nitriding services for all these steels.
In addition to its hardening effect, nitriding also imparts anti-galling, anti-welding, and anti-seizing properties to the metal part. Metals that have been nitrided can maintain their hardness in temperatures of up to 1,000° F. These features make nitrided metals well-suited for myriad applications, including:
Hardening is a metalworking process that takes place in an endothermic or vacuum atmosphere. In contrast to carburizing and nitriding, this is a through-hardening process that hardens the metal part from surface to core without changing the carbon on its surface. The process involves heating the metal above its austenitizing temperature, usually within an enclosed furnace. The austenitizing temperature, which varies according to the material, is the temperature that enables the metal’s crystal structure to transform from ferrite to austenite (aluminum, titanium and high nickel alloys have different structures but use the same principle of high temperature and then quenching) .
After the transformation to austenite has occurred, the metal is rapidly quenched in oil to change the crystal structure to martensite. The metal part is then tempered to decrease its hardness to the desired level, thereby reducing its brittleness. Steel is typically hardened through this heating and quenching process. Aluminum, which lacks carbon, can also be hardened this way. These hardened metals are used in various types of applications, from construction materials to automotive components.
Annealing is a process in which a metal part is heated to a predetermined temperature, held at that temperature, and then slowly cooled. This helps relieve residual stresses in the material that were caused by processes such as cutting, machining, or cold working. By reducing hardness, tensile strength, and yield strength, metal annealing allows for improved ductility and decreased brittleness. The main purpose of annealing is to make metals more amenable to manufacturing processes such as shaping, forming, hydroforming, stamping, bending, forging, and machining.
Stress relief involves heating a metal at a relatively low temperature, then allowing it to cool uniformly. The ideal temperature is below 1,000° F for copper or steel, and under 400° F for aluminum. Like annealing, the purpose of the stress relief treatment is to reduce internal stresses that were created during forming, machining, or rolling processes. Annealing, however, is performed at substantially higher temperatures (1,600° F or higher for steel and copper; 600° F or higher for aluminum) and is capable of relieving considerably more stress than the simpler stress relief treatment process.
Using endothermic, vacuum, and air furnaces, ThermoFusion can perform annealing and stress relieving treatments on all types of metal and plastic.
Cryogenic hardening is a metal hardening process wherein a metal part is cooled to cryogenic temperatures to relieve stress and reduce retained austenite after quenching. Cryogenic services include sub-zero cooling (usually to -100° F) and cryo treatments (-200° F or colder) to promote hardening. The internal stress relief provided by cryogenic treatment allows for tighter tolerances to be achieved during machining.
Other benefits of this treatment include:
Cryogenic treatments strengthen metals for high-performance applications in a range of industries, including aerospace, automotive, defense, and medical. For example, this process is often used to prepare aluminum for exposure to extremely cold environments such as those encountered in space. Other applications for cryogenically treated metal parts include: