Heat treatment

Heat treatment is the controlled heating of a metal part to a set temperature, holding it there and cooling it at a defined rate in order to change the internal structure of the material and thus its properties. Depending on the treatment, the same steel can be as soft as bar stock that machines easily or as hard as a tool that resists wear. Heat treatment is applied mainly to steels, but also to aluminum, copper and titanium alloys. In US shops the process is usually called heat treating.

The main processes for steel:

Annealing softens the material, refines its structure or reduces internal stresses, for example in weldments before precision machining. Hardening means heating to the temperature at which the structure transforms into austenite (usually 800 to 900 °C for common structural steels) and cooling rapidly in water, oil, a polymer quenchant or gas, which produces a hard martensitic structure. Hardened steel is brittle, so tempering follows: low-temperature tempering for high hardness (tools, sliding surfaces), high-temperature tempering for toughness. Hardening followed by high-temperature tempering is called quenching and tempering (Q&T), typical for shafts and pins made of C45 or 42CrMo4 (close to AISI 1045 and 4140).

Surface and thermochemical treatments create a hard surface layer while keeping a tough core. Surface hardening (induction, flame or laser) hardens only the surface. Carburizing, also called case hardening, enriches the surface of a low-carbon steel (for example 16MnCr5) with carbon and, after hardening and tempering, gives a case usually 0.3 to 2 mm deep with a hardness of around 58 to 62 HRC. Nitriding takes place at a lower temperature, usually around 500 to 570 °C, without quenching and therefore with minimal distortion, but the case is thinner. Age-hardenable aluminum alloys (2000, 6000 and 7000 series) are strengthened by solution heat treatment, quenching and artificial aging, resulting for example in the T6 temper.

Specify heat treatment for parts subject to wear, contact pressure or cyclic loading, where material fatigue is a concern: shafts, pins, gears, guide rails, cams, knives, punching and shearing tools and molds. Quenching and tempering increases the strength of the whole cross section, while surface hardening, carburizing and nitriding are chosen where mainly the surface has to resist and the core should stay tough.

Stress-relief annealing makes sense for weldments and complex parts that will be precision machined, and for parts from metal 3D printing, which retain significant internal stress after the build.

The steel must suit the chosen process. Low-carbon structural steels (for example S235) practically do not harden, carburizing requires case-hardening steels and nitriding requires steels with nitride-forming elements. Select the material and the treatment together and discuss them with the heat treater.

Hardening causes dimensional changes and distortion. Precision surfaces are machined with a stock allowance and finished by grinding only after heat treatment. Sharp inside corners, abrupt changes in cross section and holes close to an edge increase the risk of quench cracks. Mark surfaces that must stay soft (for example threads on carburized parts) on the drawing.

Instead of the single word "harden", state the specific requirement: the type of treatment, the hardness with a tolerance (for example 58 to 62 HRC), for case layers the case depth and the limit hardness to which it is measured (usually 550 HV for carburizing), and the measurement location. Ask the supplier for a hardness test report.