Two Purposes, One Vocabulary
Heat treatment exists to change the mechanical properties of metal, to remove residual stress and to improve machinability. Drawings and orders refer to the same operations, so the terms are standardised: ISO 4885 defines the vocabulary of heat treatment, and shop specifications such as AMS 2759 describe how each operation is performed and recorded for steel parts. The operations divide into preparatory treatment applied to a blank before machining and final treatment applied to set the properties of the finished part.
Annealing and Normalizing: Softening vs Refining
Both treatments heat the steel into the austenite region and then cool it, and the difference is the cooling rate. Annealing cools slowly in the furnace, which produces the softest, most machinable structure. Normalizing cools in still air, which is faster and leaves a finer pearlitic structure with higher strength and hardness.
The choice usually follows carbon content. For carbon and low alloy steels above about 0.5% carbon, full annealing is used because the steel must be soft enough to cut. Below about 0.5% carbon, normalizing is preferred because annealing leaves the steel too soft and gummy, causing built-up edge and poor surface finish. Both operations refine the grain and homogenise the structure in preparation for the final treatment, and both are normally placed after blank making and before rough machining. Spheroidise annealing is a separate variant that converts lamellar carbide into spheroids, giving the lowest hardness and best formability for subsequent cold working.
Aging and Stress Relief
Ageing, also called stress relieving, removes stresses left by casting, welding, straightening and machining. Dimensional stability is the aim rather than strength. General precision parts normally receive one ageing cycle before finishing; high precision parts such as coordinate boring machine housings receive two or more; slender precision parts such as long screws are aged repeatedly between roughing and semi-finishing. Any straightening or correction operation is followed by a further ageing cycle, because the correction itself introduces new stress. In alloy steels the term ageing can instead mean precipitation hardening after solution treatment, which raises strength rather than relaxing it, so the specification must be read in context.
Quenching, Tempering and Carburising
| Operation | Heating | Cooling | Result |
|---|---|---|---|
| Annealing | above the critical range | furnace, slow | soft, machinable, stress free |
| Normalizing | above the critical range | still air | finer pearlite, moderate strength |
| Quenching | austenitising temperature | water, polymer or oil | martensite, high hardness, high stress |
| Tempering | below the critical range | air | tempered sorbite, balanced properties |
| Carburising | about 900 – 930 °C in a carbon rich atmosphere | diffusion then quench | hard case on a tough core |
Quenching may be through hardened or applied only to the surface. Surface hardening by induction or flame heats a shallow layer and quenches it, giving small distortion, limited oxidation and decarburisation, a hard wear resistant case and a tough impact resistant core; it is often preceded by normalizing or by quench and temper to condition the core. Quench and temper as a pair is the standard route for parts needing strength with toughness: the quench forms martensite and the high temperature temper converts it to a uniform tempered sorbite, which also reduces distortion before any subsequent surface hardening or nitriding.
Carburising applies to low carbon and low alloy steels. The surface is enriched in carbon by holding the part in a carbon rich atmosphere around 900 to 930 °C, then quenched to give a hard case typically 0.5 to 2 mm deep over a tough core. The usual process route is material, forging, normalizing, rough and semi-finish machining, carburising, quenching and finishing. Locally carburised parts are protected elsewhere with copper plating or an anti-carburising coating, and any excess case is removed before final grinding. Case depth, surface carbon and hardness profile are verified on a sample section.
Records, Distortion and Common Mistakes
Tempering temperature selection decides the final balance: low temperature tempering retains high hardness, and higher tempering temperatures trade hardness for toughness.
Distortion is controlled by fixture design, cooling uniformity and, where necessary, by straightening followed by a stress relief cycle.
Each batch is recorded with furnace number, time, temperature, atmosphere and quench medium, and results are traceable to the parts processed.
Hardness alone does not prove a successful treatment; case depth, microstructure and where specified impact results are also required.
The most common error is to normalise a high carbon steel and expect it to machine like an annealed one, which costs tool life, or to anneal a low carbon steel and then fight the resulting gummy cut. The second is to treat ageing as optional, when in practice skipping it allows residual stress to move a finished part out of tolerance weeks after machining.
Frequently Asked Questions
Q: When should normalizing be chosen instead of annealing?
A: For carbon and low alloy steels below about 0.5% carbon, where annealing would leave the material too soft to machine cleanly and normalizing gives a better finish.
Q: Is tempering always required after quenching?
A: Yes for structural parts. As-quenched martensite is hard but brittle and internally stressed, and tempering converts it to a tougher tempered structure.
Q: What is the difference between ageing and annealing?
A: Annealing heats above the critical range to soften and refine the structure. Ageing is a low temperature hold that relieves residual stress and stabilises dimensions.
Q: How deep is a carburised case?
A: Typically between 0.5 mm and 2 mm, set by the diffusion time and temperature, and confirmed by hardness traverse on a section from the batch.
Q: How is heat treatment documented?
A: Each load records time, temperature, atmosphere, quench medium and the measured hardness, and the specification used is defined by standards such as ISO 4885 and AMS 2759.
