MaterialsReading time 12 min
Heat Treatment Notes: What the Shop Must Actually Do
A short heat-treatment note can change the whole route: when to machine, how much stock to leave and how to manage distortion. Read the hardness, depth and process requirements before planning the job.
01Common heat treatments
| Name | What happens | Purpose | Typical hardness |
|---|---|---|---|
| Annealing | Heat, then cool in the furnace | Soften, relieve internal stress, improve machinability | HB 150 ～ 200 |
| Normalising | Heat, then cool in air | Refine grain, even out the structure | HB 170 ～ 250 |
| Quenching | Heat, then cool rapidly | Produce martensite; the highest hardness | HRC 55 ～ 65 |
| Tempering | Reheat and hold after quenching | Reduce brittleness, stabilise dimensions | Depends on temperature |
| Quench and temper | Quenching plus high-temperature tempering | Balance of strength and toughness | HRC 25 ～ 35 |
02Hard case, tough core
Gears, cams and shafts often need a hard wearing surface while keeping a tougher core. That is why the drawing may call for case hardening rather than through hardening. The required depth and hardness determine both the treatment route and how much machining must follow.
| Method | Principle | Case depth | Materials |
|---|---|---|---|
| Carburise and quench | Diffuse carbon into the surface, then quench | 0.3 ～ 2.0 mm | Low-carbon steels such as SCM415, SCr420 |
| Nitriding | Diffuse nitrogen into the surface; no quench needed | 0.1 ～ 0.6 mm | Nitriding steels containing Al, Cr or Mo |
| Induction hardening | Induction-heat the surface layer, then quench | 0.5 ～ 5 mm | Medium-carbon steels such as S45C, SCM440 |
| Flame hardening | Flame-heat the surface layer, then quench | 1 ～ 6 mm | Medium-carbon steel, cast iron |
A note such as “carburize, quench to HRC 58–62, effective case depth 0.8–1.2 mm” specifies three different things: process, surface hardness and depth. Hardness alone does not say how far the wear-resistant layer extends.
03Reading hardness scales
| Scale | Indenter and load | Typical use | Conversion |
|---|---|---|---|
| HRC | 120° diamond cone, 150 kgf | Hardened steel, HRC 20 to 70 | The most common |
| HRB | 1.588 mm steel ball, 100 kgf | Mild steel, copper alloys | HRB 100 ≈ HRC 20 |
| HV (Vickers) | Square-based diamond pyramid | Thin parts, case layers, plating | HV 550 ≈ HRC 52 |
| HB (Brinell) | Steel or carbide ball | Castings, large parts, coarse grain | HB 300 ≈ HRC 32 |
04Where treatment fits in the route
The same drawing can require a different route depending on whether final machining comes before or after heat treatment. Plan stock allowance and inspection points around that sequence.
- Before finishing. Distortion from heat treatment gets machined out afterwards, so dimensional accuracy is good. But the material is now hard and only grinding or hard turning will touch it: expensive and slow.
- After finishing. Ordinary cutting can be used and it is efficient. But distortion cannot be corrected, so the drawing tolerances must be wide enough to absorb it.
- Two stages. Rough machine, quench and temper, finish machine, case harden, grind. The standard route for high-accuracy gears and shafts: the longest and the most expensive.
A tight form tolerance alongside a high post-treatment hardness may call for grinding or another finishing process after heat treatment. Confirm the route during quoting, not after the parts have been quenched.
05Allowing for distortion
Quenching can change dimensions and shape through phase transformation and uneven cooling. Allow for growth, bow and ovality, and verify the actual distortion on trial parts rather than assuming a fixed percentage.
- First: leave grinding stock before heat treatment, usually 0.1 to 0.3 mm per side.
- Second: design symmetrically and avoid abrupt changes in thickness, to reduce uneven cooling.
- Third: use fixture quenching or press quenching to restrain the direction of distortion.
- Fourth: switch to a method that distorts less, nitriding instead of carburising and quenching.