MaterialsReading time 12 min

That one line of heat treatment on the drawing, what does the shop actually do?

Heat treatment is the shortest line on a drawing and the one with the biggest effect on cost. It decides the order of operations, the tools you can use, and whether the part will distort.

01Five common 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

Quenching must always be followed by tempering, a quenched-only part is too brittle to use.

02Case hardening: hard outside, tough inside

Many parts need a wear-resistant surface with a tough core, gears, cams and shafts all do. That calls for case hardening rather than through hardening.

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

Nitriding runs at a low temperature, around 500 °C, distorts least, and suits parts that are already finished.

The drawing normally reads “carburise and quench HRC58-62, effective case depth 0.8–1.2 mm”. All three pieces of information are needed: method, surface hardness, case depth. Hardness without depth does not specify wear life at all.

03How to read 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

Conversion between scales is empirical (ISO 18265), not exact, measure on whichever scale the drawing asks for.

04Where heat treatment sits in the routing changes everything

The same part is a completely different job depending on whether heat treatment comes before or after finishing.

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.

So when “HRC58-62” and “cylindricity 0.005” appear on the same drawing, grinding after heat treatment has effectively been specified. Miss that at quoting and the margin on the whole order is gone.

05Dimensions change too

Martensite has a larger specific volume than austenite, so parts grow after quenching, typically between +0.05% and +0.15% linearly in carbon steel. A 200 mm shaft may be 0.1 to 0.3 mm longer after quenching, on top of bowing and ovality from uneven cooling.

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.