Heat treatment of steels

Two parts, same steel, same drawing. One lasts the life of the vehicle and one snaps. The difference is not on the drawing’s dimension lines — it is the thermal history the part went through, and that is what heat treatment specifies.

The one mechanism underneath all of it

Heat a steel above its transformation temperature and the carbon dissolves into austenite. What happens next depends entirely on how fast you cool it:

slow   (furnace)  ->  annealed     soft, coarse, very machinable
medium (still air) ->  normalised   uniform fine grain
fast   (oil/water) ->  martensite   very hard, very brittle, highly stressed

Cool slowly and the carbon has time to come out as soft carbides. Cool fast enough and it is trapped in a distorted lattice with nowhere to go. That trapped, strained structure is martensite, and it is hard for exactly that reason.

The four operations

Operation	Cool how	Result	Used for
Annealing	Furnace, very slow	Softest, coarsest	Machinability, formability, supplied stock
Normalising	Still air	Uniform fine grain	Recovering from forging or casting
Quenching	Oil or water	Martensite — hard, brittle, stressed	Making it hard
Tempering	Reheat below transformation	Hardness traded for toughness	Making it usable

Hardenability is not hardness

This is the distinction that separates people who specify steels from people who copy them.

• How hard it can get is set almost entirely by carbon content.
• How deep that hardness reaches is set by alloying — chromium, nickel, manganese, molybdenum — which slow the transformation and let a milder quench still form martensite.

That second property is hardenability. A plain carbon steel and an alloy steel with the same carbon can reach the same surface hardness and behave entirely differently in a 60 mm shaft: the plain steel is hard for a few millimetres over a soft core, the alloy steel is hard most of the way through.

Distortion and quench cracking

Martensite occupies more volume than the austenite it came from. So a surface that transforms while the core is still hot puts the part into severe internal tension — and the material at that moment is at its most brittle.

Which is why the machining comes first

The standard route for a hardened precision part is:

rough machine  ->  harden and temper  ->  grind the features that matter

Machining hardened steel is slow and expensive, and hardening moves the part anyway. So the finishing allowance is planned before hardening and removed by grinding afterwards — see machining processes for why grinding is what takes over once the part is hard.

Where only the surface needs to be hard and the core must stay tough, the answer is a different family of processes entirely — case hardening and surface engineering.

What to take away

• All of it comes down to how fast the carbon is cooled out of austenite.
• Annealing softens, normalising refines grain, quenching hardens, tempering makes it usable.
• Never leave a part as quenched — it is brittle and stressed and can crack unattended.
• Tempering temperature is the hardness against toughness dial.
• Carbon sets how hard. Alloying sets how deep. That is hardenability.
• Choose the grade from the section thickness, not from the hardness number.
• Martensite expands, so quenching puts the part in tension where it is most brittle.
• Radii, mild quench, sensible fixturing — in that order of effect.
• Machine soft, harden, grind. Plan the allowance before the part goes in the furnace.