Lesson 12.1TreatChapter 12 · Heat treatment and surface hardening

Heat treatment (hardening, tempering, annealing)

Heat, quench and temper (or slow-cool) the whole part

Heat treatment changes how hard or tough a metal is by heating and cooling it in a controlled way, without changing its shape. The same steel can be made soft enough to machine, or hard enough to cut other metals. A kitchen knife, a spring and a gear all owe their behavior to it.

Photo: Goodwin Steel Castings, CC BY-SA 2.0, via Wikimedia Commons

Quench and temper versus annealing: schematic temperature over timeThe solid line is the hardening route; circled numbers match the steps below. The dashed line is annealing, the slow-cool alternative.

How it works

Heat up. The part goes into a furnace and is heated until it glows red, typically about 800–900 °C for common steels. At that heat the atoms rearrange into a form (austenite) that can dissolve the carbon in the steel.
Hold (soak). The part stays at temperature until it is hot all the way through. Thick parts need longer, often around an hour per 25 mm of thickness as a rule of thumb.
Quench. The hot part is plunged into water, oil or a polymer solution and cools in seconds. The carbon gets trapped, forming martensite (a very hard but brittle structure).
Temper, then cool. The part is reheated to a much lower temperature, typically about 150–650 °C, held, then cooled. This trades a little hardness for much more toughness (resistance to cracking); the higher the temper, the tougher and softer the part.

Annealing takes the other road: after the hold, the part cools very slowly inside the furnace and ends up soft and easy to machine or bend. Normalizing is in between: the part cools in still air.

Key facts

Materials: Mainly carbon and alloy steels and tool steels; also cast iron, titanium and aluminum alloys (with their own recipes)
Typical parts: Gears, shafts, axles, springs, knives, tools, bolts, bearings
Part size: From small pins to parts of several tons, limited by the furnace and quench tank
Precision: Shape is kept, but parts can distort or grow slightly, often tenths of a millimeter, so critical faces are ground afterwards
Surface finish: A dark oxide scale forms in air; vacuum or protective-gas furnaces keep parts bright
Batch size it suits: Single parts to continuous furnaces running thousands of parts a day
Tooling and machine cost: Small furnaces from a few thousand dollars; industrial lines with quench tanks and gas control from hundreds of thousands to millions
Speed: Hours per cycle (heat, hold, quench, temper); batches of many parts at once

Strengths and limits

Strengths

One steel can be made soft, tough or very hard to suit the job
Treats the whole part, through its thickness
Large batches at once, so low cost per part
Can undo earlier effects: annealing softens work-hardened metal

Limits

Quenching can warp or crack parts, especially thin or uneven ones
Low-carbon steels barely harden; the alloy must suit the treatment
Scale and distortion often mean extra grinding or cleaning
Energy-hungry furnaces and long cycle times

Where it's used

AutomotiveGears, crankshafts, axles, springs
ToolsDrill bits, chisels, knives, dies and molds
AerospaceLanding gear, engine shafts, high-strength fasteners
Energy and heavy industryTurbine shafts, mining and earth-moving parts
ConstructionHigh-strength bolts, rebar, wire rope

How to recognize it

Rainbow temper colors (straw, bronze, blue) near ground edges, typical of tools and knives.
Dark gray or black oxide scale on unground surfaces.
A file skates off a hardened surface instead of biting in.
Ground bearing faces next to darker, untouched surfaces on the same part.