Hardening, Tempering and Annealing

§1The iron-carbon idea

Heat treatment works because steel changes its internal structure with temperature. Heated past a critical point it becomes austenite, in which carbon dissolves freely; how that austenite is then cooled decides everything.

The key landmark is the eutectoid: at 0.77 % carbon and 727 °C, austenite transforms on slow cooling directly into pearlite, a fine layered mix of soft ferrite and hard iron carbide. Cool slowly and you get this soft, layered structure; cool fast and the carbon is trapped before it can separate, producing hard martensite instead. Every heat treatment on this page is a way of steering that cooling — slow to soften, fast to harden, then a reheat to balance. The partial iron-carbon diagram in the hero shows the austenite field and the eutectoid point that anchors it.

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§2The lever rule

Below the eutectoid, a steel is a mix of two constituents, and their proportions are read off the diagram by the lever rule — a simple weighted balance across the composition.

fraction pearlite = C − 0.0220.77 − 0.022 (ferrite takes the remainder)

Example 1 — the make-up of a 0.40 % carbon steel

A medium-carbon steel at 0.40 % C, cooled slowly, is pro-eutectoid ferrite plus pearlite. The pearlite fraction = (0.40 − 0.022)/(0.77 − 0.022) = 0.378/0.748 = 50.5 %, so the steel is very nearly half ferrite, half pearlite — soft ferrite for toughness, hard pearlite for strength, in balance. Drop to 0.20 % C and the pearlite falls to 23.8 % (softer, tougher); rise to the eutectoid 0.77 % C and it is 100 % pearlite. The lever rule turns the carbon figure straight into the structure, and the structure into the properties.

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§3Hardening by quench

To harden a steel, heat it into the austenite field and then cool it fast enough to trap the carbon — forming martensite, the hard, brittle structure that gives quenched steel its edge.

The sequence is heat, soak, quench. Heating dissolves the carbon into austenite; quenching in water, oil or air (the medium set by the steel’s hardenability, per the tool-steel page) cools it so fast that the carbon cannot diffuse out, leaving a strained, extremely hard martensite. The catch is that as-quenched martensite is also brittle — hard enough to be useful, but too brittle to trust under shock — and the quench itself sets up internal stresses that can distort or crack the part. A freshly hardened steel is therefore rarely used as-is; it is almost always tempered next.

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§4Tempering

Tempering reheats the hardened steel to a moderate temperature to trade a little hardness for a large gain in toughness — the essential second step of hardening.

By warming quenched steel to somewhere between about 150 °C and 650 °C and holding, some of the trapped carbon precipitates and the martensite relaxes: hardness falls a little, brittleness falls a great deal, and the internal quench stresses are relieved. The temperature is the control — a low temper keeps most of the hardness for a knife edge or a cutting tool, a high temper gives up more hardness for the toughness a shaft or a gear needs. “Hardened and tempered” is thus a single decision about where on the hardness-toughness line the part should sit, and it is why every quench is followed by a temper. Carbon steels tempered much above 200 °C begin to lose their edge — the very limit that red-hardness tool steels are designed to beat.

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§5Annealing and normalising

The opposite of hardening: heat treatments that soften steel, relieve stress and refine its grain, by cooling slowly rather than fast.

Annealing heats the steel into the austenite range and cools it very slowly, usually in the furnace, producing the softest, most ductile, most machinable and stress-free condition — the state to work a steel in before a final hardening. Normalising does much the same but cools in still air, a little faster, giving a slightly harder and stronger result with a fine, uniform grain — often the condition a part is supplied and used in. Stress-relieving uses a lower temperature simply to relax the internal stresses left by welding, machining or forming, without changing the structure. Where hardening traps and strains, these treatments release and refine — the same transformations run gently in reverse.

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§6Surface hardening

Many parts need a hard, wear-resisting skin over a tough, shock-absorbing core — a gear tooth, a cam, a shaft journal. Surface hardening delivers exactly that split.

Two families do it. Case hardening (carburising, nitriding) diffuses carbon or nitrogen into the surface of a low-carbon steel at high temperature, so only the enriched skin hardens on quenching. The depth follows a diffusion law — case depth grows with the square root of time.

Example 2 — carburising case depth

If a process reaches a 0.5 mm case in 4 hours, the depth constant is 0.5/√4 = 0.25 mm/√h. To double the case to 1.0 mm needs t = (1.0/0.25)² = 16 hours — four times as long for twice the depth, the hallmark of a √t diffusion process.

Selective hardening (flame, induction) instead heats just the surface of an already medium-carbon steel fast enough to austenitise the skin alone, then quenches — hardening the surface by where the heat goes rather than by adding carbon. Both leave the core soft and tough, which is precisely what a loaded gear or shaft needs.

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§7Quick reference

The working core of the page on one card rack.

Eutectoid

0.77 % C · 727 °C

→ pearlite (slow cool)

Lever rule

%pearlite = (C−0.022)/(0.77−0.022)

0.40 % C → ~50/50

Harden

austenitise + quench

→ hard, brittle martensite

Temper

reheat 150–650 °C

↓ hardness, ↑ toughness

Soften

anneal (furnace) · normalise (air)

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Handbook application: from concept to controlled practice

Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Hardening, Tempering and Annealing. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.

The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.

Apply Hardening, Tempering and Annealing by beginning with the duty, not the component or software command. Convert the key ideas—hardening, tempering, annealing, iron-carbon, quench—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.

Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.

Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.

Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.

Step-by-step operating method

Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.

Illustrative design review record

Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.

Evidence class	Question	Release expectation
Requirement	What must the design do and under which conditions?	Approved and traceable
Input	Where did the load, property, tolerance or process limit come from?	Source, unit and revision recorded
Analysis	Which model and assumptions connect input to result?	Checkable calculation or simulation
Verification	How will conformity be demonstrated?	Method and acceptance criterion agreed
Validation	Will the solution work for intended users and conditions?	Representative use evidence

Common failure modes and recovery actions

1. Watch for

Starting detailed design before interfaces and operating limits are agreed.

Recovery: Return to the governing definition or requirement and restate the decision in one sentence.

2. Watch for

Using catalogue or typical values as though they were certified project inputs.

Recovery: Separate evidence from assumption, assign an owner and set a date for validation.

3. Watch for

Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.

Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.

4. Watch for

Confusing verification of requirements with validation of user need.

Recovery: Record the consequence, decision and rationale, then update the controlled baseline.

5. Watch for

Releasing drawings or procedures without configuration, inspection and change controls.

Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.

Review checklist

What function and failure consequence govern this decision?
Which inputs are measured, specified, assumed or illustrative?
How will conformity be demonstrated and recorded?
What change would invalidate the current evidence?
Are mandatory requirements distinguished from recommendations and illustrative values?
Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
Is there a named owner and a trigger for review, escalation, change or retirement?

Questions for deeper application

What is the most important distinction a practitioner must preserve when applying Hardening, Tempering and Annealing?