Heat Treatment
Heat treatment cycles metal through controlled heating and cooling to change its hardness, strength, ductility or internal stress state.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Metal
- Typical volumes
- 1 piece to millions; furnace loads are priced by weight, so batching drives the unit cost
- Lead time
- 3–10 business days for standard atmosphere hardening and tempering; longer for carburizing, and 1–3 weeks for long nitriding cycles
- Materials
- Metal
What it is
Heat treatment changes a metal's properties without changing its shape, by cycling it through controlled heating, holding, and cooling. In steel it works by manipulating phase transformations: heating above the austenitizing temperature dissolves carbon into solution, and how fast the part is then cooled determines whether it ends up soft pearlite or hard martensite.
The lever is enormous. The same 4140 bar can be delivered soft enough to machine at roughly 20 HRC or hardened and tempered to over 50 HRC, from identical chemistry. Aluminum alloys gain most of their strength from solution treatment and aging: 6061 is solution treated near 985°F (530°C), quenched, and aged around 350°F (175°C) to reach T6.
It is a service operation, not a shaping process, and it sits between machining steps. Everything about designing for heat treatment is about the two side effects: distortion, and dimensional change.
How it works
The main cycles
Annealing heats steel above its transformation range and cools it slowly in the furnace, producing the softest, most machinable, most ductile condition.
Normalizing austenitizes and then air cools, giving a uniform fine grain structure and relieving the effects of prior forging or casting.
Stress relieving heats below the transformation range — commonly 1,100–1,250°F (595–675°C) for steel — and cools slowly, relaxing residual stress from machining, welding, or forming without changing the microstructure. It is what you do between roughing and finishing on a part that must stay flat.
Hardening and tempering is the two-step production sequence. Medium-carbon and alloy steels are austenitized around 1,500–1,600°F (815–870°C), quenched in water, oil, polymer, or gas, and then immediately tempered. As-quenched martensite is hard and brittle; tempering trades hardness for toughness in a predictable way. Tempering 4140 near 400°F leaves it in the mid-50s HRC, while tempering near 1,000–1,200°F brings it down toward the low 30s.
Case hardening puts a hard skin on a tough core. Carburizing diffuses carbon into low-carbon steel at roughly 1,650–1,750°F (900–955°C), then quenches, producing a surface around 58–62 HRC over a case typically 0.010–0.060 in (0.25–1.5 mm) deep. Nitriding diffuses nitrogen at 950–1,050°F (510–565°C) — below the transformation temperature, so there is no quench and very little distortion — giving a thinner case, typically 0.005–0.025 in (0.13–0.6 mm), at very high surface hardness on nitriding-grade alloys. Induction hardening heats only the surface with an induction coil and quenches immediately, which is how gear teeth and shaft journals are hardened locally.
Solution treatment and aging is the aluminum, 17-4PH stainless, and superalloy route: dissolve the strengthening elements at high temperature, quench to trap them in solution, then hold at a lower temperature so fine precipitates form.
Why hardenability matters
Only the metal that cools fast enough transforms to martensite. In a plain carbon steel, that is a thin skin on anything but a small section; alloying elements such as chromium, molybdenum, and nickel slow the transformation and let thicker sections harden through. This is why 4140 and 4340 exist, and why section size has to be part of the specification.
Design guidelines
Specify hardness properly
A drawing note reading "harden" is not a specification. State the hardness range (a band, not a single number), where it is measured, the test scale, and for case hardening the case depth and whether it is total or effective. Convert scales carefully — hardness taken on a thin case with a Rockwell C indenter is not valid.
Leave grinding stock
Parts move during heat treatment. Rough machine, heat treat, then finish grind, leaving 0.010–0.020 in (0.25–0.5 mm) of stock on precision surfaces — more on long, thin, or asymmetric parts. Designing a part that must be finish machined before hardening is designing in a tolerance problem. See the surface finish chart for what grinding delivers and ISO 286 fits and tolerances for the fits those finish operations have to hit.
Design for symmetry
Distortion is driven by uneven cooling and by residual stress released at temperature. Symmetric sections, uniform wall thickness, and generous radii all reduce it. Sharp internal corners are quench-crack initiation sites, and a part with a heavy hub and a thin web will distort no matter how carefully it is processed.
Choose the quench for the geometry
Water and brine are fastest and cause the most distortion and the most quench cracks; oil is the general-purpose middle ground; gas quenching in a vacuum furnace is the gentlest and is standard for tool steels and precision parts. A part that will not survive an oil quench may need a more hardenable alloy so it can be quenched more slowly.
Watch for decarburization and scale
Heating steel in air pulls carbon out of the surface, leaving a soft skin, and forms scale. Controlled-atmosphere, vacuum, and salt bath furnaces avoid it. If a part is processed in air, enough stock has to remain to machine the decarburized layer off.
Account for size change
Through hardening changes dimensions — martensite occupies more volume than the structure it replaced — and carburizing adds carbon to the surface. Size change is repeatable for a given part and process and is normally absorbed in the finishing allowance, but it must be anticipated when heat treating a finished-size feature such as a bore.
| Consideration | Recommended practice | Why |
|---|---|---|
| Hardness callout | Range, location, scale, and case depth | A single number is unverifiable |
| Grinding stock | 0.010–0.020 in (0.25–0.5 mm) | Parts distort; finishing follows hardening |
| Internal corners | Radiused, no sharp changes of section | Sharp corners initiate quench cracks |
| Section uniformity | Symmetric, uniform walls | Uneven cooling drives distortion |
| Alloy vs section size | Match hardenability to thickness | Only metal that cools fast enough hardens |
| Quench medium | Oil or gas for precision parts | Water quenching maximizes distortion risk |
| Furnace atmosphere | Controlled atmosphere or vacuum | Air heating decarburizes the surface |
| Tempering | Immediately after quench | As-quenched martensite is brittle and cracks |
Cost drivers
Heat treatment is priced by furnace time and load weight, so it is a batch economy: a full basket costs little more to run than a half-empty one. What drives cost is the process, not the part. Atmosphere-controlled batch and belt furnace work is the commodity end; vacuum and salt bath processing, long carburizing cycles, and nitriding runs measured in tens of hours cost substantially more per pound.
The larger cost is usually indirect. Distortion means extra grinding stock and an extra finishing operation. Scrap from quench cracking is total — a cracked part cannot be reworked. And heat treatment is almost always an outside service, so it inserts shipping time and a queue into the middle of the manufacturing route.
- Batch the work. Furnace charges are priced by load; combining part numbers with the same cycle spreads the setup.
- Pick the least aggressive process that meets the spec. Induction hardening a journal is cheaper and lower-distortion than through hardening the whole shaft; nitriding avoids the quench entirely.
- Design out the distortion instead of grinding it out. Symmetry and uniform sections cost nothing at design time and save a finishing operation on every part.
Questions
6 questionsWhat is the difference between hardening and tempering?
Hardening austenitizes the steel — typically 1,500–1,600°F (815–870°C) for medium-carbon and alloy grades — and quenches it to form martensite, which is hard but brittle. Tempering then reheats below the transformation range to trade some hardness for toughness. The two are always done together; as-quenched parts crack.
How much distortion should I allow for heat treatment?
Enough to grind out. Leave 0.010–0.020 in (0.25–0.5 mm) of stock on precision surfaces, and more on long, thin, or asymmetric parts. Rough machine, heat treat, then finish grind. A part that must be finish machined before hardening will not hold tolerance.
What is the difference between carburizing and nitriding?
Carburizing diffuses carbon at 1,650–1,750°F (900–955°C) and then quenches, giving a case typically 0.010–0.060 in deep at roughly 58–62 HRC, with real distortion from the quench. Nitriding diffuses nitrogen at 950–1,050°F (510–565°C) below the transformation temperature, so there is no quench, distortion is minimal, and the case is thinner at 0.005–0.025 in.
Why does alloy steel harden deeper than plain carbon steel?
Because alloying elements such as chromium, molybdenum, and nickel slow the transformation from austenite, so martensite can form at slower cooling rates. Plain carbon steel only hardens where it cools very fast, which on any substantial section is a thin skin. This is why section thickness has to be considered when selecting the grade.
How should hardness be specified on a drawing?
As a range rather than a single value, with the test scale, the location of measurement, and — for case hardening — the case depth and whether it is total or effective. Hardness measured on a thin case with a Rockwell C indenter is invalid because the indenter punches through into the soft core.
What causes quench cracking?
Sharp internal corners, abrupt section changes, an overly aggressive quench medium, and delaying the temper after quenching. Water and brine quench fastest and crack most; oil is the general-purpose choice; vacuum gas quenching is gentlest. A cracked part cannot be reworked, so the design changes are worth making up front.