Heat Treat Temperature Guide
Searchable normalize, anneal, harden, temper, and quench data for 70+ steel alloys.
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| Grade ↑ | Category | Norm. | Anneal | Harden | Temper | Quench | Notes |
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All temperatures in °F
How to use this guide
This table compiles published heat treatment ranges for the most commonly specified carbon, alloy, tool, and stainless steel grades. Use the search box to find a grade by number (for example, 4140), by category (tool), or by a keyword in the notes column (bearing, case harden). The category filter buttons narrow the list to a single family. Click any column header to sort — click again to reverse the sort direction.
The values shown are typical ranges drawn from ASM Handbook Volume 4: Heat Treating, AISI/SAE specifications, and tool steel manufacturer datasheets (Crucible Industries, Bohler-Uddeholm, Carpenter Technology). They represent the starting point for a heat treat recipe, not a finished one. Section thickness, furnace atmosphere, prior microstructure, and the specific hardness target you are aiming for all change the optimal temperatures and times for your part.
The four core processes
Normalizing
Normalizing heats steel to roughly 50–100°F above its upper critical temperature (Ac3), holds long enough to fully austenitize, and then cools the part in still air. The result is a refined, uniform grain structure with a mix of pearlite and ferrite (or pearlite and cementite for hypereutectoid grades). It is typically used to homogenize forgings, castings, or weldments before further machining or heat treatment, and to remove the coarse or banded structure that develops after hot working. Normalized parts are stronger and harder than annealed parts of the same composition because the faster air cool produces finer pearlite spacing.
Annealing
Full annealing heats the steel above its upper critical temperature, then cools it slowly in the furnace (typically 25–50°F per hour through the transformation range) to produce coarse, soft pearlite. The goal is the lowest practical hardness and the most ductile, machinable, and formable structure the alloy can be put into. Some grades — especially high-carbon and high-alloy steels — respond better to spheroidize annealing, which holds just below the lower critical for many hours so the carbides ball up into discrete particles. Bearing steel 52100 is almost always supplied spheroidize-annealed for this reason.
Hardening (austenitize and quench)
Hardening heats the steel into the austenite region, holds long enough to dissolve the desired amount of carbon and alloy into solution, and then quenches fast enough to suppress diffusion-controlled transformations (pearlite, bainite) and form martensite. The required cooling rate is set by hardenability — a function of carbon content, alloy content, and grain size. Per ASM Handbook Vol 4, plain carbon steels need a water or brine quench to make martensite in anything but a thin section, while medium-alloy grades like 4140 and 4340 harden through in oil, and high-alloy tool steels like A2, D2, and H13 harden in still air for sections up to several inches.
Tempering
As-quenched martensite is brittle and contains residual stresses; tempering trades a measured amount of hardness for toughness and dimensional stability. For most structural and tool steels, parts should be transferred from the quench to the tempering furnace before they reach room temperature (typically when they are still warm to the hand, around 150–200°F) to avoid quench cracking. Choose the tempering temperature based on the final hardness or service condition you want, not by tradition. Higher tempering temperature means lower hardness and higher toughness. Many tool steels require multiple tempers — H13, M2, and the 440-series stainless among them — because retained austenite transforms to fresh martensite during the first temper and must itself be tempered.
Quench medium selection
The quench medium controls the cooling rate at the part surface. Picking the wrong medium is one of the most common and most expensive heat-treat mistakes — quench too slow and you miss the hardness target; quench too fast and you crack the part or distort it past usable tolerance.
- Brine (~10% salt water): The fastest practical quench. Reserved for very shallow-hardening grades like W1, W2, and plain carbon steels in thin sections. Too aggressive for anything with appreciable alloy content.
- Water: Used for shallow-hardening carbon steels (1018, 1045, 1095) and some carburized cases. High distortion and crack risk on complex shapes.
- Polymer (PAG, etc.): Adjustable cooling rate between water and oil by changing concentration. Common in production heat treating where the supplier wants one tank to cover several alloys.
- Oil: The most common quench for medium-carbon alloy steels (4140, 4340, 8620 case), oil-hardening tool steels (O1, O2, L6), and many martensitic stainless steels. Fast oils cool faster than slow oils — check the supplier's GM Quenchometer rating.
- Air (still or fan): Air-hardening tool steels (A2, D2, H13, S7) develop full hardness with no liquid quench. Distortion is minimal; section size limits are large.
- Vacuum / inert gas: High-pressure nitrogen or helium in a vacuum furnace. Produces clean parts with no scale and quench rates from slow (1 bar) to oil-equivalent (10–20 bar). The default for tooling and aerospace work.
Note that which medium is appropriate is a property of the steel grade and the section size, not a matter of preference. Quenching a water-hardening grade like W1 in oil leaves you below the M50 line and the part comes out soft. Quenching an oil-hardening grade like O1 in water cracks it, often catastrophically.
Other variables that change the result
Section thickness and ruling section
The hardenability of a grade is reported for a specific bar diameter (the “ruling section” in the spec). A 1/2-inch round of 4140 oil-quenches to full hardness through the cross-section; a 4-inch round of the same grade does not — the core cools too slowly to make martensite and ends up with a softer bainitic or pearlitic core under a hardened skin. For sections beyond the published hardenability of a grade, either step up to a deeper-hardening alloy (4340 instead of 4140, for example) or accept a lower core hardness.
Soak time
A common rule of thumb is 1 hour per inch of cross-section at the austenitizing temperature, with a minimum of about 30 minutes once the part has reached temperature throughout. Tool steels with high carbide content (D2, M2, the CPM grades) may require longer soaks to dissolve enough carbide into solution; the manufacturer's datasheet is authoritative. Don't confuse furnace recovery time (time for the chamber to return to setpoint after loading) with soak time (time the part itself is at temperature). Use a thermocouple in or on a load piece if you can.
Furnace atmosphere
Heating bare steel in air above ~1200°F causes oxidation (scale) and decarburization (loss of surface carbon), both of which ruin the surface that is about to become a working edge or a sealing face. Options to control this include endothermic gas atmospheres, neutral salt baths, vacuum furnaces, and stainless-foil wraps for small batches in a non-atmosphere furnace. For knives, dies, and other parts that depend on surface hardness, atmosphere control is not optional.
Part geometry
Sharp internal corners, abrupt section changes, blind holes, and unbalanced mass concentrate stress during the quench. The cooling rate is highest at the surface and slowest at the core; the resulting strain is what causes distortion and quench cracks. Where possible, design for radii at re-entrant corners, equalize section thickness, plug or pack blind holes, and orient parts so that critical surfaces face into the quench flow. For high-distortion grades, marquenching (interrupting the quench just above Ms, then air-cooling) drastically reduces strain at the cost of slightly less hardness depth.
Frequently asked questions
What's the difference between normalizing and annealing?
Both processes heat the steel above its upper critical temperature, but the cooling step differs. Normalizing cools in still air, producing fine pearlite and a moderate increase in strength and hardness over the as-rolled condition. Annealing cools slowly in the furnace, producing coarse pearlite (or, in spheroidize annealing, balled-up carbides) and the lowest hardness the alloy is capable of.
Practically: normalize when you want a uniform, refined starting structure for further machining or heat treatment. Anneal when you need the part as soft and ductile as possible — for cold forming, deep machining, or to recover a part that has been case-hardened or work-hardened.
How do I pick a tempering temperature?
Pick by the final hardness or service condition you need, then verify against the manufacturer's tempering chart for that exact grade. Higher temper temperature gives lower hardness and higher toughness; the tradeoff is non-linear and grade-specific. For most medium-carbon alloy steels (4140, 4340, 8640) a useful first cut is: 400–500°F for maximum hardness with some toughness recovery, 700–900°F for a balance of strength and toughness, and 1000–1200°F when you need yield strength below ~150 ksi with high impact resistance.
Be aware of tempered martensite embrittlement (TME, around 500–700°F in many alloy steels) and temper embrittlement (around 700–1100°F with slow cool-down in chromium-bearing steels). Avoid tempering in those windows for parts that see impact.
What's the recommended hold time at temperature?
The widely cited rule is one hour per inch of section thickness at the austenitizing temperature, with a minimum of ~30 minutes after the part reaches temperature. Tempering is typically held at temperature for at least 1–2 hours per cycle, and for tool steels the temper is often repeated 2–3 times for stability. Highly alloyed grades with stable carbides — D2, M2, CPM-3V, CPM-10V — may need longer austenitizing soaks (30–60 minutes at temperature) to dissolve enough carbide into solution. Always defer to the steel supplier's published datasheet for tool steels and PH stainless grades.
How do I convert these temperatures to Celsius?
The exact formula is °C = (°F − 32) × 5/9. Useful round numbers for heat treating: 1000°F ≈ 538°C, 1500°F ≈ 816°C, 1750°F ≈ 954°C, 1850°F ≈ 1010°C, 2000°F ≈ 1093°C. A 50°F differential equals about 28°C, so a tempering range like 400–500°F maps to roughly 205–260°C.
Why do parts crack during quenching?
Quench cracks are caused by the volumetric expansion of austenite-to-martensite transformation combined with the thermal contraction of cooling from the quench temperature, both happening unevenly across the section. Sharp internal corners, abrupt section changes, drilled holes near edges, and stamped marks all act as stress raisers. Common contributors are: too aggressive a quench medium for the grade (oil-hardening steel quenched in water is the classic case), a part allowed to cool to room temperature before tempering, prior microstructural defects, or austenitizing too far above the recommended range and getting coarse grain.
To reduce cracking: pick the slowest quench that still makes hardness, temper immediately after the part is cool enough to handle, radius re-entrant corners, and consider marquenching for tricky geometries.
Can stainless steels be hardened by quenching?
It depends on the family. Austenitic stainless (300-series like 304, 316) cannot be hardened by quenching — it is hardened only by cold work. The "anneal" temperatures shown for these grades are solution anneal values, where the steel is heated to dissolve precipitated carbides and then quenched fast enough to prevent re-precipitation (a softening operation, not a hardening one). Ferritic stainless (430, 446) similarly does not harden by quenching. Martensitic grades (410, 416, 420, 440-series) and precipitation-hardening grades (17-4 PH, 15-5 PH, 17-7 PH) do harden through heat treatment, although by very different mechanisms — martensite formation for the 4-series, and aging precipitates for the PH grades.
Is decarburization a real problem at these temperatures?
Yes, especially above ~1400°F in an air atmosphere. Decarburization removes carbon from the surface layer, leaving a soft skin that does not respond to subsequent hardening. On a knife edge or a precision die feature, even 0.005–0.010” of decarb can ruin the part. Mitigations include heat-treating in stainless tool wrap, using neutral salt baths, working in a controlled-atmosphere or vacuum furnace, or grinding the affected layer off after heat treatment. For shop-floor heat treats on tool steels, stainless foil with a small piece of paper inside (to consume oxygen) is a low-cost, well-proven approach.
Why are some tool steels tempered three times?
High-alloy tool steels — especially the high-speed steels (M2, M4, T1) and air-hardening grades like H13 — retain a significant fraction of austenite after quenching. The first temper transforms much of that retained austenite into fresh, untempered martensite, which is itself brittle. The second temper tempers that new martensite. A third temper provides additional dimensional stability and converts any small remaining pockets of retained austenite. For service-critical tooling, three tempers at the recommended temperature, with full cool to room temperature between cycles, is standard practice.