Heat Treating
Also: heat treatment, heat treat, hardening and tempering, quench and temper, Q&T, through hardening, neutral hardening, normalizing, solution heat treatment, precipitation hardening, age hardening
Heat treating changes hardness, strength and internal stress with controlled heating and cooling: quench and temper, normalize, or solution treat and age.
3,387 finishing and heat treating companies in the Noramark directory
Heat treating heats a finished or semi-finished part to a set temperature, holds it, and cools it at a controlled rate to change its microstructure. For carbon and alloy steels the everyday cycle is quench and temper: austenitize at roughly 815-870 °C (1500-1600 °F), quench in oil, polymer, water or gas to form martensite, then temper at 150-650 °C (300-1200 °F) to trade some of that hardness back for toughness. Normalizing heats above the critical range and cools in still air to refine the grain of forgings and weldments; it gives a uniform, moderately strong starting condition rather than a hard one.
Non-ferrous and precipitation-hardening alloys work differently. Aluminum (6061, 7075), 17-4 PH stainless, Inconel 718 and beryllium copper are solution treated, cooled quickly to hold the alloying elements in solution, then aged at a lower temperature so fine precipitates form. That is how 6061-O becomes 6061-T6 and 17-4 PH Condition A becomes H900 or H1025. Aging alone moves the part so little that 17-4 PH is routinely machined in Condition A and aged afterwards.
Aerospace work is called out to AMS 2759 for steel parts, with a slash sheet for each family (/1 carbon and low-alloy, /3 precipitation-hardening stainless, /5 martensitic stainless, /7 carburizing, /11 stress relief), and AMS 2770 for wrought aluminum parts, with furnace pyrometry to AMS 2750 and Nadcap accreditation. Commercial and automotive work follows the drawing hardness, sometimes under CQI-9 process controls. The limits of the process are distortion and size change on quenching, scale and decarburization in air or poorly controlled atmospheres, and hardenability: plain 1045 through-hardens only in thin sections, while 4340 hardens through heavy ones.
At a glance
| Typical tolerances | Hardness is specified as a range. A 4-point Rockwell C band (for example 38-42 HRC, which is ±2 HRC about the nominal) is the standard commercial window; a 3-point band is achievable with care on one heat of steel, and anything narrower is inside the repeatability of the hardness test. Surface-hardening processes specify case depth as a range too, typically a window 0.25-0.5 mm (0.010-0.020") wide. Through-hardened steel moves: tool steels change size by roughly 0.05-0.1 percent on hardening, and thin, long or asymmetric parts bow, twist or go out of round, so leave 0.25-0.5 mm (0.010-0.020") per side on precision surfaces to grind after hardening. 17-4 PH shrinks about 0.05 percent (0.0005 in/in) when aged to H900, small and predictable enough to machine before aging. Aluminum solution treat and quench distorts the most; parts are straightened or finish machined afterwards. |
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| Size limits | Batch integral-quench furnaces, the workhorse of commercial heat treaters, commonly have work zones from about 600 x 900 x 600 mm (24 x 36 x 24") to 900 x 1,200 x 750 mm (36 x 48 x 30") and gross loads of roughly 450-1,800 kg (1,000-4,000 lb). Pit furnaces take long shafts hung vertically, commonly 1.5-3 m (5-10 ft) deep and more at large shops, which keeps them straighter than lying flat. Car-bottom and box furnaces handle weldments, castings and forgings of several tonnes for normalizing and stress relief, but not for a liquid quench. Aluminum solution treating is limited by the quench tank, commonly taking parts about 2-4 m (7-13 ft) long. |
| Surface finish | Depends on the furnace atmosphere. Parts hardened in an endothermic or nitrogen-methanol protective atmosphere come out dull grey to black with a light oxide. Parts heated in air get scale and a soft decarburized skin, commonly 0.05-0.25 mm (0.002-0.010") deep, that must be ground off where hardness matters. Vacuum furnaces return bright, clean parts. Oil-quenched parts are washed, and most parts are bead or shot blasted before plating, painting or inspection. Precision ground surfaces are finished after heat treatment in any case. |
| Lead time | Commercial heat treaters turn standard quench and temper, normalizing, stress relief and aging in 3-7 business days, often less for loads that match a running cycle; 24-48 hour rush service is common at a premium. Aerospace work with certification, and long cycles such as deep carburizing or gas nitriding, take 1-3 weeks. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Steels with enough carbon to harden: 1045, 1095, 4130, 4140, 4340, 52100 and the tool steels (O1, A2, D2, S7, H13, M2) are quenched and tempered. Low-carbon grades such as 1018, A36 and 8620 do not through-harden usefully and are carburized instead. Martensitic stainless (410, 420, 440C) hardens like tool steel; austenitic 304 and 316 cannot be hardened by heat treatment at all, only annealed or stress relieved. Precipitation-hardening alloys (17-4 PH, 15-5 PH, 6061, 7075, Inconel 718, Ti-6Al-4V, beryllium copper) are solution treated and aged. Titanium goes in vacuum or argon to avoid a brittle oxygen-rich surface layer (alpha case).
What drives the cost
- Furnace time: long soaks, slow cools and multi-step cycles (double tempers, cryogenic treatment, two-step aging) tie up a furnace
- Minimum lot charge: a few small parts pay for a full furnace cycle unless they ride with other work
- Weight and loading: most commercial work is priced by the pound with a per-load minimum, and parts that must be hung or fixtured individually cost more
- Atmosphere: vacuum and inert-gas cycles cost more than endothermic atmosphere or air
- Specification and documentation: AMS 2759 or AMS 2770 work at a Nadcap-accredited shop, with test coupons, hardness records and certs
- Straightening, grinding and cleaning after the quench, which can cost more than the heat treatment itself
- Hardness verification: tensile coupons, microhardness traverses or 100 percent hardness testing
When to use it
- A part needs more strength or wear resistance than annealed or as-rolled bar gives, and the alloy can harden
- Tool steel dies, punches, cutters and gauges, which must be hard to work at all
- Precipitation-hardening alloys (17-4 PH, 6061, 7075, Inconel 718) machined soft that need their final temper
- Forgings, castings and weldments that need a uniform, refined structure before machining (normalizing)
- A property the drawing can state and an inspector can check: a hardness range tested on receipt
When not to
- Prehardened bar will do: 4140 at 28-32 HRC machines as bought and skips a heat-treat step and its distortion
- Only a surface needs to be hard: carburizing, nitriding or induction hardening keep a tough core and move less
- Austenitic stainless (304, 316) and non-heat-treatable aluminum (5052, 3003) cannot be hardened thermally; they gain strength only from cold work
- Thin, flat or long, slender parts held to tight flatness, where quench distortion would scrap them, unless the drawing allows grind stock or the shop can press quench
Design tips
- Call out the alloy, the result and the spec: for example "AISI 4140, harden and temper to 38-42 HRC per AMS 2759/1". Say where on the part hardness is to be tested if it matters.
- Give hardness as a range at least 4 HRC points wide, not a single number.
- Keep sections as uniform as practical and avoid sharp internal corners, holes close to an edge and abrupt changes in thickness: they concentrate quench stress and are where quench cracks start.
- Leave 0.25-0.5 mm (0.010-0.020") per side of grind stock on bores, bearing journals and datum faces that must hold tight tolerances after hardening.
- Cut threads, keyways and small holes before hardening; drilling and tapping above about 40 HRC is slow and expensive.
- For 17-4 PH, machine in Condition A and age afterwards, or buy bar already aged to a machinable condition such as H1150, and state the condition on the drawing.
- If a part above about 36-40 HRC will be electroplated, call for a hydrogen embrittlement relief bake after plating (AMS 2759/9 or ASTM B850).
- Ask for a certificate listing the hardness readings, the specification and the load, and for test coupons if the drawing calls for tensile properties.
Heat Treating by material
Frequently asked questions
- Should I machine before or after heat treating?
- Rough machine before and finish after, for anything that must hold tight tolerances. Soft steel machines fastest, so shops rough the part, leave grind stock on critical surfaces, harden, then grind. Up to about 32 HRC, prehardened bar machines well enough that many parts skip heat treatment entirely, and 17-4 PH is machined soft and aged last because it barely moves.
- How do I call out heat treatment on a drawing?
- State the alloy, the condition to be reached and how it is checked: "4140, harden and temper to 38-42 HRC" or "17-4 PH, age to H1025 per AMS 2759/3". Add the hardness test location if it matters, any surfaces that must stay soft, and whether a certification is required. Leave the time and temperature to the heat treater unless you have a reason to fix them.
- What hardness tolerance can a heat treater hold?
- A 4-point Rockwell C range, such as 28-32 or 58-62 HRC, is the normal commercial band. Three points is possible on one heat of steel in a well-controlled furnace. Asking for ±1 HRC is asking for less than the test itself repeats to, and it will cost more or be declined.
- Does heat treating cause distortion?
- Any process with a quench moves the part. Through-hardened steel changes size and can bow, twist or go out of round, and solution-treated aluminum distorts more than steel. Gas quenching, marquenching, press quenching and balanced geometry reduce it; stress relieving after rough machining and leaving finish stock on critical features are the usual cures.
- What is the difference between normalizing and quench and temper?
- Normalizing cools in still air from above the critical temperature, giving a uniform, moderately strong structure; it refines forgings and weldments before machining. Quench and temper cools fast enough to form martensite and then tempers it, giving much higher hardness and strength in the same steel.
