Reference Guide · Heat Treatment
Heat Treatment Terms Explained: Annealing, Q&T, Case Hardening, Precipitation Hardening
Almost every metal drawing carries at least one heat-treatment instruction, and most of the vocabulary (normalize, Q&T to 30 HRC, carburize and harden, solution treat and age, H900) is shorthand that assumes the reader already knows the process behind it. This glossary explains each term the way a machine shop uses it: what happens to the metal, what it does to machinability and cost, and how to write the callout so a heat treater and a machinist both read it the same way. Entries are grouped by what they do, and every one is linkable from the other articles in this library.
In This Guide
- 01Why Heat Treatment Exists (and Why It Changes the Quote)
- 02Softening & Conditioning: Annealing, Normalizing, Stress Relief
- 03Hardening Steel: Quench & Temper, Through vs Case Hardening
- 04Surface Hardening: Carburizing, Nitriding, Induction
- 05Precipitation (Age) Hardening: Aluminum, 17-4 PH, Superalloys
- 06Heat Treatment Around Welding
- 07Where Heat Treat Goes in the Machining Sequence
- 08Writing the Callout
- 09Quick Reference Card
Why Heat Treatment Exists (and Why It Changes the Quote)
Heat treatment is any controlled heating-and-cooling cycle used to change a metal's internal structure without changing its shape. The same bar of 4140 can be soft enough to drill with a hand drill or hard enough to wear a carbide insert flat in minutes; the difference is entirely heat treatment. For steels the lever is the transformation between ferrite/pearlite (soft), austenite (the high-temperature phase), and martensite (hard); for aluminum, 17-4 PH, and nickel superalloys the lever is precipitation: dissolving alloying elements at high temperature and then letting them re-form as fine particles that pin the structure.
From the shop's side, heat treatment shows up in the quote three ways. First, it is usually an outside process: the parts ship to a certified heat treater and come back days later, so it adds calendar time and a purchased-service line. Second, it decides when machining happens, because metal that is hard is slow and expensive to cut (see section 07). Third, it moves the part: quenching distorts, aging shrinks, and stress relief lets locked-in stresses out, so finish dimensions are almost always cut after the treatment, not before.
One sentence to remember
Heat treatment trades machinability for properties. The harder and stronger the finished part needs to be, the more of the machining we try to do before the treatment and the less we leave for after it.
Softening & Conditioning: Annealing, Normalizing, Stress Relief
Annealing (full anneal, spheroidize anneal, solution anneal)
Heating to above the transformation range and cooling slowly (in the furnace) so the structure ends up as coarse, soft pearlite and ferrite. Annealed is the softest, most machinable condition for most steels and the condition in which alloy steels such as 4140/4340 are usually bought when they will be heat treated after machining. Spheroidize annealing is a longer sub-critical cycle used on high-carbon steels to make them machinable. Solution annealing is the stainless and nickel-alloy version: heat to dissolve carbides or precipitates, then cool fast enough to keep them in solution (it is the "annealed" condition of 304/316, Inconel 625, and the starting condition AMS 5662 for Inconel 718).
Normalizing
Heating above the transformation range and cooling in still air. Faster than a furnace cool, so the structure is finer and slightly harder and stronger than annealed, but uniform and stress-free. Normalized is the standard delivery condition for many structural and alloy steel bar products and the reference condition the 4140 vs 4340 vs 4130 comparison uses. It is also the usual "reset" after forging or heavy welding.
Stress relief (stress-relieved, SR)
A low-temperature hold, typically 1,000–1,250°F for steels and 350–650°F for aluminum, below any transformation temperature, followed by slow cooling. It does not change hardness; it relaxes the residual stresses left by cold drawing, welding, heavy machining, or quenching so the part does not move when the next operation removes material. Cold-drawn bar in particular carries enough locked-in stress that a long keyway or an off-center bore will bow it; a stress relieve before finishing (or buying stress-relieved bar, designated SR) is the cure. Aluminum plate tempers with a second 5 (T651, T7451) are stress-relieved by controlled stretching rather than heat, which is why thick 7050-T7451 plate machines flat.
Shop note
If a part has a thin web, a long slot, or an asymmetric pocket, ask for a stress relief between roughing and finishing, or specify stress-relieved stock. The treatment costs less than the part you'd otherwise scrap for flatness.
Hardening Steel: Quench & Temper, Through vs Case Hardening
Quench and temper (Q&T, quenched and tempered, hardened and tempered)
The workhorse process for medium-carbon and alloy steels (1045, 4140, 4340, 4130, 8620 core). Austenitize (heat to roughly 1,500–1,600°F), quench in oil, water, or polymer to form hard, brittle martensite, then temper at 400–1,200°F to trade some of that hardness for toughness. The tempering temperature sets the final properties, which is why a Q&T callout must state the result (a hardness range or a minimum tensile) rather than a temperature. Higher tempering temperature means lower hardness and higher ductility; the curve is continuous, and the 4140 article has an interactive strength-vs-temper chart.
| Typical Q&T callout | Hardness | Tensile (approx.) | What it is used for |
|---|---|---|---|
| 4140 Q&T 28–32 HRC ("pre-hard") | 28–32 HRC | 130–150 ksi | Shafts, fixtures, tooling blanks. Bought pre-hardened, machined complete, no further treatment. |
| 4140 Q&T 36–40 HRC | 36–40 HRC | 165–185 ksi | High-strength pins, arbors. Rough, treat, finish grind or hard-mill. |
| 4340 Q&T 40–44 HRC | 40–44 HRC | 190–210 ksi | Aircraft and heavy-equipment structural parts needing toughness at strength. |
| 1045 Q&T 22–28 HRC | 22–28 HRC | 110–130 ksi | Economy shafting and machine parts where 1018 is too soft. |
Through hardening vs case hardening
Through (thru) hardened means the whole cross-section transforms to martensite: the part is hard all the way through. It requires enough carbon (roughly 0.30% and up) and enough alloy content to carry the hardening depth; that is what "hardenability" means, and why 4340 through-hardens in 4" bar while 1045 only hardens a shell on the same size. Case hardened means only the outer skin (the case) is hard, usually 0.010–0.060" deep, over a soft, tough core. Case hardening is the route for low-carbon steels (1018, 8620, 9310), which cannot through-harden on their own, and for parts that need a wear surface without the brittleness of a fully hard section: gears, cam followers, pins, bushings.
Do
"4140, Q&T to 28–32 HRC, prior to machining" — tells the shop to buy pre-hard stock and machine complete. Fast and predictable.
Avoid
"Harden" with no grade-appropriate method, no hardness, and no sequence. The estimator has to guess whether you mean through, case, or induction, and will price the worst case.
Surface Hardening: Carburizing, Nitriding, Induction
Carburizing (carburize and harden)
Diffusing carbon into the surface of a low-carbon steel at 1,600–1,750°F, then quenching. The carbon-rich case hardens to 58–62 HRC while the low-carbon core stays tough. Specified by case depth (effective or total), surface hardness, and core hardness. Parts distort during the quench, so critical features are ground afterward and the RFQ should say which features.
Nitriding (gas nitride, ion/plasma nitride, ferritic nitrocarburize)
Diffusing nitrogen at a low temperature (900–1,050°F) with no quench, which makes it the lowest-distortion way to put a hard, wear- and galling-resistant skin on 4140, 4340, Nitralloy, tool steels, and some stainless grades. The case is thin (0.002–0.025") but very hard, and the part is already finish-machined when it goes in; only a light polish follows. Ideal when you need a hard surface on a part whose dimensions cannot move.
Induction hardening
Heating only a selected surface with an induction coil and quenching it immediately, so a shaft journal, a gear tooth, or a wear pad hardens while the rest of the part stays soft. Needs a medium-carbon or alloy steel (1045, 4140, 4340: the carbon has to be there already). Specified by location, case depth, and surface hardness. 1045 shafting is the classic induction-hardened material; 1018 cannot be induction hardened usefully because it lacks carbon.
Hardened surfaces and your tolerances
Anything that hardens above roughly 45 HRC is finished by grinding or hard-turning, not conventional milling. If a hardened feature carries a tight tolerance, the drawing should say the tolerance applies after hardening and grinding, and the model should leave grind stock on that feature. Our DFM guide covers the tolerance side.
Precipitation (Age) Hardening: Aluminum, 17-4 PH, Superalloys
Steels harden by forming martensite. Aluminum alloys, precipitation-hardening stainless (17-4 PH, 15-5 PH), and gamma-prime/gamma-double-prime nickel superalloys (Inconel 718, Waspaloy, Haynes 282) harden by a different mechanism: precipitation. Alloying elements are dissolved at high temperature (solution treatment), frozen in solution by a fast quench, and then allowed to come out of solution as very fine particles during a lower-temperature hold (aging). The particles block dislocation motion, so the metal gets stronger and harder. Because the hardening happens at moderate temperature with no martensitic quench, precipitation hardening distorts far less than Q&T, which is why so much precision aerospace hardware is made from these alloys.
Aluminum: T-tempers
Aluminum's temper designations encode the treatment: T4 = solution treated and naturally aged at room temperature; T6 = solution treated and artificially aged (the strength peak for 6061 and 7075); T651/T6511 = T6 plus stress relief by stretching; T73/T7351 = overaged for stress-corrosion resistance at a small strength penalty; T7451 = the stress-relieved overage used for thick 7050 plate. The aluminum guide has a note on temper designations, and AMS 2770 / AMS 2772 govern the heat treatment of aluminum parts and raw material.
17-4 PH: H-conditions
17-4 PH is bought in Condition A (solution annealed, machinable at ~32 HRC) and aged by the shop or a heat treater to a named H-condition: H900 (900°F, 1 hr: ~44 HRC, 190 ksi, the strongest), H1025, H1075, H1100, H1150 (1150°F, 4 hr: ~33 HRC, 135 ksi, the toughest and most corrosion-resistant), and double-aged H1150M. The number is the aging temperature in °F. Since aging is a single low-temperature hold with almost no distortion (H900 shrinks about 0.0004–0.0006 in/in), parts are normally machined complete in Condition A and aged afterward. The stainless guide tabulates every condition.
Nickel superalloys: solution + age, double aging, gamma prime
Inconel 718 is the textbook case: solution anneal (AMS 5662 condition), then a double aging cycle per AMS 2774 (1,325°F / 8 h, furnace cool to 1,150°F / 8 h, air cool) to reach AMS 5664 properties. The strengthening precipitate in 718 is gamma double prime (γ″, Ni₃Nb); in Waspaloy, René 41, and Haynes 282 it is gamma prime (γ′, Ni₃(Al,Ti)), which is why those are called gamma-prime strengthened or gamma-age hardened alloys. Age hardened and precipitation hardened are the same thing said two ways. Solid-solution alloys such as Inconel 625 and Hastelloy X do not respond to aging at all; their strength is in the matrix chemistry. The details, and what the condition does to machining, are in Inconel AMS specs and machining Inconel.
Heat Treatment Around Welding
Post-weld heat treatment (PWHT)
Welding leaves a hard, stressed heat-affected zone. For hardenable steels (anything above ~0.30% carbon, and alloy grades like 4140) a post-weld stress relief or full re-heat-treat restores toughness and stops cracking. In the nickel-alloy world, precipitation-hardening grades are welded in the solution-annealed condition and then aged after welding so the weld ages with the base metal. Fabricated assemblies that will be heat treated after fabrication need that stated in the RFQ, because the whole weldment goes to the furnace and distortion has to be planned for.
Strain-age cracking
A failure specific to gamma-prime superalloys (Waspaloy, René 41, Inconel X-750): if a welded part is aged while it still carries welding stresses, the precipitation reaction and the stress relief happen at the same time and the heat-affected zone cracks. Inconel 718 was designed with a sluggish γ″ reaction precisely to avoid this, which is one reason it is the most-welded superalloy. Haynes 282 was likewise designed for weldability with a slower aging response than Waspaloy.
Where Heat Treat Goes in the Machining Sequence
Three patterns cover almost every part:
1. Buy it already treated, machine complete. Pre-hard 4140 (28–32 HRC), 6061-T651, 7075-T7351, 17-4 PH H1150 bar. No outside process, one machining cycle, slightly slower cutting than annealed. The default for most precision parts up to the mid-30s HRC.
2. Machine complete, then treat. Low-distortion treatments: aging of 17-4 PH or aluminum, nitriding, stress relief, some carburize-and-grind jobs where only a few features are ground after. Cheapest cutting, one handling loop.
3. Rough → treat → finish. High-hardness Q&T, through-hardened tool steel, aged Inconel 718 above ~40 HRC. Rough soft with 0.030–0.060" stock, heat treat, finish hard (grind or hard-mill). Two machining setups and a shipping loop, but the only way to hit tolerance on hard parts. The Inconel machining article walks through this sequence.
2. Machine complete, then treat. Low-distortion treatments: aging of 17-4 PH or aluminum, nitriding, stress relief, some carburize-and-grind jobs where only a few features are ground after. Cheapest cutting, one handling loop.
3. Rough → treat → finish. High-hardness Q&T, through-hardened tool steel, aged Inconel 718 above ~40 HRC. Rough soft with 0.030–0.060" stock, heat treat, finish hard (grind or hard-mill). Two machining setups and a shipping loop, but the only way to hit tolerance on hard parts. The Inconel machining article walks through this sequence.
Tell us which one you mean
The same callout, "4140 HT 40–44 HRC," is a pattern-1 part if we can buy it pre-hard (we usually can't at that hardness) and a pattern-3 part if we can't. Adding "machine complete from pre-hard stock" or "finish after heat treat" to the drawing note removes the ambiguity, and the quote difference can be 30–50%.
Writing the Callout
A complete heat-treatment note names the process, the governing spec, the result (hardness range, condition, or minimum properties), and the sequence relative to machining. Examples that a heat treater can execute and an inspector can verify:
| Callout | Why it works |
|---|---|
| HEAT TREAT PER AMS 2759/1, Q&T TO 36–40 HRC. FINISH MACHINE AFTER HEAT TREAT. | Spec, result as a hardness range, sequence. |
| CARBURIZE AND HARDEN PER AMS 2759/7: EFFECTIVE CASE DEPTH .030–.040, SURFACE 58–62 HRC, CORE 30–40 HRC. GRIND Ø.7500 AFTER. | Case depth type, surface and core hardness, which feature is ground. |
| AGE TO CONDITION H900 PER AMS 2759/3 AFTER MACHINING. | Condition name carries the temperature and time; sequence stated. |
| SOLUTION TREAT AND AGE PER AMS 2774 TO AMS 5664. | Process spec plus the material spec whose properties define acceptance. |
| STRESS RELIEVE PER AMS 2759/11 AFTER ROUGH MACHINING. | Sequence matters most for stress relief; the spec fixes temperature and soak. |
| NITRIDE PER AMS 2759/6, CASE .008–.012, SURFACE 60 HRC MIN, MASK THREADS. | Depth, hardness, and masking of features that must stay soft. |
The AMS 2759 family (slash sheets /1 through /12) is the aerospace standard for heat treating steels; AMS 2770/2772 cover aluminum; AMS 2774 covers nickel alloys. Commercial work often cites ASTM A1033 or simply a hardness result. If your drawing has no spec, we will quote to the hardness or condition you name and say which spec we used. What the spec numbers themselves mean is covered in AMS vs ASTM vs SAE.
Quick Reference Card
Anneal / Normalize
Soften and homogenize. Anneal = furnace cool (softest); normalize = air cool (uniform, slightly stronger).
Machinability ↑, strength ↓
Stress Relief
Low-temp hold, no hardness change. Do it between rough and finish on thin or asymmetric parts.
Steel 1,000–1,250°F
Quench & Temper
Harden medium-carbon / alloy steel, then temper to the hardness you need. Call out the result, not the temperature.
4140: 28–32 HRC pre-hard
Case Hardening
Hard skin, tough core. Carburize (low-C steels, quench), nitride (low distortion), induction (local).
Case .010–.060 typical
Precipitation Hardening
Solution treat + age. Aluminum T6, 17-4 PH H900–H1150, Inconel 718 AMS 5664. Low distortion.
Machine first, age after
Sequence
Above ~40 HRC: rough → treat → finish. Otherwise buy pre-treated or treat after machining.
Say it on the drawing
Have a part with a heat-treat callout?
Send the model and drawing through the contact page and an engineer will map out the machining-and-treatment sequence with you before anything is quoted.
Sources & References
[1]SAE AMS 2759 and slash sheets /1–/12, Heat Treatment of Steel Parts; AMS 2770, Heat Treatment of Wrought Aluminum Alloy Parts; AMS 2774, Heat Treatment of Wrought Nickel Alloy and Cobalt Alloy Parts.
[2]ASM Handbook, Volume 4: Heat Treating. ASM International. Process definitions, hardenability, and case-hardening practice.
[3]ANSI H35.1, American National Standard Alloy and Temper Designation Systems for Aluminum. Temper designation definitions (T4, T6, T651, T73, T7451).
[4]AK Steel / Cleveland-Cliffs, 17-4 PH Stainless Steel Product Data Bulletin. H-condition properties and dimensional change on aging.
[5]Special Metals Corporation, INCONEL alloy 718 data sheet; Haynes International, HAYNES 282 alloy brochure. Aging cycles, γ′/γ″ strengthening, strain-age cracking resistance.