Engineering · Heat Treatment

Heat treatment reference.

What each process does, when to use it, and how the typical alloys in this reference respond. The guide for designers who need to spec heat treatment thoughtfully.

Why heat-treat

Same alloy, different part.

The same chemical composition can behave like soft butter or like a knife-edge — depending entirely on its thermal history. Heat treatment is how you tune properties to match the application: harder for wear, tougher for impact, more ductile for forming, more dimensionally stable for inspection.

The right heat treatment is usually decided early in the design phase — and called out explicitly on the drawing. Specifying "heat treat to 38–42 HRC" without naming the process leaves real ambiguity. Use the process names below.

The processes

Eight processes, distinct outcomes.

Process	Goal	Typical temp	Cool from temp	Best for
Annealing	Soften, relieve all stress	705 – 870 °C	Slow furnace cool	Ferrous alloys before machining
Normalizing	Refine grain, uniform structure	815 – 925 °C	Air cool	Castings, forgings, weldments
Stress relief	Remove residual stress only	550 – 675 °C	Slow	Post-machining, post-welding
Hardening (quench)	Form martensite, max hardness	815 – 870 °C	Oil/water/polymer quench	4140, 4340, low-alloy steels
Tempering	Trade hardness for toughness	150 – 650 °C	Air cool	Always after hardening
Solution treatment	Dissolve precipitates uniformly	Material-specific	Water/air (alloy-specific)	PH stainless, aluminum, Inconel
Precipitation aging	Form strengthening precipitates	120 – 760 °C	Air cool	17-4PH, 6061, 7075, Inconel 718
Case hardening	Hard surface, tough core	850 – 950 °C + diffusion	Oil quench, then temper	Gears, shafts, low-carbon steels

Tempering curve · 4140 steel

Hardness drops as tempering temp rises.

Quenched 4140 hits ~57 HRC. That’s glass-brittle — useless without tempering. Each 100 °F of tempering trades roughly 1–5 HRC for toughness (about 3 HRC on average, steepest between 800–1000 °F). Pick the temper that gives the hardness AND toughness your application needs.

4140 hardness vs tempering temperature (1 hr soak, oil quench from 845 °C)

Source: SAE AMS 6382, Carpenter datasheets. Other tempering times (2 hr, double-temper) shift the curve slightly. For 4340 or other Cr-Mo steels, the shape is similar with hardness 1–3 HRC higher across the range.

Solution treatment + aging

How precipitation-hardening alloys reach their strength.

Precipitation-hardening alloys (17-4PH, 6061, 7075, Inconel 718) follow a two-step thermal sequence: solution treat at high temperature (everything dissolves into solid solution), then age at moderate temperature (controlled precipitates form, locking strength in). Different aging temperatures for the same alloy yield different strength/ductility balances.

Material	Solution temp	Quench	Common aging condition	Result
17-4PH stainless	1040 °C	Air or oil	H900 — 482 °C, 1 hr	UTS 1310 MPa, 44 HRC
17-4PH (tougher)	1040 °C	Air or oil	H1150 — 621 °C, 4 hr	UTS 930 MPa, 33 HRC
Aluminum 6061	529 °C	Water	T6 — 175 °C, 8 hr	UTS 310 MPa
Aluminum 7075	466 °C	Water	T6 — 121 °C, 24 hr	UTS 572 MPa
Aluminum 7075 (SCC-resistant)	466 °C	Water	T73 — 107 °C/177 °C	UTS 505 MPa, better SCC
Inconel 718	955 °C	Air	AMS 5663 — 720/620 °C	UTS 1380 MPa
Ti-6Al-4V (STA)	955 °C	Water	STA — 540 °C, 4 hr	UTS 1170 MPa

Process selection

Pick by what the part needs to do.

Need wear resistance

Through-hardened tool steel (A2, D2, S7) or surface-hardened low-carbon steel (carburized, nitrided). For corrosion + wear: hardened 17-4PH or 440C.

Hardness target — 50–62 HRC typical

Need impact toughness

Quenched and tempered alloy steel at moderate temper (4140 H, 4340). Avoid over-aging on PH alloys. Charpy V-notch is the right test.

Tempered above 540 °C / 1000 °F

Need fatigue resistance

Solution-treated and aged alloys. Avoid welds in fatigue zones. Surface-conditioning treatments (shot peen, ultrasonic peening) compound the gain.

Low residual surface stress; smooth finish

Need dimensional stability

Stress-relieve before final machining. For PH alloys, age before final tolerance grinding. For aluminum plate, T651 (stretch-relieved) is the right pick.

Stress relief 540–675 °C, slow cool

Need surface hardness with tough core

Carburize (low-carbon steel) or nitride (Cr-Mo alloy steel like 4140). Hardness 60+ HRC at the surface, core stays at the as-quenched-and-tempered hardness.

Case depth callout — typical 0.020–0.060″

Need maximum strength period

PH alloys at peak aging condition (17-4PH H900, 7075-T6, Inconel 718 STA). Note: peak strength = lowest ductility.

Lowest aging temp; full peak conditions

Watch out for

Heat treatment moves your part.

Every thermal cycle changes part dimensions. Quenching causes the most dramatic distortion (rapid, asymmetric thermal contraction). Aging is more predictable but still real. The general planning rules:

Rough machine before HT
Leave 0.005–0.020″ stock on critical surfaces. Finish-machine or grind after HT.
Symmetric geometry distorts less
Asymmetric clamping during quench amplifies distortion. Through-holes, flat plates, and thin-wall parts are the worst offenders.
Polymer quench beats water
For 4140-class steels, polymer quench gives ~80% the hardness of water with ~½ the distortion. Worth specifying.
Aging is your friend
PH alloys change size predictably during aging — ~0.0005″/inch for 17-4PH H900, ~0.0009–0.0012″/inch for H1150. (17-7PH TH1050 is the exception: the 1400 °F conditioning step grows parts ~0.0045″/inch, ~0.004″/inch net after aging.) Tighter tolerances can be held through aging vs. quenching.
Stress-relieve before tight tolerance ops
Especially for thin-wall machining of plate stock. Aluminum plate that hasn't been stress-relieved (T6 vs T651) will spring after machining.

Spec syntax

How to call out heat treatment.

Don’t leave the spec ambiguous. Pick the AMS, MIL, or ASTM spec that matches your alloy and condition, and call out specific hardness/strength targets. Examples:

17-4PH per AMS 5643, condition H1025

→ Aged at 1025 °F. UTS 1070 MPa, 35–40 HRC.

4140 per AMS 6382, 28–32 HRC after tempering

→ Quench and tempered to a hardness window.

6061-T651 per AMS 4027

→ Solution treated, stretch-relieved, artificially aged.

Stress relieve at 595 °C ± 14 °C, 1 hr, slow cool

→ Process callout when no canonical AMS spec applies.

Keep exploring

Related tools & references

17-4PH conditions→

H900 to H1150 — the most-specified heat-treat trade-off we machine.

Hardness conversion→

Convert between HRC, HRB, HV, and HB for your callout.

Case-depth calculator→

Estimate effective case depth for carburized and nitrided parts.

AMS spec cross-reference→

Find the AMS number for your alloy and condition.

Coatings & finishes→

The finishing step that usually follows heat treatment.

Need help speccing heat treatment?

Tell us about your part and we'll get back to you promptly. Aerospace, medical, defense, and semiconductor production work welcome.Email CAD models and drawings to sales@digitalmachine.com, including ITAR, EAR, CUI and AS9100 work. Keep controlled technical data out of web forms.

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