Heat treatment selection guide
Through-harden, case harden, anneal or age: how to pick the family, the treatment within it, and the drawing call-out a heat treater can inspect to.
Through-harden, case harden, anneal or age
Choose the required behavior first: bulk strength, a wear-resistant surface, easier forming or improved dimensional stability. Then check whether the exact alloy and starting condition can deliver it.
Bulk properties
Harden and temper a suitable steel
Carbon content limits achievable hardness; alloy hardenability, section and cooling determine how deeply the response develops. Select the hardness/toughness balance for the load instead of maximizing the surface reading.
Surface + core
Choose the hardening mechanism
Carburizing enriches the surface before hardening. Nitriding forms a nitrogen-enriched layer without a subsequent quench-hardening step. Induction hardening transforms a selected region of suitable steel without adding carbon. Their depth ranges and core effects are not interchangeable.
Workability / stability
Separate annealing from stress relief
Annealing changes the material condition for a defined purpose. Stress relief reduces residual stress through a compatible thermal cycle. Neither name guarantees unchanged dimensions or properties; check the material and its prior processing.
Alloy-specific aging
Specify the actual condition
Precipitation-hardening alloys include selected aluminum, PH stainless, titanium, nickel and copper alloys. Each has its own treatment and condition system. T6, T651 and T73 are aluminum temper designations, not instructions for titanium, nickel or beryllium copper.
Start with the material already available in the required condition when that meets the design. Adding a thermal cycle to a finished component creates a new dimensional and property-control task.
The eight families in one table
Use the catalog to narrow the process family, then open the specific treatment and grade notes. A family-wide temperature or hardness envelope is a comparison aid, not a cycle prescription for a part.
| Family | What it does | Temperature | Hardness |
|---|---|---|---|
| Annealing & softening5 treatments | Full, process, spheroidize and solution anneals, plus the O temper on aluminum — the treatments that reset a work-hardened or as-rolled part to its softest, most machinable and most formable state. | 300–1,175 °C | No target |
| Stress relief1 treatment | A sub-critical hold that relaxes residual stress from welding, machining and forming without changing hardness — the cheapest insurance against distortion on the next operation. | 480–680 °C | No target |
| Through-hardening8 treatments | Normalize, quench and temper, austemper, martemper and the 17-4 PH conditions — transformations that set the strength and toughness of the whole section. | 477–955 °C | 25–62 HRC |
| Tempering3 treatments | The second half of every quench: a controlled reheat that trades a few points of hardness for the toughness a hardened part needs to survive service. | 150–650 °C | 22–60 HRC |
| Case & surface hardening7 treatments | Carburize, carbonitride, nitride, nitrocarburize, flame and induction — a hard, wear-resistant skin over a tough core, measured as case depth to a hardness criterion. | 400–955 °C | 45–67 HRC |
| Solution treat & age6 treatments | Precipitation hardening for aluminum (T6, T651, T73), titanium, Inconel 718 and beryllium copper — strength from a fine dispersion of second-phase particles, not from martensite. | 105–970 °C | 25–44 HRC |
| Cryogenic treatment1 treatment | A deep-cold soak after quenching that converts retained austenite and stabilizes dimensions on tool steels, bearings and gauges. | −196 to −80 °C | No target |
| HIP & densification1 treatment | Hot isostatic pressing closes internal porosity in castings and metal 3D prints — fatigue life and pressure tightness that no surface treatment can add. | 510–1,220 °C | No target |
Temperature and hardness are the window across every treatment in the family as published in the catalog; "No target" means no HRC range is represented in this family summary, not that hardness cannot change. Open a family for the treatments compared on cycle, case depth, distortion risk and cost tier.
Pick by what the part needs
Four scores, each 0–100 against every treatment in the catalog. Hardness is derived from the HRC target (20 HRC = 0, 70 HRC = 100); toughness, stability and cost are FabDigit’s engineering assessment. The bar is the score; the figure beside it is the catalog value behind it.
| Treatment | Score | Figure |
|---|---|---|
| Plasma (Ion) Nitride | 85/100 | 58–67 HRC |
| Gas Nitride | 85/100 | 58–67 HRC |
| Flame Hardening | 68/100 | 45–65 HRC |
| Nitrocarburize (Ferritic) | 68/100 | 45–65 HRC |
| Case Harden (Carburize) | 82/100 | 58–64 HRC |
Peak hardness achieved, from the HRC target (surface for case treatments)
| Treatment | Score | Figure |
|---|---|---|
| Full Anneal | 95/100 | — |
| Anneal to O Temper (Aluminum) | 95/100 | — |
| Process Anneal | 90/100 | — |
| Solution Anneal | 90/100 | — |
| Spheroidize Anneal | 85/100 | — |
Impact and ductility retained after the treatment
| Treatment | Score | Figure |
|---|---|---|
| Stress Relief | 95/100 | 480–680 °C (896–1,256 °F) |
| Cryogenic Treatment | 90/100 | −196 to −80 °C (−321 to −112 °F) |
| Plasma (Ion) Nitride | 88/100 | 400–580 °C (752–1,076 °F) |
| Q+T H1150 | 88/100 | 616–627 °C (1,141–1,161 °F) |
| Nitrocarburize (Ferritic) | 88/100 | 560–590 °C (1,040–1,094 °F) |
Dimensional stability — what to pick on a finish-machined part
| Treatment | Score | Figure |
|---|---|---|
| Normalize | 90/100 | $ |
| Stress Relief | 85/100 | $ |
| Process Anneal | 80/100 | $ |
| Anneal to O Temper (Aluminum) | 75/100 | $$ |
| Low Temper | 75/100 | $ |
Relative process cost per part; shorter cycles and no quench are cheaper
Rankings are computed from the live catalog each hour. Scores compare catalog entries; they do not predict a part’s service life, measured toughness or achievable tolerance. Open a treatment for its cycle, case depth and call-out, or compare every treatment in the explorer.
Check the treatment against the alloy
Choose by grade and condition, not just “steel” or “aluminum.” A low-carbon carburizing steel, a quenched-and-tempered alloy steel and a precipitation-hardening stainless need different decisions.
| Alloy family | Rated "Yes" | "With care" | Hardest treatments rated "Yes" |
|---|---|---|---|
| Steel | 20 | 0 | Gas nitride, Carburize, Flame harden |
| Stainless Steel | 7 | 8 | H900, H1025, H1075 |
| Aluminum | 5 | 1 | Al T6, Al T651, HIP |
| Titanium | 3 | 0 | Ti STA, Stress relief, HIP |
| Nickel Alloy | 4 | 0 | 718 age, Solution anneal, Stress relief |
| Copper Alloy | 1 | 2 | C17200 age |
| Brass | 1 | 1 | Stress relief |
| Bronze | 0 | 1 | — |
| Magnesium Alloy | 0 | 2 | — |
| DMLS / SLM | 7 | 21 | Solution anneal, Stress relief, HIP |
Counts are the catalog’s verdicts per alloy family; the last column lists the treatments with the highest hardness target among them. A family-level rating does not approve every grade, condition or geometry. The full treatment × alloy matrix — every verdict, with the grade notes — lives in the library explorer.
Separate surface hardness, depth and core properties
A depth needs a criterion
A hardness profile can cross a specified acceptance threshold before it reaches the unaffected core. Effective depth, total depth and compound-layer thickness describe different boundaries.
Finish removal changes what remains
Grinding removes part of a hardened layer. State whether the required depth applies after final machining, and verify the remaining case and surface condition at the required location.
Illustrative removal check: suppose the required remaining effective depth is 0.50 mm after grinding, and the plan removes 0.15 mm from the treated surface. A 0.50 mm case before grinding cannot meet that remaining-depth requirement merely by keeping the same surface-hardness reading. In a simple geometric removal model, 0.50 − 0.15 = 0.35 mm remains. Plan the original depth and removal allowance together, then verify the final condition under the chosen method. This example does not specify a treatment recipe.
| Question | What to specify | What it does not establish |
|---|---|---|
| How hard is the surface? | Suitable hardness scale/load, preparation and test location. | A reading does not by itself establish the depth of the hardened layer. |
| How deep is the required response? | Applicable case-depth method and criterion, with a permitted range. | Effective depth, total affected depth and compound-layer thickness are different quantities. |
| What supports the surface? | Required core condition, hardness or mechanical properties at a defined location. | A hard case cannot compensate automatically for an unsuitable core. |
| Which area is treated? | Hardened pattern, protected areas and permissible transition. | A process name does not locate a selective induction-hardened region or mask a thread. |
ISO 18203:2026 distinguishes case hardening depth, surface hardening depth, nitriding hardness depth and total thickness. Use the edition and criterion required by the contract; do not translate all of them into one generic “case depth.”
Plan heat treatment with machining and finishing
Reserve the final surfaces
For a route requiring finish machining after hardening, retain material on the specified faces and plan how the hardened part will be located. Allowance follows the part and process, not a universal value.
Inspect the delivered condition
Heat-treatment verification and final dimensional inspection answer different questions. A hardness result alone does not prove final size, case depth, straightness or the condition of a ground surface.
| Route | A sequence to evaluate | Decision that must stay explicit |
|---|---|---|
| Quenched-and-tempered steel | Rough machine → required thermal cycle → final machining/grinding → inspect. | Retained stock, workholding, scale/decarburization control and final hardened geometry. |
| Nitrided component | Establish the core condition → stabilize as needed → machine → nitride → allowed finish operations. | Dimensional growth, protected areas and how much layer any polishing may remove. |
| Age-hardened alloy | Confirm starting condition → machine at the chosen stage → age → finish/inspect as required. | Material-specific size change and whether machining before or after aging is preferable. |
| Stress relief between operations | Rough machine → compatible stress relief → measure → finish machine. | The prior temper/age condition and enough material to correct any released movement. |
| Coated or plated part | Coordinate thermal processing, machining and coating as one released route. | Coating cure or required post-plating bake may interact with material properties; do not reorder a specified process casually. |
Agree allowances for the actual geometry and route. A fixed 0.2–0.5 mm grind allowance is not a general rule. See CNC machining design for accessible final datums and finish selection for the delivered surface.
Low distortion does not mean zero dimensional change
Separate predictable material size change from shape distortion. Both can affect a fit, and neither has one reliable limit across all alloys and geometries.
Geometry
Locate the vulnerable features
Thin sections, large changes in section, long shafts, asymmetric stock removal and interrupted shapes deserve a specific plan for support and final correction.
History
Carry the starting condition forward
Retained stock stresses, cold work, welding and earlier treatments affect movement. A nominal alloy name alone does not describe that history.
Evidence
Measure before committing a batch
For a critical repeat part, agree trial geometry, the measurement state and acceptance. Record the actual movement before setting a repeatable final-machining allowance.
A source-specific example: Carpenter reports a linear contraction of 0.0004–0.0006 mm/mm when its 15Cr-5Ni Project 70+ alloy is aged from the solution-annealed condition to H900. Over an illustrative 100 mm length, that corresponds to 0.04–0.06 mm contraction. This is one named material and condition, not a prediction for all PH stainless, aluminum or finished parts.
Stress-relief temperature must be compatible with the prior tempering or aging treatment. Nitriding avoids a subsequent quench-hardening step, but stress relaxation and layer formation can still change size or shape.
Write the treatment and acceptance plan together
The drawing needs enough information to connect material, treatment, result and final inspection. A copied call-out is a starting point only when its scope matches the part.
For example, AMS 2759/1 has a defined carbon/low-alloy-steel scope and strength range; it is not the default for every hardening job. AMS 2770, AMS 2801 and AMS 2774 address other material families. Select the applicable process document before filling in hardness or condition.
| Element | Specify | Resolve before ordering |
|---|---|---|
| Governing document | Applicable specification, edition, class and any purchaser exceptions. | A steel process specification may have strength, section-size and material limits. Confirm its scope. |
| Alloy and starting condition | Grade, product form, prior temper and material traceability. | Mill condition, welding, cold work and earlier heat cycles affect the next operation. |
| Required result | Condition designation and/or property range appropriate to the material. | Hardness alone may not establish tensile strength, toughness, corrosion resistance or fatigue performance. |
| Test and location | Method, scale/load, test surface, sampling and reporting. | Decide whether a production part, representative coupon or sectioned specimen is needed. |
| Case or hardened pattern | Depth type, criterion, range, location and condition after final removal. | Include protected zones, transition regions and core requirements. |
| Delivered geometry and records | Final dimensions, subsequent operations and required process/test records. | Identify who approves the treatment route and any proposed straightening or rework. |
A useful drawing-note structure is: MATERIAL/STARTING CONDITION → PROCESS SPECIFICATION AND CLASS → REQUIRED CONDITION/PROPERTY RANGE → TEST METHOD AND LOCATION → FINAL MACHINING/PROTECTED ZONES → REQUIRED RECORDS. Use the specification index (41 documents) to find the document, then verify its scope and revision.
Request evidence for the requirement that matters
| Requirement | Potential evidence | Define in the order |
|---|---|---|
| Material condition | Material identity, starting condition and traceable process record. | Lot/part identification and the documents to be supplied. |
| Hardness | Appropriate test on a suitable surface or approved specimen. | Scale, force, preparation, position, sampling and permitted marks. |
| Case or selective pattern | Hardness traverse and/or metallographic examination as required. | Depth criterion, section location and representative specimen agreement. |
| Tensile or toughness requirement | Specified mechanical test under the governing material/process standard. | Test specimen, orientation, sampling and acceptance; hardness is not an automatic substitute. |
| Final fit | Dimensional inspection after the required finishing operations. | Datums, straightness/flatness, size, temperature and free or restrained state. |
| Surface integrity | Required examination for cracks, grinding damage or surface-condition limits. | Applicable method, coverage and acceptance criteria. |
Specify any accreditation, approved-processor list, furnace-control record or special test before quotation. Availability and included documentation are confirmed in the agreed quote; the guide does not promise a certification with every treatment.
Heat treatment selection — FAQ
What is the difference between hardening, tempering and annealing?
For a suitable steel, quench hardening develops a hard structure and tempering adjusts the resulting strength, hardness and toughness. Annealing is a family of treatments used to change structure or condition, often for forming or machining; it is not one universal above-transformation slow-cooling cycle for all metals.
Case hardening or through-hardening — which does my part need?
Start with the load and wear requirement. Through-hardening requires sufficient hardenability for the actual section and quench; a hard surface does not prove a hard core. Surface hardening can provide a wear-resistant region with different core properties. Carburizing, nitriding and induction hardening use different mechanisms, alloys and depth criteria.
Will 8620 through-harden?
8620 is commonly selected for carburizing because its low-carbon core and enriched surface can serve different functions. Saying it cannot harden anywhere through a section is too absolute: quench response depends on carbon, alloy hardenability, section and cooling. Specify the required core properties as well as the case; do not expect the untreated core to match the carburized surface hardness.
How much will heat treatment distort a part?
There is no universal dimensional-change limit. Geometry, residual stress, material condition, fixturing, transformation and cooling all matter. Lower-distortion routes such as nitriding can still change dimensions. Allow for the selected process, measure trial parts when needed, and identify final machining and inspection requirements.
Does stress relieving leave hardness unchanged?
A compatible cycle aims to reduce residual stress while preserving the required properties. Temperature and time must be checked against the prior tempering or aging condition. An unsuitable stress-relief cycle can overtemper or otherwise alter the material; dimensional movement during stress release is possible.
Does T651 mean a finished aluminum part can simply be re-aged?
No. The stress-relief suffix refers to controlled processing of the product, including stretching where specified. Reheating a machined T651 component does not reproduce that history. Identify the alloy, product form, starting condition and final requirements before any additional heat treatment.
What does effective case depth mean?
It is a depth evaluated against a specified criterion, usually a defined hardness threshold under the governing method. It is not automatically the full diffusion depth or the thickness of a nitrided compound layer. State the method, criterion, location and whether acceptance applies after final grinding.
Are certification and special tests included automatically?
List required process specifications, test results, traceability and any processor approvals in the request for quotation. The agreed quote and order define the required records and scope; selecting a treatment name alone does not specify every test or certification.
Sources and further reading
- Bodycote — hardening and temperingDistinct routes and the hardness/toughness purpose of tempering.
- Bodycote — stress relievingSequence after rough machining and compatibility with previous tempering.
- Bodycote — gas nitridingMaterial response and a lower-distortion process without a subsequent hardening operation.
- Ovako — steel heat-treatment guideHardenability and core response depend on composition, section and quench.
- Carpenter Technology — 15Cr-5Ni Project 70+ data sheetH900 contraction example is specific to this alloy and starting condition.
- ISO 18203:2026 — thickness of surface-hardened layersPublic scope distinguishes several hardened-depth measurements.
- SAE — AMS2759/1 carbon and low-alloy steel scopePublic scope example; select the contractually required revision, not this historical edition by default.
- Hydro — 6082 alloy and temper definitionsAluminum T6 and stress-relieved stretching tempers are product-processing designations.
Pick the treatment, then price it with the part.
Upload the part with its material condition, treatment requirement, critical dimensions and required records so the quotation covers the delivered result.