Quench hardening
Selecting and qualifying steel austenitizing, quenching and tempering with section-size, microstructure and final-property limits.
Tags: processes, heat-treatment, quenching, hardening, tempering
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Uddeholm's Dievar, Edition 14, March 2022, gives 679 HV10 after an 800–500°C interval of 15 seconds and 515 HV10 after 5,200 seconds, from its CCT diagram at 1025°C for 30 minutes austenitizing. Match the full material and cooling history, and verify the specified tempered properties separately. Dievar sheet, printed pages 5–6
Neutral hardening austenitizes, quenches and tempers suitable steel without intentional surface enrichment. Carburizing and carbonitriding add a composition profile with separate case and core requirements. Bodycote neutral-hardening guidance
This page addresses hardenable steels. Quenching an aluminium alloy from a solution treatment, for example, is not the same metallurgical process. Nor does “quenched” alone establish an accepted material condition: alloy, section, prior processing, cooling and subsequent tempering must be identified.
Hardenability is not maximum hardness
Hardness is a measured property under a specified test. Hardenability concerns how readily the material develops hardening through a section under particular cooling conditions. Bodycote distinguishes the maximum hardness associated with steel carbon content from the influence of alloying on hardenability. A high-hardenability steel may form the intended structure with slower cooling; that does not automatically mean its final hardness is greater than that of every lower-hardenability steel. Bodycote technical glossary
Surface and core follow different thermal histories. A surface reading cannot, by itself, demonstrate through-hardening of a large section. Review the largest relevant section, abrupt transitions, the material's transformation behavior and the actual quench installation before promising core properties.
Ovako's undated Heat Treatment Guide uses chemistry and cooling behavior to estimate CCT/Jominy behavior, bar-core hardness and tempering response. It expressly labels calculated results as guidance without guaranteed accuracy. Use such tools for screening and process discussion, then validate the production geometry and condition. A modeled bar is not a tested component. Ovako Heat Treatment Guide
Austenitize, transfer and cool
The austenitizing stage must establish the required temperature and matrix condition throughout the relevant material. Furnace setpoint and elapsed loading time are not interchangeable with time at temperature in a component's core. Bodycote's process description separates heating, equalization/transformation and quenching, and relates quenchant selection to steel and part dimensions. Bodycote neutral hardening
Specify or qualify the load arrangement, temperature assessment, transfer route and cooling system together. Review surface protection, decarburization and oxidation where surface properties matter. A successful small coupon does not establish the thermal history of a heavy part shielded within a dense load.
Uddeholm's Heat Treatment of Tool Steels, Edition 10, April 2018, describes the competing requirements: rapid cooling supports the desired microstructure and performance, while slower, more uniform cooling can reduce distortion. Cooling too slowly in heavy sections can permit unwanted transformations. The guide also relates martensitic volume change and unequal cooling to stresses and cracking. These are reasons to qualify a cooling path, not instructions to maximize or minimize quench severity indiscriminately. Uddeholm guide, printed pages 7 and 11
Quench medium and installation
| Medium/system | What to evaluate |
|---|---|
| Water-based quench | Suitability for required cooling, thermal gradients, agitation and crack/distortion sensitivity |
| Oil | Product cooling characteristics, working condition, agitation, heat load and subsequent cleaning |
| Aqueous polymer | Specific formulation, concentration, temperature, agitation and monitoring method |
| Air or pressurized gas | Alloy hardenability, section, gas flow and furnace/load cooling capability |
| Interrupted or staged cooling | Validated transformation path and equalization, rather than merely pausing a direct-quench cycle |
The A3TS process sheet on quenching-fluid selection treats the fluid and installation as a combined problem. It identifies steel, geometry, surface/core targets, agitation, bath temperature and polymer concentration as relevant variables, with subsequent washing and operations also part of selection. Accordingly, “polymer quench” does not define one cooling curve, and a pressure rating alone does not establish gas-quench capability for every load. The sheet has no verified year used here. A3TS, Selection of Quenching Fluids
Follow the selected equipment and quenchant supplier's current instructions and SDS for handling, monitoring and maintenance. Establish operating limits and response to contamination or drift through the approved process, rather than substituting a generic fluid temperature, concentration or agitation recipe.
Tempering and retained austenite
Final acceptance should refer to the specified tempered condition. Tempering is not simply a discretionary reduction in hardness: it is part of developing the required combination of structure, hardness and toughness. Ovako notes that low/medium-alloy steels generally soften on tempering, while some tool-steel or other alloy systems have different responses. A universal rule that every higher tempering temperature gives the same property trend is inappropriate. Ovako guide
Linde's Neutral Hardening and Annealing brochure, ©2017, explains that martensite-start and finish temperatures depend on composition. In high-carbon or high-alloy cases, transformation may remain incomplete at room temperature, leaving retained austenite. Do not assume cooling to room temperature establishes a completely martensitic structure. If retained austenite, dimensional stability or subzero treatment matters, specify and qualify those requirements for the alloy and sequence rather than assuming every hardened part needs the same treatment. Linde brochure, printed page 11
Bounded producer example
Uddeholm Dievar, Edition 14, March 2022, provides a CCT example austenitized at 1025°C for 30 minutes. Its listed cooling curves include 679 HV10 with an 800–500°C cooling time of 15 seconds, and 515 HV10 with a time of 5200 seconds. These are results associated with the stated diagram conditions, not final tempered acceptance values or a universal hardness-versus-time equation. The sheet separately specifies tempering and cautions about distortion/cracking during rapid cooling. Dievar sheet, printed pages 5–6
The example illustrates why cooling history belongs with the material data. Its two hardness values should not be interpolated into an arbitrary production recipe, transferred to another steel or treated as evidence that the surface and core of a large die cooled identically.
Original cooling-interval calculation
Assume two thermocouples on a trial component record the following illustrative histories:
- Surface: 800°C at 10 seconds, 500°C at 30 seconds; interval 20 seconds.
- Core: 800°C at 25 seconds, 500°C at 85 seconds; interval 60 seconds.
The average cooling rates over that interval are (800 − 500)/20 = 15°C/s and (800 − 500)/60 = 5°C/s. The core's interval is three times longer. This calculation describes measured-time differences only; it does not predict hardness or prove a constant cooling rate. Transformation assessment needs the full relevant thermal path and suitable material data. These invented histories are not the Dievar cooling curves above.
Final inspection and handoff
Specify alloy/product condition, final hardness range and method, test locations, any core or microstructure requirement, permitted retained austenite where relevant, crack inspection and dimensional limits. Define the final inspection stage after tempering and applicable finishing. Include critical bores, straightness, roundness and transitions rather than checking only an accessible flat face.
Agree representative trials, sacrificial sections or coupons and their relationship to the load. Record austenitizing, transfer, cooling and tempering controls separately from the final measured properties. Subsequent grinding, welding or heating requires review against the accepted condition. A hardness result at one location is useful evidence, but it cannot replace every other functional requirement.