Martempering

An example bath allows 300 seconds from immersion and a representative load meets the qualified temperature-equalization criterion in 240 seconds, leaving 60 seconds nominal margin. A larger section taking 330 seconds exceeds the limit by 30 seconds. Use a common start event and validated part-temperature evidence. Account for bath recovery, uncertainty and transfer variation.

Martempering holds for temperature equalization before martensitic transformation. Austempering holds for isothermal transformation. Qualify the cooling sequence and subsequent tempering.

The practical question is whether the actual section can cool and equalize within the available metallurgical time window. A bath temperature and a nominal holding time alone do not answer that question.

Definition and distinction from austempering

ISO 4885:2026, edition 4, published July 2026, defines martempering/marquenching in clause 3.3.16 as cooling austenite to just above Ms, equalizing temperature, and then cooling further to form martensite. Its notes identify air as the usual final cooling medium and specify subsequent tempering. By contrast, clause 3.3.3 defines austempering around an isothermal transformation to lower bainite; it does not require subsequent tempering. These definitions describe different transformation objectives. ISO 4885:2026 catalog · Official vocabulary

Despite its name, martempering is not the final tempering operation. Likewise, a supplier's broader use of “interrupted” or “step” quenching should be resolved into an actual thermal sequence rather than assumed to mean the same standardized process.

Bodycote describes delaying cooling to equalize temperature as the basis for reducing distortion, cracking, and residual stresses. These are process objectives, not guarantees that a complicated part will stay within finished dimensional tolerances. Bodycote: martempering/marquenching

Alloy, Ms, section size, and time window

Ms identifies the start of martensitic transformation for the relevant austenite condition. It is not one fixed temperature for all steels. Select the austenitizing treatment and staging conditions from material-specific transformation data and the qualified process. A treatment that produces the required surface hardness can still leave an unacceptable core structure.

ISO's through-hardening terminology explicitly notes that core martensite content depends on hardenability, geometry, and hardening conditions. Similar surface hardness therefore does not establish equivalent through-section response. ISO 4885:2026, clauses 3.3.8, 3.3.17, and 3.3.27

Paulo's process description emphasizes holding only long enough to equalize core and surface temperatures, followed by cooling and tempering. Its application discussion favors highly hardenable steels and identifies bearings, gears, tooling, dies, and shafts. Its illustrative bath temperatures and hold times are commercial guidance rather than limits for every alloy or section. The page is undated. Paulo: martempering

The process must satisfy two competing requirements: the load needs enough time to equalize, while the cooling path and residence time must preserve the intended transformation route. Extending the hold arbitrarily can change the microstructure instead of simply improving uniformity. Assess the applicable time-temperature-transformation and continuous-cooling data with actual transfer, cooling, and load conditions. A constant-temperature transformation diagram is not a direct record of a nonuniform production part's cooling history.

Bounded producer example: Uddeholm Arne

Uddeholm's Arne, Edition 11, April 2019, gives different maximum bath times depending on the preceding austenitizing temperature and bath temperature. For 825 °C austenitizing, its martempering table lists:

These are maxima for the named producer steel and specified route, not interchangeable minimum soak times. The brochure separately requires tempering and warns that tool size and actual treatment parameters influence hardness. Uddeholm Arne, page 4

Its dimensional table uses a 100 × 100 × 25 mm sample plate and reports nonzero changes after martempering. It also says hardening and tempering dimensional changes must be combined. That evidence supports leaving qualified machining allowance; it does not justify a promise of zero growth or a universal percentage allowance. The PDF's printed edition is 04.2019, distinct from its 2024 upload path. Arne, pages 2 and 5

Original example: equalization margin

Use the hypothetical 300-second maximum residence and 240-second equalization time from the same immersion event.

The nominal timing margin is:

300 − 240 = 60 seconds

For a section requiring 330 seconds, the 300-second removal misses equalization and the 330-second removal violates the residence limit. Change and requalify the route, load or material.

Account for measurement uncertainty, load variation, bath recovery and transfer repeatability before approving a timing margin. Evaluate transformation with separate material evidence.

Tempering, distortion, and acceptance

Aalberts' hardening guidance notes that lattice transformations cause changes in component shape and size, and that subsequent tempering establishes the required hardness/toughness combination. Its description reinforces why final acceptance belongs after the specified complete route. The page is undated. Aalberts: hardening and tempering

Qualify dimensions at a defined reference temperature after thermal equilibration. Compare like-for-like datums and measurement setups; separate uniform size change from bow, twist, or out-of-roundness. A lower average distortion than an oil-quench comparison does not establish that every part meets the drawing. Where required, verify hardness at meaningful locations, core response, microstructure, cracks, and the mechanical properties relevant to service.

Review surface protection, salt removal where applicable, fixture contact, bath compatibility, and finishing stock with the processor. Martempering does not repair pre-existing cracks, replace the required austenitizing treatment, or automatically eliminate retained austenite and later dimensional drift.

A useful handoff contains four groups of information:

1. Material and geometry: producer grade, condition, prior work, lot, section transitions, and critical features.
2. Complete route: austenitizing, transfer, staging temperature, maximum residence, equalization criterion, final cooling, and tempering.
3. Load controls: bath/equipment capability, loading arrangement, temperature evidence, cleaning, and permitted process variation.
4. Acceptance: hardness and microstructure locations, mechanical requirements, dimensional limits, crack inspection, traceability, and deviation disposition.

Related reference

Grades, tempers, and conditions