Tempering
Tempering hardened steel: microstructural changes, strength and toughness, retained austenite, secondary hardening, embrittlement, and qualified treatment sequences.
Tags: processes, heat-treatment, tempering
On this page
An example qualified instruction calls for three separate two-hour tempers. At 45 minutes heat-through and 60 minutes cooling per cycle, the model totals 11.25 h, including 6 h of qualifying holds. Preserve the specified intermediate cooling and reheating. Use actual load evidence for the start of each hold.
Tempering develops properties after hardening. Qualify alloy, austenitizing, quench, cooling before temper and repeat cycles. Verify toughness and stability separately from hardness.
This page concerns metallurgical tempering of steel. It does not describe tempered glass or use “temper” as a generic synonym for every aluminum material condition.
Definition and neighboring processes
ISO 4885:2026, edition 4, July 2026, defines tempering in clause 3.4.8 as heating below A1, soaking one or more times, and cooling appropriately, generally after quench hardening to obtain required properties. Its definition of a tempering curve explicitly includes the specified tempering times: a graph is not a temperature-only material constant. ISO 4885:2026 · Official vocabulary
| Process | Primary distinction |
|---|---|
| Quench hardening | Establishes the hardened starting structure through the specified cooling route |
| Tempering | Modifies the hardened structure through subsequent controlled heating and cooling |
| Martempering/marquenching | A staged hardening route with temperature equalization; it is followed by tempering |
| Precipitation aging | Develops strengthening precipitates from the appropriate starting state; it is not interchangeable with a martensitic tempering instruction |
| Stress relief | Targets residual stresses; its compatibility with an existing hardened condition must be established |
TWI's technical overview distinguishes these treatments and explains that tempering reduces hardness and strength while improving ductility/toughness in its conventional ferritic-steel context. That general trend is not a universal monotonic rule for highly alloyed tool steels. TWI heat-treatment methods
What changes during tempering
Uddeholm's Heat Treatment of Uddeholm Tool Steels, Edition 9, October 2017, describes the as-quenched structure as martensite, retained austenite, and carbides, with inherent stresses. It explains that high-temperature tempering can affect retained austenite, and cooling after a temper can produce new martensite requiring another temper. Its discussion of secondary hardening attributes a hardness increase to newly precipitated carbides and transformation effects in applicable high-alloy/tool steels. Equal hardness can therefore arise from different microstructures and routes. Uddeholm guide, page 5
For process selection, evaluate the property that controls service: wear, chipping, impact, fatigue, hot strength, corrosion resistance, or dimensional drift. Do not choose a hotter or colder temper solely because a hardness chart crosses the desired number. Identify the chart's austenitizing temperature, quenching conditions, specimen dimensions, hold time, and number of tempers before comparing results.
Embrittlement and cooling matter
ISO 4885 distinguishes irreversible and reversible temper embrittlement in certain hardened steels. The definitions identify different temperature/exposure behavior; reversible embrittlement can also develop during slow cooling through a susceptible range. These are alloy-dependent phenomena, not proof that every steel must avoid one universal temperature band. Use the grade-specific qualified route and required toughness evidence. ISO 4885:2026, clauses 3.9.16–3.9.16.2
A lower hardness does not by itself demonstrate that embrittlement was avoided. If the application requires impact or fracture performance, establish those requirements and their test conditions separately. Likewise, a temper does not repair a quench crack. Suspect cracking, unexpected hardness, and out-of-tolerance distortion require investigation against the complete history before approving rework.
Named producer examples
BÖHLER K110: current sheet and multiple tempers
The live K110 sheet prints K110 EN_GB – 09.2026 on its final page. Its tempering guidance calls for slow heating immediately after hardening, one hour per 20 mm thickness with a two-hour minimum, and recommends at least three tempers above the secondary-hardness maximum, cooling in air to room temperature after each step. Its chart identifies 20 mm square specimens. The sheet also states that the presented data refer exclusively to long products; other product variants may differ.
These statements support a producer-specific route, not a blanket time-per-thickness rule for every steel. Select the actual tempering temperature and process controls from the applicable K110 requirements and qualified load practice. BÖHLER K110, pages 1, 3, and 6
Uddeholm Arne: different grade, different instruction
Arne's Edition 11, April 2019 brochure specifies two tempers, intermediate cooling to room temperature, and at least two hours at temperature. Its tempering curves were generated using 15 × 15 × 40 mm oil-quenched specimens, and it cautions that actual tools can achieve lower hardness because of size and treatment conditions. Do not transfer a K110 chart or repetition count to Arne because both are tool steels. Uddeholm Arne, pages 2 and 4
Timing, load temperature, and an original calculation
Uddeholm's general tool-steel guide distinguishes heating through from the subsequent hold, and emphasizes prompt tempering after quenching. Its guidance remains tool-steel-specific; the permissible delay, start temperature, and any preliminary temper must follow the selected route rather than an improvised universal rule. Uddeholm guide, page 10
Use the hypothetical three-cycle instruction, including the heat-through and cooling for the final temper:
| Time category | Calculation | Total |
|---|---|---|
| Qualifying holds | 3 × 2 h | 6 h |
| Heating through | 3 × 0.75 h | 2.25 h |
| Cooling | 3 × 1 h | 3 h |
| Complete modeled schedule | 6 + 2.25 + 3 h | 11.25 h |
Include intermediate cooling and reheating in the route. This model excludes transfers, furnace recovery, queue time and uncertainty.
Acceptance and downstream exposure
Plan inspection after the complete specified route and temperature equilibration. Record hardness method and locations, surface preparation, sampling, and limits. Add microstructure, retained-austenite measurement, tensile or toughness tests, dimensional checks, and crack inspection where the application and governing documents require them. Avoid converting a producer's approximate chart into a contractual minimum without agreement.
Review later welding, nitriding, coating, grinding, and service exposure against the established heat-treated condition. A later thermal operation can change that condition. Declare relevant prior tempering temperatures and approved rework limits to the downstream processor; “already hardened” is insufficient information.
A useful order contains four groups of information:
- Input identity: alloy/producer, product form, lot, dimensions, austenitizing/quench history, and any cold treatment or previous temper.
- Complete sequence: temperature/time tolerances, hold-time basis, maximum delays, heating/cooling requirements, and repetition count.
- Load controls: arrangement, temperature evidence, atmosphere/surface requirements, and permitted process variation.
- Acceptance: required properties and test conditions, dimensions, traceability, downstream exposure limits, and deviation disposition.