Austemper
Through-hardening · Through-hardening
Austempering forms bainite by quenching austenitized steel or ductile iron into a hot bath and holding isothermally.
Austemper at a glance
Four scores, each 0–100 against every heat treatment in this catalog. Hardness is derived from the HRC target (20 HRC = 0, 70 HRC = 100) — treatments without a hardness target (anneals, stress relief, HIP, cryo) are not rated. Toughness, dimensional stability and cost are FabDigit's engineering assessment.
What is Austemper?
Austempering forms bainite by quenching austenitized steel or ductile iron into a hot bath and holding isothermally. It can provide high toughness and wear resistance with less distortion than a conventional cold quench.
How it works
The part is austenitized and transferred rapidly to a salt bath above the martensite-start temperature but below the pearlite range. The bath removes heat quickly enough to avoid pearlite, then holds the section nearly isothermal while austenite transforms to upper or lower bainite. Because transformation occurs at one controlled temperature and no martensitic quench through the full section is required, thermal gradients and cracking tendency are reduced. The process only works when the section can cool past the pearlite nose before transformation begins; hardenability and section thickness therefore set strict size limits.
Why specify it
- Produces approximately 25–55 HRC bainitic structures
- Reduces quench distortion relative to conventional oil or water Q&T
- Combines wear resistance with useful impact toughness
Where it falls short
- Section size is limited by the need to bypass the pearlite nose
- Molten-salt handling and washing add process cost
- Not every steel has a practical austemper transformation window
The Austemper cycle
Typical furnace parameters from the governing specifications and shop practice; the heat treater sets the exact recipe from the alloy, section size and required result.
| Temperature | 790–900 °C (1,454–1,652 °F) — Austenitizing range; subsequent isothermal bath is commonly 230–400 °C. |
|---|---|
| Soak | 0.5–2 h — Isothermal hold commonly 0.5–4 h according to transformation data. |
| Cooling | Salt bath |
| Atmosphere | Salt bath |
| Door to door | 4–10 h — Transformation hold depends on grade and bath temperature. |
Stages
- 1Austenitize at the grade-specific temperature
- 2Quench rapidly into a 230–400 °C salt bath
- 3Hold to complete bainite transformation, then air cool
What Austemper does to the part
| Hardness | 25–55 HRC (250–550 HB) — Lower bainite is generally harder than upper bainite; grade controls. |
|---|---|
| Section affected | The whole section transforms. |
| Tensile strength | 850–1,600 MPa — Broad bainitic steel and ADI range; material specification governs. |
| Toughness | Bainite commonly provides better impact resistance than untempered martensite at comparable hardness. |
| Distortion | Low risk. Isothermal transformation reduces thermal and transformation gradients, but transfer and fixture design remain critical. |
| Surface after treatment | Salt residue must be washed completely; protective austenitizing atmosphere controls scale and decarburization. |
| Machining sequence | Rough machine before treatment; finish grind only critical features because bainitic material is difficult to cut. |
Specifications for Austemper
The documents a drawing cites to define this treatment. Each links to a page explaining the standard, its classes and the tests it invokes.
| Standard | Role | Classes used | Status |
|---|---|---|---|
Heat Treatment of Steel Parts, General Requirements | Governing | — | Active |
Test and related methods: ASTM E10 (Standard Test Method for Brinell Hardness of Metallic Materials), ASTM E18 (Standard Test Methods for Rockwell Hardness of Metallic Materials), ASTM A1033 (Standard Practice for Quantitative Measurement and Reporting of Hypoeutectoid Carbon and Low-Alloy Steel Phase Transformations), ISO 683-1 (Heat-treatable steels, alloy steels and free-cutting steels — Part 1: Non-alloy steels for quenching and tempering), ISO 683-2 (Heat-treatable steels, alloy steels and free-cutting steels — Part 2: Alloy steels for quenching and tempering).
Drawing call-out
AUSTEMPER AISI 4140 TO 40–45 HRC; AUSTENITIZE AND ISOTHERMAL HOLD PER APPROVED AMS 2759 PROCEDURE; TEST ASTM E18 AT LOCATION SHOWNWhich alloys take Austemper?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library. Typical grades: 1095, 4140, 5160, 52100, ADI Grade 1, ADI Grade 3.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | The cycle is established for carbon, alloy, and tool steels within grade-specific temperature limits.
| |
| With care | Sintered steel can be treated after density and residual porosity are qualified.
| |
| With care | Printed steel may use the cycle after density, chemistry, and build-orientation effects are qualified.
| |
| Limited | Only selected hardenable stainless grades have a qualified austemper route. | |
| No | Aluminum does not use this ferrous transformation cycle. | |
| No | Brass requires lower-temperature stress relief or recrystallization annealing. | |
| No | Bronze requires composition-specific annealing or precipitation treatment. | |
| No | Cobalt-chrome requires alloy-specific solution or aging practice. | |
| No | Copper alloys require alloy-specific annealing or precipitation-hardening cycles. | |
| No | Magnesium requires tightly controlled alloy-specific cycles. | |
| No | Nickel alloys require alloy-specific solution, anneal, or age cycles. | |
| No | Precious metals require alloy-specific annealing or age-hardening practice. | |
| No | Refractory metals require material-specific vacuum annealing practice. | |
| No | Titanium requires alloy-specific annealing or solution-and-age cycles. | |
| No | Tungsten heavy alloys require supplier-specific sintering and stress-relief practice. | |
| No | Zinc alloys melt or lose properties at temperatures used by this process. |
Design rules for Austemper
What to decide on the drawing before the part goes to the heat treater.
- Identify the alloy designation and incoming condition on the purchase order.A temperature that is correct for one grade can overtemper, sensitize, or fail to transform another grade.
- Place hardness test locations at least 3 mm from an edge where geometry permits.Edge decarburization, local cooling, and indentation spacing can bias the reported result.
- Leave 0.2–0.5 mm finish stock on critical ground surfaces when a quench or scale-producing atmosphere is used.Final grinding removes oxide, decarburization, and dimensional movement after the thermal cycle.
- Allow 4–8 working days beyond machining.Furnace scheduling, controlled cooling, hardness testing, and certification extend beyond the soak time.
- Qualify the maximum section thickness before release.The center must cool through the pearlite nose before transformation, or mixed pearlite-bainite properties result.
Austemper versus the alternatives
Treatments engineers weigh against Austemper, and when each wins.
| Treatment | Hardness | Toughness | Stability | Cost | Choose it when |
|---|---|---|---|---|---|
Austemper this page | 34 | 75 | 75 | $$$ | — |
| 38 | 55 | 45 | $$ | when section thickness exceeds the qualified austemper limit | |
| 53 | 55 | 70 | $$$ | when tempered martensite rather than bainite is required |
Cost, lead time and applications
Relative cost
Austemper is a premium-cost treatment ($$$) relative to the other heat treatments in this catalog.
Heat treatment is priced per lot with a furnace minimum. The exact figure appears on the instant quote once the part is uploaded.
Lead time
| Added lead time | 4–8 working days |
|---|---|
| Furnace time | 4–10 h |
What Austemper is used for
- Spring clips
- Automotive gears
- Austempered ductile-iron brackets
- Wear-resistant chain components
Austemper — frequently asked
What temperature is used for Austemper?
The representative process window is 790–900 °C, but the exact setpoint follows the alloy and governing specification. The listed soak is 0.5–2 h after the load reaches temperature.
What hardness does Austemper produce?
The catalog result is 25–55 HRC. Hardness must be verified on the part or a qualified witness coupon using the test method called out on the drawing.
How long does Austemper take?
The thermal cycle is typically 4–10 h door to door. Commercial lead time is normally 4–8 additional working days because loading, certification, and inspection are separate from furnace time.
Will Austemper distort the part?
The catalog rates distortion risk as low. Reserve machining stock and verify critical datums after treatment when tolerances are tighter than 0.05 mm.
How should Austemper be specified on a drawing?
State the governing process specification, alloy condition, required result, and inspection location. Report hardness to ASTM E18 or ASTM E10 as applicable, and do not substitute a furnace recipe for an acceptance requirement.