Flame Hardening
Case & surface hardening · Surface hardening
Flame hardening rapidly austenitizes selected steel surfaces with an oxy-fuel flame and immediately quenches them to martensite.
Flame harden 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 Flame harden?
Flame hardening rapidly austenitizes selected steel surfaces with an oxy-fuel flame and immediately quenches them to martensite. Designers use it for large or irregular wear surfaces when a furnace cycle is impractical and the core must remain tough.
How it works
A shaped oxy-fuel flame traverses the specified surface and raises only a shallow layer above the steel's austenitizing temperature. A following water or polymer spray quenches that layer fast enough to form martensite while the colder interior remains in its prior condition. Carbon is not added, so attainable hardness comes from the base steel; medium-carbon steels are preferred. Flame pattern, traverse speed, burner distance, preheat, and quench delay control the hardness profile. A low temper may follow to reduce quench stress without materially softening the case.
Why specify it
- Hardens very large parts that do not fit conventional furnaces
- Produces 45–65 HRC on suitable medium-carbon steels
- Targets wear surfaces while retaining the original core properties
- Can produce effective hardened depths from about 0.5 to 6.0 mm
Where it falls short
- Lower pattern repeatability than a dedicated induction coil
- Open-flame heating can oxidize or decarburize the surface
- Water-spray quenching can distort thin or asymmetric sections
- Steels below about 0.30% carbon cannot develop a high-hardness martensitic case
The Flame harden 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 | 800–950 °C (1,472–1,742 °F) — Surface austenitizing range; the qualified setpoint depends on steel composition. |
|---|---|
| Soak | 0.001–0.05 h — Seconds to a few minutes of local heating, expressed in hours. |
| Cooling | Water quench |
| Atmosphere | Open flame |
| Door to door | 0.1–2 h — Typical local processing time; large raceways can require longer. |
Stages
- 1Preclean and locate the hardening boundary
- 2Traverse the oxy-fuel flame to austenitize the surface
- 3Immediately spray quench
- 4Low temper when specified
What Flame harden does to the part
| Hardness | 45–65 HRC — Water-quenched medium-carbon and alloy steels; carbon content limits the maximum. |
|---|---|
| Case depth | 0.5–6 mm — Effective case depth measured by hardness traverse to the drawing-specified threshold, commonly 50 HRC; SAE J423 requires the threshold to match carbon content where 50 HRC is unsuitable. |
| Toughness | The unaffected core retains its prior toughness, but an untempered martensitic surface is notch-sensitive. |
| Distortion | High risk. Intense asymmetric heating and spray quenching can move long or thin parts; qualify the scan path and leave finish stock. |
| Surface after treatment | Visible heat tint and scale are normal; grinding may be required where finish or decarburization limits matter. |
| Machining sequence | Finish machine unaffected features first; leave 0.2–0.5 mm grind stock on precision hardened faces and avoid cutting the specified effective case. |
Specifications for Flame harden
The documents a drawing cites to define this treatment. Each links to a page explaining the standard, its classes and the tests it invokes.
Test and related methods: ASTM E18 (Standard Test Methods for Rockwell Hardness of Metallic Materials), ASTM E384 (Standard Test Method for Microindentation Hardness of Materials), SAE J423 (Methods of Measuring Case Depth), AMS 2759 (Heat Treatment of Steel Parts, General Requirements).
Drawing call-out
FLAME HARDEN AREA SHOWN TO 50–58 HRC; 1.5–2.5 MM EFFECTIVE CASE DEPTH TO 50 HRC PER SAE J423; SURFACE HARDNESS PER ASTM E18Which alloys take Flame harden?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library. Typical grades: 1045, 1050, 4140, 4340, 5150, Ductile iron ASTM A536.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | Best on medium-carbon or alloy steels with adequate hardenability.
| |
| With care | Only fully sintered steel with verified density and carbon content should be considered. | |
| With care | Only printable martensitic steels may respond, after density and cracking risk are qualified.
| |
| Limited | Martensitic grades can respond; austenitic grades cannot.
| |
| No | Aluminum does not form martensite by flame quenching. | |
| No | Brass cannot be martensitically flame hardened. | |
| No | Bronze cannot be martensitically flame hardened. | |
| No | Cobalt-chrome does not use this martensitic surface-hardening route. | |
| No | Copper alloys do not respond to this steel transformation treatment. | |
| No | Open-flame processing is unsafe and metallurgically inapplicable. | |
| No | Nickel superalloys are not flame surface hardened by martensitic quenching. | |
| No | Precious metals do not form a martensitic case by flame quenching. | |
| No | Refractory metals require protected high-temperature processing. | |
| No | Titanium requires alloy-specific thermal cycles and strict atmosphere control. | |
| No | Tungsten heavy alloys are not suitable for local flame quench hardening. | |
| No | The alloy would melt far below the steel austenitizing range. |
Design rules for Flame harden
What to decide on the drawing before the part goes to the heat treater.
- Specify the hardened boundary on the drawing.Flame position defines where the martensitic case starts and stops.
- State effective case depth with its hardness criterion.A depth without the threshold is not inspectable under SAE J423.
- Use at least about 0.30% carbon for a high-hardness case.The process adds no carbon and depends on base-metal hardenability.
- Add generous transition zones instead of stopping at a sharp notch.An abrupt hardness boundary at a stress raiser promotes cracking.
- Leave grind stock on precision wear faces.Scale and quench movement are expected from open-flame processing.
Flame harden versus the alternatives
Treatments engineers weigh against Flame harden, and when each wins.
| Treatment | Hardness | Toughness | Stability | Cost | Choose it when |
|---|---|---|---|---|---|
Flame harden this page | 68 | 45 | 25 | $$ | — |
| 67 | 50 | 60 | $$ | when tighter pattern control and repeatable production are required | |
| 82 | 55 | 35 | $$$ | when a deep continuous case is required on low-carbon steel |
Cost, lead time and applications
Relative cost
Flame Hardening is a standard-cost treatment ($$) 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 | 3–7 working days |
|---|---|
| Furnace time | 0.1–2 h |
What Flame harden is used for
- Large gear teeth
- Machine-tool ways
- Crane wheels and sheaves
- Large shaft journals
- Track and rail wear faces
Flame harden — frequently asked
How deep is flame hardening?
Commercial hardened depths are commonly about 0.5–6.0 mm. The drawing must state an effective case-depth threshold, commonly 50 HRC, because total transformed depth is a different measurement.
What hardness can flame hardening reach?
Suitable medium-carbon and alloy steels commonly finish at 45–65 HRC. A 1045 part generally cannot match the maximum hardness of a higher-carbon grade under an otherwise similar quench.
Does flame hardening add carbon?
No. It transforms carbon already present in the steel, so low-carbon 1018 is a poor candidate for a high-hardness case.
Will the core properties change?
Normally the heated layer is shallow and the core remains below the transformation range. A small section or slow scan can heat through, so the process must be qualified on representative geometry.
How is case depth inspected?
A polished cross section is traversed with microhardness indents under SAE J423 or ASTM E384. The reported depth is where hardness falls to the drawing's stated criterion.