Case Harden (Carburize)
Case & surface hardening · Thermochemical diffusion
Carburizing diffuses carbon into low-carbon steel at austenitizing temperature, then quenches and tempers the enriched surface into a hard martensitic case.
Carburize 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 Carburize?
Carburizing diffuses carbon into low-carbon steel at austenitizing temperature, then quenches and tempers the enriched surface into a hard martensitic case. It is specified for gears, shafts, and other contact-loaded parts that need 58–64 HRC over a tough core.
How it works
The part is held in a carbon-rich gas or low-pressure vacuum atmosphere, commonly around 900–955 °C. Carbon activity at the surface is controlled while carbon diffuses inward; depth grows approximately with the square root of time. The part is direct quenched or cooled, reheated, and quenched so the high-carbon case transforms to martensite while the low-carbon core remains tougher. A 150–205 °C temper relieves quench stress. Carbon potential, boost-and-diffuse timing, alloy hardenability, retained austenite, carbide control, and quench severity determine the final hardness profile and distortion.
Why specify it
- Provides 58–64 HRC contact surfaces
- Supports effective case depths from about 0.5 to 2.5 mm in routine production
- Combines a wear-resistant case with a 28–40 HRC 8620-type core
- Improves gear pitting and rolling-contact fatigue resistance
Where it falls short
- Long high-temperature cycles and quenching create distortion
- Not suitable for already high-carbon through-hardening steel
- Excess surface carbon can form carbides and retained austenite
- Blind holes and stop-off boundaries require explicit masking instructions
The Carburize 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 | 900–955 °C (1,652–1,751 °F) — Gas-carburizing range for common carburizing steels; AMS 2759/7 controls the qualified procedure. |
|---|---|
| Soak | 2–30 h — About 4 h can produce roughly 0.76 mm ECD at 927 °C, while deep cases can exceed 16 h. |
| Cooling | Oil quench |
| Atmosphere | Endothermic gas / vacuum |
| Door to door | 8–40 h — Includes heat-up, boost/diffuse, quench, wash, and temper. |
Stages
- 1Preclean and apply stop-off
- 2Carburize with controlled carbon potential
- 3Diffuse to the required profile
- 4Direct quench or reheat and oil quench
- 5Temper at 150–205 °C
What Carburize does to the part
| Hardness | 58–64 HRC — Typical finished carburized case on 8620, 9310, and comparable steels. |
|---|---|
| Case depth | 0.5–2.5 mm — Effective case depth to 50 HRC per SAE J423; ISO 2639 uses 550 HV1 unless another limit is agreed. |
| Toughness | A low-carbon alloy core remains substantially tougher than the high-carbon martensitic case; core strength depends on grade, section, and quench. |
| Distortion | High risk. Austenitizing plus oil quench can change runout, tooth geometry, and bore size; use press quenching or finish grinding where necessary. |
| Surface after treatment | Gas carburizing leaves dark oxide after quench; low-pressure carburizing can produce a bright, oxide-free surface. |
| Machining sequence | Machine before carburizing, protect soft-machined regions with stop-off, then grind critical flanks, bores, and journals without violating minimum case depth. |
Specifications for Carburize
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 |
|---|---|---|---|
Carburizing and Heat Treatment of Carburizing Grade Steel Parts | Governing | Class 1, Class 2, Class 3 | Active |
Steels — Determination and verification of the depth of carburized and hardened cases | Alternative | — | Active |
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 2750 (Pyrometry), ISO 683-3 (Heat-treatable steels, alloy steels and free-cutting steels — Part 3: Case-hardening steels).
Drawing call-out
CARBURIZE AND HEAT TREAT PER AMS 2759/7 TO 60–63 HRC; 0.8–1.2 MM EFFECTIVE CASE DEPTH TO 50 HRC PER SAE J423 ON GEAR FLANKS; CORE 30–40 HRCWhich alloys take Carburize?
Verdict per material family, with the grades that behave differently. Each family links to its datasheets in the material library. Typical grades: 1018, 1020, 4118, 4620, 4820, 8620, 9310, 16MnCr5, 20MnCr5.
| Material family | Verdict | Notes |
|---|---|---|
| Yes | Low-carbon carburizing grades are designed for this process.
| |
| With care | Fully sintered low-carbon steel may be carburized, but porosity and carbon control must be qualified. | |
| Limited | AM carburizing steel is possible only with validated chemistry, density, and post-build route.
| |
| No | Aluminum cannot be carburized and quenched into a steel case. | |
| No | Brass cannot tolerate or respond to the steel carburizing cycle. | |
| No | Bronze does not form a carburized martensitic case. | |
| No | Cobalt-chrome does not use this steel carburizing route. | |
| No | Copper alloys do not form a carburized martensitic case. | |
| No | The process temperature exceeds magnesium alloy capability. | |
| No | Nickel alloys do not use conventional steel carburizing. | |
| No | Precious metals do not form a carburized steel case. | |
| No | Carbon reactions in refractory metals require different controlled processes. | |
| No | Conventional carburizing damages corrosion performance and is outside AMS 2759/7 scope. | |
| No | Carbon pickup embrittles titanium rather than creating a steel-type case. | |
| No | Tungsten heavy alloys do not form the required carburized martensite. | |
| No | Zinc alloys melt below the process temperature. |
Design rules for Carburize
What to decide on the drawing before the part goes to the heat treater.
- Specify effective case depth to 50 HRC or CHD to 550 HV1.SAE J423 and ISO 2639 use different conventional criteria.
- Specify surface and core hardness separately.The load path depends on both the martensitic case and supporting core.
- Keep sharp internal corners out of the carburized region.Carbon enrichment plus quench stress raises crack sensitivity.
- Call out soft areas and stop-off boundaries.Threads, weld lands, and later machining zones may need carbon exclusion.
- Leave grind stock but preserve the minimum case after grinding.Final grinding corrects quench movement while consuming case depth.
- Provide sacrificial coupons for inaccessible case-depth locations.A microhardness traverse is destructive.
Carburize versus the alternatives
Treatments engineers weigh against Carburize, and when each wins.
| Treatment | Hardness | Toughness | Stability | Cost | Choose it when |
|---|---|---|---|---|---|
Carburize this page | 82 | 55 | 35 | $$$ | — |
| 74 | 50 | 50 | $$ | when small low-carbon parts need a shallower case and lower processing temperature | |
| 85 | 50 | 85 | $$$$ | when low distortion and no quench outweigh the need for a deep case |
Cost, lead time and applications
Relative cost
Case Harden (Carburize) is a premium-cost treatment ($$$) 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 | 5–10 working days |
|---|---|
| Furnace time | 8–40 h |
What Carburize is used for
- Transmission gears
- Aerospace gears
- Cam followers
- Shaft splines
- Bearing rollers
- Heavy-duty pins
Carburize — frequently asked
How hard is a carburized case?
Suitable carburizing steels commonly finish at 58–64 HRC. The core of carburized 8620 is often about 28–40 HRC, depending on section and quench.
How long does carburizing take?
At about 927 °C, roughly 0.76 mm effective depth may take around 4 hours of carburizing plus diffusion, while 1.5 mm can require about 16 hours. Total furnace time is longer because heating, quenching, washing, and tempering are included.
What is effective case depth?
SAE J423 normally reports the perpendicular depth to 50 HRC for a carburized case. ISO 2639 instead defines CHD at 550 HV1 unless another limit is agreed.
Why use 8620 instead of 4140?
8620 has about 0.20% carbon, so carburizing can create a 60 HRC surface while retaining a tough core. 4140 is normally through hardened or induction hardened rather than selected as a carburizing grade.
Does carburizing distort parts?
Yes. The 900–955 °C exposure and oil quench can move bores, gear teeth, and long shafts, so finishing allowances and quench strategy belong in the process plan.
Can stainless steel be carburized?
Conventional AMS 2759/7 carburizing is for carburizing-grade steels, not corrosion-resistant stainless. Specialized low-temperature stainless carburizing exists but is a different qualified process.