Carburizing
Also: case hardening, case carburizing, carburize and harden, gas carburizing, vacuum carburizing, low-pressure carburizing, LPC, carbonitriding, pack carburizing
Carburizing diffuses carbon into low-carbon steel (8620, 1018), then quenches it: a 58-62 HRC wear case over a tough core for gears, pins and shafts.
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Carburizing heats low-carbon steel, typically 0.10-0.25 percent carbon, to about 900-955 °C (1650-1750 °F) in a carbon-rich atmosphere so carbon diffuses into the surface. The part is then quenched and tempered: the high-carbon skin becomes martensite of 58-62 HRC, while the low-carbon core stays tough, at roughly 25-40 HRC in alloy grades like 8620 depending on section size and softer in plain 1018. The hard case resists wear and contact fatigue and the core carries bending and shock without cracking, which is why most gears, cam followers, pins and bearing races made from low-carbon steel are carburized.
Case depth is set by time and temperature. Carbon diffusion depth grows with the square root of time, so doubling the case takes about four times as long. Drawings specify effective case depth (ECD), the depth at which hardness falls to 50 HRC under SAE J423 (ISO 2639 uses 550 HV), measured on a sectioned part or coupon, as a range such as 0.75-1.0 mm (0.030-0.040"). Typical commercial cases run 0.5-1.5 mm (0.020-0.060"); heavily loaded gearing goes deeper at the cost of very long cycles.
Gas carburizing in an endothermic atmosphere with an integral oil quench is the commercial standard. Vacuum or low-pressure carburizing (LPC) feeds acetylene or propane into a vacuum furnace, avoids the intergranular oxidation that gas atmospheres leave in the surface, gives more even cases in blind holes and gear roots, and is usually paired with a high-pressure gas quench that distorts less than oil. Carbonitriding adds ammonia to put nitrogen in the case as well; it runs cooler and suits thin cases on plain carbon parts. Areas that must stay soft, such as threads or a zone to be drilled later, are masked with copper plate or stop-off paint, or left with extra stock that is machined off after carburizing and before hardening.
At a glance
| Typical tolerances | Effective case depth is specified as a range: a 0.25 mm (0.010") window is standard for cases up to about 1 mm (0.040"), for example 0.50-0.75 mm (0.020-0.030"), and wider windows are normal for deeper cases. Surface hardness is 58-62 or 58-63 HRC after temper; core hardness depends on section size and is specified as a minimum or a range such as 28-40 HRC when it matters. Carburized parts grow and distort in the quench: gears lose some lead and profile accuracy and shafts bow, so bearing journals and gear teeth are ground after hardening with about 0.1-0.25 mm (0.004-0.010") of stock per side. LPC with a gas quench moves less but still moves. Grinding removes case, so state whether ECD applies before or after grind. |
|---|---|
| Size limits | Integral-quench batch furnaces commonly take loads within about 600 x 900 x 600 mm (24 x 36 x 24") to 900 x 1,200 x 900 mm (36 x 48 x 36") and roughly 450-1,800 kg (1,000-4,000 lb) per load. Pit carburizers take long shafts and large ring gears, commonly 1.5-3 m (5-10 ft) deep at larger shops. Vacuum carburizing furnaces are usually smaller, around 600 x 600 x 900 mm (24 x 24 x 36"). Thin webs, sharp edges and small teeth carburize through and turn brittle if the case is deep relative to the section, so size the case to the thinnest section. |
| Surface finish | Gas-carburized, oil-quenched parts come out dull grey to black with oil residue and a light oxide; they are washed and usually shot or bead blasted. Gas atmospheres also leave intergranular oxidation, a shallow altered layer up to about 25 µm (0.001") deep that lowers fatigue strength unless it is ground off or shot peened. LPC parts come out clean and nearly bright with no intergranular oxidation. Roughness is essentially unchanged by carburizing; precision surfaces are ground after hardening. |
| Lead time | Commercial gas carburizing in 5-10 business days for standard cases. Cases over about 1.5 mm (0.060") and aerospace work to AMS 2759/7 take 2-3 weeks. Copper masking adds a plating and stripping step and a few days. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Low-carbon steels. 8620 is the default alloy carburizing grade, with 4320, 4620, 9310 and similar grades for heavier-duty and aerospace gearing. Plain 1018 and 1020 carburize well for light-duty pins, bushings and wear plates, but their core stays soft and the case needs a water or brine quench to harden fully in anything but thin sections, so they distort more. Leaded 12L14 is carburized for small, lightly loaded screw-machine parts. Medium-carbon steels such as 4140 and 1045 are not normally carburized: they already harden on their own, and extra surface carbon brings retained austenite and cracking, so they are induction hardened or nitrided instead. Stainless needs special low-temperature processes; aluminum, titanium and copper alloys are not carburized.
What drives the cost
- Case depth: time at temperature rises with the square of the depth, so a 1.5 mm case costs far more furnace time than a 0.5 mm case
- Selective masking: copper plating and stripping, stop-off paint, or extra machining of sacrificial stock
- Process: vacuum carburizing with gas quench costs more per load than endothermic gas carburizing with oil quench
- Grinding and straightening after hardening to recover distortion
- Testing: sectioned coupons or sacrificial parts for case-depth traverses, plus certs
- Fixturing: gears and shafts hung or fixtured individually to control distortion
When to use it
- Gears, splines, cams, pins and bearing surfaces that need a hard, wear- and fatigue-resistant surface over a core that survives shock loads
- Parts in low-carbon steel (8620, 1018) that cannot through-harden
- Case depths from about 0.5 to 2 mm (0.020-0.080"), deeper than nitriding reaches
- Heavily loaded gearing where contact stress calls for a deep, hard case
- Selective hardening where threads or holes to be drilled later must stay soft
When not to
- Finished parts that cannot tolerate quench distortion and will not be ground afterwards: nitriding runs below the transformation range and moves far less
- Medium-carbon steels such as 4140 and 1045: induction harden or nitride them
- Only a band or journal on a long shaft needs hardening: induction hardening is faster and cheaper
- Stainless, aluminum and titanium, which need other surface treatments
Design tips
- Call out ECD as a range with the hardness that defines it, surface hardness, core hardness if it matters, and whether the depth is after grind: for example "carburize and harden, ECD 0.75-1.0 mm at 50 HRC after grind, surface 58-62 HRC".
- Mark soft areas on the drawing with a clear boundary and allow a transition zone of a few millimeters at the edge of a masked area.
- If threads must stay soft, mask them, or leave stock, carburize, cool slowly, machine the case off and cut the thread, then harden.
- Radius the roots of teeth, shoulders and edges; sharp corners over-carburize and chip.
- Leave 0.1-0.25 mm (0.004-0.010") of grind stock per side on bearing journals and gear flanks, and add it to the case depth so the ground surface still has full case.
- Choose 8620 over 1018 when the part sees bending or impact loads: its core hardens and it quenches in oil with less distortion.
- Ask for a coupon of the same alloy and similar section to run with the load for the case-depth test, so good parts are not cut up.
Carburizing by material
Frequently asked questions
- How deep is a carburized case?
- Most commercial cases are 0.5-1.5 mm (0.020-0.060") effective case depth, measured to 50 HRC. Light-duty pins and bushings use 0.25-0.5 mm (0.010-0.020"); heavily loaded gears go to 2 mm (0.080") or more. Deeper cases take much longer, because depth grows with the square root of time at temperature.
- What is effective case depth?
- The depth below the surface at which hardness drops to a stated value: 50 HRC for carburized steel under SAE J423, or 550 HV under ISO 2639. It is measured with a microhardness traverse on a sectioned part or coupon. Total case depth, to where carbon returns to the core level, is deeper and less useful for design.
- Can 4140 be carburized?
- It can be, but it rarely should be. 4140 already has about 0.40 percent carbon and hardens on its own; adding surface carbon raises the risk of retained austenite and quench cracking, and the core ends up harder and less tough than a carburizing grade. For a hard surface on 4140, use induction hardening or nitriding.
- Carburizing or nitriding: which is better?
- Carburizing gives a deeper case (0.5-2 mm) and higher load capacity but needs a quench, so parts distort and are usually ground. Nitriding gives a thinner, harder case (about 0.1-0.6 mm) at a lower temperature with no quench, so parts barely move and can be finished first. Heavily loaded gears are carburized; dies, precision shafts and parts that cannot be ground afterwards are nitrided.
- How do you mask areas from carburizing?
- Copper plating, typically about 0.013-0.025 mm (0.0005-0.001") thick, blocks carbon and is the aerospace standard; stop-off paints are cheaper for commercial work. The other route is to leave extra stock on the soft area, carburize, machine the case off, then harden. Mark the soft areas on the drawing and expect a transition of a few millimeters at the edge of a mask.
