Overview
8620 is the canonical carburizing alloy steel — formulated specifically for case hardening. The chemistry strikes a balance:
- Low carbon (~0.20%) keeps the core soft and tough during service — protects against impact loading and prevents brittle failure
- Alloying (Ni-Cr-Mo) provides hardenability of the carburized surface case after quenching, reaching 58–62 HRC reliably
- Excellent core toughness at moderate Q&T tempers
The result is a part with hard wear-resistant surface (58–62 HRC) over a tough ductile core (28–35 HRC). This is the gear and bearing material model: contact surfaces don’t wear, gear teeth don’t bend or fatigue, but the bulk of the part has enough toughness to absorb shock loading.
The position in the alloy steel landscape:
| Need | Choose |
|---|---|
| Carburizing gears, bearings | 8620 (standard) |
| Through-hardening | 4140 |
| Premium through-hardening | 4340 |
| Welded structural | 4130 |
| Spring service | 5160 |
| Free-machining | 12L14 |
| Aerospace gears (premium) | 9310 (higher Ni for tougher core) |
Carburizing — the design space
The defining 8620 heat treatment:
- Machine components in annealed/normalized condition
- Carburize in carbon-rich atmosphere (gas, vacuum, salt, or pack) at 900–925°C for 4–12 hours (depending on case depth)
- Quench in oil (sometimes salt bath quench)
- Temper at 150–200°C for stress relief
- Cryogenic treatment (optional, for precision/aerospace)
- Shot peen (for aerospace and high-performance)
- Finish grind precision surfaces to remove distortion
Case depth selection:
| Application | Case depth |
|---|---|
| Light automotive transmission gears | 0.5 mm |
| Standard automotive transmission | 0.8 mm |
| Medium industrial gear reducer | 1.0 mm |
| Heavy-duty industrial gears | 1.5 mm |
| Mining/quarry/heavy equipment | 2.0 mm |
| Very heavy gear teeth (large modules) | 2.5 mm |
Carburize process selection:
- Gas carburizing (endothermic atmosphere): Standard for high- volume automotive and industrial gears. Cost-effective. Some intergranular oxidation in the case.
- Vacuum (low-pressure) carburizing: Premium aerospace gears. Cleaner case with better fatigue. More expensive.
- Salt bath carburizing: Older technology. Faster than gas but environmental concerns; declining usage.
- Pack carburizing: Legacy, low-volume specialty work. Used for tool steel modifications and antique restoration.
Machining notes
Annealed/normalized 8620 machines well — better than 4140 due to lower carbon:
- Coated carbide (TiAlN preferred)
- Speed: 100–200 SFM
- Feed: 0.005–0.020 in/rev
- Standard cutting fluid
- Tool life ~70% of A36
- Machinability rating ~65%
The standard production sequence:
- Rough machine 8620 normalized bar to leave grinding stock (typically 0.2–0.5 mm per surface for non-critical, 1+ mm for precision gear teeth)
- Carburize + quench + temper to develop hard case
- Grind precision surfaces — gear teeth (using gear grinders or honing), bearing races (using internal grinders), shaft diameters (using cylindrical grinders)
- Inspect for case depth, surface hardness, dimensional accuracy
Direct machining of carburized 8620 above ~60 HRC is impractical by conventional methods. Grinding is the production finish process.
Heat treatment alternatives
While carburizing is the canonical 8620 process, other options:
- Through Q&T without carburize: Limited to ~33 HRC by 0.20% carbon. Useful for moderate-strength shafts where surface hardening isn’t needed. Most applications choose 4140 instead for higher core strength.
- Carbonitriding: Carburize + nitride combination. Adds nitrogen to the case for higher wear resistance. Less distortion than carburizing alone. Used for some automotive applications.
- Induction hardening of carburized 8620: Localized re-heat- treatment for surface hardness restoration after grinding. Specialized aerospace gear processing.
Welding considerations
8620 in annealed/normalized condition welds well — much better than 4140 due to lower carbon (0.20% vs 0.43%):
- TIG, MIG, stick all work routinely
- Preheat generally unnecessary for thin sections
- Thicker sections (>25 mm) may benefit from 150–200°C preheat
- Low-hydrogen electrodes recommended for critical structural welds
Standard practice: weld in annealed/normalized condition, then carburize the assembly. This is the production sequence for welded gear cluster assemblies.
Do not weld carburized 8620 in service. The case-hardened HAZ is unpredictable — fusion welding can produce:
- Brittle martensite in the HAZ
- Loss of case hardness in adjacent regions
- Cracking from quenched-state stresses
- Re-carburizing would be needed to restore properties
For welded assemblies requiring case hardness, design the weld zones away from carburized surfaces.
Corrosion considerations
Same baseline as A36 / 1018 / 4140 — none. The alloy chemistry is for hardenability, not corrosion. 8620 rusts in any moist environment.
For gear and bearing service, corrosion management is primarily through:
- Lubrication — gear oils and bearing lubricants contain corrosion inhibitors
- Sealed enclosures — gearbox housings exclude moisture and contamination
- Oil bath operation — submerged components don’t corrode significantly
- Coatings on non-functional surfaces — paint or phosphate + paint on gearbox housing exteriors
For gear surfaces themselves (functional teeth and bearing races), no coating is applied — surface texture and friction characteristics must be controlled. Corrosion control comes from lubrication management.
Aerospace gear processing — the premium application
Helicopter transmissions, aircraft accessory drives, and military aviation gear systems use 8620 (or higher-Ni variants like 9310) with rigorous processing:
- Vacuum-arc remelted (VAR) base stock — minimum inclusion content for fatigue
- Forge to near-net shape with controlled grain flow
- Anneal for machinability
- Rough machine with grinding allowance
- Vacuum carburize at 900–925°C in low-pressure environment
- Oil quench (or pressurized gas quench for some processes)
- Cryogenic treatment at -80°C or -196°C to eliminate retained austenite
- Temper at 150–175°C
- Shot peen with controlled intensity and coverage
- Grind teeth to AGMA Class 10–14 precision
- Hone teeth for final surface finish (sometimes)
- Honing/lapping for bearing races
- NDT inspection — magnetic particle, eddy current, dye penetrant
- Final inspection — case depth verification, hardness profile, surface finish, dimensional
The total processing cost for an aerospace 8620 gear is many times the raw material cost — but the result is gears that run for tens of thousands of hours in fatigue service.
Applications by industry
- Automotive (the dominant industry) — transmission gears (manual and automatic), differential gears, axle pinions, ring gears, planet gears. The standard gear material for passenger cars, light trucks, and commercial vehicles.
- Industrial gear reducers — gearbox gears for industrial machinery, mining and quarry equipment, paper mill drives, conveyor drives. Sized for case depth based on tooth module and load.
- Aerospace — helicopter main transmission gears (often with vacuum carburize), aircraft accessory drives, landing gear geared mechanisms. Premium processing including VAR base stock and rigorous QA.
- Agricultural equipment — tractor transmission gears, PTO shaft gears, harvester drive gears.
- Heavy machinery — mining equipment gears, construction equipment final drives, forestry harvester gears. Heavy case depths for tooth durability.
- Wind turbine gearboxes — main planetary gears, high-speed stage gears. Premium processing for the 20+ year service life.
- Marine propulsion — gearbox gears for marine engines.
- Bearing manufacturing — bearing races and rollers (especially large bearings; smaller bearings often use 52100 high-carbon bearing steel instead).
Failure modes worth designing around
Inadequate case depth is the most common gear failure mode. Too thin a case fails by case crushing (the case collapses into the core under contact load) or pitting/spalling (case fatigue from rolling contact). Mitigations:
- Specify case depth appropriate to tooth size (rule of thumb: case depth ≈ tooth module / 4–6)
- Verify case depth by microhardness traverse on production samples
- Account for grinding stock removal in carburize time calculation
Retained austenite in the carburized case — slow cooling after carburize + quench leaves untransformed austenite that destabilizes over time, causing:
- Dimensional growth in service (austenite → martensite transformation increases volume)
- Surface hardness degradation over time
- Possible cracking from transformation stresses
Mitigation: cryogenic treatment after quench to convert retained austenite to martensite. Standard for aerospace gears; optional for commercial automotive.
Gear tooth bending fatigue at root fillets — even with hard case + tough core, root fillet stress concentration initiates fatigue cracks. Mitigations:
- Generous root radius (per AGMA gear design standards)
- Shot peen after carburizing for compressive surface stress
- Grind tooth profile to remove decarb and surface defects
Atmospheric corrosion of exposed gear teeth and bearing races in storage or non-operating service. Corrosion pits initiate fatigue cracks in subsequent service. Mitigations:
- Oil all gears and bearings during storage
- Sealed enclosures for service
- VCI (vapor corrosion inhibitor) bags for shipping/storage
Quench distortion during carburize + quench — gear teeth and precision features distort during rapid cooling. Mandatory finish- grind after heat treatment. Distortion budget typically 0.05–0.20% on diametrical dimensions; tighter for precision aerospace.
Specifying 8620 for through-hardening is a common designer error. 8620’s 0.20% carbon caps through-hardness at ~33 HRC by Q&T alone — much lower than 4140 (50+ HRC) at similar cost. For through-hardened parts, specify 4140 from the start; 8620 is specifically for case hardening.
Welding carburized 8620 in service — produces brittle, unpredictable HAZ. Don’t weld case-hardened components. For repair, replacement is the standard approach.
Hydrogen embrittlement in high-hardness Q&T core conditions during plating. Bake-out after plating mandatory for high-strength service applications.
Decarburization during heat treatment without protective atmosphere — the entire point of 8620 is the case carbon content. Decarburization in the protective atmosphere undoes the carburizing work. Always use proper protective atmosphere (endothermic for gas carburizing, controlled atmosphere for any heat treatment). This is the #1 8620 heat treatment QA issue.
Wear failure from inadequate lubrication — gear teeth and bearing surfaces must be properly lubricated. Boundary lubrication failure, gear oil degradation, or contamination causes wear that exceeds the case depth. Lubrication management is as important as material selection for gear service life.
Specifying 8620 for impact-loaded structural service — the 0.20% carbon means low core strength. For impact-loaded heavy structural service, 4340 (better core toughness) or 4140 (higher core strength) are usually better choices.