Carburizing AISI 8620: Case Depth, Core Hardness, and Applications
AISI 8620 is a nickel-chromium-molybdenum low-carbon alloy steel that is among the most widely specified carburizing grades in North American gear, pinion, shaft, and pin production. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. Its nominal 0.18-0.23% carbon core composition is deliberately low so the core remains tough after carburizing, while the Ni-Cr-Mo alloy content (0.40-0.70% Ni, 0.40-0.60% Cr, 0.15-0.25% Mo) provides enough hardenability that the carburized case reaches 58-62 HRC and the core reaches 30-40 HRC on oil quench. This article describes the metallurgy of 8620, the typical carburized case-depth and hardness outcomes, comparison with induction-hardening options on medium-carbon grades, and guidance for buyers who need to source carburized 8620 parts, since atmosphere carburizing is a specialty service that requires dedicated sealed-atmosphere or vacuum equipment not available at every commercial heat treater.
What is AISI 8620 and why is it used for carburizing?
AISI 8620 is a low-carbon triple-alloy steel specified in ASTM A29/A29M and SAE J404 with nominal chemistry 0.18-0.23% C, 0.70-0.90% Mn, 0.40-0.70% Ni, 0.40-0.60% Cr, and 0.15-0.25% Mo. The low carbon content is the reason 8620 is chosen as a carburizing grade: in the as-delivered state the material is soft and machinable (typically 149-179 HB annealed, equivalent to about 10-13 HRC), yet the nickel, chromium, and molybdenum provide enough hardenability that after carbon diffuses into the surface and the part is quenched, the high-carbon surface layer transforms to hard martensite while the low-carbon core transforms to a tougher mix of martensite, bainite, and ferrite. This combination of soft machinability before heat treatment and a 58-62 HRC case over a 30-40 HRC tough core after treatment is difficult to match with any other processing route, which is why 8620 dominates mass-produced gears, pinions, camshaft lobes, pins, bushings, and similar wear-plus-shock components (ASM Handbook, Vol. 4A, ASM International, 2013; SAE J404). The grade's hardenability band is formally published in SAE J1268 as 8620H, which lets a heat treater verify the material's end-quench response before processing.
What carburizing cycle parameters does 8620 typically require?
Atmosphere gas carburizing of 8620 runs at 1,650-1,750 °F (900-955 °C) in an endothermic atmosphere whose carbon potential is set between 0.9 and 1.1% by adjustment of the enrichment hydrocarbon; vacuum (low-pressure) carburizing uses acetylene or propane at similar or slightly higher temperatures in the 1,650-1,800 °F range. Time at temperature sets the case depth per the diffusion relationship in which depth scales roughly with the square root of time: reaching a 0.030 inch effective case at 1,700 °F takes approximately 4 hours of carburize time plus a diffusion step, while 0.060 inch requires on the order of 16 hours total, and 0.100 inch can exceed 30 hours. After carburizing, parts are either direct-quenched from the carburizing temperature into oil or reheated to a lower austenitizing temperature near 1,525-1,550 °F and then oil quenched for improved grain refinement and core toughness; quench oil agitation is controlled to limit distortion on long or thin sections. A post-quench temper at 300-375 °F for 2 hours relieves quench stresses without meaningfully reducing case hardness, typically producing the finished case in the 58-62 HRC range (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995). UTEC Industrial does not operate carburizing atmosphere or vacuum carburizing equipment; buyers who need carburized 8620 parts should source from a heat treater specializing in gas or vacuum carburizing.
What case depth and hardness should be expected on carburized 8620?
Typical outcomes on correctly processed 8620 are a surface hardness of 58-62 HRC measured by Rockwell C on the finished part, an effective case depth (depth to 50 HRC per the conventional definition) anywhere from 0.020 to 0.080 inch depending on the cycle, and a core hardness of 28-40 HRC depending on section size and quench severity (Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995). Smaller sections (under 1 inch through-thickness) oil-quenched aggressively can produce core hardness toward the upper end of that range, 36-40 HRC, because 8620's hardenability is sufficient to harden the full section at those sizes. Larger sections (over 2 inches) tend to produce core hardness in the lower 28-32 HRC range because the quench cooling rate at the core falls below the critical rate needed to form fully martensitic structure. When specifications call for a minimum core hardness above 35 HRC on sections heavier than about 2 inches, a higher-hardenability carburizing grade such as 4320 or 8822 is usually more reliable than 8620. Surface hardness is measured by Rockwell C per ASTM E18 on the finished ground surface; effective case depth is verified by microhardness traverse on a sectioned test coupon per ASTM E384.
How does carburized 8620 compare to induction-hardened 4140 or 4340?
The two processing routes produce superficially similar results — a hard wear surface over a tougher core — but the metallurgy and the applications they suit are different. Carburized 8620 produces a graded composition gradient: the surface has elevated carbon (roughly 0.80-0.90%), the sub-surface carbon falls continuously to the 0.20% core composition over the carburized depth, and the hard case and tough core are therefore metallurgically distinct layers bonded by a smooth transition. Induction hardened 4140 or 4340 produces a uniform-composition part whose surface has been austenitized by localized heating and quenched while the core stayed below transformation temperature; the hard zone and the core have the same chemistry, and the transition between them is determined by how deep the induction field heated the surface above Ac3 (ASM Handbook, Vol. 4C, ASM International, 2014). Carburizing is the right choice when the part must be made from a low-carbon grade for weldability, formability, or cold-heading reasons, when the case depth needed is shallow (under 0.050 inch) and the geometry is complex enough that an induction coil cannot reach all the surfaces, or when the application requires the specific rolling-contact-fatigue resistance that the carbon gradient in a carburized case provides. Induction hardening is the better choice on rotationally symmetric parts — shafts, rollers, crane wheel treads, splines — where a dedicated coil can be designed to the geometry, and where a medium-carbon grade (4140, 4340, 1045) provides enough core hardenability that a graded composition is not needed. UTEC Industrial performs induction hardening on qualifying geometries at its Spokane facility, but does not perform carburizing; parts that genuinely require carburizing are a different sourcing question.
What applications specify carburized 8620, and why?
Carburized 8620 is the default material specification for a large fraction of general-purpose industrial gears — spur gears, helical gears, bevel gears, pinions, and worm gear wheels — produced for gearboxes, reducers, speed changers, and drive assemblies across automotive, agricultural, industrial machinery, and off-highway equipment (ASM Handbook, Vol. 4D, ASM International, 2014). The combination of 58-62 HRC case hardness on the gear teeth flanks for wear and rolling-contact-fatigue resistance and 30-40 HRC core for bending fatigue strength and shock absorption is the canonical case-hardened gear profile. Camshaft lobes, rocker arm pads, roller-follower pins, and wrist-pin-class pins use the same profile in engine and compressor applications where the surface sees cyclic Hertzian contact loading. Chain sprockets and sprocket teeth in heavy industrial chain drives — conveyors, mining equipment, material handling — frequently specify carburized 8620 teeth where the combination of wear, impact, and high contact stress in service would fatigue a through-hardened part or wear out an induction-hardened part. The named failure modes that drive the 8620-carburize specification are tooth-flank pitting (surface-initiated contact fatigue, resisted by high case hardness), tooth-root bending fatigue (resisted by the tough core and by compressive residual stress in the case left by the quench), and tooth-tip wear (resisted by the high case hardness).
How should carburized 8620 parts be specified on a drawing?
A complete case-hardening callout for an 8620 part names the process, the case depth with tolerance, the surface hardness with tolerance, the core hardness where it matters to function, and the test location. A representative callout reads approximately: "Material: AISI 8620 per ASTM A29/A29M. Gas carburize, oil quench, and temper at 325 °F. Effective case depth 0.040-0.060 in to 50 HRC, measured on sectioned sample per ASTM E384. Surface hardness 58-62 HRC per ASTM E18 on finished ground tooth flank. Core hardness 30-38 HRC at mid-tooth, measured per ASTM E18 on sectioned sample." Callouts that omit the effective case depth, omit the test location, or omit the tempering requirement are under-specified — the heat treater will have to ask, and the interpretation can vary between shops. For gears in particular, AGMA gear-rating standards and OEM internal standards often specify a case-depth range scaled to the module or diametral pitch of the gear, because an over-depth case on a small-tooth gear will cause brittle tooth-tip cracking under shock loading, while an under-depth case allows pitting before the expected service life (ASM Handbook, Vol. 4D, ASM International, 2014; SAE J1397). For non-gear parts, the specification principle is the same — state every parameter the processor needs to meet the function.
Where do buyers source carburized 8620, and what should they look for in a heat treater?
Atmosphere gas carburizing and vacuum (low-pressure) carburizing require sealed, atmosphere-controlled furnaces that are specialty equipment: most commercial heat treaters that run carbon-steel annealing, stress relief, normalizing, quench-and-temper, and induction hardening do not operate carburizing atmosphere furnaces. Gas carburizing capacity in North America is concentrated at dedicated captive gear-plant heat-treat departments (automotive, off-highway OEM) and at commercial carburizing houses that run batch and continuous carburizing lines; vacuum carburizing is concentrated at aerospace-qualified heat treaters and at the newer generation of cold-box-quench gear-plant heat-treat installations. When selecting a carburizing source, buyers should confirm the shop operates the correct process type for the drawing specification (gas, vacuum, or specialty), holds the required pyrometry compliance for the customer's quality system (AMS 2750 for aerospace work, or the customer's own internal furnace-class rating for commercial work), can verify surface hardness per ASTM E18 and effective case depth per ASTM E384 on sectioned coupons, and ships the thermocouple chart plus hardness record with the job. For buyers in the Pacific Northwest who need through-hardening, stress relief, annealing, or induction hardening on parts that will not be carburized, UTEC Industrial's 6' x 10' x 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief cover that scope in the Spokane, WA area; buyers whose parts specifically require carburizing must continue to source from a carburizing specialist (ASM Handbook, Vol. 4A, ASM International, 2013; AMS 2759; AMS 2750).
References
- ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes, ASM International, 2013.
- ASM Handbook, Volume 4C: Induction Heating and Heat Treatment, ASM International, 2014.
- ASM Handbook, Volume 4D: Heat Treating of Irons and Steels, ASM International, 2014.
- Heat Treater's Guide: Practices and Procedures for Irons and Steels, 2nd edition, ASM International, 1995.
- ASTM A29/A29M, Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought, ASTM International.
- ASTM E18, Standard Test Methods for Rockwell Hardness of Metallic Materials, ASTM International.
- ASTM E384, Standard Test Method for Microindentation Hardness of Materials, ASTM International.
- SAE J404, Chemical Compositions of SAE Alloy Steels, SAE International.
- SAE J1268, Hardenability Bands for Carbon and Alloy H Steels, SAE International.
- SAE J1397, Estimated Mechanical Properties and Machinability of Steel Bars, SAE International.
- AMS 2750, Pyrometry, SAE Aerospace.
- AMS 2759, Heat Treatment of Steel Parts, General Requirements, SAE Aerospace.
Need In-House Heat Treating for Heavy Industrial Parts?
UTEC Industrial operates a 6' × 10' × 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief at our Spokane, WA facility. Weldment stress relief, annealing, quench and temper, and induction hardening — all under one roof, with full documentation on every job.
Questions? Call (509) 922-1832 or email sales@utec.co