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Heat Treatment of Heavy Industrial Gearing: Through- and Surface Hardening

Heavy industrial gearing — open gears on mining draglines and shovels, ball-mill trunnion gears and their driving pinions, cement-kiln girth gears, and large slow-turning reduction gears — sits at the intersection of three heat-treatment disciplines: through-hardening and tempering of the gear blank for core strength, selective surface hardening of the tooth flanks for wear and pitting resistance, and (for the highest-duty gears) carburizing at specialty heat treaters equipped for the process. 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. This article covers the options, the trade-offs, the applicable AGMA and ISO references, and where to source capabilities that a general-purpose heat treater does not provide.

What are the main heat-treatment routes for heavy industrial gearing?​

Three routes cover the heavy-gear population. The first is through-hardening by quench and temper on a medium-carbon alloy steel blank (typically 4140 or 4340), delivering a uniform hardness across the tooth and body in the 28–38 HRC range and producing an "as-hardened" gear that can be cut or finished to final profile afterward. Through-hardened gears are the standard choice for slow-turning, high-torque, moderate-pitting-load applications — trunnion gears on ball mills and rotary kilns, heavy reduction gears in mining processing, and large open gears in cement and mineral industries. The second is surface hardening of the tooth flanks by induction (or, in a few specialty cases, flame) after the gear body is through-hardened and the teeth are finish-cut — this produces a hard, wear-resistant surface (50–58 HRC) on the loaded faces of the teeth with a tough through-hardened core beneath. The third is carburizing, which produces the highest-performance gears by diffusing carbon into the surface of a low-alloy case-hardening steel (typically 8620, 9310, or 4320) during an atmosphere or vacuum thermal cycle — the resulting 58–62 HRC case on a 30–40 HRC core is the standard for premium OEM industrial gears in high-speed enclosed drives and for aerospace gearing. Carburizing requires atmosphere (endothermic gas) or vacuum furnace capability that general-purpose car-bottom heat treaters do not operate; buyers needing carburized gears should specify a specialty heat treater with atmosphere or vacuum carburizing capability (AGMA 923; AGMA 2001; ISO 6336; ASM Handbook, Vol. 4A, ASM International, 2013).

When is through-hardening the right choice for a heavy gear?​

Through-hardening is the right specification for heavy slow-turning gears where the combination of bending fatigue strength at the tooth root and moderate contact fatigue on the tooth flank is the governing criterion, and the cost of carburizing or the dimensional constraints of a surface-hardening process do not favor the case-hardened alternative. Typical examples: ball-mill trunnion gears and their driving pinions, rotary-kiln girth gears and drive pinions, mining-shovel swing gears and dragline-walk gears, heavy-bucket-wheel excavator gearing, and large open-gear drives on crushers and conveyors. The hardness target is typically 32–38 HRC for 4340 quench-and-tempered gears, falling in the middle of the 4340 Q&T range — high enough to deliver the bending fatigue strength needed for repeated tooth loading, low enough to retain the toughness and machinability for post-HT gear cutting or grinding. Through-hardened 4340 is typically austenitized at 1,475–1,525 °F, oil quenched, and tempered at 1,000–1,050 °F for 2 hours per inch of section. The gear blank is heat-treated before teeth are cut in most heavy-gear practice, because cutting teeth after hardening requires either profile grinding (slow and expensive on large teeth) or carbide broaching (limited to specific tooth geometries). For the largest gears where cutting after hardening is impractical, the through-hardening is performed first at a somewhat lower hardness (28–32 HRC), teeth are cut, and then a final tempering or stress-relief cycle is performed to address cutting residual stress (AGMA 923; AGMA 2001-D04; ASM Handbook, Vol. 4A, ASM International, 2013; SAE J1268).

How is induction hardening applied to the tooth flanks of a heavy gear?​

Induction hardening of gear teeth comes in two geometries: single-tooth hardening, where a profile-matched coil tracks along one tooth flank at a time, and spin hardening, where the entire gear rotates through a coil that encircles or scans the gear radially and all teeth are hardened simultaneously. Single-tooth hardening produces the most accurate case-depth control and the best metallurgical result — the case follows the tooth profile at constant depth, and the root (the critical region for bending fatigue) is hardened continuously with the flank. Spin hardening is faster on smaller gears but tends to deliver a shallower case at the root than at the tip if the coil-gear geometry is not carefully optimized, and is generally not used for the heaviest gears. Typical case depth on an induction-hardened heavy-gear tooth is 0.080–0.250 inch at the flank with 50–58 HRC surface hardness; the through-hardened core of the gear body remains at 30–38 HRC. Post-induction temper at 350–450 °F for 2 hours is standard to convert brittle untempered martensite to tempered martensite and relieve surface tensile stress. Induction hardening is appropriate for gears where the geometry permits a well-fitted coil and the case-depth requirement is within the reach of the process (roughly 0.08–0.50 inch depending on diametral pitch). UTEC Industrial performs induction hardening on qualifying gear geometries — the coil and tooth-profile match must be verified on a per-job basis, and the gear must fit within the induction station's envelope (AGMA 946; ASM Handbook, Vol. 4C, ASM International, 2014; ASTM E18; ASTM E384).

When is carburizing required, and where should buyers source it?​

Carburizing is the specification of choice for enclosed high-speed industrial gearing in the AGMA 2001 "high-hardness" grades, for aerospace and defense gearing, for racing and high-performance drive train gears, and for precision industrial gears where contact-fatigue life under heavy loading is the governing criterion. The process diffuses carbon into the surface of a low-alloy case-hardening steel (commonly 8620, 9310, 4320, 17CrNiMo6, or similar) at 1,600–1,750 °F in an atmosphere (endothermic gas, methanol-based, or propane-based) or vacuum carburizing furnace, typically for 2–40 hours depending on the required case depth. The resulting carbon-enriched surface layer hardens on quench to 58–62 HRC while the core stays at 28–40 HRC, giving the gear a hard wear-resistant surface on a tough core. Required case depth typically runs 0.030–0.100 inch for automotive and industrial reducer gears and 0.100–0.300 inch for the largest industrial gears. Carburizing is not part of a general-purpose car-bottom furnace heat treater's process list — it requires an atmosphere or vacuum furnace with controlled carbon potential and dedicated quench facilities that only specialty heat treaters maintain. Buyers requiring carburized gears should source from a heat treater with atmosphere or vacuum carburizing capability, a current AGMA or ISO gear-heat-treatment audit, and Nadcap accreditation if the gear is aerospace-class. For non-aerospace industrial carburized gears, regional heat treaters with endothermic-gas generators and integral-quench furnaces are typical sources (AGMA 923; AGMA 2001; AGMA 2004; ISO 6336-5; ASM Handbook, Vol. 4A, ASM International, 2013).

What AGMA and ISO standards govern gear heat treatment, and what do they specify?​

Four reference standards appear consistently on heavy-gear drawings. AGMA 923-C22, Metallurgical Specifications for Steel Gearing, is the umbrella document covering through-hardening, carburizing, nitriding, and induction hardening quality requirements for gears — it specifies the hardness and case-depth grades (typically Grade 1, 2, and 3 in order of increasing quality requirements), the acceptable metallurgical microstructure for each process, and the inspection methods. AGMA 946-A21, Recommended Practice for Carburized Aerospace Gearing, focuses specifically on carburized case quality for high-cycle aerospace and high-performance gearing. AGMA 2001-D04 (along with its metric companion 2101-D04), Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth, defines the bending and pitting fatigue strength factors that translate hardness and case depth into rated torque capacity — the gear hardness specification traces back to the required torque rating through this document. ISO 6336-5 provides the international equivalent, defining the same hardness grades (ML, MQ, ME for minimum, medium, and enhanced quality) with their associated minimum hardness, case depth, and microstructure. Heavy-gear drawings typically call out the applicable hardness grade, the case-depth range if surface-hardened, the core hardness, and the applicable standard — for example: "Through-harden to 32–38 HRC per AGMA 923, Grade 2" or "Carburize and harden to Grade MQ per ISO 6336-5, case depth 0.060–0.090 inch, core hardness 30–40 HRC". The heat treater is responsible for processing to the called-out grade and documenting the metallurgy; the buyer is responsible for translating the application's torque rating into the correct grade specification (AGMA 923-C22; AGMA 946-A21; AGMA 2001-D04; ISO 6336-5).

What documentation should accompany a heat-treated heavy gear?​

A heavy gear's heat-treatment documentation package has more content than a simple shaft or plate weldment because of the multiple processes involved and the AGMA/ISO grade requirements the gear is normally specified to. The standard package covers: material certificate for the forging or billet, with chemistry analysis; austenitizing cycle parameters and the furnace temperature record; quench medium, agitation, and actual cooling-rate data where available; temper parameters and temperature record; hardness verification at specified positions (for through-hardened gears, typically on a prolongation or a sample from the same heat-treat lot; for through-hardened gear rims, at a test plug location on the blank rim); for induction-hardened teeth, the case-depth verification by microhardness traverse on a sample tooth and per-tooth surface hardness readings; for carburized gears, the carbon-profile data from test coupons, the quench-medium record, and microstructure evaluation; and applicable NDE records (magnetic-particle or eddy-current for through-hardened gears, ultrasonic for the blank before cutting). For AGMA Grade 2 or Grade 3 gearing — and for any gear subject to ISO 6336-5 ME (enhanced) grading — the documentation must substantiate the grade claim: a Grade 2 carburized gear, for example, requires documented core-hardness minimums, effective-case-depth range, carbon-profile shape within specification, and microstructure free of excessive retained austenite or intergranular oxidation. Without this documentation, the gear cannot be certified to the grade called out on the drawing regardless of whether the processing was actually performed to that level (AGMA 923-C22; ASTM E18; ASTM E384; AMS 2750; ASTM E709).

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References​

  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
  • ASM International. (2014). ASM Handbook, Volume 4C: Induction Heating and Heat Treatment. ASM International.
  • AGMA 923-C22: Metallurgical Specifications for Steel Gearing. American Gear Manufacturers Association.
  • AGMA 946-A21: Recommended Practice for Carburized Aerospace Gearing. American Gear Manufacturers Association.
  • AGMA 2001-D04: Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth. American Gear Manufacturers Association.
  • AGMA 2004-C08: Gear Materials, Heat Treatment and Processing Manual. American Gear Manufacturers Association.
  • ISO 6336-5: Calculation of Load Capacity of Spur and Helical Gears — Part 5: Strength and Quality of Materials. International Organization for Standardization.
  • 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.
  • ASTM E709: Standard Guide for Magnetic Particle Testing. ASTM International.
  • SAE J1268: Hardenability Bands for Carbon and Alloy H Steels. SAE International.
  • AMS 2750: Pyrometry. 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.

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