Choosing carburizing for gear applications often creates a dilemma: you need hard surfaces for wear resistance, but worry about creating brittle cores that crack under shock loads. With extensive experience in gear heat treatment across aerospace and industrial applications, understanding the balance between surface hardness and core toughness is critical for avoiding costly failures in high-stress assemblies.
No, carburizing will not make your gear core too brittle when properly executed. Correct carburizing maintains a tough, ductile core (25-40 HRC) while creating a hard surface layer (58-62 HRC). Core brittleness only occurs from over-carburizing, using inappropriate steels, or inadequate tempering cycles.
Learn how to avoid brittle gear cores during carburizing with the right material, case depth, and quality checks—based on real production experience.
Table of Contents
When does carburizing make sense for my gear design?
Carburizing makes sense when your gears face heavy contact stress, abrasive wear, or long service life requirements while maintaining core toughness for shock loads. Best for applications requiring surface hardness above 58 HRC while preserving ductile cores, typically in industrial drives, automotive transmissions, and precision equipment.
Use carburizing when:
- High-cycle applications (100,000+ cycles)
- Contaminated or abrasive environments
- Surface wear is the primary failure mode
- Standard tooth profiles with adequate machining stock
Avoid carburizing when:
- High-shock, low-cycle applications
- Fine-pitch gears (below 2 module)
- Complex internal features requiring precise post-machining
- Through-toughness matters more than surface hardness
From our machining experience, carburized gears work well in servo motor gearboxes (1,000-3,000 RPM), industrial mixer drives, and conveyor systems. Carburized gears achieve 58-62 HRC surface hardness with 25-40 HRC core hardness, providing wear resistance while maintaining impact toughness. Simple spur and helical gears machine well post-carburizing to ±0.0002″ tolerances, while complex geometries require careful planning due to heat treatment distortion.
Design Takeaway: Choose carburizing for high-cycle, wear-dominated applications in harsh environments. Ensure your design allows standard tooth profiles and sufficient machining stock for post-heat treatment finishing.
Which gear steels work best with carburizing?
8620 steel offers the best balance of carburizing response and post-heat treat machinability for most gear applications. 8620 has excellent machinability and provides good core toughness while achieving high surface hardness after carburizing, making finish machining operations more predictable.
Machining difficulty ranking (1=easiest, 5=hardest):
- 8620 – Difficulty: 2/5, best overall machinability after carburizing
- 4320 – Difficulty: 3/5, medium nickel content provides modest strength increase but harder on tools
- 9310 – Difficulty: 4/5, superior hardenability and core strength but requires premium tooling
- 4140 – Difficulty: 3/5, can be carburized but typically harder to machine than 8620
Decision matrix:
- Choose 8620 if: Final tolerance requirements ≤ ±0.05mm, standard carbide tooling
- Choose 4320 if: Moderate load increase needed, tolerance requirements ≤ ±0.03mm
- Choose 9310 if: Maximum strength required, have ceramic/CBN tooling capability
From our experience, 8620 steel is easier to machine in the annealed state than 4320 or 9310 and maintains this advantage post-carburizing. After heat treatment, machining should be minimal to retain heat treatment results, making steel selection critical for final operations.
Design Takeaway: Start with 8620 for standard applications requiring post-carburizing machining to ±0.05mm. Upgrade only when strength requirements exceed 8620’s capability and you can accommodate the additional machining complexity.
How does carburizing compare to nitriding for gear applications?
From a machining perspective, nitriding allows finish machining before heat treatment while carburizing requires significant post-processing operations. Nitriding is often performed on finish-machined components with little distortion and usually requires no post-heat treatment machining.
Machining workflow comparison:
Carburizing workflow:
- Rough machine → carburize → finish machine/grind to final dimensions
- Gears must be ground after carburizing to achieve final shape and tolerance due to deformations from high process temperatures Requires hard machining capabilities and premium tooling
Nitriding workflow:
- Finish machine → stress relieve → nitride (minimal final operations)
- Nitriding eliminates or reduces several finishing operations including hard finish machining
- Gears can be machined undersize and grown into tolerance during nitriding
Decision criteria:
- Choose nitriding if: Final tolerance requirements < ±0.02mm, want to avoid hard machining
- Choose carburizing if: Maximum surface durability needed, tolerance budget > ±0.05mm allows post-machining
- Choose nitriding if: Complex geometries that are difficult to machine after hardening
Design Takeaway: Nitriding eliminates post-heat treat machining challenges but limits case depth. Carburizing provides deeper cases but requires planning for significant finish machining operations and associated costs.
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What design risks should I flag before carburizing?
All dimensions change during carburizing, requiring careful stock allowances and tolerance planning. Expect distortion of 10% of case depth – for 0.040″-0.060″ case depth, parts can move 0.004″-0.006″ during the carburizing process.
Stock allowance calculator:
- Standard features: Add 0.2-0.3mm stock on all machined surfaces
- Precision bores: Add 0.1-0.2mm diameter, plan for post-carburizing boring
- Gear teeth: Plan for finish grinding – typically 0.1-0.15mm per flank
- Threading: Use stop-off paints to prevent carburizing threads as they’ll be significantly out of tolerance
Go/No-Go decision criteria:
- Avoid carburizing if: Total tolerance budget < 0.3mm across all features
- Avoid carburizing if: Internal threads or precision bores cannot be masked
- Plan carefully if: Multiple datum surfaces need re-establishing after heat treatment
- Avoid carburizing if: Part geometry prevents adequate fixturing for hard machining
Critical machining risks:
- Material grows and distorts past critical temperature, with quenching adding to distortion
- 8620 can be dimensionally unstable during carburization with unpredictable movement
- Thin sections may through-harden, eliminating machinable core for finish operations
Design Takeaway: Budget 0.3mm minimum total tolerance allowance for carburizing. Design all critical features to allow post-heat treat machining access, and plan fixturing strategy for hard case operations before committing to carburizing.
What case depth should I specify for high-load carburized gears?
Specify 0.8-1.2mm effective case depth for high-load gears when post-carburizing machining is required. Finished case depth of 0.025-0.035″ (0.6-0.9mm) after grinding provides adequate performance while allowing our standard tooling to achieve final tolerances cost-effectively.
Machining cost decision matrix:
- 0.8-1.2mm: Standard tooling, ±0.01mm achievable, 2x machining cost
- 1.2-1.5mm: Industrial/heavy truck range 0.045-0.065″ (1.1-1.7mm) – ceramic tools required, 3x cost
- >1.5mm: Grinding typically required, 5x cost
Vendor capability assessment: Ask your machining vendor: “Can you finish machine 58-62 HRC surfaces to ±0.01mm?” If yes, they have premium tooling. If they only guarantee ±0.02mm on hardened surfaces, limit case depth to 1.0mm. Avoid through-hardening thin sections as they become very brittle and difficult to machine.
Choose cases under 1.2mm for conventional machining. Tool life decreases exponentially above 1.2mm – expect 3-5x longer cycle times. Cases to 0.200″ (5.0mm) are possible but require specialized grinding operations.
Design Takeaway: For conventional post-machining, stay within 0.8-1.2mm case depth. Always verify your vendor’s hardened surface machining capabilities before specifying case depth.
How does carburizing affect gear performance under loads?
Carburizing improves both impact and fatigue performance, but gears optimized for maximum fatigue life require deeper cases that significantly increase post-machining costs. Fatigue strength has an increase-decrease trend with increasing effective case depth, creating a trade-off between performance and machinability.
Load type assessment:
- Fatigue-dominated: >100,000 cycles, steady loads, surface wear primary concern
- Impact-dominated: Shock loads >3x normal operating load, sudden starts/stops
- Mixed service: Both conditions present
Application decision criteria:
- Fatigue-optimized (1.2-1.5mm): Maximum cycle life but 300% machining cost increase
- Impact-optimized (0.8-1.0mm): Good shock resistance with baseline machining cost
- Balanced approach (1.0mm): 80% of maximum performance, 2x machining cost
Performance validation: The “80% performance” rule means accepting 20% shorter fatigue life for 60% lower machining costs. For most industrial applications, this trade-off improves overall value. Fatigue limit increased from 651.36 MPa to 918.88 MPa after treatment, but achieving maximum requires cases that increase finishing costs by 200-400%.
Impact fatigue resistance improved with increasing nickel content – steels like 4320/9310 allow shallower cases while maintaining impact resistance.
Design Takeaway: If your gear sees primarily steady loads with occasional shock, choose impact-optimized design (0.8-1.0mm case) to minimize post-machining complexity and co
How much will my gear distort during carburizing?
Gears typically grow 0.1-0.3mm on external diameters and shrink on internal features, requiring 0.2-0.3mm machining stock allowance on all critical surfaces. Size change is growth or shrinkage from original machined dimensions, and our finishing operations must remove this stock while maintaining tight tolerances.
Stock allowance calculator for your gear:
- Standard tolerance (±0.05mm): 0.2mm stock allowance
- Precision tolerance (±0.02mm): 0.3mm stock allowance
- Gear diameter >200mm: Add additional 0.1mm for size-related distortion
Drawing specification guidance: “Re-establishing datums” means all your reference surfaces will change dimensions. Specify on drawings: “Post-carburizing machining allowance included” and identify which surfaces will be finish-machined. Avoid tight positional tolerances between carburized and non-carburized features.
Large gears deform in drum shape with expansion at ends, middle section shrinkage, while inner splines show stable shrinking. Helix angles change from 15° to 14° after heat treat – specify this change in your gear tooth design.
Process selection impact: Low-pressure carburizing reduced helix variation by 59% for left flank and 48% for right flank compared to conventional processing. If distortion control is critical, specify vacuum/low-pressure carburizing to reduce stock allowance to 0.15mm.
Design Takeaway: Add 0.3mm to all critical dimensions in your CAD model, and specify which surfaces require post-carburizing machining on your drawings. Simple geometries reduce finishing complexity by 50%.
Will carburizing cause uneven hardness on complex gear shapes?
Yes, complex gear shapes often develop uneven case hardness that increases your total machining costs by 30-50% due to required adaptive finishing strategies. The pitch to root ratio of effective case depth after conventional carburizing is often uneven, with case depth in tooth roots sometimes only half the depth at mid-pitch location.
Go/No-Go complexity assessment:
- Avoid carburizing if: Multiple internal corners <1.5mm radius, thin-to-thick ratios >3:1
- Plan for higher costs if: Internal splines, deep keyways, or complex transitions
- Good for standard carburizing: Simple external gears, generous radii ≥1.5mm
Specific geometry cost impacts:
- Sharp corners (<1.5mm radius): Hand finishing required, +2-3 days
- Internal splines/keyways: Different cooling rates cause excessive stresses, specialized tooling needed
- Thin-to-thick transitions: Create hardness gradients, double finishing time
- Deep tooth roots: Half the case depth of pitch line, uneven grinding required
Cost reduction options: Specify minimum 1.5mm radii on internal corners. Vacuum carburizing achieves better carbon profiles in gear tooth roots – adds 15-20% to heat treatment cost but reduces finishing costs by 25-40%.
Design Takeaway: Budget 30-50% extra machining cost for complex geometries. Specify generous radii (≥1.5mm) and gradual transitions to minimize hardness variations.
What tolerances are achievable after carburizing and finishing grinding?
Grinding after carburizing achieves ±0.005-0.01mm on gear teeth and ±0.002-0.005mm on cylindrical surfaces, but tighter tolerances significantly increase your project costs. Grinding achieves finishes of 8 to 32 microinches (0.2 to 0.8 micrometers) and tolerances up to ±0.0001 inch (±0.0025mm) on hardened carburized surfaces.
Tolerance vs cost/schedule decision matrix:
- ±0.01mm: Standard grinding, baseline cost, 3-5 days
- ±0.005mm: Precision grinding, 2x cost, 7-10 days
- ±0.002mm: Ultra-precision capability, 3-4x cost, 2-3 weeks
Critical planning rule: 40% of case depth must remain after grinding to maintain performance. If your case depth is 1.0mm, plan for 0.4mm minimum remaining.
Avoid unnecessarily tight tolerances – excessive grinding reduces surface compressive stresses and gear performance. Stress relief after final grinding adds 2-4 hours processing time.
Design Takeaway: Specify ±0.01mm unless your application truly requires tighter tolerances. Each level of precision roughly doubles your grinding costs and extends lead times significantly.
What post-processing steps are needed after carburizing gears?
Budget 40-60% additional cost and 5-10 days extra lead time for standard post-carburizing operations: stress relief, final machining, and inspection. Processing flow includes carburizing and quenching → shot peening → finish turning → gear grinding.
Standard operations cost breakdown:
- Stress relief: Done before final finishing, +1-2 days, +10-15% cost
- Final machining: Bearing surfaces, keyways, critical features, +3-5 days, +25-35% cost
- Quality inspection: CMM measurements with 2-micron accuracy, +1 day, +5-10% cost
Cost reduction strategies:
- Choose vacuum carburizing: Eliminates shot peening operations, reduces costs by 15-20%
- Simplify design features: Avoid hand finishing requirements
- Eliminate non-critical tolerances: Reduce inspection time
Optional operations:
- Shot peening: +30% fatigue life, +20% cost, may require subsequent machining
- Superfinishing: 0.5 microinch finishes for sealing, +40-60% finishing cost
Total timeline: 7-12 days standard operations, 15-20 days with optional operations.
Design Takeaway: Plan for 50% cost increase and 7-10 days additional lead time. Design with standard operations in mind to control project costs and schedules.
What design risks should I flag before carburizing?
Flag designs with thin walls under 3mm, sharp internal corners under 1.5mm radius, or features that cannot accommodate 0.2-0.3mm machining allowance – these create expensive rework and potential part failure. Improper treatment can lead to excessive surface carburization, making the tooth tip brittle and prone to chipping, with susceptibility to cracking during grinding operations.
Design risk assessment checklist:
- High-risk features: Thin walls <3mm, sharp corners <1.5mm radius, unsupported spans >50mm
- Medium-risk features: Complex internal splines, multiple keyways, abrupt thickness changes
- Low-risk features: Simple external gears, generous radii ≥2mm, uniform cross-sections
The biggest cost risk is designing features that are through-hardened completely. Different cross sections may have different cooling rates causing excessive stresses and breakage, leading to 100% scrap rates. Sharp internal corners concentrate stress and higher carburizing temperature and longer time increase degree of over-carburization on surface, making subsequent machining extremely difficult.
Material preparation also affects costs. The blank must be normalized or annealed at temperatures that exceed planned carburizing temperature by 50 degrees F or expect increased distortion. Insufficient time or temperature during normalizing results in greater distortion during subsequent heat treatment steps, adding rework costs.
Plan support strategies for your geometry since gravity can cause sagging and bending if sections are not properly supported during high temperatures and long processing times. Large, flat sections are particularly vulnerable and may require custom fixturing that adds 20-30% to processing costs.
Design Takeaway: Use the risk checklist to identify problematic features early in design. High-risk features typically double your post-carburizing machining costs and may require process upgrades or design changes to avoid failure.
How much does carburizing add to gear production time and cost?
Carburizing typically adds 15-40% to total gear production cost and 3-7 days to lead time, depending on case depth and process selection. Boost-diffuse carburizing achieves 25% time savings compared to single carbon potential carburizing, making process selection critical for project economics.
Time varies dramatically by case depth: Parts are heated for 2 to 36 hours depending on depth of case required. Shallow cases (0.5-0.8mm) typically need 8-16 hours, while deep cases (1.5-2.0mm) require 24-48 hours. Thick case depths take days of carburizing cycle time, significantly impacting project schedules.
Process selection creates major time differences. Advanced microwave carburizing reduced cycle time from 272 minutes to 212 minutes for equivalent case depth. Vacuum carburizing enables much faster times than atmospheric carburizing due to higher operating temperatures, though increasing temperature costs more in alloy and fixture usage.
Material and heat treatment has the largest cost impact in automotive gear units, making carburizing optimization crucial for competitiveness. Plasma carburizing offers shorter processing times and reduced running costs but requires higher equipment investment.
Quality factors also affect schedule – pack carburizing case depth variation often exceeds 0.25mm, requiring additional quality control time. Gas carburizing methods are faster and lend themselves to mechanized material handling, reducing total cycle time compared to manual processes.
Design Takeaway: Budget 15-40% cost increase and 3-7 days for carburizing. Minimize case depth to required levels and consider boost-diffuse or vacuum processes for 20-25% time savings on high-volume projects.
Conclusion
Carburizing significantly improves gear performance but requires careful planning for post-machining operations, distortion control, and tolerance management. Smart design choices minimize finishing complexity and costs while maximizing gear durability. Contact us to explore manufacturing solutions tailored to your carburized gear requirements – we’ll help optimize your design for both performance and machinability.
Frequently Asked Questions
Yes, we provide free design reviews focusing on post-carburizing machinability. We’ll identify potential issues like inadequate machining allowances, difficult-to-fixture geometries, or features that may cause uneven hardness. This prevents costly surprises after heat treatment.
Provide STEP files showing both pre-carburizing and final dimensions, plus drawings specifying which surfaces need post-carburizing machining. Include case depth specifications and any critical tolerances. This helps us accurately estimate machining complexity and costs.
Standard finishing operations take 5-10 days after receiving carburized parts. Complex geometries or tight tolerances may require 10-15 days. Rush service is available for critical projects with 2-3 day turnaround at premium pricing.
We typically handle orders from single prototypes to 500+ pieces. Carburized gear finishing is cost-effective even for small quantities since the setup complexity is similar regardless of volume. Prototype quantities help validate your design before committing to larger production runs.
Yes, we regularly finish gears carburized by other suppliers. However, we need to verify case depth and hardness distribution first, as poor carburizing quality can damage our tooling. Send us a sample piece or hardness report for evaluation before quoting.
We can often salvage parts with moderate distortion through adaptive machining strategies, though this increases costs by 30-50%. For future orders, we’ll recommend design changes or process improvements to prevent recurrence. Severe distortion may require redesign or re-carburizing.
