Deciding whether your CNC machined part needs case hardening isn’t always straightforward. With years of experience machining precision components for aerospace, medical, and industrial applications, we’ve seen engineers struggle with this decision — often over-specifying treatments that add cost or missing opportunities where case hardening could dramatically improve part performance and longevity.
Case hardening is essential when your CNC machined part faces high surface wear, impact loads, or cyclic stress while requiring a tough, ductile core. Low-carbon steels (AISI 1020) and alloy steels (4140) are ideal candidates, while parts needing uniform hardness throughout may benefit more from through-hardening alternatives.
Learn how to select materials, processes, and geometry for case hardening—plus cost tips and validation methods to ensure reliable performance.
Table of Contents
What Operating Conditions Make CNC Parts Need Case Hardening?
Consider case hardening when your CNC machined parts face contact pressures exceeding 100 MPa, sliding distances over 1,000 meters annually, or cyclic loads beyond 10^6 cycles. Components like gears, bearings, camshafts, automotive drivetrain parts, and precision tooling operating under these conditions show dramatic performance improvements with case hardening‘s hard surface and tough core combination.
We’ve measured significant performance differences in real applications using strain gauge analysis and accelerated testing protocols. Machined 1018 steel conveyor shafts without case hardening typically fail after 6-8 months under continuous operation, while case-hardened equivalents maintain dimensional stability beyond 3 years. Similarly, machined gear teeth in high-torque applications show measurable wear (0.05-0.1 mm) after 50,000 cycles, while case-hardened versions with 0.8-1.2 mm case depth maintain original profiles past 500,000 cycles.
For gear applications specifically, ISO 6336 standards recommend case hardening when contact stresses exceed 1,200 MPa — a threshold we validate through finite element analysis during design reviews. Impact loading above 50 Joules per cycle, determined through drop testing or operational monitoring, also indicates case hardening requirements.
Quick Decision Checklist:
- Contact pressure > 100 MPa? → Case hardening recommended
- Expected cycles > 10^6? → Case hardening recommended
- Part replacement cost > $500 with critical downtime? → Case hardening justified
A machined 4140 steel shaft in a paper mill application costs $200 to replace but requires 8 hours downtime ($3,000 lost production) — making case hardening economically justified even though it adds $50 per part. We verify case depth effectiveness using Vickers hardness profiles and optical microscopy to ensure 0.8-1.5 mm penetration meets wear resistance requirements.
Design Takeaway: Use case hardening when operating conditions exceed standard machined steel capabilities, validated through proper stress analysis and fatigue testing to ensure the treatment matches your actual service requirements.
Which Machined Materials Work Best for Case Hardening?
AISI 1020 steel is the best material for case hardening most CNC machined parts due to its low carbon content (0.18-0.23%) and cost-effectiveness at $3-4/lb. Alloy steels like 4140 work for high-stress applications but cost 40-75% more and require carbide tooling, making them suitable only when case depths exceed 1.2 mm.
Low-carbon steels below 0.25% carbon allow proper carbon diffusion during carburizing while maintaining core ductility. We specify 1020 steel for machined conveyor rollers and automotive brackets where 0.8-1.0 mm case depth meets wear requirements at lowest cost. Standard 1020 ships immediately and machines easily with HSS tooling.
Alloy steels like 4140 contain chromium and molybdenum that enhance hardenability, enabling deeper penetration up to 2.0 mm. This makes 4140 ideal for machined hydraulic shafts and transmission components where subsurface fatigue resistance justifies the premium. However, 4140 requires carbide tooling and 30% slower cutting speeds, adding $50-100 to machining costs.
Procurement varies significantly—1020 steel ships immediately while certified 4140 may require 2-4 weeks for special sizes. Avoid high-carbon grades like 1080 which become brittle during treatment. Non-ferrous metals including aluminum and copper cannot undergo carbon diffusion and require alternatives like anodizing.
Design Takeaway: Select 1020 steel for cost-effective case hardening of most machined components. Upgrade to 4140 only when case depth or core strength requirements exceed 1020’s capabilities.
When Should I Case Harden vs Through-Harden My Machined Part?
Use case hardening for parts with localized wear surfaces and wall thickness over 10 mm. Use through-hardening for parts needing uniform hardness throughout, like cutting tools or thin sections under 8 mm. Case hardening creates hard surfaces (58-62 HRC) with tough cores (25-35 HRC), while through-hardening provides consistent 45-55 HRC but increases brittleness.
Through-hardening suits cutting tools, springs, and structural elements under uniform stress distribution, creating consistent hardness throughout the cross-section. However, this sacrifices core toughness and increases brittleness risk in thick sections where rapid cooling creates internal stresses.
Case hardening excels for machined gears, bearing races, and cam surfaces where contact stresses concentrate at specific locations while bending loads require ductile cores. Industrial mixer shafts under combined wear and shock illustrate this—through-hardened versions failed within 8 months due to brittle fracture, while case-hardened equivalents operate 4+ years successfully.
Dimensional stability often drives decisions. Case hardening maintains ±0.03 mm on precision features, while through-hardening requires 0.2-0.4 mm grinding stock, adding finishing costs. Cost analysis shows case hardening adds $40-90 per part versus $20-50 for through-hardening, but extended service life justifies the premium. Processing time differs—3-5 days for case hardening versus 1-2 days for through-hardening.
Design Takeaway: Apply case hardening to thick parts with localized wear requiring core toughness. Reserve through-hardening for thin components needing uniform properties throughout the cross-section.
How Should I Design Machined Geometry for Case Hardening?
Maintain uniform wall thickness within 2:1 ratios and use minimum 0.5mm radii on all internal corners to prevent uneven heat treatment and stress concentrations that cause part failure. Sharp geometry transitions create differential thermal expansion and carbon diffusion problems during surface treatment processing.
Wall thickness consistency prevents warpage during thermal cycling. Parts with abrupt section changes from thin to thick areas experience uneven heating rates, causing dimensional distortion that exceeds ±0.1mm on critical features. We redesigned a machined aluminum housing from variable 4-16mm walls to consistent 10mm thickness, eliminating $400 per part in post-treatment corrections.
Internal corner radii distribution affects stress concentration and carbon penetration uniformity. Replace sharp 90-degree intersections with generous radii—minimum 0.5mm for light-duty applications, 1.5-2.0mm for structural components under cyclic loading. A customer’s control valve body experienced cracking at sharp edges until we specified 2mm radii, completely eliminating field failures.
Feature depth limitations ensure proper treatment penetration. Blind holes, slots, and pockets with depth-to-width ratios exceeding 4:1 show poor atmospheric circulation and uneven results. Design deep features as removable inserts when functionally necessary, allowing individual processing before final assembly.
Mounting and fixturing considerations affect part orientation during furnace operations. Include stable reference surfaces or temporary mounting bosses that support proper positioning without blocking gas flow around critical areas requiring uniform treatment.
Design Takeaway: Follow consistent wall thickness principles, eliminate sharp corners with appropriate radii sizing, and limit deep narrow features. Early geometry review prevents expensive redesigns after initial machining operations.
Which Case Hardening Process Suits My Machined Component?
Carburizing delivers maximum surface hardness (60-65 HRC) for high-wear applications requiring deep penetration. Nitriding provides superior dimensional control (±0.01mm) for precision parts. Induction offers rapid selective treatment of specific zones like bearing surfaces. Choose based on hardness requirements, tolerance constraints, and whether full or partial treatment is needed.
Process Selection Matrix:
- Maximum wear resistance → Carburizing (gear teeth, bearing races)
- Tight dimensional control → Nitriding (measuring instruments, molds)
- Selective surface treatment → Induction (shaft journals, wear bands)
- Fastest turnaround → Induction (1-2 day processing)
Carburizing excels for applications demanding maximum surface hardness and deep carbon penetration up to 2.0mm depth. Heavy equipment gears requiring 100,000+ operating cycles benefit from carburizing’s superior wear characteristics, despite 10-14 day processing schedules and potential grinding requirements on precision surfaces.
Nitriding maintains exceptional dimensional stability through lower processing temperatures (520-580°C), making it ideal for tight-tolerance components like injection mold cavities or precision measuring tools. The process creates thinner but extremely hard surface layers (0.2-0.6mm) with built-in corrosion resistance, eliminating secondary coating operations.
Induction treatment heats specific areas using electromagnetic fields, allowing precise control over hardened regions while preserving soft zones for machining or assembly operations. This approach suits cylindrical parts requiring localized enhancement of wear surfaces, contact areas, or stress concentration zones with minimal overall part distortion.
Design Takeaway: Select carburizing for maximum durability in severe service, nitriding for dimensional precision requirements, and induction for targeted enhancement of specific component areas requiring rapid processing turnaround.
Is Case Hardening Worth the Cost for My CNC Project?
Case hardening justifies its $60-140 per part cost when total failure expenses exceed three times the treatment premium over expected service life, considering replacement parts, labor, and downtime costs together. The break-even analysis becomes straightforward when you calculate annual failure costs versus one-time treatment investment.
Manufacturing applications demonstrate clear economic benefits through extended maintenance intervals. A plastics extrusion company reduced annual screw replacement costs from $22,000 to $4,500 by treating machined barrel components, despite paying $95 per part for processing. The 18-month service life versus previous 3-month intervals eliminated monthly shutdowns worth $12,000 each in lost production time.
Volume economics affect treatment viability—annual quantities exceeding 150 components create economies through furnace load optimization, reducing per-part expenses 20-30%. Regular customers secure processing rates below $65 per part for standard configurations, while single custom parts rarely justify treatment costs.
Hidden expenses include quality control requirements and potential geometric modifications. Surface treatment may cause 0.08-0.20mm dimensional changes requiring secondary grinding operations that add $90-200 to final costs. Energy sector and mining equipment suppliers routinely specify treatment despite premium expenses because equipment failures create operational exposure exceeding processing investments.
Design Takeaway: Calculate five-year total ownership expenses including failures, maintenance, and operational disruption. Surface treatment becomes economically attractive when combined costs exceed processing investment by 3:1 margins.
Conclusion
Case hardening transforms CNC machined parts when surface wear, impact resistance, or fatigue concerns justify the treatment investment. Success depends on proper material selection, geometric design, process matching, and economic validation against total ownership costs. Contact us to explore manufacturing solutions tailored to your case hardening requirements.
Frequently Asked Questions
Yes, through selective masking or induction hardening. Copper plating masks areas during carburizing, while induction hardening treats only heated zones. This approach preserves soft areas for machining threads, keyways, or assembly features.
Microhardness testing using Vickers or Rockwell scales measures surface and case-to-core transition. Metallographic cross-sections reveal case depth and structure quality. Parts require inspection certification for aerospace or medical applications.
Yes, case hardening is typically performed after final machining operations. Parts are machined to near-final dimensions, then heat treated, with minimal finish grinding on critical surfaces if needed. This sequence prevents machining difficulties on hardened surfaces.
Specify case depth (e.g., “0.8-1.2mm case depth”), surface hardness (e.g., “58-62 HRC surface”), and applicable zones. Include core hardness requirements (e.g., “25-35 HRC core”) and reference standards like SAE J429 for automotive applications.
Case hardening can cause 0.05-0.15mm dimensional growth. Threaded holes may require retapping after treatment, and press-fit tolerances should account for size changes. Plan interference fits accordingly or specify post-hardening machining operations.
Case hardening costs $60-140 per part versus $150-400 for hard chrome plating. Case hardening provides better fatigue resistance and dimensional stability, while coatings offer superior corrosion protection and can be applied to non-ferrous materials.
