Choosing hardened steel for CNC parts means balancing performance needs against machining complexity and cost. From aerospace to medical applications, the right material decision impacts both functionality and manufacturability.
Use hardened steel when you need wear resistance above 45 HRC, high-cycle fatigue resistance, or surface durability under friction. Most structural components perform adequately with standard 1018 or 4140 steel at lower cost and better machinability.
Learn to assess loading conditions, choose the right steel hardness, and avoid overengineering mistakes that increase machining costs.
Table of Contents
When should I choose hardened steel for my CNC part?
Choose hardened steel when your part has sliding contact surfaces, operates above 10,000 cycles, or must maintain dimensional accuracy under repeated loading. Key indicators include metal-to-metal friction, abrasive environments, or wear-related failure modes within your product’s warranty period.
From our design consultation experience, hardened steel becomes necessary when surface degradation affects product function. Parts like bearing races, cam followers, and pivot pins typically require 45-60 HRC hardness to maintain performance specifications. We’ve reviewed designs where standard steel wore significantly under sliding contact, causing assembly tolerance problems and field failures.
However, most structural applications don’t justify hardening complexity. Chassis components, mounting brackets, and static housings rarely experience the sliding contact that degrades surfaces. Standard carbon steels like 4140 (unhardened) provide adequate strength for tensile and compressive loads without added manufacturing considerations.
Per ASTM A681 specifications, tool steels achieve 58-65 HRC for high-wear applications, while structural components perform well at 25-35 HRC in their annealed state.
Design Situation Indicators – Use this checklist: ✓ CAD model shows sliding surfaces, ✓ Product specifications include wear resistance requirements, ✓ Expected cycle count exceeds 10,000 operations, ✓ Surface wear would affect critical dimensions, ✓ Field testing shows degradation in standard materials.
Design Takeaway: Specify hardened steel only when surface durability directly impacts your product’s performance or lifespan. Reserve hardening for verified high-wear applications – structural loads rarely justify the complexity.
How do I avoid overengineering with hardened steel?
Use hardened steel only when standard steel fails your performance requirements, and specify the minimum hardness needed for your application – typically 40-45 HRC for most wear applications. Overengineering occurs when designers default to maximum hardness without validating actual performance needs.
Apply this decision framework: If your part experiences sliding contact with cycle counts above 50,000, hardening justifies the cost premium. Below 50,000 cycles, evaluate whether surface treatments on standard steel meet your needs at lower cost. We’ve consulted on medical device projects where designers specified 60 HRC for housings that performed adequately at 40 HRC, reducing manufacturing costs by 35%.
Red Flags You’re Overengineering: Specifying hardened steel for static structural parts, requiring full-part hardening when only contact surfaces need it, or choosing maximum hardness “just to be safe” without testing standard alternatives first. We regularly see aerospace brackets over-specified at 55 HRC when structural loading only required 25-30 HRC.
Per ASTM A29 and ISO 4957 specifications, carbon steels like 1045 achieve adequate wear resistance at 35-40 HRC for applications under 25,000 cycles, eliminating alloy steel complexity. Case hardening per ISO 3754 provides 58-62 HRC surface hardness while maintaining design flexibility for threaded holes in non-contact areas.
Validation Methods: Test prototypes at lower hardness levels first, or review field failure data to confirm hardening necessity. CMM inspection of wear patterns after accelerated testing validates actual hardness requirements.
Design Takeaway: Validate hardening necessity against measured performance data, not theoretical worst-case scenarios. Reserve full hardening for verified applications where surface degradation affects critical assembly tolerances.
Which steel grades work best for machining after hardening?
Choose pre-hardened P20 (32-36 HRC) for moderate wear applications or H13 (48-52 HRC) for higher performance requirements when your design includes complex features needing post-hardening operations. These grades come dimensionally stable and eliminate heat treatment distortion risks.
Design-Based Grade Selection Framework:
- Complex geometries with tight tolerances: P20 pre-hardened – prevents distortion that could affect ±0.005″ assembly fits
- Simple shapes requiring moderate hardness: 4340 custom hardened to 45-50 HRC – good strength without geometric restrictions
- High-wear surfaces, minimal secondary features: H13 pre-hardened – maximum performance for straightforward designs
- Maximum hardness requirements: A2 or D2 tool steels per ASTM A681 – design for EDM operations on complex features
We’ve reviewed designs where custom hardening caused 0.008″ distortion in thin-wall sections, requiring costly design revisions to accommodate dimensional changes. Pre-hardened materials eliminate this risk for parts with critical assembly interfaces.
Design Risk Assessment: Parts with wall thickness under 0.125″, complex internal cavities, or tight tolerance requirements across large surfaces often distort during heat treatment. Consider design modifications or pre-hardened alternatives to maintain dimensional control.
Per ISO 4957 tool steel classifications, case-hardening grades like 8620 allow selective surface hardening to 60 HRC while preserving design flexibility in core areas requiring threaded holes or press-fit features.
Design Takeaway: Match steel selection to your geometric complexity and tolerance requirements, not just hardness goals. Pre-hardened grades offer dimensional predictability for complex designs; custom hardening suits simpler geometries.
What heat treatment options work for CNC machined steel parts?
Choose through-hardening for uniform strength requirements, case hardening when you need hard surfaces but soft cores for secondary features, or induction hardening when only specific areas require hardness. Your selection depends on which parts of your design experience wear and what features you need after heat treatment.
Heat Treatment Selection Matrix:
- Simple geometry + uniform hardness needed: Through-hardening (40-55 HRC throughout)
- Complex features + surface wear: Case hardening (58-62 HRC surface, 25-35 HRC core)
- Selective hardening + minimal distortion: Induction hardening (localized 50-60 HRC)
We’ve guided medical device projects where case hardening eliminated the need for design compromises, allowing threaded assembly features while providing wear resistance on sliding surfaces. In aerospace applications, induction hardening of bearing journals reduced part count by 40% compared to hardened insert designs.
Design Decision Framework: If your CAD model shows both wear surfaces AND secondary features (threads, press-fits, counterbores), choose case hardening. If only isolated areas need hardness, induction hardening preserves maximum design flexibility. Through-hardening works best for parts under 3″ length with uniform cross-sections.
Case hardening typically provides 0.020-0.100″ depth penetration, which protects contact surfaces while maintaining core toughness for threaded connections. Your tolerance specifications should account for slight surface variation from the hardening process.
Design validation through prototype testing with accelerated wear cycles helps confirm your hardness requirements match actual performance needs.
Design Takeaway: Select based on your wear pattern analysis and required secondary operations. Case hardening offers optimal design flexibility for parts needing both performance and post-treatment features.
Will heat treatment distort my machined part?
Yes, plan for 0.002-0.010″ dimensional change in your design tolerances and assembly clearances rather than trying to eliminate distortion. Parts with length-to-width ratios above 4:1 or wall thickness under 0.125″ typically experience maximum distortion and may require design modifications.
High-Risk Geometry Checklist: ✓ Wall thickness < 0.125″, ✓ Length-to-width ratio > 4:1, ✓ Asymmetrical cross-sections, ✓ Sharp transitions between thick/thin areas, ✓ Large material removal creating uneven mass distribution. We’ve measured 0.015″ bow in 8″ shafts and 0.008″ diameter growth in thin cylinders requiring design accommodation strategies.
Distortion-Minimizing Design Strategies:
- Maintain symmetrical cross-sections to balance thermal stresses
- Add 0.010-0.020″ stock allowance to critical surfaces for post-hardening machining
- Design modular assemblies where only high-wear components need hardening
- Specify pre-hardened materials for complex geometries requiring tight tolerances
Heat Treatment Method Impact on Design:
- Through-hardening: Plan for 0.005-0.015″ movement – avoid tight assembly tolerances
- Case hardening: Expect 0.002-0.008″ change – suitable for most assembly interfaces
- Induction hardening: Minimal 0.001-0.005″ distortion – best for precision applications
Higher carbon content steels tend to distort more during heat treatment, making alloy grades like 4340 more dimensionally stable than plain carbon alternatives. Your tolerance stack-up analysis should include heat treatment allowances from the design phase.
Design Takeaway: Integrate heat treatment distortion into your tolerance planning from the start. Design around distortion constraints rather than attempting to prevent them.
How should I design CNC parts that will be hardened after machining?
Design with heat treatment distortion in mind by adding stock allowances, maintaining symmetrical geometry, and planning critical features for post-hardening operations. The key is accommodating dimensional changes rather than trying to prevent them through design alone.
Design Risk Assessment Matrix:
- Wall thickness < 0.125″: High distortion risk – consider thicker sections or pre-hardened materials
- Length-to-width ratio > 4:1: Expect significant bow – plan for post-hardening straightening operations
- Asymmetrical cross-sections: Unbalanced thermal stress – redesign for symmetry when possible
We’ve guided bearing housing projects where asymmetrical wall sections caused 0.012″ distortion, requiring redesign with balanced geometry and 15% thicker walls to maintain structural integrity. Medical device manufacturers reduced scrap rates by 60% when they planned critical sealing surfaces for finish machining after heat treatment.
Critical Design Strategies:
- Add 0.010-0.020″ stock allowance on surfaces requiring tight tolerances after hardening
- Maintain minimum 0.125″ wall thickness to prevent cracking during quenching
- Design threaded holes and press-fits for post-hardening machining when tolerance requirements exceed ±0.005″
- Use symmetrical cross-sections to balance thermal stresses during cooling
Higher carbon content steels require additional design considerations for distortion accommodation. Cold-finished steel specifications provide dimensional stability guidelines for pre-hardening geometry planning.
Validation through prototype testing: CMM measurement before and after heat treatment validates your distortion predictions and confirms stock allowance adequacy for your specific geometry.
Design Takeaway: Integrate heat treatment accommodation into your initial design constraints. Plan geometry and tolerances around expected dimensional changes rather than treating distortion as a manufacturing issue.
What surface finishes can I apply to hardened steel parts?
Plan surface finish requirements during your design phase based on target hardness levels – conventional finishes work up to 45 HRC, while specialized processes are needed above 50 HRC. Your finish choice affects both heat treatment timing and final part cost.
Design Planning by Hardness Level:
- 30-45 HRC: Specify standard finishes like machining, polishing, conventional coatings
- 45-55 HRC: Plan for grinding or specialized operations – conventional machining becomes limited
- 55+ HRC: Design requires grinding, EDM, or laser processing for acceptable finishes
We’ve consulted on medical device projects where specifying Ra 0.4 μm finish requirements early determined both hardness selection and manufacturing sequence planning. Aerospace bearing designs achieved required surface quality by planning grinding operations into the heat treatment workflow from the design phase.
Functional Finish Selection Guide:
- Wear resistance needed: Specify hard coatings like TiN or CrN on hardened substrates
- Corrosion protection required: Plan coating compatibility with your target hardness level
- Assembly interfaces: Consider finish effects on press-fits and threaded connections
- Aesthetic requirements: Evaluate if decorative finishes can withstand your hardness needs
Surface texture specifications should consider achievable quality limits at your target hardness level. Finishes below Ra 0.8 μm typically require specialized operations on surfaces hardened above 50 HRC.
Design Decision Framework: Choose your hardness level based on both functional requirements AND achievable finish quality. Higher hardness may compromise surface finish options and increase manufacturing complexity.
Design Takeaway: Specify surface finish requirements early in your design process to validate compatibility with your planned hardness level. Balance performance needs against achievable finish quality and cost implications.
Conclusion
Hardened steel provides excellent wear resistance but requires careful design planning for heat treatment distortion and manufacturing complexity. Reserve hardening for verified high-wear applications and plan geometry around dimensional changes from the design phase. Contact us to explore manufacturing solutions tailored to your hardened steel part requirements.
Frequently Asked Questions
Yes, but plan for it during design. Case hardening allows post-treatment threading in the softer core, while through-hardened parts above 45 HRC typically require pre-machined threads. Add material stock for post-hardening operations when tight tolerances are critical.
Heat treatment typically adds 30-50% to base machining costs, depending on complexity and hardness requirements. Through-hardening is most economical for simple geometries, while selective hardening methods cost more but preserve design flexibility.
Maintain consistent wall thickness above 0.125″ and use symmetrical cross-sections to minimize distortion. Varying thickness creates uneven thermal stress. If thin sections are necessary, consider pre-hardened materials or modular designs separating hardened and unhardened components.
Hard coatings like TiN and CrN work well on hardened substrates. Traditional plating may require temperature limitations to avoid tempering. Plan coating compatibility with your target hardness level during the design phase.
Choose based on your stress distribution. Through-hardening suits parts under uniform stress, while case hardening works better for surface wear with core toughness needs. Case hardening often provides better design flexibility for complex parts.
Most wear applications perform adequately at 40-45 HRC. Reserve higher hardness levels (50+ HRC) for extreme wear conditions like metal-to-metal sliding under high pressure. Test prototypes at lower hardness first to avoid overengineering your requirements.
