Designing parts from hardened steel isn’t just about specifying the right HRC value — it’s about understanding how heat treatment fundamentally changes your machining options. After 15+ years machining everything from 60 HRC tool steel components to heat-treated aerospace brackets, we’ve learned that the biggest cost overruns happen when product developers don’t account for hard material constraints early in the design phase.
Most CNC shops can handle materials up to 45-50 HRC with standard tooling, but beyond that range, you’re looking at specialized hard-turning capabilities, carbide tooling, and 2-3x higher costs due to slower cutting speeds and frequent tool changes.
Learn 7 key design choices for cost-effective hardened parts—covering tolerance limits, geometry constraints, and pre vs post-hardening machining.
Table of Contents
What Design Features Should I Evaluate Before Machining Hardened Parts?
Start by identifying which features actually need hardness — often only wear surfaces, bearing areas, or cutting edges require full hardening. Consider splitting complex parts into soft and hard sections, or using selective hardening techniques like case hardening or nitriding to keep the bulk material machinable.
Use this quick assessment for your part:
- Features deeper than 3x width → Machine before hardening
- Tolerances tighter than ±0.05mm → Plan post-hardening finishing
- Internal threads smaller than M6 → Consider EDM or pre-hardening
- Thin walls under 2mm → Evaluate warping risk during heat treatment
- Sharp internal corners → Increase radii to 0.5mm minimum for hard machining
Ask This: Do more than 30% of my features fall into the categories above? If yes, consider splitting into soft/hard components or selective hardening.
We regularly see product developers save 40-60% on machining costs by redesigning one complex hardened part into two simpler pieces — one soft structural component and one small hardened insert. For medical device housings, we often recommend hardening only the sealing surfaces while leaving the bulk geometry in soft 316 stainless. Audio equipment faceplates follow a similar pattern — complex mounting geometry stays soft, while wear surfaces get selective case hardening.
Common redesign opportunities include deep blind holes, complex internal pockets, perpendicular intersections, and undercuts. These geometries work fine in soft materials but become cost-prohibitive once hardened above 45 HRC.
Design Takeaway: Map out which specific surfaces need hardness for function, then design around keeping those features simple and accessible. Consider hybrid approaches like hardened inserts, selective case hardening, or two-piece assemblies before committing to machining complex geometry in fully hardened materials.
Should You Machine Before or After Hardening Your Steel Parts?
For most hardened steel parts, machining before heat treatment offers better geometry flexibility and lower tooling costs. However, any features requiring tight tolerances (±0.025mm or better) typically need finish machining after hardening to compensate for heat treatment distortion. The choice depends on your tolerance requirements and feature complexity.
Pre-hardening works best when:
- Complex internal geometry (pockets, undercuts, intersections)
- Deep features with high length-to-diameter ratios
- Multiple setups or difficult tool access required
- Cost is a primary concern
Post-hardening works best when:
- Simple external features only (bearing surfaces, seal grooves)
- Tight final tolerances required (±0.01-0.025mm)
- Dimensional stability is critical
- Small batch sizes where setup costs matter less
Pre-hardening machining allows you to create complex internal geometries that would be nearly impossible once the material reaches 50+ HRC. We routinely machine complex aerospace brackets and medical housings in soft 4140 or tool steel, then send them for heat treatment. The trade-off is that critical dimensions often shift by 0.05-0.15mm during hardening, requiring stock allowance for post-heat-treat finishing on functional surfaces.
In medical device housings, we often see hybrid approaches where bulk geometry is machined soft, then critical sealing surfaces are finish-machined after hardening to hold ±0.01mm flatness requirements. Audio equipment faceplates follow a similar pattern — rough machine the complex geometry, harden for durability, then finish-machine the critical mounting surfaces.
Design Takeaway: Reserve post-hardening machining for simple external features and critical dimensions only. Design complex internal geometry assuming pre-hardening machining, and plan for 0.1-0.2mm stock removal on surfaces that need tight final tolerances.
Can My Hardened Material Still Be Machined After Heat Treatment?
Materials above 45-50 HRC require carbide tooling and specialized hard-turning capabilities. Most CNC shops can handle up to 50 HRC with proper setup, but beyond 55 HRC, you’ll need shops with dedicated hard machining equipment. Only about 20-30% of general CNC providers offer hard-turning capabilities.
Vendor qualification checklist for your RFQ:
- Ask: “What’s your maximum workable hardness?”
- Confirm: “Do you have ceramic or CBN tooling capabilities?”
- Verify: “Can you handle hard-turning operations above 50 HRC?”
- Request: “Show examples of similar hardness work you’ve completed”
Machinability by hardness range:
- 30-45 HRC: Most standard CNC shops, normal tooling
- 45-55 HRC: Specialized shops required, carbide tooling mandatory
- 55-62 HRC: Limited shop options, ceramic/CBN tooling needed
- Above 62 HRC: EDM often required, very few capable shops
Tool steel grades like A2, D2, and O1 machine reasonably well even when hardened, while high-carbon steels become extremely challenging above 50 HRC. We’ve successfully machined 58 HRC tool steel components, but expect to quote 5-8 shops since many will decline the work. Heat treatment also introduces residual stresses that can cause dimensional shifts during machining, requiring careful work holding strategies.
Design Takeaway: Plan to contact 2-3x more shops than usual for hardened work and specify hardness ranges (e.g., “50-55 HRC”) rather than exact values to give shops processing flexibility. Always verify hard-turning capabilities during vendor qualification to avoid project delays.
Do I Need to Avoid Tight Tolerances or Small Features in Hardened Parts?
Heat treatment causes dimensional changes of 0.05-0.15mm, making tolerances tighter than ±0.05mm expensive and requiring post-hardening finishing operations. Small features like holes under 6mm diameter, thin walls under 2mm, and sharp internal corners become difficult or impossible to machine economically in hardened materials above 50 HRC.
Tolerance guidelines by hardness:
- 30-45 HRC: ±0.025mm achievable with standard tooling
- 45-55 HRC: ±0.05mm practical limit without grinding
- 55+ HRC: ±0.1mm standard, tighter specs require grinding/EDM
Small feature limitations:
- Holes under 6mm: Difficult to drill, often need EDM
- Wall thickness under 2mm: High warping risk during heat treatment
- Internal corners under 0.5mm radius: Tool breakage risk increases significantly
- Thread pitch finer than M6: Often requires tapping before hardening
We regularly see parts designed with ±0.01mm tolerances throughout, when only bearing surfaces actually need that precision. A medical device housing we optimized went from requiring full grinding operations to standard hard-turning by relaxing non-critical tolerances from ±0.01mm to ±0.05mm — reducing cost by 60% with no functional impact.
Heat treatment distortion affects different geometries differently. Long, thin parts tend to bow, while complex shapes with varying wall thickness often twist or develop localized stress points. This is why aerospace components often use “net shape” forging followed by minimal finish machining rather than machining complex geometry from solid hardened bar stock.
Design Takeaway: Reserve tight tolerances (±0.025mm or better) for functional surfaces only, and increase minimum feature sizes when possible. Consider specifying “±0.05mm unless noted” as your general tolerance, with tight callouts only where assembly or function demands it.
Do I Need to Adjust Surface Finish Expectations for Hardened Materials?
Achieving Ra below 1.6 μm on hardened materials often requires grinding or specialized tooling, while standard hard-turning typically delivers Ra 3.2-6.3 μm finishes. Plan for secondary operations like grinding or polishing if your application requires fine finishes for sealing or cosmetic requirements.
Evaluate your surface finish needs:
- Does this surface seal fluids? → Ra ≤ 0.8 μm required
- Does this surface slide against another part? → Ra 0.8-1.6 μm recommended
- Is this a cosmetic face users see? → Ra 1.6 μm usually sufficient
- Is this a structural/mounting surface? → Ra 3.2-6.3 μm acceptable
Surface finish capabilities by process:
- Hard turning (50-60 HRC): Ra 3.2-6.3 μm, standard process
- Hard milling (45-55 HRC): Ra 1.6-3.2 μm, requires sharp carbide
- Grinding (any hardness): Ra 0.4-1.6 μm, secondary operation required
- Polishing (post-grind): Ra 0.1-0.4 μm, tertiary operation required
Medical device components with sealing requirements typically specify Ra 0.8 μm, while hydraulic fittings often require Ra 0.4 μm for leak-proof seals. Audio equipment faceplates might specify Ra 1.6 μm for aesthetics, but the internal mounting features work perfectly with Ra 6.3 μm from standard hard turning.
How to specify on drawings: Use “Ra 3.2 unless noted” as your general callout, then add specific finish symbols (like ∇∇∇) directly on critical surfaces. Avoid blanket fine finish specifications that drive up complexity unnecessarily.
Design Takeaway: Specify surface finishes based on function, not aesthetics. Reserve Ra ≤ 1.6 μm callouts for sealing surfaces, bearing areas, or user-facing cosmetic features only. Standard Ra 3.2-6.3 μm from hard machining works for most structural and mounting
Will Machining Hardened Steel Blow Up My Budget or Tool Life?
Expect 2-3x higher costs due to slower cutting speeds, frequent tool changes, and carbide tooling requirements. Cycle times often double compared to soft materials, and specialized tooling can cost 5-10x more than standard HSS tools. Budget accordingly and get quotes early to avoid project surprises.
Cost drivers in hardened material machining:
- Tool costs: Carbide and ceramic tools cost 5-10x more than HSS
- Cutting speeds: 50-75% slower feeds to prevent tool breakage
- Tool life: Expect 3-5x more frequent tool changes
- Setup time: Additional fixturing and work holding requirements
- Secondary operations: Grinding or EDM for features unmachineable by turning/milling
The hardness level directly impacts your budget. At 45 HRC, we typically see 2x cost increases over soft steel. At 55+ HRC, costs can reach 4-5x due to extremely slow material removal rates and premium tooling requirements. A simple shaft that costs $50 to turn in soft 1018 steel might cost $150-200 when made from 60 HRC tool steel.
Tool breakage becomes a significant risk factor above 50 HRC. A broken $200 ceramic insert in the middle of a long machining cycle can add hours to your timeline and hundreds to your cost. This is why many shops add 10-20% contingency to hardened material quotes to account for unexpected tooling failures.
Lead times also increase substantially. What might be a 3-day job in soft material often becomes 1-2 weeks for hardened parts due to slower cutting speeds, more cautious programming, and potential rework if tools fail mid-operation.
Design Takeaway: Get multiple quotes specifically for hardened material work, as pricing varies dramatically between shops. Factor in 2-3x cost increases for budget planning, and add 1-2 weeks to your timeline expectations. Consider value engineering to reduce hardened material volume where possible.
What's the Best Way to Call Out Hardness Specs in My RFQ or Drawing?
Specify hardness range (e.g., 58-62 HRC), heat treatment method, and critical dimensions that need post-hardening verification to ensure accurate quotes and proper documentation. If unsure about hardness requirements, include “or equivalent performance” to allow shops to recommend alternatives based on your application.
Choose your hardness level by application:
- Cutting tools and precision gears: 58-62 HRC typical
- Wear plates and sliding surfaces: 45-55 HRC often sufficient
- Structural parts with wear areas: 40-50 HRC adequate
- High-impact components: 35-45 HRC for toughness
Select your heat treatment method:
- Through hardening: When entire part needs strength and wear resistance
- Case hardening (0.5-2mm depth): Tough core with hard surface for impact resistance
- Selective hardening: Only specific contact surfaces need hardness
Essential callouts for your drawing:
- Hardness range: “58-62 HRC” rather than “60 HRC ±1”
- Heat treatment method: “Through harden” vs “Case harden 0.5mm depth”
- Verification requirements: “Rockwell test required, report with parts”
- Critical dimensions: Note which features need post-hardening inspection
Heat treatment vendors need flexibility to account for part geometry and material chemistry variations. Specifying “60 HRC exactly” often requires expensive trial runs, while “58-62 HRC” allows standard processing. For regulated industries, specify documentation clearly: “AS9102 First Article required” or “ISO 13485 heat treatment certification.”
Design Takeaway: Start with application-based hardness selection, use ranges instead of exact values, and include “or shop recommendation” language if uncertain. Clear specifications prevent costly misunderstandings and ensure vendors can suggest optimizations during quoting.
Conclusion
Hardened material machining requires careful design planning to balance performance with cost-effectiveness. Focus on selective hardening, realistic tolerances, and proper vendor qualification to avoid budget overruns. Most design challenges have cost-effective solutions when addressed early in the development process.
Contact us to explore manufacturing solutions tailored to your hardened material requirements.
Frequently Asked Questions
Yes, but consider the hardness mismatch. Standard steel screws (30-40 HRC) may wear against hardened surfaces (60 HRC). For critical applications, specify hardened fasteners or use inserts to protect threads in the hardened component.
Add 2-4 weeks to standard machining timelines for heat treatment and any required post-hardening operations. Also plan to contact 2-3x more suppliers since many shops don’t handle hardened materials. Budget 2-5x higher costs depending on complexity.
Most existing designs work with minor modifications. The key changes are typically increasing corner radii to 0.5mm minimum, avoiding deep narrow pockets, and loosening non-critical tolerances. Your design intent usually stays intact with these adjustments.
Often yes. Consider hardened inserts for wear areas, separate hardened plates for contact surfaces, or case hardening only critical zones. Two-piece designs frequently cost less than machining complex geometry from fully hardened material.
Focus on the functional requirement: “wear-resistant surface for 10,000 cycles” rather than “60 HRC ±2.” This allows your manufacturing partner to recommend the most cost-effective solution, whether that’s through hardening, case hardening, or coating alternatives.
Ask yourself: what surfaces will experience wear, impact, or contact with other parts? Often only bearing surfaces, cutting edges, or wear zones need hardening. Case hardening or selective hardening can give you the performance you need while keeping manufacturing costs down.
