Steel CNC Machining
The all-rounder in mechanical engineering — from structural to tool steel.
Steel — The #1 Material in Mechanical Engineering
Steel is the most important structural material in the world. In CNC machining, steel covers the broadest spectrum: from soft structural steel (S235) to easily machinable free-machining steel (11SMnPb30) to high-strength quenched & tempered steel (42CrMo4).
Choosing the right steel grade directly impacts machining time, tooling costs, and part quality. We're happy to advise you on the optimal material selection for your application.
Steel Grades Overview
| Category | Grades | Tensile Str. | Application |
|---|---|---|---|
| Structural Steel | S235JR, S355J2 | 360–510 MPa | Machine frames, fixtures, brackets |
| Free-machining Steel | 11SMnPb30, 11SMn30 | ~400 MPa | Serial turned parts, bolts, bushings |
| Case-hardening Steel | C15, 16MnCr5 | ~500 MPa | Gears, shafts (after case hardening) |
| Quenched & Tempered Steel | C45, 42CrMo4 | 700–1100 MPa | Highly stressed shafts, axles, bolts |
| Tool Steel | 1.2379, 1.2312 | up to 2000 MPa | Tools, molds, dies |
Cutting Data by Steel Group
| Steel Group | vc Milling | vc Turning | Note |
|---|---|---|---|
| Structural Steel | 150–250 m/min | 150–250 m/min | Straightforward, moderate edge loading |
| Free-machining Steel | 180–300 m/min | 200–350 m/min | Ideal for turned parts, short-breaking chips |
| Q&T Steel | 80–150 m/min | 100–180 m/min | High cutting forces, stable clamping needed |
| Tool Steel | 40–100 m/min | 60–120 m/min | CBN or ceramic when hardened |
Pro tip: Free-machining steel (11SMnPb30) is the most cost-effective turning material. The lead additives dramatically improve chip breaking. For new projects, always check whether free-machining steel is sufficient.
Heat Treatment and Post-Processing
Many steel components require heat treatment after or between CNC machining steps:
- Quenching & Tempering — Hardening + tempering for defined strength (e.g., C45 to 58–62 HRC)
- Case Hardening — Carburizing + hardening of the surface layer (0.5–1.5 mm). Core remains tough.
- Nitriding — Nitrogen diffusion for surface hardness without distortion
- Stress Relieving — After roughing, before finishing (500–600°C)
We coordinate heat treatments through our partner network and finish-machine afterwards — everything from a single source.
Surface Treatment and Corrosion Protection
Steel rusts without protection — the right surface treatment is essential:
| Process | Layer Thickness | Application |
|---|---|---|
| Black Oxide (Burnishing) | 1–2 µm | Light corrosion protection + dark finish. Most affordable option. |
| Zinc Plating (galvanic) | 5–25 µm | Standard corrosion protection for machine parts. With chromate passivation (blue, yellow, black). |
| Electroless Nickel Plating | 10–50 µm | Uniform coating even inside bores. Wear protection + dimensional accuracy. |
| Powder Coating | 60–120 µm | RAL colors, robust, UV-resistant. For housings and covers. |
| Phosphating | 5–15 µm | Primer for paints, light corrosion protection, lubricity. |
| Hard Chrome Plating | 10–100 µm | Extremely wear-resistant (HV 1000). For shafts, pistons, guides. |
Important: For parts with tight tolerances (h6, H7), account for coating thickness before coating in CNC machining. We adjust final dimensions to the planned coating.
Material Selection Guide
The right steel grade saves machining time and costs:
| Application | Recommended Steel | Why? |
|---|---|---|
| Serial turned parts (bolts, bushings) | 11SMnPb30 | Free-machining steel: best machinability, short chips |
| Highly stressed shafts | 42CrMo4 | Q&T steel: highest strength + toughness |
| Gears (hardened) | 16MnCr5 | Case-hardening steel: hard surface, tough core |
| General structural parts | S355J2 | Structural steel: good value, weldable |
| Knurled sleeves, pins | C45 | Q&T steel: hardenable, affordable, versatile |
| Tools, dies | 1.2379 | Tool steel: extremely wear-resistant, hardenable to 62 HRC |
Tip: If the function permits, use free-machining steel instead of structural steel. The significantly better machinability saves 20–40% machining time — compensating for the slightly higher material cost.
Industries and Application Examples
- Mechanical Engineering — Shafts, axles, bolts, bushings, frame parts. The foundation of every machine is steel.
- Automotive Industry — Transmission parts, drive shafts, crankshafts. Q&T steel for highest loads.
- Toolmaking — Dies, matrices, cutting plates, guide pillars. Tool steel for extreme wear resistance.
- Construction — Fasteners, anchor plates, brackets. Structural steel for cost-effective solutions.
- Energy Technology — Generator shafts, pump parts, valve seats. High-strength steel for continuous operation.
- Agricultural Machinery — Bolts, axles, wear parts. Robust parts for harsh operating conditions.
Frequently asked Questions
Which steel is best for CNC turned parts?
Free-machining steel 11SMnPb30 — it can be turned at high cutting speeds and produces short, well-breaking chips. Ideal for serial parts on lathes with bar feeders.
What is the difference between S235 and S355?
S235 has a minimum yield strength of 235 MPa, S355 of 355 MPa. S355 is stronger but also more expensive and slightly harder to machine. For purely static components, S235 is often sufficient.
Can hardened steel be CNC machined?
Yes, up to about 62 HRC with CBN tools (hard turning/hard milling). Above 62 HRC, grinding or EDM is necessary. Hard turning can replace grinding in many cases.
How much does CNC machining of steel cost?
Steel is the most cost-effective metal material. Typical unit prices for turned parts: €15–100 (one-off), €5–40 (small batch). Free-machining steel is cheapest, tool steel most expensive. Request a specific quote.
What corrosion protection for steel parts?
Most common: galvanic zinc plating (blue/yellow/black chromated). For higher requirements: electroless nickel plating or powder coating. For wear protection: hard chrome plating or nitriding.
What is the difference between C45 and 42CrMo4?
C45 is an unalloyed Q&T steel (700 MPa), affordable and versatile. 42CrMo4 is alloyed (1000+ MPa), tougher at equal hardness and suitable for highest dynamic loads. 42CrMo4 costs ~30% more but is worthwhile for highly stressed parts.
What tolerances are achievable for steel turned parts?
Standard: ±0.05 mm. IT7 fits (h7, H7) as standard. IT6 fits (h6) for shafts and bearing seats. On our DMG MORI NLX 2000, tolerances down to IT5 are reproducibly achievable.
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