Introduction
Machining low-carbon structural steels such as S355 and the alloyed grade 42CrMo4 (often used for shafts and gears) is a common task in fabrication and CNC shops. Although both are machinable, their metallurgical differences demand distinct tooling choices and cutting strategies. This post gives practical, shop-ready guidance for buyers, CNC operators and procurement managers to optimize cycle time, surface finish and tool life.
Technical comparison: S355 vs 42CrMo4
| Characteristic | S355 (low-carbon structural) | 42CrMo4 (alloyed, quenched/tempered) |
|---|---|---|
| Chemistry / microstructure | Low C, higher Mn; ferrite/pearlite; generally softer and more ductile | Higher C/Cr/Mo; heat-treatable; stronger and tougher after hardening |
| Typical hardness (annealed/normalized) | ~120–180 HB (soft/normalized) | ~180–260 HB (varies with heat treatment) |
| Machinability | Good — easy chip formation and low cutting forces | Moderate to challenging — higher forces, potential work hardening |
Key machining considerations
- Workholding & rigidity: Both materials require rigid setups. 42CrMo4 benefits most from minimized overhang and solid fixturing due to higher cutting forces.
- Thermal behavior: 42CrMo4 retains heat and can harden locally; use appropriate coolant strategy to avoid thermal shock and discoloration.
- Chip control: Low-carbon steels make long, ductile chips—use positive tool geometry and chipbreakers. For 42CrMo4, chips may be thicker and stiffer; consider interrupted-cut capable tooling for roughing.
- Surface integrity: Watch for built-up edge (BUE) on low-carbon steels and hardening beneath the surface on 42CrMo4 when improperly cut or when feeds are too low.
Recommended tooling & geometry
- Turning: Use indexable carbide inserts with positive rake and an effective chipbreaker for S355. For 42CrMo4 choose tougher carbide substrates (with less aggressive rake) and robust holders to handle higher forces.
- Milling: Solid carbide end mills with variable helix and polished flutes work well on S355. For 42CrMo4 consider coated carbide with a tougher substrate and reduced helix to resist chipping.
- Drilling: Use coated twist drills or carbide drills with good coolant-through capability for higher strength alloys. Spot and peck strategies help evacuate chips in deep holes.
- Coatings: PVD coatings (TiN, TiAlN variants) are excellent for low-carbon work with stable temperatures; for tougher alloyed steel, consider CVD-style or tougher PVD stacks designed to resist abrasion and adhesion.
- Toolholder choice: Use high-rigidity holders, minimized stick-out, and balanced milling holders at high speeds.
Practical speeds, feeds & depth of cut (starting points)
Actual values depend on machine power, tool geometry, and coolant. Use these conservative starting ranges and optimize from there.
| Operation | S355 (starting point) | 42CrMo4 (starting point) |
|---|---|---|
| Turning (carbide) | Cutting speed: 160–300 m/min; Feed: 0.15–0.4 mm/rev; DOC: 1–4 mm (roughing) | Cutting speed: 100–220 m/min; Feed: 0.12–0.35 mm/rev; DOC: 0.5–3 mm (roughing) |
| Milling (solid carbide) | Cutting speed: 180–280 m/min; Feed per tooth: 0.04–0.2 mm/z | Cutting speed: 120–200 m/min; Feed per tooth: 0.03–0.15 mm/z |
Cooling & lubrication
- For S355, flood coolant or high-pressure mist often improves chip control and reduces built-up edge.
- For 42CrMo4, use flood coolant or through-tool coolant for deep cuts and drilling; coolant helps reduce tool wear and part hardening.
- In finishing of low-carbon steels, dry machining is sometimes acceptable with the right coating and high cutting speeds — but validate surface integrity and tool life first.
Recommended Products
If you’re sourcing tooling for these steels, prioritize:
- Indexable carbide turning inserts with chipbreaker geometry suited to steel and matching toolholders.
- Solid carbide end mills with variable helix and polished flutes for better chip evacuation.
- Tougher substrate carbide inserts or coated grades for alloyed, heat-treatable steels.
- High-pressure through-tool coolant holders and carbide drills for deep-hole work.
- Balanced milling chucks and rigid, zero-backlash toolholders for finish passes.
Talk to tooling suppliers about tooling families optimized for steel (look for supplier data sheets that list recommended speeds & feeds by material).
Troubleshooting & tips
- Long, stringy chips: Increase feed slightly, use a chipbreaker, or add coolant to reduce adhesion.
- Built-up edge (BUE): Increase cutting speed, change coating, or use negative rake/edge prep to minimize adhesion.
- Rapid tool wear on 42CrMo4: Reduce cutting speed, increase coolant flow, or switch to tougher substrate/coating combination.
- Surface hardening: If the material work-hardens under the tool, increase feed and reduce rubbing (ensure the tool is sharp and rake is appropriate).
- Vibration or chatter: Reduce overhang, lower radial immersion for milling, increase feed, or improve clamping.
Frequently Asked Questions
1. Is 42CrMo4 harder to machine than S355?
Yes—42CrMo4 is alloyed for strength and toughness, so it typically produces higher cutting forces and requires tougher tooling and sometimes slower cutting speeds compared with S355.
2. Can I use the same insert/coating for both steels?
For general shop convenience you can select a versatile carbide grade with a steel-optimized coating, but for high-volume or critical parts it’s better to match tooling specifically (tougher grades for 42CrMo4, more aggressive/coated grades for S355).
3. Should I use dry machining?
Dry machining can work for some finishing operations on low-carbon steels with the right coatings and rigid setups, but for 42CrMo4 and heavy cuts, coolant is recommended to control temperature and chip evacuation.
4. How do I avoid work hardening on 42CrMo4?
Avoid light rubbing cuts at low speeds. Use sharp tools, sufficient depth of cut to engage the material, and adequate coolant. Validate by sampling and checking subsurface hardness where critical.
5. Where should I start when optimizing a new part?
Start with conservative speeds and feeds, verify chip formation and surface finish, then increase feed or speed incrementally until you reach an acceptable balance of cycle time and tool life.
Call to action
Need cutting data tailored to your machine, tooling brand or part geometry? Contact our tooling specialists with your part drawing, material certificate and machine details for a free application review and recommended tooling list. We can provide tooling bundles, cutting data sheets and sample runs to help you hit target cycle time and part quality.