Steel & Stainless Steel
Steel is the most machined material on Earth. But "steel" covers a huge range — from soft, free-cutting 1215 that machines like butter, to hardened H13 tool steel that needs CBN inserts. This page breaks down the major steel families and how to machine each one.
Carbon Steel
Carbon steel is iron with up to ~0.5% carbon. More carbon = harder and stronger, but less ductile.
| Grade | Carbon % | Typical use | Machinability |
|---|---|---|---|
| 1018 / 1020 | 0.18–0.20% | General shafts, brackets, fasteners | Easy — but gummy, needs chip breaker |
| 1045 | 0.45% | Shafts, gears, axles | Good — medium hardness, nice chips |
| 1215 (free-machining) | 0.15% + lead/sulfur | High-volume screw machine parts | Excellent — the 100% baseline |
Low-carbon steel challenge: 1018 is soft and ductile — it forms long, stringy chips that wrap around the tool. Use a chip-breaker insert and run slightly higher feed to force chip breaking. Aluminum-like speeds (Vc = 150–250 m/min with carbide) work well.
Alloy Steel
Alloy steels add chromium, molybdenum, nickel, or vanadium to improve strength and toughness. The most common in CNC shops: 4140 and 4340.
| Grade | Properties | Typical condition | Vc (carbide) |
|---|---|---|---|
| 4140 | Cr-Mo alloy, good toughness | Annealed HB 200, or pre-hard HRC 28–32 | 180–250 m/min annealed, 100–150 pre-hard |
| 4340 | Ni-Cr-Mo, high strength | Quenched & tempered HRC 30–38 | 80–120 m/min |
| 4150 | Higher carbon, springs | HRC 40–45 | 60–100 m/min |
Calculate: Turning Speed & Feed · Spindle Speed
Tool Steel
Tool steels are alloyed for hardness and wear resistance after heat treatment. They come annealed (soft, machinable) and are hardened after machining.
| Grade | Common name | Use | Machining condition |
|---|---|---|---|
| P20 | Pre-hard mold steel | Injection mold bases | Pre-hard HRC 28–32, machine as delivered |
| H13 | Hot work tool steel | Die casting dies, forging dies | Annealed HB 200, machine then harden to HRC 48–52 |
| D2 | Cold work tool steel | Blanking dies, punches | Annealed HB 220, machine then harden to HRC 60 |
| A2 | Air-hardening tool steel | Precision gauges, punches | Annealed, low distortion on hardening |
| S7 | Shock-resisting steel | Chisels, forming tools | Tough, impact-resistant |
Machining hardened steel (HRC 45+): once steel is hardened above HRC 45, standard carbide inserts struggle. Options: (1) use CBN or ceramic inserts at low speed, (2) hardmill with advanced coated carbide at 60–100 m/min, or (3) leave stock for grinding. Don't try to rough HRC 50+ with regular carbide — you'll burn through inserts fast.
Stainless Steel
Stainless steel contains at least 10.5% chromium, which forms a passive oxide layer that resists corrosion. But that oxide layer, combined with low thermal conductivity and work-hardening, makes stainless one of the hardest common materials to machine.
Stainless families
| Family | Common grades | Magnetic? | Machinability |
|---|---|---|---|
| Austenitic (300 series) | 304, 316, 321 | No | Worst — work-hardens, gummy, low heat conduction |
| Ferritic (400 series) | 430, 409 | Yes | Better — closer to mild steel |
| Martensitic | 410, 420, 440C | Yes | Hardenable, abrasive |
| Precipitation hardening | 17-4 PH, 15-5 PH | Yes | Good in solution-treated condition |
Machining austenitic stainless (304/316) — the rules
- Use sharp tools: a honed, sharp edge cuts cleanly. A worn edge rubs and work-hardens the surface.
- Positive rake angle: 15–20° rake shears the material instead of pushing it.
- Adequate feed: don't feed too light. Feed below 0.05 mm/rev causes rubbing → work hardening → built-up edge. Keep f = 0.1–0.2 mm/rev.
- Lower speed: Vc = 80–120 m/min with coated carbide (vs 200+ for mild steel). Low thermal conductivity means heat stays at the edge.
- Flood coolant: copious flood coolant or through-tool coolant. Heat is the enemy of stainless machining.
- Chip breaker: stainless forms long stringy chips — use a chip-breaker geometry to keep chips short and controllable.
The #1 stainless mistake: running too slow and feeding too light. Operators see it "work hardening" and slow down — but that makes it worse. The tool starts rubbing instead of cutting, which work-hardens the surface further. Correct response: keep feed up (0.1–0.15 mm/rev), use a sharp positive-rake insert, and let the chip breaker do its job.
Quick Reference: Steel Cutting Speeds
| Material | Carbide Vc (m/min) | Feed rough (mm/rev) | Coolant |
|---|---|---|---|
| 1018 low carbon | 180–280 | 0.2–0.3 | Optional |
| 1045 / 4140 annealed | 150–250 | 0.2–0.4 | Flood |
| 4140 pre-hard (HRC 28–32) | 100–160 | 0.15–0.3 | Flood |
| P20 mold steel | 120–180 | 0.15–0.3 | Flood |
| H13 / D2 annealed | 80–120 | 0.1–0.25 | Flood |
| 304 / 316 stainless | 80–120 | 0.1–0.2 | Heavy flood |
| 17-4 PH | 100–150 | 0.1–0.25 | Flood |
Full table: Material Property Reference