Steel & Stainless Guide
Carbon steels, alloy steels and stainless families: strength, machinability and corrosion
Plain carbon steels (1018, 1045) are the cheapest structural metals and machine well; alloy steels (4140, 4340) add strength and hardenability; stainless steels — austenitic 303/304/316, martensitic 410/440, precipitation-hardening 17-4 — trade cost and machinability for corrosion resistance. Grade selection is a four-way trade: strength, corrosion, cost and machinability.
1. Carbon and alloy steels
Low-carbon steels (1018: UTS ≈440 MPa, yield ≈370 MPa) are soft, weld easily and machine with good finish — the default for brackets and non-critical parts. Medium-carbon (1045: UTS ≈585 MPa) and alloy grades (4140: UTS ≈655 MPa, 4340: UTS ≈745 MPa) add strength and respond to heat treatment for gears, shafts and structural components.
Free-machining 12L14 (leaded, resulfurized) cuts with short chips at high speed but has reduced toughness and is not weldable; it is the default for high-volume turned parts when corrosion resistance is not needed. Structural plate A36 (yield 250 MPa) is specified for weldments and frames.
2. Stainless families
Austenitic 304/316 are the workhorses: 304 (UTS ≈586 MPa, yield ≈241 MPa) is general-purpose; 316 adds molybdenum for chloride/marine resistance (UTS ≈579 MPa). Both work-harden during machining — use sharp tools and positive feeds. 303 is the free-machining austenitic grade (UTS ≈690 MPa) for production turning, at a small corrosion penalty.
Martensitic 410/440 offer high hardness (440C to ~58–60 HRC) for bearings and cutters; precipitation-hardening 17-4 PH (H900: UTS ≈1255 MPa, yield ≈1170 MPa) combines stainless corrosion resistance with high strength — the standard for medical and aerospace fittings.
3. Machining and heat treatment
Carbon and alloy steels machine at moderate speeds with coolant; hardness below ~35 HRC keeps tool life economic. Heat-treatable grades are specified by hardness rather than strength: 4140 Q&T to 28–32 HRC for shafts, 4340 for high-fatigue components, tool steels (O1, A2, D2) hardened to 55–62 HRC for dies and cutters.
Surface treatments: zinc plating (corrosion, ~5–12 µm), black oxide (cosmetic, thin), nickel plating (hardness and wear), phosphate + oil, and for stainless, passivation (nitric/citric) restores the chromium oxide layer after machining.
4. Selection guidance
Start with 1018 or 12L14 for machined parts, A36 for weldments, 304 for food/medical/corrosion, 316 for marine and chloride, 4140/4340 for high-strength components, 17-4 for high-strength stainless. Stainless costs roughly 3–5× carbon steel per kilogram and machines slower — avoid it unless corrosion or hygiene demands it.
Watch galvanic corrosion when coupling stainless with aluminum or carbon steel; specify passivation on all machined stainless to prevent embedded-iron rust spots.
Quick reference
| Grade | Type | UTS MPa | Yield MPa | HRC / note | Best for |
|---|---|---|---|---|---|
| 1018 | Low carbon | 440 | 370 | 126 HB | Brackets, general |
| 12L14 | Free-machining | 540 | 415 | 163 HB | High-volume turned parts |
| 1045 | Medium carbon | 585 | 450 | 197 HB | Shafts, gears |
| 4140 | Alloy | 655 | 417 | Q&T 28–32 | High-strength components |
| A36 | Structural | 400 | 250 | – | Weldments |
| 303 | Austenitic SS | 690 | 550 | – | Production machining |
| 304 | Austenitic SS | 586 | 241 | – | Food, medical, general |
| 316 | Austenitic SS | 579 | 241 | – | Marine, chloride |
| 17-4 H900 | PH SS | 1255 | 1170 | – | High-strength stainless |
Sources & references
- ASTM A36 — Structural steel
- ASTM A240 — Stainless plate
- ASTM A276 — Stainless bar
- ASTM A311 — Free-machining steel
- EN 10083 — Quenched and tempered steels
- Xometry Learning Center — Steel & Stainless Guide (public engineering reference)