Decoding Steel Numbers
Every time you see a steel callout like “4140” or “1018,” there’s a system behind those digits. The AISI/SAE numbering system tells you the steel’s composition at a glance — once you know how to read it.
How the System Works
Steel designations are typically 4 digits: XXYY
- First digit (X) — major alloying element
- Second digit (X) — percentage of that alloying element (approximate)
- Last two digits (YY) — carbon content in hundredths of a percent
Example: 4140
- 4 = Molybdenum (Cr-Mo series)
- 1 = ~1% alloying content
- 40 = 0.40% carbon
Major Series
| First Digit | Series | Alloying Element(s) | Common Grades |
|---|---|---|---|
| 1 | 1XXX | Carbon steel (plain) | 1018, 1020, 1045, 1095 |
| 1 | 11XX | Free-machining (resulfurized) | 1117, 1141, 1144 |
| 1 | 12XX | Free-machining (resulf. + rephosph.) | 1212, 1215, 12L14 |
| 1 | 13XX | Manganese (1.75%) | 1330, 1340 |
| 2 | 2XXX | Nickel | 2317, 2515 |
| 3 | 3XXX | Nickel-Chromium | 3140, 3310 |
| 4 | 40XX | Molybdenum | 4023, 4042 |
| 4 | 41XX | Chromium-Molybdenum | 4130, 4140, 4150 |
| 4 | 43XX | Ni-Cr-Mo | 4340 |
| 4 | 46XX | Nickel-Molybdenum | 4620 |
| 5 | 51XX | Chromium | 5120, 5140, 5160 |
| 5 | 52XX | Chromium (bearing) | 52100 |
| 6 | 61XX | Chromium-Vanadium | 6150 |
| 8 | 86XX | Ni-Cr-Mo | 8620, 8640 |
| 9 | 92XX | Silicon-Manganese | 9255, 9260 |
Most Common Grades — Quick Reference
| Grade | Carbon % | Use Case | Machinability | Weldability |
|---|---|---|---|---|
| 1018 | 0.18 | General purpose, case hardening, shafts | Good | Excellent |
| 1020 | 0.20 | Structural, non-critical parts | Good | Excellent |
| 1045 | 0.45 | Gears, shafts, axles (medium strength) | Fair | Fair |
| 1095 | 0.95 | Springs, knives, high-carbon tooling | Poor | Poor |
| 4130 | 0.30 | Aircraft tubing, roll cages | Good | Good (preheat) |
| 4140 | 0.40 | High-strength shafts, gears, tooling | Fair | Fair (preheat) |
| 4340 | 0.40 | Landing gear, crankshafts, highest strength | Fair | Poor |
| 8620 | 0.20 | Case-hardened gears, pinions | Good | Good |
| 12L14 | 0.15 | Screw machine parts (easiest to machine) | Excellent | Poor (lead) |
Carbon Content Rules of Thumb
- Low carbon (< 0.30%) — weldable, formable, can be case-hardened. Not through-hardenable.
- Medium carbon (0.30–0.60%) — heat-treatable, good balance of strength and toughness. Preheat before welding.
- High carbon (> 0.60%) — very hard, used for springs, tools, cutting edges. Difficult to weld.
Related: International Steel Grade Equivalents | Hardness & Tensile Strength | Machinability Ratings