AISI 420 and AISI 440C are the two ends of the same family: both are martensitic stainless steels and both are used in the hardened condition. The difference comes down to how far the carbon and chromium contents are raised.
Carbon rises fourfold
In AISI 420 carbon is 0.16–0.25% and chromium 12–14%. In AISI 440C carbon is 0.95–1.20% and chromium 16.0–18.0%. Carbon rises roughly fourfold.
Carbon that high leads to the formation of a great many chromium carbides in the structure during hardening. Those carbides are extremely hard and give the material outstanding wear resistance. According to our datasheet, 440C reaches a hardness of 58–62 HRC when austenitised at 1010–1065 °C and tempered at 150–250 °C.
The price is toughness. The elongation of 440C is in the 2–5% range — the material is hard but brittle. Because part of the chromium is tied up in carbides, its corrosion resistance stays behind that of the austenitic grades despite the high chromium content.
Comparison in numbers
| PROPERTY | AISI 420 | AISI 440C |
|---|---|---|
| Carbon (C) | 0.16 – 0.25% | 0.95 – 1.20% |
| Chromium (Cr) | 12 – 14% | 16.0 – 18.0% |
| Molybdenum (Mo) | — | max 0.75% |
| Manganese (Mn) | max 1.00% | max 1.0% |
| Hardness | max 196 HB (annealed) | 58 – 62 HRC (hardened) |
| Tensile strength Rm | 655 MPa (annealed) | 760 – 1950 MPa (depending on condition) |
| Yield strength Rp0.2 | 345 MPa (annealed) | 450 – 1700 MPa |
| Elongation | 25% (annealed) | 2 – 5% |
| Austenitising | — | 1010 – 1065 °C |
| Tempering | — | 150 – 250 °C |
| Wear resistance | Moderate | Very high |
| UNS | S42000 | S44004 |
| W.Nr (DIN/EN) | 1.4021 | 1.4125 |
| AMS | 5506 · 5620 · 5621 | 5618 · 5630 · 5880 |
Values are taken from our product datasheets. The AISI 420 values are for the annealed condition; the values after hardening depend on the regime applied.
Which one, and when?
AISI 420 is sufficient — plastic injection moulds, general purpose knives, surgical hand instruments, shafts and pins, parts with a moderate wear load. It is more economical and far easier to machine.
AISI 440C is required — stainless bearing balls and races, valve components, high-quality knife edges, pump wear rings, precision parts working in abrasive environments. If the specification calls out AMS 5618 or AMS 5630, the grade being asked for is 440C.
The decisive question in the choice is this: is the part exposed to impact, or to wear? If there is impact, the 420 family is the right side; if it is pure wear, 440C is.
Intermediate grades
For hardness requirements between the two there are AISI 420B and AISI 420C, and for a tougher martensitic grade AISI 431. For the entry grade of the family, see our AISI 410 – AISI 420 comparison.
Technical datasheets
AISI 420 — chemical composition, mechanical values, standard equivalents
AISI 440C — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.