440C and Chinese 9Cr18Mo are the same class of steel: a high-carbon, high-chromium martensitic stainless used for bearing rings and balls. It is chosen for hardness and wear resistance, not for maximum corrosion resistance — 440C resists corrosion less well than 304 or 316. It is magnetic, it is heat-treatable to around 58–60 HRC, and its corrosion resistance depends on staying below a tempering temperature ceiling.
440C and 9Cr18Mo are the same class of steel
If a Chinese supplier quotes 9Cr18Mo against your 440C drawing, they are not substituting a cheaper material. The two designations cover the same steel family: high-carbon (around 1 % C), high-chromium (around 17 % Cr) martensitic stainless, with molybdenum added to raise hardness and tempering stability.
| Designation | Where it is used |
|---|---|
| 440C | AISI / ASTM (US) |
| 9Cr18Mo | GB (China) |
| 1.4125 | EN / DIN (Europe) |
| SUS440C | JIS (Japan) |
| 95Cr18 / 9Cr18 | GB, the molybdenum-free variant |
⚠️ "Equivalent" is not "identical". The composition ranges in the Chinese and American grade standards overlap but are not defined identically, and 9Cr18 without the Mo is a genuinely different steel from 9Cr18Mo. Before a contract, compare the actual grade standards rather than the shorthand names — particularly if your specification names a heat-treatment condition or a qualification standard.
The most expensive misconception — stainless does not mean rust-proof
This is the single most common mistake we see on stainless bearing enquiries, and it is worth stating plainly:
440C has lower corrosion resistance than 304 or 316.
The reason is chemistry. What makes a stainless steel corrosion-resistant is chromium dissolved in the matrix, available to form a passive oxide film. 440C carries around 1 % carbon, and much of that carbon ties up chromium as chromium carbides. Those carbides are exactly what gives 440C its hardness and wear resistance — and every one of them is a little chromium removed from corrosion duty.
So the trade is explicit:
| 440C / 9Cr18Mo | 304 / 316 | |
|---|---|---|
| Structure | Martensitic | Austenitic |
| Hardenable by heat treatment | Yes, to roughly 58–60 HRC | No |
| Load capacity as a bearing | Full — it is a bearing steel | Low — soft raceways deform |
| Corrosion resistance | Moderate | High (316 highest) |
| Magnetic | Yes | Generally no, or weakly |
| Typical use | Bearings for wet, washdown, mildly corrosive duty | Bearings for chemical and marine duty where load is light |
Read that table as a decision, not a ranking. If your bearing has to carry load, you need a hardenable steel, and that means 440C — accepting moderate corrosion resistance. If your bearing sits in seawater or aggressive chemicals and carries very little load, a 316 bearing is the right answer even though its raceways are soft.
The failure mode we are asked about most often is a 440C bearing installed in a continuously wet or chloride-rich environment, rusting, and being reported back as a material defect. It usually is not one. It is the wrong grade for the environment.
The temperature ceiling nobody mentions
Most stainless bearing pages give an operating temperature range and stop. The more useful number is a processing limit that determines the corrosion resistance you receive.
Carpenter's published data for Type 440C states that the steel is not usually recommended for elevated-temperature service, because corrosion resistance is reduced when the steel is used in the annealed condition or hardened and tempered above about 427 °C (800 °F); for best corrosion resistance, tempering should stay below that figure. Taylor Special Steels' data sheet puts the caution lower still, noting that tempering above 370 °C may cause some loss of corrosion resistance.
⚠️ Source: published material data sheets from Carpenter (Type 440C) and Taylor Special Steels, retrieved 2026-09-13 — indicative until checked against the mill certificate for the actual heat. Verification path: request the original data sheet and the actual tempering records from your steel supplier or heat treater.
Two practical consequences:
- A 440C bearing is normally tempered low — in the region of 150–180 °C — precisely to keep both hardness and corrosion resistance. That low tempering temperature is also why the steel should not run hot in service: exceed the tempering temperature and you begin tempering the part further, losing hardness permanently.
- "What temperature can a stainless bearing take?" is really two questions. One is the seal and grease limit, which is usually what binds first — a nitrile seal gives out long before the steel does. The other is the steel's own limit, set by its tempering condition. Neither is a single catalogue number, and the answer changes if the bearing is open, shielded or sealed.
If your application runs hot, tell your supplier the actual temperature and let the heat treatment and the seal material be chosen against it, rather than reading a range off a table.
When 440C is the right choice — and when it is not
Choose 440C / 9Cr18Mo when:
- The bearing carries real load and must stay hard
- The environment is wet, humid, washdown, food-related or mildly corrosive
- Dimensional stability matters and the duty temperature is moderate
Choose 304 or 316 instead when:
- The environment is seawater, chlorides or aggressive chemicals
- Load is light and speed is low
- Non-magnetic behaviour is required
Consider something else entirely when:
- The duty is continuously hot — the tempering ceiling and the seal both become binding
- Full non-magnetic behaviour plus load capacity is required — this usually leads to ceramic or hybrid designs rather than a stainless steel
Stainless is not one decision, it is three: the grade, the heat-treatment condition, and the seal. Send us the environment, the operating temperature, the load and speed, and whether non-magnetic behaviour is required, and we will tell you which grade the application actually needs — including the cases where the answer is not 440C.
FAQ
Is 9Cr18Mo the same as 440C?
They are the same class of steel — high-carbon, high-chromium martensitic stainless — with 9Cr18Mo the Chinese GB designation and 440C the AISI one. The composition ranges in the two standards overlap but are not defined identically, so compare the actual grade standards before contracting. Note that 9Cr18 without molybdenum is a different steel from 9Cr18Mo.
Do 440C stainless bearings rust?
They can. 440C has moderate corrosion resistance — lower than 304 or 316 — because much of its chromium is tied up in carbides that give it hardness. It suits wet, humid and washdown environments, not seawater or chloride-rich chemical service.
Are stainless steel bearings magnetic?
440C and 9Cr18Mo are martensitic and therefore magnetic. 304 and 316 bearings are austenitic and generally non-magnetic or only weakly magnetic. If your application requires non-magnetic behaviour, 440C is not a candidate.
What temperature can a 440C bearing run at?
There are two limits. The steel's own limit is set by its tempering condition — published material data advises keeping tempering below roughly 427 °C for best corrosion resistance, and 440C is normally tempered far lower than that. In a sealed or shielded bearing, however, the seal material and grease usually reach their limit first. State your actual operating temperature rather than selecting from a range.
How hard is a 440C bearing?
Heat-treated 440C typically reaches around 58–60 HRC, which is what allows it to carry loaded raceways. Austenitic grades such as 304 and 316 cannot be hardened by heat treatment, which is why stainless bearings made from them have a much lower load rating.
What should I check before contracting, beyond the grade name?
Compare the actual grade standards rather than the shorthand names, because the composition ranges overlap but are not identical — and confirm whether your specification means 9Cr18Mo or the molybdenum-free 9Cr18. Then state the heat-treatment condition (the tempering temperature, against the corrosion ceiling above) and, for sealed bearings, the seal material and grease, which usually reach their limit before the steel does.