M50 is the steel used when a bearing has to keep its hardness at 300 °C and run at dn values that soften ordinary chrome steel. It is a vacuum-melted, high-molybdenum tool steel — and it is priced accordingly.
| Designations | M50 (AMS 6490) · M50 NiL (AMS 6278) · 1.3551 (similar) |
|---|---|
| Chemistry | ≈0.85% C · 4.0% Cr · 4.25% Mo · 1.0% V |
| Hardness | 60–64 HRC (M50 NiL: 60–64 HRC case, 35–45 HRC core) |
| Max service temperature | 315 °C continuous — ≈430 °C short excursions |
| Melting practice | Vacuum induction melted, vacuum arc remelted (VIM-VAR) |
| Corrosion resistance | None |
| Magnetic | Yes |
| Relative cost | 6–10× 100Cr6, with long lead times |
| Typical duty | Aero engine shafts, APUs, high-dn machine tool spindles |
Molybdenum and vanadium form stable carbides that resist coarsening at temperature, so M50 retains hardness and dimensional stability well above the 120 °C ceiling of 100Cr6. That is why it appears in engine bearings, where the oil may be at 200 °C and the ring itself hotter still, and in high-speed spindles where the limiting factor is heat generated in the contact rather than external load.
M50 NiL takes the idea further: a low-carbon core that stays tough, with a carburised and hardened case. The soft core arrests cracks and tolerates debris dents that would spall a through-hardened ring. It is the standard for aero-engine bearings expected to survive FOD.
M50 is not a drop-in improvement for an ordinary application. It costs six to ten times as much, is produced in small heats with long lead times, is harder to grind and is less tolerant of shock than a carburising grade. Below 250 °C it offers no life advantage over 100Cr6 — the fatigue limit at equal hardness is comparable, and the price is not.
| Property | M50 | Stabilised 100Cr6 (S3) | Silicon nitride hybrid |
|---|---|---|---|
| Hardness at 20 °C | 60–64 HRC | ≈59–62 HRC | ≈1500 HV balls / steel rings |
| Hardness at 300 °C | ≈58 HRC | ≈55–57 HRC | Unchanged (ceramic) |
| Max continuous temp | 315 °C | 300 °C | 400 °C |
| Shock tolerance | Moderate (M50 NiL better) | Good | Low — brittle |
| Availability | Made to order | Catalogue | Catalogue, limited sizes |
| Relative cost | 6–10× | 1.2–1.5× | 8–15× |
The usual M50 housing: aero engine and high-speed spindle positions.
Read →Main shaft bearings in engine and gearbox duty.
Read →Where contact heat, not external load, sets the material.
Read →M50 is usually paired with a high precision class and a matched set.
Read →M50 is through-hardened to 60–64 HRC. M50 NiL is a low-carbon version that is carburised: hard case, tough core. NiL tolerates shock and debris far better and is preferred where FOD resistance matters.
Not as a catalogue swap. M50 bearings are made to drawing, with different heat treatment and usually different cages and clearance. The boundary dimensions can match; nothing else is guaranteed to.
None, in the ordinary sense. At that temperature the lubricant is usually a formulated ester or PFPE grease rated for the interval, or an oil-jet / mist system. The lubricant, not the steel, sets the maintenance interval.
Yes — high-speed machine tool spindles, some turbo-machinery and racing applications. It remains a specialist choice driven by temperature and dn, not by general preference.
| Industry | Duty | Where it fits | |
|---|---|---|---|
| Electric Motors & Drives | continuous speed, long grease life and shaft voltages from PWM drives | Hot housings | Detail → |
| Machine Tools & Spindles | speed, stiffness and thermal growth of the spindle | High speed spindles | Detail → |
| Steel & Metallurgy | very high load, heat and cooling water washing the grease out | Hot roll necks | Detail → |
Model numbers, quantities and the application — we reply with brand options, price and lead time within 30 minutes on working days.