440C Stainless Steel
Also: AISI 440C, UNS S44004, 1.4125, X105CrMo17, SUS440C, SAE 51440C, bearing stainless, 440C ESR
440C is the hardest standard stainless: 1% carbon, 17% chromium, 58-60 HRC hardened, for bearings, blades and valve parts. Machine annealed, grind hard. Corrosion modest.
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At a glance
| Density | 7.7 g/cm³ (0.278 lb/in³) |
|---|---|
| Tensile strength | Hardened and tempered at 150-200 °C (300-400 °F) about 1970 MPa (285 ksi); annealed about 760 MPa (110 ksi) |
| Yield strength | Hardened and tempered low about 1900 MPa (275 ksi); annealed about 450 MPa (65 ksi) |
| Hardness | Hardened and tempered at 150-200 °C (300-400 °F) 58-60 HRC; annealed about 230 HB (ASTM A276 maximum 269 HB) |
| Elongation | Hardened about 2%; annealed about 14% |
| Modulus of elasticity | 200 GPa (29 Msi) |
| Thermal | Melts at 1370-1480 °C (2500-2700 °F); thermal conductivity ~24 W/m·K; expansion ~10.2 um/m·K; hardened parts hold full hardness to about 200 °C (400 °F) |
| Composition | Fe, 16-18% Cr, 0.95-1.20% C, up to 0.75% Mo, up to 1% Mn, up to 1% Si |
Typical values, Hardened from 1040 °C (1900 °F), oil quenched, sub-zero treated and tempered at 150-200 °C (300-400 °F); annealed figures as noted. Confirm against the mill certificate and the purchase spec.
- MachinabilityFair
- About 40% of B1112 annealed: hard carbides wear tools, so use coated carbide, rigid setups and generous coolant, and leave grind stock. Above 58 HRC it is ground, lapped or EDM cut, not machined.
- WeldabilityNot recommended
- One percent carbon martensite cracks on cooling; no filler or preheat schedule makes a reliable joint. Join 440C parts mechanically or braze them before hardening.
- FormabilityPoor
- Annealed bar and plate take only gentle bends; there is no formed-sheet use. Parts are machined, hardened and ground.
- Corrosion resistanceFair
- Hardened, tempered low and polished it resists fresh water, steam, food, alcohol and mild acids; it pits in chlorides and salt spray and rusts if left unpassivated after grinding. Tempering above 370 °C (700 °F) makes it worse.
440C is a high-carbon (0.95-1.20%), 17% chromium martensitic stainless that hardens to 58-60 HRC, the highest of the standard stainless grades. That hardness, with the chromium carbides that come with it, gives the wear resistance of a tool steel and enough corrosion resistance for bearings, valve seats, nozzles, blades and surgical tools that see water, steam, food and mild chemicals. Bearing-quality 440C (ASTM A756, AMS 5618 premium melted) is the standard corrosion-resistant ball and roller bearing steel.
It is bought annealed at about 230 HB, machined with the same care as a tool steel (roughly 40% of B1112), then hardened from 1010-1065 °C (1850-1950 °F) with an oil or air quench, often a sub-zero hold, and a low temper. Distortion is small because sections air harden. After hardening, only grinding, lapping and EDM change it. Hardened parts keep their hardness to about 200 °C (400 °F), after which the temper softens them.
Its corrosion resistance is modest: better than 420 and about like 410 in the hardened, polished state, well short of 304, and poor in chlorides where pits start at the carbides. It cannot be welded, does not form, and is brittle at full hardness (about 2% elongation), so design against impact and stress concentration. For a tougher hard stainless use 420 or 17-4 PH H900; for a harder or more corrosion-resistant blade steel, the powder-metallurgy grades.
Heat treatment
Anneal at 845-900 °C (1550-1650 °F) with a slow furnace cool to about 230 HB. Harden by preheating to 760-790 °C (1400-1450 °F), austenitizing at 1010-1065 °C (1850-1950 °F), oil quench or air cool (air for sections under about 50 mm / 2"), then a sub-zero hold at -75 °C (-100 °F) to transform retained austenite, then temper at 150-200 °C (300-400 °F) for 58-60 HRC or 315-370 °C (600-700 °F) for 56-57 HRC with a little more toughness. Never temper at 425-565 °C (800-1050 °F): toughness and corrosion resistance both drop. Vacuum hardening with gas quench is standard for bearings and precision parts.
Tempers and conditions
- Annealed (as purchased)
- Hardened and tempered, 58-60 HRC
- Hardened and tempered, 56-57 HRC (higher temper)
- Precision ground and polished bar and shafting
Stock forms
- Round bar and precision ground shafting
- Flat bar and plate (limited)
- Wire and balls
- Forgings
- Bearing-quality bar (ASTM A756, AMS 5618)
Relative cost
Tier 4 of 5. About 1.5-2x 304 bar per pound; bearing-quality and premium-melted AMS 5618 stock costs more. Add hardening, sub-zero and grinding: a finished 440C part is a tool-steel-priced part.
Typical applications
- Ball and roller bearings, races and balls for corrosive or clean environments
- Valve seats, balls, needles and nozzles
- Knife blades, scalpels and surgical cutting tools
- Pump shafts, plungers and wear sleeves
- Gauge blocks, pins and measuring tools
- Molds and dies for corrosive plastics
- Chisels, punches and rolls in food and chemical plants
Process compatibility
How 440C takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| CNC Machining | FairFair | Machine annealed with coated carbide and leave 0.1-0.3 mm (0.004-0.012") grind stock; every feature that matters is finished after hardening. |
| CNC Turning | FairFair | Shafts, plungers and seats turn annealed at tool-steel speeds; expect carbide wear from the chromium carbides. |
| Heat Treating | ExcellentExcellent | Hardens to 58-60 HRC with an oil or air quench, sub-zero and low temper; state the HRC and forbid the 425-565 °C temper band. Can 440C stainless be hardened? |
| Vacuum Heat Treating | ExcellentExcellent | The standard for bearings and precision parts: bright, low-distortion hardening with gas quench, sub-zero and temper in one cycle. |
| CNC Grinding | ExcellentExcellent | Hardened 440C grinds and laps to bearing tolerances and mirror finishes; the normal finishing route. |
| Centerless Grinding | ExcellentExcellent | Precision shafting and pins are centerless ground after hardening to a few micrometres of size and roundness. |
| Wire EDM | GoodGood | Cuts hardened blades and inserts without softening them; temper after cutting and remove the recast layer on fatigue-loaded parts. |
| Passivation | GoodGood | ASTM A967 citric after final grinding and polishing; nitric baths need a low concentration on 440C or they etch. Needed to keep a ground surface from rusting in storage. |
| Electropolishing | FairFair | Possible on surgical parts for a smooth passive surface, but the carbides leave a duller finish than austenitic grades; mechanical polish first. |
| Black Oxide | FairFair | MIL-DTL-13924 Class 3 blackens hardened 440C for glare control and mild rust protection under oil. |
| Investment Casting | FairFair | Castable, but the coarse as-cast carbides give poor toughness; wrought bar is used for anything loaded. |
| Welding | Not recommendedNot recommended | Cracks on cooling in any process; braze or fasten instead. |
| Press Brake Forming | Not recommendedNot recommended | No useful ductility even annealed; parts are machined and ground, never bent. |
Design and sourcing tips
- Specify "440C per ASTM A276 (or AMS 5630), annealed; harden and temper to 58-60 HRC" and call the temper band (150-200 °C / 300-400 °F) so the shop does not soften or embrittle it.
- Order bearing-quality or premium-melted stock (ASTM A756, AMS 5618) for bearings, seats and polished surfaces; ordinary bar has carbide stringers that show up as pits.
- Design for grinding after hardening: generous radii, no sharp internal corners, grind stock left on every toleranced feature, and no threads finer than necessary.
- Treat it as brittle: 2% elongation at 60 HRC. Avoid impact loads, press fits with high interference and thin unsupported sections.
- Passivate (ASTM A967) after the last grind; a freshly ground 440C surface rusts in humid storage within days.
- For salt water or chemicals choose 17-4 PH or 316 and give up hardness, or a nitrogen-alloyed corrosion-resistant bearing steel; 440C pits in chlorides.
Frequently asked questions
- Is 440C stainless steel magnetic?
- Yes, in every condition. It is a martensitic chromium steel; a magnet holds to it like carbon steel. Non-magnetic bearings use ceramic or an austenitic grade.
- How hard is 440C stainless?
- 58-60 HRC when hardened from 1010-1065 °C (1850-1950 °F), sub-zero treated and tempered at 150-200 °C (300-400 °F). Tempering at 315-370 °C (600-700 °F) gives 56-57 HRC with slightly better toughness. Annealed, it is about 230 HB, roughly 20 HRC.
- Does 440C rust?
- It can. Hardened, polished and passivated, it resists fresh water, steam, food and mild chemicals well. Salt water, chloride cleaners and marine air pit it, a ground surface left unpassivated will spot in humid storage, and a temper above 370 °C (700 °F) lowers its resistance. It is a hard stainless, not a corrosion-resistant one.
- Is 440C good for knives?
- Yes; it was the premium stainless blade steel for decades and still makes a good working knife: 58-60 HRC, fine edge, easy to sharpen, decent rust resistance. Modern powder-metallurgy stainless steels hold an edge longer and are tougher at the same hardness, at several times the price.
- What is the difference between 440A, 440B and 440C?
- Carbon: 440A has 0.60-0.75%, 440B 0.75-0.95%, 440C 0.95-1.20%. Less carbon means lower attainable hardness (about 55 HRC for 440A) but better toughness and corrosion resistance. 440C is the bearing and blade grade; 440A shows up in low-cost cutlery and 440B is rarely stocked.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM A276 and ASTM A756 (bearing quality)
- AMS 5630 and AMS 5618
- ASM Handbook, Volume 4D: Heat Treating of Irons and Steels
- Producer data sheets for 440C bar (Carpenter, Crucible, Latrobe)
