AISI 440C Martensitic Stainless Steel: High Hardness Bearing & Tooling Grade
ASTM A276/A756 · Published: 2026-06-24 · Updated: 2026-06-25
AISI 440C is the highest-carbon stainless steel in common use, containing 0.95-1.20% carbon and 16-18% chromium. It achieves the highest hardness of any stainless steel — up to 60 HRC when properly heat treated — making it the stainless grade of...
AISI 440C is the highest-carbon stainless steel in common use, containing 0.95-1.20% carbon and 16-18% chromium. It achieves the highest hardness of any stainless steel — up to 60 HRC when properly heat treated — making it the stainless grade of choice for bearings, valve seats, surgical instruments, and high-end knife blades where both corrosion resistance and wear resistance are required.
Unlike austenitic stainless steels (304, 316), 440C is martensitic: it can be hardened by heat treatment. The hardening process involves austenitizing at 1,010-1,065°C, oil quenching, and tempering at 150-370°C depending on the desired balance of hardness and toughness. Sub-zero treatment (-75°C) between austenitizing and tempering transforms retained austenite to martensite, maximizing final hardness.
Quick Facts
| Category | |
| Standard | ASTM A276/A756 |
| Density | 7.80 g/cm³ |
| Yield Strength | 450 MPa (annealed) / 1,860 MPa (hardened) |
| Tensile Strength | 760 MPa (annealed) / 1,970 MPa (hardened) |
Global Equivalents & Cross-Reference
Related Materials
🧮 Material Weight Calculator
Calculate the weight based on this material's density: 7.80 g/cm³
Frequently Asked Questions
Is 440C magnetic?
Yes. Unlike 304 and 316 which are non-magnetic (austenitic), 440C is martensitic and strongly magnetic in all conditions — annealed, hardened, and tempered. This magnetic property is inherent to its crystal structure and does not change with heat treatment. This makes 440C suitable for applications where magnetic detection or magnetic workholding is used.
Can 440C be welded?
Yes, but with significant precautions. 440C has high carbon equivalent and is highly hardenable, making it susceptible to cold cracking in the HAZ. Preheat to 200-300°C, use austenitic stainless filler (309 or 312), maintain interpass temperature, and perform post-weld heat treatment immediately. For most applications, consider using 440A or 440B (lower carbon variants) or a precipitation-hardening stainless (17-4PH) if welding is required.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu
Stainless & Corrosion-Resistant Steel — Engineering Reference
Stainless steels are defined by their chromium content (minimum 10.5%), which forms a self-healing passive layer of chromium oxide that resists corrosion. Grades are classified by microstructure: austenitic (300 series), ferritic (400 series), martensitic, and duplex.
ASTM A240/A276/A312, EN 10088, JIS G4304/G4305, GB/T 3280/4237
Food processing equipment, medical implants, chemical tanks, architectural cladding, marine hardware, pharmaceutical piping
For welded stainless fabrications, specify the low-carbon variant (304L, 316L) to prevent chromium carbide precipitation at grain boundaries, which causes intergranular corrosion (weld decay).