Stainless Steel 440: Ultra-Hard Martensitic Alloy for High-Wear Precision Components

Stainless Steel 440 is a family of high-carbon martensitic grades (440A/B/C/F) renowned for their extreme hardness and wear resistance, making them the gold standard for industrial bearings, premium cutlery, and precision watch components. With 16–18% chromium and up to 1.20% carbon, it delivers the highest hardness of any common stainless steel grade.

Chemical Composition (Key, % per ASTM A240)

Carbon (C): 0.60–1.20

Chromium (Cr): 16.00–18.00

Molybdenum (Mo): ≤0.75 (440C only)

Manganese (Mn): ≤1.00

Mechanical Properties (Heat-Treated)

Tensile Strength: ≥860 MPa

Yield Strength: ≥620 MPa

Elongation in 50mm: ≥8%

Hardness: Up to 60 HRC (440C)

Performance Advantages

Industry-Leading Hardness: 440C reaches 60 HRC, ideal for wear-resistant parts.

Superior Edge Retention: The top choice for premium knife blades and surgical scalpels.

Dimensional Stability: Maintains precise tolerances after heat treatment for watch and bearing components.

Applications

High-end knife blades and industrial cutters

Ball bearings and roller bearings

Watch gears and precision mold components

Equivalent Grades

EN 1.4125 (440C), JIS SUS440A/B/C, DIN X90CrMoV18 (440C)

5 Common Questions & Answers

What are the differences between 440A, 440B, 440C, and 440F subtypes?The 440 family varies by carbon and molybdenum content: 440A (0.60–0.75% C, no Mo) is the most ductile, suited for scissors and low-wear cutlery; 440B (0.75–0.95% C, no Mo) balances hardness and toughness for hunting knives; 440C (0.95–1.20% C, 0.40–0.75% Mo) is the hardest (up to 60 HRC) and most wear-resistant, used for bearings and industrial cutters; 440F is a free-machining variant of 440C with added sulfur (0.15–0.30%) for easier precision machining of watch gears. Molybdenum in 440C improves corrosion resistance and hardness, while sulfur in 440F reduces machining friction without sacrificing core properties.

Why is 440C the top choice for industrial bearings?440C's ultra-high hardness (60 HRC) provides exceptional wear resistance, critical for bearings that undergo constant rolling contact with minimal lubrication. Its molybdenum content enhances corrosion resistance to lubricant breakdown products and moisture, preventing bearing seizure and extending service life. Unlike carbon steel bearings, 440C does not rust, making it suitable for outdoor or wet bearing applications (e.g., marine propeller shafts). Its dimensional stability after heat treatment ensures precise bearing clearances, which is essential for smooth, low-noise operation in machinery and automotive systems. No other common stainless steel grade can match 440C's combination of hardness and corrosion resistance for bearing use.

How does heat treatment optimize 440C's properties for different uses?440C is heat-treated via a three-step process: austenitization (heating to 980–1040°C to dissolve carbides), quenching in oil (to form hard martensite), and tempering (to balance hardness and toughness). Low-temperature tempering (150–200°C) retains 60 HRC hardness for non-impact parts like bearings, but leaves the steel brittle. Medium-temperature tempering (300–400°C) reduces hardness to 50–55 HRC while increasing toughness, making it suitable for knife blades that may withstand minor impacts. Over-tempering above 400°C causes excessive softening (below 50 HRC), so strict temperature control is critical to match the steel's properties to the application's needs.

What are the corrosion resistance limitations of 440 grades?440 grades have lower corrosion resistance than austenitic grades like 304 due to their high carbon content, which reduces the continuity of the protective chromium oxide layer. 440A/B are prone to rust in humid, salty environments and will corrode in contact with dilute acids, while 440C's molybdenum content provides modest improvement in chloride resistance but still falls far short of 316. Proper passivation (acid treatment to remove surface iron contaminants) can enhance the oxide layer and improve corrosion resistance, but 440 grades should still be avoided in marine or chemical environments. For these harsh settings, 316 bearings or tools are preferred, even though they are softer than 440C.

Why is 440F preferred for precision machining of watch components?440F contains sulfur (0.15–0.30%), which acts as an internal lubricant during machining, reducing friction between the cutting tool and workpiece. This allows for faster machining speeds, smoother surface finishes, and longer tool life compared to standard 440C, which is notoriously difficult to machine due to its high hardness. 440F retains 440C's core properties (up to 58 HRC after heat treatment), so it still delivers the wear resistance needed for watch gears and precision components. Without sulfur, machining 440C would require frequent tool changes and slow speeds, increasing production costs and reducing part precision-making 440F the only practical choice for high-volume precision machining of 440-series parts.