440C Steel Technical Overview

440C Steel Technical Overview: 440C steel is a high-carbon, high-chromium martensitic stainless steel with a typical composition of 0.95-1.20% Carbon and 16.0-18.0% Chromium. This yields high hardness, reaching approximately Rockwell C 60, making the grade suitable for applications requiring excellent wear resistance, such as cutlery and ball bearings. It provides good corrosion resistance.

1. Chemical Composition

The standard chemical composition limits are detailed below:

Component	%
Carbon (C)	0.95 – 1.20
Chromium (Cr)	16.0 – 18.0
Manganese (Mn)	1.00 max
Silicon (Si)	1.00 max
Molybdenum (Mo)	0.75 max
Phosphorus (P)	0.040 max
Sulfur (S)	0.030 max

2. Mechanical Properties:

• This steel achieves high hardness, typically 56-58 HRC annealed, reaching up to 58 HRC after heat treatment.
• Quenching from 1050°C in oil yields 60 HRC. A subsequent cold treatment at -75 °C for 1 hour can increase this to 61.5 HRC.
• Tempering reduces hardness: 150°C tempering results in 61 HRC; 200°C tempering yields 59 HRC.
• Per ASTM A 276-03, the maximum hardness for annealed (A condition) cold-finished 440B and 440C bar/shape is 285 HB.
• JIS G 4303:1998 specifies a minimum hardness of 56 HRC for hot-finished (HF) SUS440B (Q ≤ 75) and 58 HRC for SUS440C under the same conditions.

3. Physical Properties:

Key physical properties are listed here:

Property	Value
Approximate Critical Temperatures
Ac1 (start of austenite formation on heating)	815-865°C
Ar1 (start of ferrite/carbide formation on cooling)	765-665°C
Ms (start of martensite formation)	145°C
Thermal Conductivity (at 20°C)	29.3 W/(m·K)
Coefficient of Linear Expansion (20 to 100°C)	10.5 μm/m·K
Coefficient of Linear Expansion (20 to 200°C)	10.5 μm/m·K
Coefficient of Linear Expansion (20 to 300°C)	10.5 μm/m·K
Coefficient of Linear Expansion (20 to 500°C)	11.7 μm/m·K

4. Applications

• Cutlery: High hardness allows for a durable sharp edge, while corrosion resistance withstands food and washing. Therefore, it is common for high-quality knife blades.
• Bearings: Its high hardness and wear resistance make it suitable for ball bearings and components, especially where corrosion resistance is also required. It is specified as a wear—and corrosion-resistant bearing steel.
• Surgical and Dental Instruments: The combination of high hardness (edge retention), wear resistance (longevity), and good corrosion resistance (withstanding sterilization/bodily fluids) makes 440C appropriate for surgical and dental tools.
• Valve Components: 440C’s corrosion resistance against various media and wear resistance against fluid flow support its use in nozzles and valve parts.
• Plastic Molds: High strength, hardness, and corrosion resistance make 440C a common choice for plastic molds.
• Marine Applications: High chromium levels provide sufficient corrosion resistance for use in marine atmospheres or seawater environments, though limitations exist.
• Applications Requiring High Wear Resistance: Generally, the high carbon content makes 440C a preferred stainless steel option where higher wear resistance is primary.
• Components for Corrosive Environments: AISI 440C is widely used where corrosion resistance is a key consideration.

5. Heat Treatment

Recommended heat treatment process for 440C steel:

5.1Forging

Post-forging, higher-carbon grades like 440C require slow, careful cooling to prevent cracking. An interrupted cooling cycle might be employed: air cool to 150-250°C, reheat to ~650°C, then finally cool. This minimizes excessive grain-boundary carbide formation.

5.2 Austenitizing

Heat to a suitable temperature, typically 1038-1040°C (1900°F), to transform the microstructure to austenite and achieve optimal hardness/corrosion resistance. Hold at temperature to ensure adequate carbide dissolution.

5.3 Quenching

Rapid cooling after austenitizing forms martensite. Oil quenching or air quenching are standard methods, depending on section size and desired cooling rate.

5.4 Tempering

Quenched martensite requires tempering to reduce brittleness and increase toughness while maintaining sufficient For 440C, temper between 200°C and 350°C (approx. 400°F to 660°F). The specific temperature dictates the final hardness/toughness balance.

5.5 Tempering at 204°C (400°F)after austenitizing and oil quenching is a standard practice to optimize hardness and corrosion resistance.

• Tempering between 400°C and 600°C is generally avoided as it can cause sensitization (precipitation of coarser chromium carbides), reducing corrosion resistance.
• Stress Relieving: To reduce internal stresses, perform stress relief after forging or heavy machining. For hardened material, stress relief is slightly lower (14 to 28°C or 25 to 50°F) than the last tempering temperature.

6. 440C Steel’s Corrosion Resistance

6.1 Carbide Formation & Chromium Depletion:

Coarse eutectic carbide formation (during solidification and heat treatment) binds significant chromium, reducing the ‘free’ chromium available in the martensitic matrix for the protective passive layer. This affects overall corrosion resistance compared to lower-carbon stainless steels.

6.2 Comparison to Other Martensitic Grades:

Compared to grades like 420, 440C offers higher wear resistance (due to more carbides) but potentially slightly lower corrosion resistance in some environments due to chromium depletion. It is still considered to have good corrosion resistance.

6.3 Influence of Heat Treatment: Proper heat treatment is critical.

• Austenitizing aims to dissolve carbides and maximize chromium in solution, though based on the carbon content, undissolved carbides will remain.
• Lower temperature tempering (200°C-350°C) is recommended to balance hardness and corrosion resistance.
• Avoid tempering between 400°C-600°C, as this causes sensitization(coarser chromium carbide precipitation) and impairs corrosion resistance.

6.4 Performance in Specific Environments:

• Passivated 440C showed no rusting in tap water immersion tests.
• Conventional 440C can develop rust in 3.5% NaCl solution immersion tests, unlike some newer nitrogen-alloyed steels.
• Generally, achieving good corrosion resistance through hardening steel requires at least 10-11% ‘free’ chromium in the matrix post-heat treatment. High-carbon steels like 440C need excess chromium in the initial alloy to meet this.

6.5 Applications Context

Despite the carbon effect, 440C is widely used where high hardness, wear resistance, and reasonable corrosion resistance are needed (e.g., cutlery, bearings). However, compared to other stainless grades or specialized alloys, it may not be the optimal choice for severely corrosive environments (e.g., constant saltwater exposure).

7. Is 440C a Japanese steel?

No, 440C is an American designation (AISI/ASTM).

8. Is D2 or 440C better?

• Choose D2 for maximum abrasive wear resistance where corrosion is a minor factor or the environment is mild (e.g., cold-work tooling, long-run dies). D2 offers superior wear resistance and dimensional stability for such applications.
• Choose 440C for a balance of good wear resistance and significant corrosion resistance (e.g., molds, cutlery, and bearings in moist or mildly chemical environments).

9. Is 440C steel good for a knife?

Yes, its combination of hardness, wear resistance, and corrosion resistance makes 440C a very suitable and popular material for many knives.