440C STAINLESS STEEL | 1.4125 | SUS440C

3. Mechanical Properties of 440C Stainless Steel

For applications demanding exceptional performance, particularly in high-stress and wear-intensive environments, a thorough understanding of a material’s mechanical properties is crucial. Type 440C stainless steel, a high-carbon martensitic grade, stands out for its remarkable characteristics. At Aobo Steel, we believe in empowering our clients with clear, practical information. Let’s delve into the key mechanical properties that define 440C and make it a preferred choice for specific industrial uses.

3.1 Unmatched Hardness: The Defining Trait of 440C

When it comes to hardness, 440C stainless steel is in a class of its own. This is arguably its most significant attribute.

• Annealed Condition: In its softened state, 440C typically exhibits a hardness of around 270 HBW, with a maximum of approximately 285 HB.
• Heat-Treated Condition: After appropriate heat treatment, 440C can achieve exceptionally high hardness levels. When tempered at 150°C (300°F), it commonly reaches 60 HRC. Some data even suggests it can attain up to 65 HRC. It’s widely recognized that no other stainless steel grade achieves a higher hardness than Type 440C.

• Note on P/M Versions: Powder Metallurgy (P/M) versions of 440C might show different hardness values, for instance, around 43 HRC after heat treatment, but this is highly dependent on the specific processing and density achieved.

The high carbon content (typically 0.95% to 1.20%) in 440C is a primary contributor to its superior hardness compared to other grades in the 440 series (like 440A and 440B).

3.2 Strength and Performance Under Load

The impressive hardness of 440C stainless steel directly translates to high strength, especially after heat treatment.

• Hardened Condition: In its hardened state, the 0.2% yield strength of 440C can exceed 1000 MPa and reach values as high as 1900 MPa. This makes it capable of withstanding significant stresses.
• Powder Metallurgy (P/M) Considerations: For P/M processed 440C, typical heat-treated properties might include a yield strength of around 410 MPa (60 ksi) and an ultimate tensile strength of approximately 620 MPa (90 ksi). It’s important to remember that these P/M values are typical and can vary based on manufacturing specifics.

3.3 Superior Wear Resistance for Demanding Applications

The combination of extreme hardness and the presence of chromium carbides within its microstructure gives 440C stainless steel excellent wear resistance. This makes it an ideal material for components subjected to abrasive or high-friction conditions. The addition of molybdenum (typically 0.40% to 0.75%) can further enhance mechanical resistance through the precipitation of Mo2C carbides, contributing to secondary hardening.

3.4 Ductility and Toughness: Important Considerations

While 440C excels in hardness and strength, these properties often come with trade-offs in ductility and toughness, which are characteristic of high-carbon martensitic stainless steels.

• Ductility: 440C generally exhibits low ductility. Elongation values can be as low as 2% in its hardened state. P/M heat-treated 440C also shows similarly low elongation, typically around 2%.
• Toughness: Martensitic stainless steels, including 440C, tend to have lower room temperature impact strength. For conventionally produced 440C (via ingot casting), the presence of coarse eutectic carbides can be a factor. These carbides can potentially act as initiation sites for cracks under stress, which might affect fatigue life and overall toughness.

3.5 Corrosion Resistance in Various Environments

Type 440C offers good corrosion resistance, especially for a hardenable stainless steel. It’s often selected when resistance to corrosion is a key requirement alongside high hardness. It performs suitably in environments like marine atmospheres or when in contact with seawater. However, its corrosion resistance is considered moderate when compared to some other stainless steel grades, primarily due to the alloy composition necessary to maintain its hardenable martensitic structure.

• Optimizing Corrosion Resistance: For the best corrosion performance, tempering of 440C should be carried out either below 400°C (750°F) or above 600°C (1100°F).

3.6 Hardenability and Optimizing Properties Through Heat Treatment

440C stainless steel is known for its excellent hardenability. It can achieve full hardness through a cross-section of up to 38.1 mm (1-1/2 inches) square.

The final mechanical properties of 440C are heavily influenced by the heat treatment process, particularly tempering:

• Maximizing Strength: To achieve maximum strength and hardness in 440C, tempering is typically performed at low temperatures, generally between 200°C and 350°C (or below 330°C / 625°F).
• Effect of Higher Tempering Temperatures: Using higher tempering temperatures will result in lower hardness and strength but will concurrently increase ductility and toughness.

3.7 Key Mechanical Properties of 440C Stainless Steel at a Glance