If you need a stainless steel that combines extreme hardness, wear resistance, and decent corrosion protection, AISI 440C stainless steel is the gold standard. As a high-carbon martensitic grade, it’s the go-to for parts that demand long life in tough conditions—from surgical tools to industrial bearings. This guide breaks down everything you need to know to work with this powerful material.
1. AISI 440C Stainless Steel: Overview & Key Specifications
Let’s start with the fundamentals of AISI 440C stainless steel—its makeup, standards, and core properties.
Chemical Composition
The AISI 440C composition is what makes it stand out. It has a high 440C carbon 0.95–1.20% (the highest among common martensitic grades) and 440C chromium 16–18% for corrosion resistance. Small amounts of manganese (max 1.0%), silicon (max 1.0%), and molybdenum (0.4–0.6%) boost strength and wear resistance. This blend creates a metal that hardens to exceptional levels.
Industry Standards & Equivalents
AISI 440C is regulated by global standards to ensure quality:
- 440C UNS S44004: The official identifier in the Unified Numbering System.
- 440C ASTM A276/A580: ASTM A276 covers bars and wire; ASTM A580 applies to sheets and strips.
- 440C EN 1.4125 equivalent: The European standard match, used in EU manufacturing.
Physical & Mechanical Properties
Below is a table of critical properties that make AISI 440C stainless steel ideal for high-wear applications:
| Property | Value |
|---|---|
| 440C density | 7.75 g/cm³ |
| 440C melting point | 1450 °C |
| 440C yield strength | 450 MPa |
| 440C tensile strength | 760–1030 MPa |
| 440C Brinell hardness | 270–285 HB (annealed); up to 60 HRC (hardened) |
| 440C martensitic microstructure | Enables extreme hardening when heat-treated |
Example: A bearing manufacturer relies on AISI 440C stainless steel for raceways. Its hardened hardness (up to 60 HRC) resists wear, even under 10,000+ hours of use—far longer than softer grades like 410.
2. Corrosion Resistance & Environmental Performance
While AISI 440C stainless steel is known for hardness, it also offers solid corrosion protection in mild environments.
Key Corrosion Behaviors
- 440C atmospheric corrosion: Excellent resistance to dry and slightly humid air. It’s used for outdoor fixtures like gate hinges that don’t face saltwater.
- 440C mild chloride tolerance: Can handle low chloride levels (e.g., indoor plumbing) but avoid coastal areas or saltwater—its 440C marine environment limits mean it may rust in high-salt settings.
- 440C oxidation resistance up to 800 °C: Maintains strength and resists scaling at high temperatures, making it suitable for high-heat parts like valve seats.
- 440C stress corrosion cracking susceptibility: Low risk in most applications, but avoid prolonged exposure to high temperatures and chlorides together.
Testing & Improvement Methods
- 440C salt spray performance ASTM B117: In this test, it typically shows minimal rust after 48–72 hours (vs. 304’s 100+ hours).
- 440C passivation treatment: Applying a nitric acid solution thickens the chromium oxide layer, boosting corrosion resistance by 40–60%.
- 440C rust prevention: Regular cleaning and lubrication (for moving parts like bearings) extend its life in harsh environments.
- 440C vs 304 corrosion comparison: 304 offers better corrosion resistance (especially in saltwater), but 440C is harder—choose based on your priority (wear vs. corrosion).
Case Study: A medical device maker uses AISI 440C stainless steel for surgical scalpels. They apply passivation treatment to prevent rust from blood and cleaning chemicals, ensuring the scalpels meet FDA standards for biocompatibility.
3. Heat Treatment & Microstructure Control
Heat treatment is critical to unlocking AISI 440C stainless steel’s full hardness—here’s how to do it right.
Common Heat Treatment Processes
| Process | Temperature | Purpose |
|---|---|---|
| 440C annealing 850–900 °C slow cool | 850–900 °C (slow cool) | Reduces hardness (to 270–285 HB), improves machinability |
| 440C austenitizing 1010–1065 °C | 1010–1065 °C | Converts microstructure to austenite, preparing for hardening |
| 440C oil or air quench hardening | Oil/air quench after austenitizing | Forms hard martensite, increases hardness to 58–60 HRC |
| 440C tempering 150–370 °C | 150–370 °C (air cool) | Reduces brittleness, maintains high hardness |
Critical Considerations
- 440C hardness vs tempering curve: Tempering at 150 °C keeps hardness high (58–60 HRC) for wear parts; tempering at 370 °C lowers it to 50–52 HRC but increases toughness.
- 440C retained austenite: Quenching leaves small amounts of soft austenite—use 440C cryogenic treatment for retained austenite (-80 to -196 °C) to convert it to martensite, boosting hardness by 2–3 HRC.
- 440C carbide distribution: Proper austenitizing ensures even carbide spread, preventing weak spots in the metal.
- 440C grain size ASTM 6–9: Controlled heating keeps grain sizes small (ASTM 6–9), balancing strength and ductility.
Example: A cutlery maker heat-treats AISI 440C stainless steel blades: austenitize at 1040 °C, oil quench, temper at 200 °C, and cryogenically treat. The result? Blades that hold an edge for 500+ cuts and resist chipping.
4. Welding, Fabrication & Machining Guidelines
Working with AISI 440C stainless steel is trickier than softer grades—follow these tips to avoid issues.
Welding Tips
- 440C weldability challenges: High carbon content increases cracking risk. Preheating and post-weld tempering are mandatory.
- 440C preheat 250–300 °C: Preheating slows cooling, reducing stress in the weld.
- 440C post-weld tempering 650–700 °C: Tempering after welding restores ductility and prevents brittleness.
- 440C filler metal ER410NiMo: This filler matches 440C’s strength and corrosion resistance, avoiding weak welds.
- 440C hydrogen cracking risk: Use low-hydrogen electrodes and dry the metal before welding to eliminate hydrogen bubbles.
Machining & Fabrication
- 440C machining speeds and feeds: Use slow speeds (50–150 m/min for turning) and low feeds (0.05–0.15 mm/rev) to avoid overheating.
- 440C tool life with CBN/ceramic inserts: Cubic Boron Nitride (CBN) or ceramic tools last 5–10x longer than carbide when machining hardened 440C.
- 440C grinding burn prevention: Use coolants and slow grinding speeds (15–20 m/s) to avoid burning the metal (which weakens it).
- 440C formability limits: Poor formability when hardened—anneal first if bending or shaping is needed.
- 440C EDM parameters: Electrical Discharge Machining (EDM) works well for complex parts (e.g., mold inserts). Use low current (5–10 A) for precise cuts.
Case Study: An injection molding company machines AISI 440C stainless steel screws. They use CBN inserts and slow speeds (80 m/min), producing screws that resist wear from plastic pellets—lasting 3x longer than 430 screws.
5. Product Forms, Sizes & Supply Chain
AISI 440C stainless steel is available in specialized forms to fit high-performance applications.
Common Product Forms
- 440C stainless steel round bar stock: Diameters 6–300 mm (used for bearings, shafts, and cutlery).
- 440C sheet thicknesses: 0.5–10 mm (used for precision instruments and razor blades).
- 440C plate sizes: 10–200 mm thick, up to 2000×4000 mm (used for mold bases).
- 440C precision ground flat stock: Thicknesses 1–50 mm, with tight tolerances (±0.01 mm) for tooling.
- 440C thin foil gauges: 0.01–0.1 mm thick (used for small precision parts like watch components).
- 440C mirror finish sheets: Available in 0.5–5 mm thicknesses (used for decorative high-wear parts).
Supply Chain Tips
- 440C hollow bar suppliers: Look for suppliers with ISO 9001 certification—hollow bars require strict quality control for wall thickness.
- 440C forged blanks: Forged blanks (up to 500 kg) are ideal for large parts like valve bodies; lead times are 3–4 weeks.
- 440C metric & imperial sizes: Most suppliers offer both (e.g., 10 mm round bar = 3/8 inch), so order based on your project’s needs.
- 440C custom forgings: Suppliers can create custom shapes (e.g., bearing races) for unique applications.
6. Industry Applications & Case Studies
AISI 440C stainless steel shines in applications where hardness and wear resistance are non-negotiable.
Key Applications
- 440C surgical scalpels: Sharpens to a fine edge and resists corrosion from bodily fluids.
- 440C bearings & races: Hardness (up to 60 HRC) resists wear in high-speed rotating parts.
- 440C valve seats & balls: Withstands high temperatures (up to 800 °C) and pressure in engines.
- 440C cutlery blades: Holds an edge longer than 410 or 430, making it perfect for chef’s knives.
- 440C injection molding screws: Resists wear from abrasive plastic pellets.
- 440C firearm components: Strong and impact-resistant—used for barrels and triggers.
Real-World Example: A watchmaker uses AISI 440C stainless steel for watch cases and movements. The metal’s hardness (55–58 HRC) resists scratches from daily wear, and its corrosion resistance keeps the watch looking new for 10+ years. Customers praise the cases for holding up to bumps and moisture.
Yigu Technology’s Perspective on AISI 440C Stainless Steel
At Yigu Technology, we supply AISI 440C stainless steel to medical, automotive, and tooling clients worldwide. Its unbeatable combination of hardness and corrosion resistance makes it ideal for high-wear parts. We offer precision-ground flat stock and custom forgings, and our team advises clients on heat treatment—like cryogenic processing—to maximize hardness. For medical clients, we ensure passivation meets FDA standards, so their tools are safe and durable.
FAQ About AISI 440C Stainless Steel
- Is AISI 440C stainless steel magnetic?
Yes! Its 440C martensitic microstructure makes it strongly magnetic—unlike non-magnetic austenitic grades like 304. This is useful for parts that need to attract magnets, like some sensor components. - Can AISI 440C be used in food processing?
Yes, but only for parts that don’t touch acidic or salty foods regularly. Its mild chloride tolerance and passivation treatment let it work for food-cutting blades, but avoid using it for tanks holding tomato sauce or brine (304 is better for those). - What’s the difference between AISI 440C and 440A/440B?
440C has the highest carbon content (0.95–1.20%), so it hardens to the highest hardness (up to 60 HRC). 440A (0.60–0.75% carbon) and 440B (0.75–0.95% carbon) are softer but easier to machine—choose 440C for maximum wear resistance.
