What's The Difference of 440A And 440C Stainless Steel

The Key Differences Between 440A and 440C Stainless Steel

440A and 440C are both belong to 440 stainless steel series, they come from America steel standard ASTM A276, and they are known for high hardness and wear resistance. The primary variance between 440A and 440C lies in their carbon (C) that directly influence hardness, strength, and corrosion resistance. Their distinct properties make them suited for drastically different applications.

1. Chemical Composition: The Foundation of Differences

Carbon: 440C contains nearly twice as much carbon as 440A. Carbon is the primary hardening agent in martensitic stainless steels—higher carbon content enables greater hardness and wear resistance after heat treatment.

Chromium: Both grades have identical chromium ranges (16–18%), a key element for forming a protective oxide layer that resists rust. However, carbon can "compete" with chromium: higher carbon may reduce the amount of chromium available for oxide formation, subtly impacting corrosion resistance.

Because 440 stainless steel is very common and useful material, so you can easily find them from different steel standard, such as you can find them from Japanese JIS G4303, please check below chart:

Japanese stainless steel start from “SUS”, they are just like America standard JIS G4303, they also have A, B, C series, the only difference is C element, besides Japan standard specially ruled SUS420F with higher sulfur, it will improve the machinability.

You can also find similar steel material from EN10088-3 and GB/T1220, in these steel standards, they are not a series grade, it only stipulated the gade which is same as 440C.

2. Mechanical Properties: Hardness vs. Toughness

Mechanical performance is where 440A and 440C diverge most sharply, especially after heat treatment (a critical step for martensitic steels to achieve full hardness).

Hardness

440A: After quenching and tempering (Q&T), its Rockwell C (HRC) hardness typically ranges from 56–58 HRC. Lower carbon content limits maximum hardness but improves ductility (the ability to bend without breaking).

440C: With higher carbon, it reaches 58–62 HRC post-Q&T—among the hardest standard stainless steel grades. This makes it ideal for applications requiring extreme wear resistance (e.g., knife blades, bearings).

Toughness and Ductility

440A: More ductile and tough than 440C. It can withstand moderate impact or bending without cracking, making it suitable for parts like valve seats or surgical instruments that need both corrosion resistance and flexibility.

440C: High hardness comes at the cost of reduced toughness. It is brittle under impact or heavy bending—overloading can lead to chipping or fracture. This limits its use to low-impact, high-wear scenarios.

3. Corrosion Resistance: A Subtle But Critical Gap

Both grades offer "good" corrosion resistance for martensitic stainless steels, but 440A outperforms 440C in mild to moderate corrosive environments:

440A: The lower carbon content leaves more chromium available to form a dense oxide layer. It resists rust in moist air, freshwater, and mild chemicals (e.g., household cleaners) better than 440C.

440C: Higher carbon slightly impairs its oxide layer integrity. It still resists rust in dry or low-humidity environments but may corrode faster in saltwater, high humidity, or acidic conditions. For marine or chemical applications, 440C often requires additional coatings (e.g., passivation) to enhance protection.

Note: Neither grade matches the corrosion resistance of austenitic stainless steels (e.g., 304 or 316), which are better suited for harsh environments like seawater or industrial chemicals.

4. Heat Treatment: Processing Requirements

Heat treatment protocols differ to optimize each grade’s unique properties:

440A: Requires lower tempering temperatures (typically 200–300°C) to balance hardness and ductility. Its lower carbon content makes it easier to machine before heat treatment (annealed hardness: ~25 HRC, compared to 440C’s ~28 HRC).

440C: Needs higher tempering temperatures (300–400°C) to relieve internal stresses from high carbon content. Annealed 440C is harder to machine than 440A, requiring specialized tools or slower cutting speeds to avoid excessive wear.

5. Applications: Matching Properties to Use Cases

The choice between 440A and 440C hinges on prioritizing wear resistance (440C) or toughness/corrosion resistance (440A):

440A Applications

Valve seats and stems in plumbing systems (needs ductility to seal tightly)

Surgical tools (requires corrosion resistance and flexibility)

Food processing equipment parts (resists mild cleaning chemicals)

Small bearings in low-wear, high-humidity environments

440C Applications

High-end knife blades (extreme hardness for sharpness retention)

Rolling element bearings in high-wear machinery (e.g., automotive or industrial pumps)

Mold inserts for plastic injection molding (resists abrasion from molten plastics)

Precision gears in low-impact mechanical systems

Conclusion: Choosing the Right Grade

440A and 440C are not interchangeable

Select 440A if your application demands a balance of corrosion resistance, ductility, and moderate hardness—especially in environments with moisture or mild chemicals.

Select 440C if extreme wear resistance and maximum hardness are critical, and the application involves low impact and minimal exposure to harsh corrosives.

Understanding these differences ensures optimal performance,longer part life,and cost-effectiveness, avoiding the mistake of using a brittle 440C in a high-impact role or a softer 440A in a high-wear application.