440A vs 440B vs 440C: Martensitic Steel Hardness Spectrum

 

How do their chemical compositions vary?​

All three contain 16-18% chromium, which provides their basic corrosion resistance. The key difference is in carbon content: 440A has 0.6-0.75% carbon, 440B has 0.75-0.95% carbon, and 440C has 0.95-1.20% carbon. They also contain small amounts of manganese, silicon, phosphorus, and sulfur, with 440C sometimes having a slight molybdenum addition (up to 0.75%) to enhance wear resistance. The increasing carbon content directly influences their ability to harden during heat treatment.​

 

How does their hardness differ after heat treatment?​

After proper heat treatment (quenching and tempering), 440A achieves a hardness of 56-58 HRC, making it the softest of the three. 440B reaches 58-60 HRC, offering a balance of hardness and toughness. 440C is the hardest, with a hardness of 58-62 HRC, due to its higher carbon content. The higher the carbon content, the more carbides form during heat treatment, which contribute to increased hardness but can also reduce toughness if not properly tempered.​

 

What about their corrosion resistance?​

All three have similar corrosion resistance, thanks to their chromium content, which is sufficient to resist rust in dry air and mild environments. However, their corrosion resistance is lower than that of austenitic steels like 304 or 316. 440C, with its higher carbon and possible molybdenum, has slightly better corrosion resistance than 440A and 440B in some conditions, but the difference is minimal. They are all prone to pitting in high-chloride environments, so they are best used in dry or low-moisture settings where corrosion demands are not extreme.​

 

How do their toughness and wear resistance compare?​

Toughness decreases as carbon content increases: 440A is the most tough, making it less likely to chip or fracture under impact. 440B offers a middle ground, balancing toughness and wear resistance. 440C is the most wear-resistant due to its high hardness and more abundant carbides, but it is also the most brittle and can chip if subjected to heavy impact. This makes 440A suitable for applications needing some toughness, while 440C is ideal for parts where wear resistance is critical and impact is minimal.​

 

What are the typical applications for each?​

440A is used in bearing races, surgical instruments, and cutlery where a combination of moderate hardness and toughness is needed. It is also used in valve components that require some corrosion resistance and wear resistance. 440B finds applications in ball bearings, knife blades, and industrial tools that need higher hardness than 440A but more toughness than 440C. 440C, with its extreme hardness and wear resistance, is used in high-performance knife blades, bearing balls, valve seats, and precision measuring tools where long-term wear resistance is essential, even at the cost of some toughness.​