440A vs 440B vs 440C: Martensitic Stainless Steel Hardness and Grade Selection

AISI 440A, 440B and 440C are martensitic stainless steels built on the same corrosion-resistant base chemistry but placed at different points on the carbon scale. That single variable, carbon, decides how hard the steel can become after hardening and tempering, and how much toughness has to be surrendered to get there. For buyers specifying cutlery strip, bearing components, valve trim, surgical instruments or precision tooling, the 440A vs 440B vs 440C decision is fundamentally a hardness-versus-toughness trade-off rather than a corrosion-resistance trade-off.

Chemical Composition and Carbon Content

All three grades carry 16.0-18.0% chromium, which forms the passive surface film responsible for their basic atmospheric and mild-environment corrosion resistance. Carbon steps upward across the family: 440A at 0.60-0.75%, 440B at 0.75-0.95% and 440C at 0.95-1.20%. Manganese, silicon, phosphorus and sulfur are held to the usual martensitic limits, and 440C may carry a small molybdenum addition, typically up to 0.75%, which supports wear resistance and edge retention.

Element, %	440A	440B	440C
Carbon, C	0.60-0.75	0.75-0.95	0.95-1.20
Chromium, Cr	16.0-18.0	16.0-18.0	16.0-18.0
Molybdenum, Mo	Not specified	Not specified	Up to 0.75
Manganese, Mn	1.00 max	1.00 max	1.00 max
Silicon, Si	1.00 max	1.00 max	1.00 max
Phosphorus, P	0.040 max	0.040 max	0.040 max
Sulfur, S	0.030 max	0.030 max	0.030 max

Because chromium is identical across the family, no grade offers a meaningful corrosion advantage over the others. The real differentiator is carbide volume: as carbon rises, more chromium-rich carbides form during hardening, which is precisely what raises hardness, but also what removes chromium from solid solution and reduces the corrosion reserve.

Hardness After Hardening and Tempering

After austenitising, quenching and tempering, the three grades separate clearly on the Rockwell C scale. 440A reaches roughly 56-58 HRC and is the softest of the family. 440B delivers about 58-60 HRC and occupies the middle ground. 440C is the hardest, achieving approximately 58-62 HRC when fully hardened and tempered for cutting service.

Property	440A	440B	440C
Hardness range	56-58 HRC	58-60 HRC	58-62 HRC
Relative toughness	Highest	Intermediate	Lowest
Relative wear resistance	Lowest	Intermediate	Highest
Relative carbide volume	Moderate	Higher	Highest

Normal hardening practice for the family is austenitising in the 1010-1070 °C range, followed by an oil or controlled-atmosphere quench, then tempering between 150 °C and 315 °C. Tempering temperature is the second lever on the property balance: low-temperature tempering preserves maximum hardness, while higher tempering temperatures trade some hardness for impact toughness. Tempering in the 400-600 °C window is generally avoided for these grades because chromium carbide precipitation can lower both toughness and corrosion resistance.

Corrosion Resistance in Practice

Corrosion performance is broadly similar across 440A, 440B and 440C, and it is clearly below that of austenitic grades such as 304 or 316. The 16-18% chromium level resists rust in dry air and mild indoor service, but a hardened, heavily carbidic 440C structure leaves less chromium in solid solution, so the grade should not be treated as a chloride-resistant material. All three are prone to pitting where chlorides concentrate and are best applied in dry or low-moisture environments. In cutlery practice this is why 440 series blades are cleaned and dried after use, and why these grades are not the first choice for marine or chemical duty.

Toughness, Wear Resistance and Applications

Impact toughness falls as carbon rises. 440A is the most forgiving under shock loading, 440B balances hardness against toughness, and 440C offers the greatest wear resistance with the least resistance to chipping under heavy impact. Matching that spectrum to the application is straightforward:

440A: bearing raceways, surgical and dental instruments, valve components and cutlery that need a combination of moderate hardness and dependable toughness.

440B: ball bearings, knife blades and industrial tooling that requires more hardness than 440A while keeping more toughness than 440C.

440C: high-performance knife blades, bearing balls, valve seats and precision measuring tools where long-term wear resistance matters more than impact resistance.

Choosing Between 440A, 440B and 440C

Start from the failure mode. If a part must survive impact, chipping or bending, specify 440A or 440B and accept the lower hardness ceiling. If a part must hold a sharp edge or a precise dimension against abrasion for a long service life, specify 440C and manage brittleness through design and correct tempering. Where corrosion resistance governs the design, none of the 440 grades is the right answer and an austenitic or ferritic grade should be selected instead. Recording the required hardness range, the tempering temperature and the end application in the purchase specification removes the ambiguity that most often leads to a 440A part being supplied where 440C was intended, or the reverse.

Frequently Asked Questions

Q: What is the main difference between 440A, 440B and 440C?
Carbon content. 440A contains 0.60-0.75% carbon, 440B contains 0.75-0.95% and 440C contains 0.95-1.20%, while chromium stays at 16.0-18.0% in all three.

Q: Which of the three is the hardest?
440C, at approximately 58-62 HRC after hardening and tempering, compared with 58-60 HRC for 440B and 56-58 HRC for 440A.

Q: Are the 440 grades more corrosion resistant than 304 or 316?
No. Their 16-18% chromium gives only moderate resistance, and they pit readily in chloride-rich environments where austenitic grades perform markedly better.

Q: Can 440A be substituted for 440C?
Only where the required hardness and wear resistance can be reduced. 440A cannot be hardened to the 440C level, so edge retention and abrasion life will be lower.

Q: How are the 440 grades heat treated?
They are austenitised at about 1010-1070 °C, quenched in oil or a controlled atmosphere, then tempered between 150 °C and 315 °C to set the final hardness and toughness balance.

Q: Why does 440C chip more easily?
Its high carbon content produces the largest carbide volume and the highest hardness, which lowers impact toughness. Controlled tempering and designs that avoid sharp stress risers reduce the risk.