440A vs 440B vs 440C Martensitic Stainless Steels: Hardness Gradient for Wear Applications

The 440 Family: Hardness by Carbon Content

440A, 440B and 440C are hardenable martensitic stainless steels defined in ASTM A276 (bar) and A240 (plate). They share the same 16-18% chromium and up to 0.75% molybdenum base, and they differ in one decisive element: carbon. Higher carbon means more carbides after heat treatment, which means higher hardness, better edge retention and greater wear resistance, at the cost of lower corrosion resistance and toughness.

Chemical Composition per ASTM A276

Element (wt%)	440A UNS S44002	440B UNS S44003	440C UNS S44004
C	0.60-0.75	0.75-0.95	0.95-1.20
Mn	1.00 max	1.00 max	1.00 max
Si	1.00 max	1.00 max	1.00 max
P	0.040 max	0.040 max	0.040 max
S	0.030 max	0.030 max	0.030 max
Cr	16.00-18.00	16.00-18.00	16.00-18.00
Mo	0.75 max	0.75 max	0.75 max

Hardness and Wear Performance

After full hardening by oil quenching and low-temperature tempering, typical maximum hardness reaches about 56 HRC for 440A, 58 HRC for 440B and 60 HRC for 440C, making 440C among the hardest of the standard stainless grades. Hardness translates directly into wear resistance and edge retention: 440A is adequate for light cutting duty, 440B for moderate industrial wear, and 440C for high-stress cutting, bearing and sliding applications.

Corrosion Resistance Trade-Off

Corrosion resistance runs in the opposite direction to hardness. The higher the carbon, the more chromium carbide forms during heat treatment, and the less chromium remains in the matrix to build the passive film. 440A therefore resists rust best of the three, in dry indoor and mild outdoor service with normal maintenance. 440C is the most vulnerable, with visible pitting risk in humid or coastal environments unless coated or oiled. None of the three is suitable for marine, chemical or high-moisture service without protection.

Heat Treatment Practice

Typical practice: harden 440A from 1010-1060 C, 440B from 1020-1070 C and 440C from 1010-1070 C, quenching in oil or forced air, then temper at 150-200 C for 440A and 440C and 175-225 C for 440B to balance hardness and toughness. 440C is often triple-tempered at 150-200 C to convert retained austenite and prevent brittleness. Over-tempering at higher temperatures reduces hardness and edge retention, so tempering must be controlled to the application requirement.

Applications, Limitations and Alternatives

440A: kitchen knives, scissors, small hand tools and cutlery where occasional moisture exposure is acceptable. 440B: industrial cutting blades, woodworking tools and moderate-wear bearings. 440C: surgical scalpels, precision bearing races, valve seats and industrial saw blades demanding maximum wear resistance. All three grades have poor weldability and should not be used in welded assemblies without strict preheat and post-weld tempering. Where corrosion resistance and hardness are both required, consider the precipitation-hardening grade UNS S17400 instead.

FAQ

Which 440 grade is best for knives?

440C gives the best edge retention and wear resistance and is the classic high-end knife steel, but it needs care against corrosion. 440A holds a good edge for kitchen use and resists rust better; many household blades use 440A or 440B for that reason.

Why is 440C harder but less corrosion resistant than 440A?

Higher carbon forms more chromium carbides during hardening, which boost hardness but consume chromium that would otherwise maintain the passive film. Hardness and corrosion resistance therefore trade off against each other across the family.

Can 440 series stainless be welded?

Poorly. The martensitic structure hardens and cracks easily in welds. If welding cannot be avoided, strict preheating and immediate post-weld tempering are required, and the assembly is usually annealed afterward. Welded design is generally avoided.

What heat treatment maximizes the hardness of 440C?

Austenitize at 1010-1070 C, oil quench, then temper at 150-200 C, typically with multiple tempering cycles to eliminate retained austenite. This develops hardness toward the 60 HRC level while reducing brittleness.

Is 440 stainless magnetic?

Yes. The martensitic structure is magnetic in all conditions, annealed or hardened. This is normal for hardenable stainless steels and does not indicate a quality problem.

Are the 440 grades suitable for marine environments?

No. Even 440A has limited pitting resistance in salt water. For marine and chloride service select a molybdenum-bearing austenitic or duplex grade, or a precipitation-hardening grade such as UNS S17400 with appropriate coating.