What Is 440C Stainless Steel?

What Is 440C Stainless Steel?

440C (UNS S44004) is a high-carbon martensitic stainless steel. After hardening it delivers the highest hardness, wear resistance, and strength of all stainless steel grades, which suits components such as ball bearings, valve assemblies, nozzles, and cutting tools. Martensitic grades are hardened by quenching and tempering, and their corrosion resistance is lower than that of austenitic grades because part of the chromium is locked up in carbides. The steel is available in bar, wire, and strip form, typically supplied annealed so the buyer can machine it before hardening.

Chemical Composition

Element	Limit (%, by weight)
Carbon	0.95-1.20
Chromium	16.00-18.00
Manganese	max 1.00
Silicon	max 1.00
Nickel	max 0.60
Phosphorus	max 0.035
Sulfur	max 0.030

Heat Treatment and Achievable Hardness

Operation	Cycle
Annealing	800-920 °C, slow cooling, hardness max 269 HB
Preheating	649-816 °C
Quenching	1010-1070 °C, oil cooling
Tempering	100-180 °C, rapid cooling, hardness 58-60 HRC

Tempering above about 200 °C causes a sharp drop in hardness and corrosion resistance, so 440C parts that need maximum hardness are tempered in the low range. Because the steel has good dimensional stability at high temperature, it is also used for corrosion-resistant high-temperature bearing applications.

Mechanical Properties and Applications

In the hardened condition, typical values are tensile strength around 1960 MPa, yield strength around 1900 MPa, and elongation about 2%; annealed material shows about 560 MPa tensile strength with 18% elongation. Applications include ball and roller bearings operating in corrosive or strongly oxidizing atmospheres without lubrication, valve seats and stems, nozzles, medical scalpels and scissors, knife blades, and wear plates. The high hardness also makes 440C the standard stainless for high-end kitchen cutlery and industrial shear blades.

440A, 440B, 440C, and 440F Compared

440A (max 0.75% C) and 440B (0.75-0.95% C) are softer, tougher, and slightly more corrosion resistant than 440C. 440C offers the highest hardness and wear resistance. 440F is the free-machining variant of 440C with added sulfur for better machinability, used where production turning dominates. Choose the lowest carbon version that meets the hardness requirement, because corrosion resistance improves as carbon drops.

Common Misconceptions

'440C is as corrosion resistant as 304.' No. The high carbon content consumes chromium as carbides, so 440C has noticeably lower corrosion resistance than austenitic 304 or 316, especially in chlorides.

'440C stays hard after high-temperature tempering.' Tempering above about 200 °C drops hardness sharply; 440C is not a high-temperature strength alloy.

'440C can be used below freezing.' Martensitic grades lose ductility at sub-zero temperatures; impact-toughness requirements must be checked before specifying 440C for cold service.

Frequently Asked Questions

What is the hardness of 440C after heat treatment?

Oil quenched from 1010-1070 °C and tempered at 100-180 °C, 440C reaches 58-60 HRC, the highest of any stainless steel.

What is 440C stainless steel used for?

Ball and roller bearings, valve seats, nozzles, medical scalpels, scissors, knife blades, and wear parts that need high hardness in corrosive or oxidizing service.

Is 440C stainless steel magnetic?

Yes. 440C is martensitic and strongly magnetic in both annealed and hardened conditions, unlike austenitic grades.

Can 440C rust?

Yes, in aggressive chloride environments. Its corrosion resistance is moderate; it is not suitable for seawater or long-term outdoor exposure without protection.

What is the difference between 440C and 440A?

Carbon content: 440C has 0.95-1.20% versus max 0.75% in 440A. 440C is harder and more wear resistant; 440A is tougher and slightly more corrosion resistant.

What standards cover 440C bar?

ASTM A276 for hot-finished or cold-finished bar, with general requirements per ASTM A484; wire and strip are covered by A580 and A240 respectively where applicable.