440C Stainless Steel (UNS S44004): High-Hardness Martensitic Grade Explained

What Is 440C Stainless Steel?

440C is a high-carbon, high-chromium martensitic stainless steel, designated UNS S44004 in the American system and 1.4125 (X105CrMo17) in EN 10088. Its defining feature is a carbon content of 0.95-1.20%, the highest among the standard martensitic stainless grades. Combined with 16.0-18.0% chromium, this allows the steel to be hardened to approximately 58-60 HRC, the maximum hardness available from a conventional stainless steel.

The same carbon content that provides hardness creates the grade's limitations: reduced toughness and impact resistance, moderate corrosion resistance that is not suitable for harsh chemical or marine environments, and difficult machining after hardening. For this reason 440C is always used in the hardened and tempered condition, and nearly all machining is performed in the annealed state.

Equivalent Grades

Standard System	Designation
AISI / ASTM (USA)	440C / UNS S44004
EN (Europe)	1.4125 / X105CrMo17
JIS (Japan)	SUS440C
GB (China)	108Cr17

Chemical Composition (ASTM A276)

Element	C	Mn	P	S	Si	Cr	Mo
Content, %	0.95-1.20	1.00 max	0.040 max	0.030 max	1.00 max	16.0-18.0	0.75 max

Mechanical Properties and Hardness

Condition	Typical Hardness	Note
Annealed (for machining)	269 HB max	Soft enough for turning, milling and drilling
Hardened and tempered	58-60 HRC	Full hardness for wear and cutting applications

In the hardened condition, tensile strength is typically in the range of 1800-2000 MPa. The trade-off is reduced ductility and impact toughness, so components are designed to operate in compression or wear rather than under impact.

Heat Treatment

Achieving the full potential of 440C requires a controlled, sometimes multi-step heat treatment:

Austenitizing at 1010-1065 degrees Celsius to dissolve chromium carbides into the matrix.

Rapid quenching in oil, or interrupted quenching for thin sections, to transform the structure to martensite.

Optional sub-zero treatment at about -75 degrees Celsius to convert retained austenite and maximize hardness.

Tempering at 150-370 degrees Celsius to relieve stress and balance hardness and toughness; double tempering is common for dimensional stability.

Because the steel has high carbon and chromium, it is very susceptible to decarburization at austenitizing temperature. Protective atmosphere or vacuum heat treatment is strongly recommended to preserve the surface hardness and corrosion resistance.

Corrosion Resistance and Limitations

440C has moderate corrosion resistance. It resists atmospheric corrosion, mild chemicals, fresh water and many foodstuffs, and is far more corrosion resistant than carbon tool steels. However, it is inferior to 304 and 316 in this respect, and it is not suitable for seawater, strong acids or chloride-rich environments where pitting would occur. The steel should not be welded for structural applications; welding produces hard, brittle martensite and crack-sensitive heat-affected zones.

Typical Applications

Precision ball and roller bearings for aerospace and instrumentation.

Valve seats, valve stems and pump parts in high-wear fluid systems.

Knife blades and cutting tools where edge retention is critical.

Mold inserts, dies and gauges requiring high hardness and good polishability.

Nozzles, shafts, bushings and instrument parts in wear service.

Frequently Asked Questions

Q1. What hardness can 440C achieve?

440C is normally hardened and tempered to 58-60 HRC, and in optimized practice can reach about 60-62 HRC with sub-zero treatment. This is the highest hardness of the standard stainless steels.

Q2. Why is 440C less corrosion resistant than 304?

The high carbon content of 440C consumes chromium as carbides, leaving less chromium in solid solution to form the protective passive film. In addition, the martensitic structure has no chromium-rich grain-boundary network. Corrosion resistance is therefore lower than that of austenitic grades such as 304 or 316.

Q3. Can 440C be welded?

Welding 440C is not recommended for structural joints. The weld and heat-affected zone transform to hard, brittle untempered martensite on cooling and are prone to cracking. Where joining is unavoidable, preheating, austenitic filler and immediate tempering are required, with post-weld hardness and crack testing.

Q4. What is the difference between 440A, 440B and 440C?

The three grades share the same chromium range but differ in carbon: 440A has 0.60-0.75%, 440B has 0.75-0.95%, and 440C has 0.95-1.20%. Higher carbon means higher achievable hardness and wear resistance, but lower toughness and corrosion resistance.

Q5. Why is vacuum heat treatment recommended for 440C?

At austenitizing temperatures of 1010-1065 degrees Celsius, the surface of 440C loses carbon rapidly in an oxidizing atmosphere. Vacuum or controlled-atmosphere furnaces prevent decarburization, preserving surface hardness and corrosion resistance.

Q6. Is 440C a good choice for knife blades?

Yes, for blades where edge retention is the priority. 440C holds an edge well and resists corrosion far better than carbon steel. It is slightly harder to sharpen than softer stainless grades and can be brittle if tempered too hard, so a temper of 56-58 HRC is often preferred for knives.