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When engineers need a stainless steel that can be hardened like a tool steel, 440 is usually the first family they consider. It is the best-known group of high-carbon martensitic stainless steels, and it sits at the hard, wear-resistant end of the stainless range. It will not match the corrosion resistance of 304 or 316, but it can hold a sharp edge and resist abrasion in ways those grades cannot. This article explains what 440 stainless steel is, the differences between its grades, its properties, how it is heat treated and worked, where it is used, and how to choose it.
What Is 440 Stainless Steel?
440 stainless steel is a family of martensitic stainless steels. Martensitic means the microstructure can be transformed by heat treatment into a very hard, strong structure, much like carbon and tool steels. What sets 440 apart is its combination of:
High carbon (roughly 0.6 to 1.2 percent), which allows high hardness
About 16 to 18 percent chromium, which provides stainless behavior
Small additions of molybdenum and sometimes other elements, which improve corrosion resistance and tempering stability
The chromium forms a thin, self-healing oxide film on the surface that protects against rust. The carbon combines with chromium and other elements to form hard carbides, which give excellent wear resistance.
The Main Grades
Grade | Carbon (approx.) | Chromium (approx.) | Key character |
|---|---|---|---|
440A | 0.60 to 0.75% | 16 to 18% | Best corrosion resistance and toughness in the family, lowest hardness |
440B | 0.75 to 0.95% | 16 to 18% | Middle ground between 440A and 440C |
440C | 0.95 to 1.20% | 16 to 18% | Highest hardness and wear resistance, lowest corrosion resistance of the three |
440F | Similar to 440C | 16 to 18% | Free-machining version with added sulfur or selenium, for easier machining |
Molybdenum is typically up to about 0.75 percent. Exact limits vary by standard (ASTM A276, AISI, and others), so always check the applicable specification.
Common equivalents (approximate):
440A: UNS S44002, roughly comparable to X70CrMo15 (EN 1.4109 family) in some markets
440B: UNS S44003
440C: UNS S44004, EN 1.4125 (X105CrMo17), Japanese SUS440C
Equivalents are not always exact matches, so confirm chemistry before substituting.
Chemical Composition Summary
Element | Role |
|---|---|
Carbon | Controls hardness and carbide content; more carbon means harder but less corrosion resistant and less tough |
Chromium | Provides corrosion resistance and forms carbides |
Molybdenum | Improves pitting resistance and helps the steel resist softening during tempering |
Manganese and silicon | Deoxidizers; kept low (typically up to 1 percent each) |
Phosphorus and sulfur | Kept low, except in 440F where sulfur aids machining |
Iron | Balance |
Mechanical Properties
Properties depend strongly on heat treatment. Typical values (approximate, for guidance only):
Property | Annealed | Hardened and tempered (low temperature) |
|---|---|---|
Hardness, 440A | About 95 HRB | Up to about 55 to 57 HRC |
Hardness, 440B | About 96 HRB | Up to about 57 to 59 HRC |
Hardness, 440C | About 97 HRB | Up to about 58 to 60 HRC (some sources report up to 60 HRC or slightly higher) |
Tensile strength, 440C | About 760 MPa | Up to about 1,970 MPa |
Yield strength, 440C | About 450 MPa | Up to about 1,900 MPa |
Elongation | About 14% | Often under 5% |
Density | About 7.75 g/cm³ | Same |
Elastic modulus | About 200 GPa | Same |
Key points:
440C offers the highest hardness and wear resistance of any common stainless grade
Toughness falls as hardness rises, so 440C is brittle compared with austenitic grades
Ductility in the hardened state is low, so impact and bending loads need care
Physical Properties
Property | Typical value |
|---|---|
Melting range | About 1,370 to 1,480 °C |
Thermal conductivity | About 24 W/m·K at room temperature (lower than carbon steel, higher than austenitic grades) |
Thermal expansion | About 10 to 10.5 µm/m·°C |
Magnetism | Magnetic in all conditions |
Electrical resistivity | About 0.6 µΩ·m |
Being magnetic is a useful way to separate 440 from non-magnetic austenitic grades like 304 and 316, though it does not distinguish it from other magnetic steels.
Corrosion Resistance
440 stainless has moderate corrosion resistance. It resists:
Fresh water, steam, and mild atmospheric conditions
Food acids, many organic chemicals, and mild alkalis
Most dry or lightly humid environments
It is not recommended for:
Marine or chloride-rich environments, where pitting is likely
Strong acids and hot, aggressive chemicals
Applications needing the corrosion performance of 316
Grade differences: 440A resists corrosion best, 440C least, because more carbon ties up chromium in carbides, leaving less free chromium to form the protective film.
Condition matters: corrosion resistance is best when the steel is hardened, tempered at low temperature, and polished. Tempering in the 425 to 565 °C range can reduce corrosion resistance and toughness, so it is usually avoided. A clean, smooth, passivated surface greatly improves performance.
Heat Treatment
Heat treatment is what unlocks the hardness of 440. A typical sequence:
1. Annealing (to soften for machining)
Heat slowly to about 845 to 900 °C
Hold, then cool very slowly in the furnace (for example, at about 15 °C per hour down to about 600 °C), then air cool
Gives a soft, machinable structure
2. Hardening
Preheat in steps to avoid cracking (for example, 760 °C)
Austenitize at roughly 1,010 to 1,065 °C (the exact temperature depends on grade and the desired balance of hardness and toughness)
Hold long enough to dissolve carbides appropriately
Quench in oil or air, or use a vacuum furnace with gas quenching
Small sections can be air quenched, but oil or gas is typical for full hardness
3. Sub-zero treatment (optional but common)
Cool to about minus 70 to minus 196 °C soon after quenching
Converts retained austenite into martensite, increasing hardness and dimensional stability
Important for bearings, gauges, and precision parts
4. Tempering
Temper promptly after quenching to relieve stress and improve toughness
For maximum hardness and corrosion resistance, use a low-temperature temper at about 150 to 370 °C
Avoid the 425 to 565 °C range, which reduces toughness and corrosion resistance
Double tempering is common
Tempering temperature | Effect |
|---|---|
About 150 to 200 °C | Highest hardness, good corrosion resistance, lower toughness |
About 260 to 370 °C | Slight hardness reduction, better toughness |
About 425 to 565 °C | Tempered martensite embrittlement and corrosion risk, usually avoided |
Distortion, scale, and decarburization are risks. Vacuum or protective atmosphere furnaces reduce them.
Fabrication and Machining
Machinability
In the annealed state, 440A, 440B, and 440C machine fairly well but are harder on tools than mild steels
440F is the free-machining choice, but its sulfur reduces corrosion resistance and toughness
Machine in the annealed condition, then heat treat, and finish by grinding
Grinding and polishing
Hardened 440C grinds to a very fine, sharp edge and takes a high polish
Use controlled grinding to avoid overheating and burning, which can cause cracks and soften the edge
Forming
Cold forming is limited, particularly for 440C, because of its high carbon
Hot working is done at about 1,000 to 1,150 °C, followed by slow cooling and annealing
Welding
Generally not recommended, especially for 440C, because of cracking risk in the hard martensitic structure
If necessary: preheat to about 200 to 300 °C, use suitable filler, and temper or anneal afterwards
Often avoided in favor of mechanical joining or brazing
Applications
Cutlery and knives
Kitchen, hunting, and pocket knives, and surgical blades
440A for budget and corrosion-tolerant blades, 440C for premium edge retention
Forms a sharp, long-lasting edge that is easy to sharpen
Bearings
Ball and roller bearings, bearing races, and balls in corrosive or humid environments
Precision instrument bearings, where stainless and hardness are both needed
Valve and pump components
Valve seats, balls, stems, and nozzles
Pump shafts and wear parts for mildly corrosive fluids
Surgical and dental instruments
Scissors, scalpels, forceps, and dental tools
Needs hardness, edge retention, and sterilizability
Industrial tooling
Cutting tools, knives for food processing and plastics, and slitter knives
Molds and dies for corrosive plastics, as well as gauges and measuring tools
Other uses
Firearm components such as barrels and bolts
Injection molding screws and pins
Aerospace parts such as fasteners, pivots, and actuator components
Ball studs and spindles
Textile, paper, and food machinery wear parts
Scissors and razor blades
Advantages and Limitations
Advantages | Limitations |
|---|---|
Very high hardness for a stainless steel | Corrosion resistance lower than austenitic grades |
Excellent wear and abrasion resistance | Brittle compared with 304 and 316 |
Good edge retention and polishability | Poor weldability |
Heat-treatable to a wide range of properties | Susceptible to chloride pitting |
Magnetic and dimensionally stable when properly treated | Distortion and cracking risk during heat treatment |
Cost-effective compared with some powder metallurgy steels | Not suitable for severe corrosion or marine use |
How 440 Compares with Other Steels
Steel | Type | Corrosion resistance | Hardness potential | Typical use |
|---|---|---|---|---|
440C | Martensitic | Moderate | Very high | Bearings, premium blades |
440A | Martensitic | Fair to good | High | General blades, instruments |
420 | Martensitic | Fair | Medium | Cutlery, surgical tools, molds |
410 | Martensitic | Fair | Low to medium | Fasteners, valve parts |
304 | Austenitic | Very good | Cannot be hardened by heat treatment | Food, architecture, general use |
316 | Austenitic | Excellent | Cannot be hardened by heat treatment | Marine, chemical, medical implants |
17-4 PH | Precipitation hardening | Good | High | Aerospace, shafts, fittings |
154CM / 440C modified | Martensitic (modified) | Moderate | Very high | Premium knives |
Carbon tool steels (e.g., D2) | Not stainless | Poor | Very high | Tools, dies |
440C is sometimes compared with D2 tool steel for wear resistance, with 440C offering better stain resistance.
Selecting the Right 440 Grade
Need | Suggested grade |
|---|---|
Best corrosion resistance and some toughness | 440A |
Balance of hardness and corrosion resistance | 440B |
Maximum hardness and wear resistance | 440C |
High-volume machined parts | 440F |
Marine or highly corrosive service | Consider 316, duplex, or other non-440 grades |
Welded construction | Consider 304, 316, or 17-4 PH instead |
Ask these questions:
How corrosive is the environment?
Does the part need to hold an edge or resist wear?
Will it see impact or bending loads?
Must it be welded or formed?
What heat treatment facilities are available?
Standards and Forms
440 stainless steel is commonly supplied as:
Round bar and wire
Flat bar, plate, and strip
Forgings and billets
Balls (for bearings and valves)
Typical specifications include ASTM A276 (bars), ASTM A314 (billets and bars for forging), ASTM A580 (wire), ASTM A959 (heat-treating guidance), and AMS 5630 (440C bar, aerospace). Always check the current edition and the purchaser’s requirements.
Care and Maintenance
Keep surfaces clean and dry, especially knife blades and bearings
Wipe after contact with salt, acids, or chlorides
Avoid long soaking in water or leaving blades wet
Avoid abrasive pads that scratch the protective film
Re-passivate machined or ground parts where stainless performance is important
Oil tools and blades lightly for long-term storage
Sharpen with correct angles and avoid overheating
Common Problems and Their Causes
Problem | Likely cause |
|---|---|
Rust spots or pitting | Chloride exposure, poor surface finish, low-temperature heat treatment errors, or contamination |
Cracking during hardening | Too-rapid heating or quenching, sharp corners, no preheating |
Low hardness after heat treatment | Wrong austenitizing temperature, too much retained austenite, decarburization |
Chipped or brittle edge | Excessive hardness, low tempering temperature for the use, impact loading |
Distortion | Uneven heating, stress from machining, quench severity |
Weld cracking | No preheat, hard martensite in the heat-affected zone |
Loss of corrosion resistance after tempering | Tempering in the embrittlement range, or surface oxidation or scale |
Conclusion
440 stainless steel earns its place by doing something most stainless grades cannot: it hardens to a level where it can hold a razor edge, resist wear, and carry load in bearings, while still offering reasonable resistance to rust. The grade choice is a trade-off. 440A leans toward corrosion resistance and toughness, 440C toward hardness and wear resistance, and 440B sits in between. Success depends on correct heat treatment, careful finishing, and using it where its corrosion resistance is adequate. Values in this article are typical and approximate, so always confirm against the supplier’s certificate and the governing specification for your application.