What Is ASTM A240 SS 440C Plate?
Type 440C is the highest-carbon grade of the 440 series and is supplied as plate, sheet and strip under ASTM A240 / ASME SA240. The grade is built for one purpose above all others: hardness that survives wear. After hardening and tempering it reaches roughly 56 to 59 HRC, which is why it appears in bearings, valve trim, pump components, tooling and cutting edges rather than in general structural work.
Because the grade is martensitic, it is magnetic and it hardens by heat treatment rather than by cold work. That single fact drives most of the practical rules around buying, machining and using 440C plate.
Chemical Composition of 440C (UNS S44004)
| Element | Content, % |
|---|---|
| Carbon (C) | 0.95–1.20 |
| Chromium (Cr) | 16.00–18.00 |
| Molybdenum (Mo) | 0.75 max |
| Manganese (Mn) | 1.00 max |
| Silicon (Si) | 1.00 max |
| Phosphorus (P) | 0.040 max |
| Sulfur (S) | 0.030 max |
| Iron (Fe) | Balance |
Carbon near 1 % combined with 16–18 % chromium produces a large volume fraction of chromium carbides after hardening. That is the source of both the wear resistance and the limitation: less chromium remains in solid solution, so corrosion resistance is lower than that of a 304 or 316 grade.
Physical Properties
| Property | Value |
|---|---|
| Density | 7650 kg/m³ |
| Elastic modulus | 200 GPa |
| Mean coefficient of thermal expansion, 0–100 °C | 10.1 µm/m/°C |
| Mean coefficient of thermal expansion, 0–200 °C | 10.3 µm/m/°C |
| Mean coefficient of thermal expansion, 0–600 °C | 11.7 µm/m/°C |
| Thermal conductivity at 100 °C | 24.2 W/m·K |
| Specific heat | 460 J/kg·K |
| Electrical resistivity | 600 nΩ·m |
Mechanical Properties: Annealed and Hardened Conditions
Plate is normally supplied in the annealed condition and hardened by the user after machining, because the material machines best when soft and distorts during quenching. The table gives typical values for both conditions.
| Condition | Tensile strength, MPa | Yield strength 0.2 %, MPa | Elongation, % | Hardness |
|---|---|---|---|---|
| Annealed | 758 | 448 | 14 | 269 HB max |
| Tempered at 204 °C | 2030 | 1900 | 4 | 59 HRC |
| Tempered at 260 °C | 1960 | 1830 | 4 | 57 HRC |
| Tempered at 316 °C | 1860 | 1740 | 4 | 56 HRC |
| Tempered at 371 °C | 1790 | 1660 | 4 | 56 HRC |
Note the trade-off in the hardened rows: tempering at a higher temperature lowers hardness but improves toughness, so the tempering temperature is chosen from the wear-versus-shock balance of the finished part rather than from a single universal figure.
Heat Treatment, Machining and Corrosion Behaviour
Hardening: austenitize, quench, then temper immediately. The part should not be left in the as-quenched state because the risk of cracking is high.
Machining: carry out the bulk of machining in the annealed condition and allow for dimensional change during hardening.
Welding: 440C is not usually recommended for welded construction, because the high carbon content and martensitic structure make cracking likely. Austenitic or lower-carbon martensitic grades serve welded assemblies better.
Corrosion behaviour: adequate for mild atmospheres, fresh water, some organic media and mildly alkaline solutions. Performance falls away in chloride-bearing or strongly acidic environments, where a 316 or duplex grade is the better answer.
Magnetic response: fully ferromagnetic in both the annealed and the hardened condition.
Available Ranges and Finishes
Thickness: 0.25–200 mm, with 0.3–120 mm covering most plate orders.
Width: 1000, 1219, 1500, 1800, 2000, 2500, 3000 and 3500 mm.
Length: 2000, 2440, 3000, 5800, 6000 and 12000 mm, or cut to order.
Finishes: 2B, 2D, BA, No. 1, No. 4, No. 8, mirror, checkered, embossed, hairline, sand blast and brush.
Forms: plate, sheet, strip, flats, perforated sheet and chequered plate.
Documentation: mill test certificate to EN 10204 3.1, with heat number traceability.
Frequently Asked Questions
Q: Is 440C stainless steel magnetic?
Yes. Type 440C is a martensitic grade and is fully ferromagnetic in the annealed, hardened and tempered conditions.
Q: What hardness can 440C plate reach?
Typically about 56 to 59 HRC after hardening and tempering, with the highest hardness obtained at the lower tempering temperatures and slightly lower hardness at 371 °C.
Q: Can 440C be welded?
Welding is not normally recommended for this grade. The combination of about 1 % carbon and a martensitic microstructure makes cracking likely, so welded assemblies are usually designed with an austenitic or lower-carbon martensitic grade instead.
Q: How does 440C corrosion resistance compare with 304 or 316?
Lower. The high carbon ties up chromium as carbides, leaving less chromium in solution, so 440C is suited to mild environments and not to chloride-bearing or strongly acidic service.
Q: In what condition is 440C plate supplied?
Normally annealed, at 269 HB maximum, which is the condition that machines best. Hardening is carried out after machining to the finished dimensions.
