Stainless Steel 440C
440C stainless steel is a high-carbon martensitic stainless steel containing up to 1.20% carbon, which gives it the highest hardness and wear resistance of the entire 440 series. Its chromium content (16–18%) provides moderate corrosion resistance, while heat treatments allow hardnesses of 58 to 60 HRC to be achieved. It is primarily used in applications demanding high mechanical strength and good dimensional stability, such as bearings, molds, surgical instruments, and quality cutlery.
Chemical composition
Section titled “Chemical composition”The chemical composition of 440C steel includes a carbon content between 0.95% and 1.20%, which gives it its martensitic hardenability. The following table shows the typical composition according to international specifications.
| Element | Content (%) |
|---|---|
| Carbon (C) | 0.95 – 1.20 |
| Chromium (Cr) | 16.0 – 18.0 |
| Manganese (Mn) | ≤ 1.00 |
| Silicon (Si) | ≤ 1.00 |
| Molybdenum (Mo) | ≤ 0.75 |
| Phosphorus (P) | ≤ 0.040 |
| Sulfur (S) | ≤ 0.030 |
| Iron (Fe) | Balance |
Physical properties
Section titled “Physical properties”At room temperature, the density of 440C stainless steel is 7.80 g/cm³ (0.282 lb/in³) and its melting point is around 1483 °C (2700 °F). These physical characteristics make it suitable for components operating under demanding mechanical conditions and moderate temperatures.
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.80 g/cm³ | 0.282 lb/in³ |
| Melting point | 1483 °C | 2700 °F |
Mechanical properties
Section titled “Mechanical properties”In the annealed condition, the yield strength of 440C is 425 MPa (61.6 ksi) and its tensile strength reaches 760 MPa (110 ksi). Through quenching and tempering, values of up to 1900 MPa (276 ksi) yield strength and 1970 MPa (286 ksi) tensile strength can be achieved, with a hardness of 57 HRC or higher.
| Property | Annealed | Tempered at 315 °C |
|---|---|---|
| Tensile strength (MPa) | 760 | 1970 |
| Yield strength (0.2%, MPa) | 425 | 1900 |
| Elongation (%) | 14 | 2 |
| Rockwell hardness | 97 HRB | 57 HRC |
| Elastic modulus (GPa) | 200 | 200 |
| Shear modulus (GPa) | 83.9 | 83.9 |
| Poisson’s ratio | 0.27 – 0.30 | 0.27 – 0.30 |
Thermal properties
Section titled “Thermal properties”The linear thermal expansion coefficient of 440C is 10.2 µm/m·°C (5.67 µin/in·°F) and its thermal conductivity at 100 °C is 24.2 W/m·K (168 BTU·in/hr·ft²·°F). These values determine its behavior in applications with temperature variations and its response to welding or heat treatment processes.
| Property | Metric | Imperial |
|---|---|---|
| Coefficient of thermal expansion (0‑100 °C) | 10.2 µm/m·°C | 5.67 µin/in·°F |
| Thermal conductivity (at 100 °C) | 24.2 W/m·K | 168 BTU·in/hr·ft²·°F |
Equivalent designations
Section titled “Equivalent designations”440C stainless steel is recognized under multiple international standards. The UNS designation S44004 and the European EN 1.4125 are the most commonly used in industry. Some standardized equivalents are listed below.
| Standard / Organization | Designation |
|---|---|
| UNS | S44004 |
| DIN / EN | 1.4125 |
| AMS | 5618, 5630, 5880 |
| ASTM | A276, A314, A473, A493, A580 |
| SAE | 51440C |
| SAE J405 | 51440C |
| MIL | MIL‑S‑862 |
| Federal | QQ‑S‑763 |
Manufacturing
Section titled “Manufacturing”440C steel can undergo hot and cold forming processes, although cold workability is limited. For hot working, it is recommended to preheat the part to 760 °C (1400 °F) and forge in the range of 1038 to 1204 °C (1900 to 2200 °F), avoiding cooling below 927 °C (1700 °F). Subsequently, it should be cooled slowly to room temperature and a full anneal applied. Cold working is only possible through conventional practices with light reductions and must be carried out with the material in the annealed condition.
Heat treatment
Section titled “Heat treatment”Annealing of 440C is carried out at temperatures between 843 °C and 871 °C (1550–1600 °F), followed by slow furnace cooling to obtain a hardness of around 97 HRB. Quenching is performed by progressively heating to 760 °C (1400 °F) and then raising the temperature to 1010–1065 °C (1850–1950 °F), with final cooling in air or oil. Subsequent tempering, typically at 148 °C (300 °F) or higher temperatures depending on the desired hardness, allows adjusting toughness without losing high wear resistance.
Machining
Section titled “Machining”Machining of 440C should preferably be done in the annealed condition, where it has a hardness of 97 HRB enabling acceptable chip removal. The use of carbide or ceramic tools is recommended, along with chip breakers to control long, fibrous chips. Proper coolant application improves surface finish and extends tool life.
Welding
Section titled “Welding”Welding of 440C steel is not recommended due to its tendency to air-harden and form cold cracks. If welding is unavoidable, the part must be preheated to 260 °C (500 °F) and, after welding, a heat treatment at 732–760 °C (1350–1400 °F) for 6 hours followed by slow furnace cooling should be applied to avoid cracking.
Applications by industry
Section titled “Applications by industry”The combination of high hardness, wear resistance, and moderate corrosion resistance makes 440C an ideal material for precision components in multiple sectors.
| Industry | Typical applications |
|---|---|
| Automotive / Bearings | Bearing races and balls, ball bearings |
| Cutlery / Tools | Knives, cutting blades, dies and molds |
| Aerospace | Valves, engine components, landing gear |
| Medical / Dental | Surgical instruments, burs, needles |
| Metrology / Calibration | Gauge blocks, measuring instruments, templates |
Comparison with similar materials
Section titled “Comparison with similar materials”440C stainless steel is distinguished from its predecessors 440A and 440B mainly by its higher carbon content, which directly affects achievable hardness and wear resistance, at the expense of lower toughness and corrosion resistance.
| Property | 440A | 440B | 440C |
|---|---|---|---|
| C content (%) | 0.60 – 0.75 | 0.75 – 0.95 | 0.95 – 1.20 |
| Maximum hardness (HRC) | ~ 56 | ~ 57 | 58 – 60 |
| Corrosion resistance | Moderate-good | Moderate | Moderate-low |
| Toughness | Medium | Medium-low | Low |
| Typical applications | General-purpose cutlery, low-demand valves | Higher quality cutlery, dental instruments | Bearings, precision molds, high-performance cutlery |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum hardness that 440C stainless steel can achieve?
Section titled “What is the maximum hardness that 440C stainless steel can achieve?”With proper heat treatment, 440C can achieve a hardness of 58 to 60 HRC, placing it among the hardest martensitic stainless steels.
What is the carbon content of 440C and how does it affect its properties?
Section titled “What is the carbon content of 440C and how does it affect its properties?”440C contains between 0.95% and 1.20% carbon; this high percentage allows the formation of chromium carbides and complete martensitic transformation, achieving tensile strengths of up to 1970 MPa.
Can 440C stainless steel be welded without risk of cracking?
Section titled “Can 440C stainless steel be welded without risk of cracking?”To weld 440C, preheating to 260 °C (500 °F) and a post-weld heat treatment of 732–760 °C for 6 hours with slow cooling is required; even so, the risk of cracking remains high.
How does the corrosion resistance of 440C compare to other stainless steels?
Section titled “How does the corrosion resistance of 440C compare to other stainless steels?”440C has moderate corrosion resistance, inferior to austenitic steels such as 304 or 316, because most of the chromium combines with carbon to form carbides, reducing the free chromium available to form the passive layer.
In what temperature range can 440C be used continuously?
Section titled “In what temperature range can 440C be used continuously?”440C can operate at temperatures up to 250–300 °C continuously without significant loss of hardness; above these values, rapid softening due to overtempering begins.
What differentiates 440C from 440A and 440B in terms of carbon percentage?
Section titled “What differentiates 440C from 440A and 440B in terms of carbon percentage?”440C contains up to 1.20% carbon, while 440A is limited to 0.75% and 440B to 0.95%; this difference accounts for 440C offering 3 to 4 HRC points higher hardness.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=6846
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/stainless-steel-classifications-d_368.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=259