Stainless Steel 420
Stainless steel 420 is a high-carbon martensitic steel with a minimum chromium content of 12 %, capable of achieving the highest hardness among 12 % chromium stainless steels (up to 50 HRC). It offers good ductility in the annealed condition and excellent corrosion resistance when polished, ground, or surface hardened. Its ability to be hardened by heat treatment makes it an ideal choice for parts requiring high mechanical strength and wear resistance.
Chemical Composition
Section titled “Chemical Composition”The typical chemical composition of stainless steel 420 (UNS S42000) according to ASTM A276 is shown in the following table.
| Element | Minimum (% by weight) | Maximum (% by weight) |
|---|---|---|
| Carbon (C) | – | 0.15 |
| Manganese (Mn) | – | 1.00 |
| Silicon (Si) | – | 1.00 |
| Phosphorus (P) | – | 0.040 |
| Sulfur (S) | – | 0.030 |
| Chromium (Cr) | 12.0 | 14.0 |
| Iron (Fe) | Balance | Balance |
Mechanical Properties by Heat Treatment Condition
Section titled “Mechanical Properties by Heat Treatment Condition”The mechanical properties of 420 steel vary significantly with heat treatment, reaching a maximum tensile strength of 1620 MPa in the hardened and tempered condition at 427 °C. Tempering between 425 °C and 600 °C should be avoided due to embrittlement.
| Condition / Tempering Temperature (°C) | Tensile Strength (MPa / ksi) | Yield Strength 0.2 % (MPa / ksi) | Elongation (% in 50 mm) | Brinell Hardness (HB) | Charpy V Impact (J) |
|---|---|---|---|---|---|
| Annealed (Condition A ASTM A276) | 655 (95) | 345 (50) | 25 | 241 max. | – |
| 204 | 1600 (232) | 1360 (197) | 12 | 444 | 20 |
| 316 | 1580 (229) | 1365 (198) | 14 | 444 | 19 |
| 427 | 1620 (235) | 1420 (206) | 10 | 461 | – |
| 538 | 1305 (189) | 1095 (159) | 15 | 375 | – |
| 593 | 1035 (150) | 810 (117) | 18 | 302 | 22 |
| 650 | 895 (130) | 680 (99) | 20 | 262 | 42 |
Note: Hardness in annealed condition is the maximum specified value; the rest are typical values after hardening and tempering. Tempering between 425 °C and 600 °C (zone marked with # in the original source) causes embrittlement and should be avoided.
Physical and Thermal Properties
Section titled “Physical and Thermal Properties”The density of stainless steel 420 in the annealed condition is 7800 kg/m³ (0.282 lb/in³). Values for thermal expansion, thermal conductivity, and other properties are detailed below.
| Property | Value (SI) | Value (Imperial) |
|---|---|---|
| Density | 7800 kg/m³ | 0.282 lb/in³ |
| Elastic modulus | 200 GPa | 29 000 ksi |
| Mean coefficient of thermal expansion (0‑100 °C) | 10.3 µm/m·°C | 5.72 µin/in·°F |
| Mean coefficient of thermal expansion (0‑315 °C) | 10.8 µm/m·°C | 6.00 µin/in·°F |
| Mean coefficient of thermal expansion (0‑538 °C) | 11.7 µm/m·°C | 6.50 µin/in·°F |
| Thermal conductivity at 100 °C | 24.9 W/m·K | 14.4 Btu/(h·ft·°F) |
| Specific heat (0‑100 °C) | 460 J/kg·K | 0.110 Btu/lb·°F |
| Electrical resistivity | 550 nΩ·m (55 µΩ·cm) | – |
Corrosion Resistance
Section titled “Corrosion Resistance”The corrosion resistance of 420 steel is good in moderate service conditions when the material is polished, hardened, or ground, forming a passive chromium oxide layer. However, it is inferior to that of austenitic stainless steels such as 304 or 316, so it is not recommended for highly aggressive environments or prolonged exposure to strong acids. Controlled addition of sulfur (≥0.03 %) improves machinability but may slightly reduce corrosion resistance compared to low‑sulfur grades.
Heat Treatment
Section titled “Heat Treatment”420 steel can be hardened by oil quenching or air cooling from austenitizing temperatures of 980 °C to 1035 °C. To obtain maximum hardness (up to 50 HRC), a low tempering between 150 °C and 370 °C is recommended. Tempering between 425 °C and 600 °C should be avoided due to temper embrittlement. Annealing is performed by heating to 840‑900 °C, followed by slow furnace cooling to reduce hardness and improve machinability.
Welding
Section titled “Welding”Stainless steel 420 has limited weldability due to its high carbon content and tendency to form brittle martensite in the heat‑affected zone. To minimize cracking, preheating to 200‑300 °C and immediate post‑weld heat treatment (tempering around 650 °C) are necessary. Use of 420 type filler metal (AWS ER420) is recommended to maintain similar properties.
Machinability
Section titled “Machinability”In the annealed condition, 420 steel offers acceptable machinability, inferior to that of the 420F variant (with higher sulfur content) but superior to austenitic stainless steels. After hardening, its high hardness makes machining operations difficult, so roughing in the annealed condition and light finishing after heat treatment are recommended.
Applications by Industry
Section titled “Applications by Industry”Thanks to its combination of high hardness and moderate corrosion resistance, 420 steel is used in industrial sectors such as:
- Medical and dental: surgical instruments, scissors, forceps, scalpels.
- Cutlery and flatware: high‑quality knives, cutting blades.
- Industrial processes: valve seats, pump shafts, wear components.
- Plastics processing: injection molds requiring high abrasion resistance.
- Cutting tools: shears, cutting discs, sheet metal scissors.
- Food industry: work surfaces, equipment parts in contact with mildly aggressive foods.
Comparison with Similar Materials
Section titled “Comparison with Similar Materials”The choice of 420 steel over other grades is based on the required balance of hardness, corrosion resistance, and machinability.
| Material | Reason for selection |
|---|---|
| 410 | Can be chosen if only low hardened hardness is needed, offering better weldability and lower cost. |
| 416 | Preferred when high machinability is required, although its hardened hardness and corrosion resistance are lower than those of 420. |
| 440C | Selected if higher hardened hardness or strength than 420 can offer (up to 60 HRC) is needed. |
| 420F | Free‑machining variant with higher sulfur content, but with slightly lower corrosion resistance. |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum achievable hardness of 420 steel?
Section titled “What is the maximum achievable hardness of 420 steel?”The maximum hardness is 50 HRC, the highest among all 12 % chromium stainless steels.
What is the maximum service temperature without excessive loss of strength?
Section titled “What is the maximum service temperature without excessive loss of strength?”It can be used in continuous service up to approximately 600 °C, provided the temper embrittlement range (425‑600 °C) is avoided.
What heat treatment is recommended to obtain maximum hardness?
Section titled “What heat treatment is recommended to obtain maximum hardness?”Hardening at 1010 °C with oil quenching followed by low tempering at around 205 °C, achieving a hardness of 50 HRC.
What is the typical tensile strength in the annealed condition?
Section titled “What is the typical tensile strength in the annealed condition?”In the annealed condition, the tensile strength is 655 MPa (approximately 95 ksi).
What is the thermal conductivity at room temperature?
Section titled “What is the thermal conductivity at room temperature?”Thermal conductivity measured at 100 °C is 24.9 W/m·K, equivalent to 14.4 Btu/(h·ft·°F).
What elongation percentage does it offer in the annealed condition?
Section titled “What elongation percentage does it offer in the annealed condition?”The minimum elongation in a 50 mm gauge length is 25 %, indicating good ductility before hardening.
References
Section titled “References”- azom.com: https://www.azom.com/article.aspx?ArticleID=972
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/stainless-steel-classifications-d_368.html
- steelnumber.com: https://www.steelnumber.com/en/steel_alloy_composition_eu.php?name_id=2120