Stainless Steel 420 vs. 440 Bearing Units
Understanding the material composition of Stainless Steel 420 vs. 440 bearing units is critical for selecting the right material for industrial applications. Both steels are martensitic stainless steels, but their carbon content, alloying elements, and heat treatment responses differ, affecting their performance.
Chemical Composition of Stainless Steel 420 vs. 440
The key difference between Stainless Steel 420 and 440 lies in the carbon content and chromium levels, which influence hardness, corrosion resistance, and wear properties.
| Element |
420 Stainless Steel |
440 Stainless Steel (Primarily 440C) |
| Carbon (C) |
0.15 – 0.40% |
0.95 – 1.20% (Higher hardness) |
| Chromium (Cr) |
12.0 – 14.0% |
16.0 – 18.0% (Better corrosion resistance) |
| Nickel (Ni) |
0.5% max |
0.75% max |
| Manganese (Mn) |
1.0% max |
1.0% max |
| Silicon (Si) |
1.0% max |
1.0% max |
| Molybdenum (Mo) |
0.5% max (optional) |
0.75% max (in some 440 grades) |
| Phosphorus (P) |
0.04% max |
0.04% max |
| Sulfur (S) |
0.03% max |
0.03% max |
Key Differences in Material Composition
-
Carbon Content:
- 420 SS has lower carbon (0.15-0.40%), making it softer and easier to machine.
- 440 SS (especially 440C) has higher carbon (0.95-1.20%), making it much harder and more wear-resistant.
-
Chromium Content:
- 420 SS has 12-14% chromium, offering moderate corrosion resistance.
- 440 SS has 16-18% chromium, which improves oxidation and rust resistance.
-
Molybdenum (Mo) Content:
- Some 440 stainless steels contain molybdenum, enhancing corrosion resistance.
- 420 stainless steel rarely includes molybdenum, making it less effective in aggressive environments.
Mechanical Properties Comparison
| Property |
420 Stainless Steel |
440 Stainless Steel (Primarily 440C) |
| Hardness (HRC, Rockwell Scale) |
48-50 HRC |
58-60 HRC (higher wear resistance) |
| Tensile Strength |
700 – 850 MPa |
760 – 900 MPa |
| Yield Strength |
500 MPa |
570 MPa |
| Density |
7.75 g/cm³ |
7.70 g/cm³ |
| Elongation at Break |
15% |
13% |
| Modulus of Elasticity |
200 GPa |
200 GPa |
Key Differences in Mechanical Properties
-
Hardness & Wear Resistance
- 440 stainless steel is significantly harder (up to 60 HRC) than 420 stainless steel (50 HRC max).
- The high hardness of 440C stainless steel makes it ideal for high-load, high-speed, and high-wear environments.
-
Tensile Strength
- 440 stainless steel has a higher tensile strength than 420, meaning it withstands higher stress and heavy loads without deformation.
-
Ductility & Elongation
- 420 stainless steel is more ductile, making it easier to machine and form into bearing housings or cast components.
- 440 stainless steel is more brittle due to its higher carbon content.
Heat Treatment and Hardening
Both 420 and 440 stainless steels can be hardened through heat treatment, but 440C is capable of achieving much higher hardness.
420 Stainless Steel Heat Treatment
- Annealing: Heated to 840-900°C, then slowly cooled to soften the material.
- Hardening: Heated to 980-1035°C, then oil or air quenched.
- Tempering: Usually tempered at 150-400°C to adjust hardness and improve toughness.
440 Stainless Steel Heat Treatment
- Annealing: Heated to 843-871°C, then furnace cooled.
- Hardening: Heated to 1010-1065°C, then oil quenched for maximum hardness.
- Tempering: Usually tempered at 150-260°C to retain high hardness.
Key Takeaways: ✔ 440 stainless steel can be hardened to higher levels, making it ideal for high-wear applications.
✔ 420 stainless steel is easier to machine and process, but cannot reach the same hardness as 440 stainless steel.
Corrosion Resistance
While both materials offer some level of corrosion resistance, 440 stainless steel performs better in wet and chemically aggressive environments.
| Environment |
420 Stainless Steel |
440 Stainless Steel |
| Mild Industrial Conditions |
✅ Good |
✅ Excellent |
| High Humidity & Marine |
❌ Not recommended |
✅ Better but not as good as 316 SS |
| Acidic/Chemical Processing |
❌ Poor |
✅ Moderate resistance |
| Frequent Washdowns (Food/Pharma) |
❌ Limited |
✅ More resistant |
- 420 Stainless Steel is prone to surface rusting if exposed to prolonged moisture.
- 440 Stainless Steel has better pitting resistance, making it more suitable for wet and chemical environments.
Machinability and Welding
| Feature |
420 Stainless Steel |
440 Stainless Steel |
| Machinability |
✅ Easier to machine |
❌ Difficult to machine due to hardness |
| Grinding & Polishing |
✅ Easier to polish |
❌ Requires diamond grinding |
| Welding |
✅ Can be welded with preheating |
❌ Poor weldability, cracks easily |
- 420 stainless steel is more machinable than 440 stainless steel, making it preferred for applications that require complex shaping.
- 440 stainless steel is more challenging to machine due to its extreme hardness.
Final Comparison Summary
| Feature |
420 Stainless Steel Bearing Units |
440 Stainless Steel Bearing Units |
| Best Used In |
General machinery, automotive, dry environments |
High-wear, high-load, wet environments |
| Hardness (HRC) |
48-50 HRC |
58-60 HRC |
| Wear Resistance |
Moderate |
Excellent |
| Corrosion Resistance |
Moderate |
Higher than 420 but lower than 316 |
| Tensile Strength |
Good |
Higher than 420 |
| Machinability |
✅ Easier to machine |
❌ Hard to machine |
| Cost |
More affordable |
More expensive |
Which One Should You Choose?
