Strategic Planning and Future-Proof Bearing Unit Engineering 420 vs. 440 Stainless Steel

Published in: Food processing and pharmaceutical equipment

Material Composition and Properties AISI 420 vs. AISI 440

Published on: 16. February 2025

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.

Stainless Steel 420 vs. 440 Bearing Units

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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?

Choose Stainless Steel 420 Bearing Units If:
- You need a cost-effective solution.
- The application involves moderate corrosion conditions.
- You require easier machining and shaping.
Choose Stainless Steel 440 Bearing Units If:
- You need high wear resistance for high-speed, heavy-load applications.
- The environment involves humidity, mild chemicals, or frequent washdowns.
- You require maximum hardness and durability.