Material Composition and Properties of AISI 420 vs. AISI 440

Chemical Composition: AISI 420 vs. AISI 440

The composition of stainless steel determines its mechanical properties, corrosion resistance, and durability in specific environments.

Element	AISI 420 (% by weight)	AISI 440 (% by weight)
Chromium (Cr)	12.0 – 14.0%	16.0 – 18.0%
Carbon (C)	0.15 – 0.40%	0.95 – 1.20%
Nickel (Ni)	≤ 0.75%	≤ 0.75%
Manganese (Mn)	≤ 1.00%	≤ 1.00%
Silicon (Si)	≤ 1.00%	≤ 1.00%
Phosphorus (P)	≤ 0.04%	≤ 0.04%
Sulfur (S)	≤ 0.03%	≤ 0.03%
Molybdenum (Mo)	–	0.75% (optional in AISI 440C)

Key Differences:

• AISI 440 has significantly higher carbon (0.95–1.20%) than AISI 420 (0.15–0.40%), making it much harder and more wear-resistant.
• AISI 440 has more chromium (16–18%) than AISI 420 (12–14%), which enhances corrosion resistance.
• AISI 440C (a subtype of AISI 440) contains molybdenum, improving corrosion resistance and strength, especially in marine and chemical processing environments.

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Hardness and Strength

Hardness is an essential factor in bearing units as it affects wear resistance, load-bearing capacity, and longevity.

Property	AISI 420	AISI 440
Hardness (HRC)	50 – 55 HRC	58 – 62 HRC
Tensile Strength	700 – 900 MPa	900 – 1200 MPa
Yield Strength	450 MPa	600 MPa

Key Differences:

• AISI 440 is harder (58–62 HRC) than AISI 420 (50–55 HRC), making it more wear-resistant but also more brittle.
• AISI 420 is easier to machine and shape, making it more practical for custom-designed bearing units.
• AISI 440 can withstand higher stress and load capacities, making it ideal for high-performance industrial applications.

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Corrosion Resistance

Corrosion resistance is a critical factor in industries that expose bearings to moisture, chemicals, or extreme temperatures.

Corrosion Resistance	AISI 420	AISI 440
General Corrosion	Moderate	High
Saltwater Resistance	Poor	Excellent
Chemical Resistance	Moderate	High
Acid/Alkali Resistance	Moderate	High

Key Differences:

• AISI 440’s higher chromium content (16-18%) significantly improves corrosion resistance, making it ideal for wet, marine, and chemical exposure.
• AISI 420 is better suited for drier, low-humidity environments, where corrosion is not a primary concern.
• AISI 440C (with molybdenum) offers superior resistance to acidic and alkaline environments, making it preferable for chemical processing and pharmaceutical industries.

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Wear Resistance and Friction

Wear resistance is crucial for bearings, gears, and moving parts that undergo continuous friction.

Property	AISI 420	AISI 440
Wear Resistance	Good	Excellent
Friction Coefficient	0.20 – 0.30	0.15 – 0.25

Key Differences:

• AISI 440’s higher hardness and carbon content provide superior wear resistance.
• AISI 420 is more ductile and resistant to chipping, making it preferable for applications where impact resistance is required.
• AISI 440 bearings last longer in high-speed, high-load applications, such as automotive transmissions, heavy-duty conveyors, and precision machinery.

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Machinability and Weldability

Machinability affects how easily a material can be cut, shaped, and customized for specific bearing applications.

Property	AISI 420	AISI 440
Machinability	High	Low
Weldability	Moderate	Poor

Key Differences:

• AISI 420 is much easier to machine than AISI 440, making it preferable for custom and complex bearing designs.
• AISI 440’s high hardness makes it challenging to machine and requires specialized tools for cutting and shaping.
• AISI 420 can be welded with proper preheating, while AISI 440 is not recommended for welding due to its high carbon content, which makes it brittle.

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Heat Treatment and Performance at High Temperatures

Both AISI 420 and AISI 440 can be heat-treated to enhance their mechanical properties.

Property	AISI 420	AISI 440
Heat Treatment	Yes	Yes
Max Operating Temp	600°C	700°C
Tempering Range	200 – 550°C	200 – 600°C

Key Differences:

• AISI 440 can be heat-treated to achieve extreme hardness (~62 HRC), making it ideal for high-speed, high-load applications.
• AISI 420 remains more ductile after heat treatment, reducing the risk of cracking under stress.
• AISI 440 performs better in high-temperature environments, making it useful in aerospace, automotive, and heavy-duty manufacturing.

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Choosing Between AISI 420 and AISI 440 Bearing Units

Factor	AISI 420	AISI 440
Hardness	Moderate (50–55 HRC)	High (58–62 HRC)
Corrosion Resistance	Moderate	Excellent
Wear Resistance	Good	Superior
Machinability	Easy to machine	Difficult to machine
Weldability	Moderate	Poor
Heat Resistance	Up to 600°C	Up to 700°C
Cost	Lower cost	Higher cost

Best Uses for Each Material

• Choose AISI 420 if:

  • You need moderate corrosion resistance.
  • Your application requires easier machinability.
  • Cost is a key factor in material selection.
  • Your bearings operate in low-moisture, low-load environments.

• Choose AISI 440 if:

  • You require superior corrosion and wear resistance.
  • Your application involves high loads, friction, or exposure to chemicals.
  • You need bearings for marine, pharmaceutical, or chemical processing industries.

By understanding the material properties of AISI 420 and AISI 440, industries can optimize bearing performance, reduce maintenance costs, and improve equipment longevity.

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