Stainless Steel SUS440A

High-carbon martensitic stainless steel offering excellent hardness, wear resistance, and strength for demanding applications in aerospace, automotive, and industrial machinery.

Introduction to Stainless Steel SUS440A: High-Strength and Wear-Resistant Alloy

Stainless Steel SUS440A is a high-carbon martensitic stainless steel known for its excellent strength, hardness, and wear resistance. With a carbon content of approximately 0.60-0.75%, SUS440A perfectly balances corrosion resistance and machinability, making it ideal for demanding applications where durability and strength are critical. It is commonly used to produce bearings, cutting tools, and shafts that need to withstand high stress and wear conditions.

Thanks to its higher carbon content, SUS440A can be hardened more than other stainless steels, achieving a 58-60 HRC hardness after heat treatment. This material is frequently chosen for CNC machining applications where high strength, wear resistance, and the ability to withstand corrosion are necessary. At Neway, CNC-machined SUS440A steel parts are processed precisely, maintaining tight tolerances to meet the demands of various industrial sectors.

SUS440A Stainless Steel: Key Properties and Composition

SUS440A Stainless Steel Chemical Composition

Element

Composition (wt%)

Role/Impact

Carbon (C)

0.60–0.75%

High carbon content allows for enhanced hardness and wear resistance.

Manganese (Mn)

0.60–1.00%

Improves toughness and enhances hardenability.

Chromium (Cr)

16.00–18.00%

Provides excellent corrosion resistance, especially in acidic environments.

Phosphorus (P)

≤0.04%

Reduces impurities, optimizing machinability and mechanical properties.

Sulfur (S)

≤0.03%

Improves machinability by enhancing chip formation during CNC operations.

SUS440A Stainless Steel Physical Properties

Property

Value

Notes

Density

7.75 g/cm³

Slightly denser than other stainless steels, offering higher strength.

Melting Point

1,450–1,510°C

Suitable for high-temperature applications after heat treatment.

Thermal Conductivity

24.9 W/m·K

Relatively low heat conductivity, ideal for parts exposed to high temperatures.

Electrical Resistivity

6.9×10⁻⁶ Ω·m

Low conductivity, ensuring suitability for non-electrical components.

SUS440A Stainless Steel Mechanical Properties

Property

Value

Testing Standard/Condition

Tensile Strength

620–850 MPa

ASTM A276/A276M standard

Yield Strength

450 MPa

Ideal for parts subjected to high loads

Elongation (50mm gauge)

8–12%

Low elongation indicates high stiffness and resistance to deformation.

Brinell Hardness

250–300 HB

Hard, offering excellent resistance to wear.

Machinability Rating

60% (vs. 1212 steel at 100%)

Requires high-speed machining but provides excellent tool life.

Key Characteristics of SUS440A Stainless Steel: Benefits and Comparisons

SUS440A stainless steel is highly valued for its high hardness, corrosion resistance, and wear resistance. Below is a comparison of SUS440A with other materials such as Stainless Steel SUS304 and Stainless Steel SUS420, highlighting its unique advantages.

1. Enhanced Hardness and Wear Resistance

  • Unique Trait: High carbon content (0.60–0.75%) allows SUS440A to achieve hardness levels of 58-60 HRC, providing exceptional wear resistance.

  • Comparison:

    • vs. SUS304: SUS304, with its lower carbon content, has lower hardness (Rockwell B 92–98) and less wear resistance than SUS440A.

    • vs. SUS420: SUS440A offers superior hardness (58-60 HRC) compared to SUS420, which has a lower range of 50-55 HRC after heat treatment.

2. Corrosion Resistance

  • Unique Trait: Chromium content (16.00–18.00%) provides excellent resistance to corrosion, particularly in acidic environments.

  • Comparison:

    • vs. SUS304: SUS304 has a higher chromium content (18–20%) and better overall corrosion resistance, but it is less resistant to wear.

    • vs. SUS420: SUS440A's corrosion resistance is comparable to SUS420 but provides superior mechanical strength and wear resistance.

3. Strength and Toughness

  • Unique Trait: SUS440A’s high strength (620–850 MPa tensile strength) makes it an ideal choice for parts under high-stress conditions.

  • Comparison:

    • vs. SUS304: SUS304 is austenitic and offers good toughness but lacks the strength and hardness of SUS440A.

    • vs. SUS420: While SUS420 provides good hardness, SUS440A is stronger and more resistant to wear and fatigue.

4. Machinability

  • Unique Trait: SUS440A is highly machinable with the right tools and settings, though it requires high-speed machining and carbide tooling for best results.

  • Comparison:

    • vs. SUS304: SUS304 is more machinable due to its lower hardness, but SUS440A's hardness and wear resistance make it more suitable for demanding applications.

    • vs. SUS420: SUS420, being harder, can be difficult to machine at high speeds, whereas SUS440A offers a better balance between machinability and performance.

5. Post-Processing Flexibility

  • Unique Trait: SUS440A’s hardness makes it ideal for post-processing techniques such as polishing and coating, improving its aesthetic appeal and durability.

  • Comparison:

    • vs. SUS304: SUS304 is more easily polished but lacks the high wear resistance of SUS440A, which is ideal for high-contact parts.

    • vs. SUS420: SUS440A provides better post-processing options due to its higher hardness and wear resistance.

CNC Machining Challenges and Solutions for SUS440A Stainless Steel

Machining Challenges and Solutions

Challenge

Root Cause

Solution

Work Hardening

High carbon content and martensitic structure

Use carbide tools with TiN coatings to reduce friction and prevent work hardening.

Surface Roughness

Hardness causing material "tearing"

Optimize feed rates and utilize climb milling for smoother finishes.

Burr Formation

Hard material properties

Increase spindle speed and reduce feed rates during finishing passes.

Dimensional Inaccuracy

Residual stresses from hardening

Perform stress-relief annealing at 650°C for precision machining.

Chip Control Issues

Tough material forming continuous chips

Utilize high-pressure coolant (7–10 bar) and implement chip breakers.

Optimized Machining Strategies

Strategy

Implementation

Benefit

High-Speed Machining

Spindle speed: 900–1,200 RPM

Reduces heat buildup and improves tool life by 20%.

Climb Milling

Directional cutting path for optimal surface finish

Achieves surface finishes of Ra 1.6–3.2 µm, improving part aesthetics.

Toolpath Optimization

Use trochoidal milling for deep pockets

Reduces cutting forces by 35%, minimizing part deflection.

Stress-Relief Annealing

Preheat to 650°C for 1 hour per inch

Minimizes dimensional variation to ±0.03 mm.

Cutting Parameters for SUS440A Stainless Steel

Operation

Tool Type

Spindle Speed (RPM)

Feed Rate (mm/rev)

Depth of Cut (mm)

Notes

Rough Milling

4-flute carbide end mill

800–1,200

0.15–0.25

2.0–4.0

Use flood coolant to prevent work hardening.

Finish Milling

2-flute carbide end mill

1,200–1,500

0.05–0.10

0.5–1.0

Climb milling for smoother finishes (Ra 1.6–3.2 µm).

Drilling

135° split-point HSS drill

600–800

0.10–0.15

Full hole depth

Peck drilling for precise hole formation.

Turning

CBN or coated carbide insert

300–500

0.20–0.30

1.5–3.0

Dry machining is acceptable with air blast cooling.

Surface Treatments for CNC Machined SUS440A Stainless Steel Parts

  1. Electroplating: Adds a corrosion-resistant metallic layer, extending part life in humid environments and improving strength.

  2. Polishing: Enhances the surface finish, providing a smooth, shiny appearance ideal for visible components.

  3. Brushing: Creates a satin or matte finish, masking minor surface defects and improving aesthetic quality for architectural components.

  4. PVD Coating: Boosts wear resistance, increasing tool life and part longevity in high-contact environments.

  5. Passivation: Creates a protective oxide layer, enhancing corrosion resistance in mild environments without altering dimensions.

  6. Powder Coating: Offers high durability, UV resistance, and a smooth finish, ideal for outdoor and automotive parts.

  7. Teflon Coating: Provides non-stick and chemical-resistant properties, ideal for food processing and chemical handling components.

  8. Chrome Plating: Adds a shiny, durable finish that enhances corrosion resistance, commonly used in automotive and tooling applications.

  9. Black Oxide: Provides a corrosion-resistant black finish, ideal for parts in low-corrosion environments like gears and fasteners.

Industry Applications of CNC Machined SUS440A Stainless Steel Parts

Automotive Industry

  • Engine Mounting Brackets: Cold-rolled SUS440A steel is ideal for automotive components that require high tensile strength and durability.

Industrial Machinery

  • Hydraulic Cylinders: Stress-relieved SUS440A steel maintains precise tolerances under high-pressure environments.

Aerospace

  • Turbine Blades: SUS440A is used in aerospace for its high strength and resistance to wear, making it perfect for critical rotating components.

Technical FAQs: CNC Machined SUS440A Stainless Steel Parts & Services

  1. What are the key advantages of using SUS440A stainless steel for high-precision parts?

  2. How does SUS440A perform under welding conditions compared to other stainless steels?

  3. What are the ideal post-processing techniques for enhancing the surface finish of SUS440A steel parts?

  4. How does SUS440A compare to other stainless steels in terms of wear resistance for heavy-duty applications?

  5. What are the best practices for machining SUS440A to achieve high-precision tolerances?

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