A Guide to Stainless Steel CNC Machining

Stainless steel CNC machining is essential for producing parts requiring strength and corrosion resistance. Known for its high corrosion resistance and resistance to rust, stainless steel is one of the most widely used materials in metalworking. From machine parts to structures and building foundations, stainless steel is used in a range of applications. With over 100 grades available, engineers can select the right stainless steel based on machining requirements, performance needs, and intended use.

Characteristics of Stainless Steel

High Corrosion Resistance

Stainless steel contains at least 10.5% chromium and up to 1.2% carbon. Chromium forms a thin passive film that protects the surface, giving stainless steel a long service life and reducing the need for repairs or replacements.

While often thought of as “rust-proof,” steel is not immune to corrosion. In typical indoor environments, this is rarely an issue, but in outdoor environments with moisture or salt exposure, corrosion can develop gradually.

High Heat Resistance

Stainless steel maintains its tensile strength up to 500°C (932°F). Beyond this point, strength decreases, particularly in martensitic and ferritic grades.

High Strength

Stainless steel is stronger than iron and is widely used where high strength is required, such as in construction and civil engineering foundations and frameworks of structures and buildings.

Low Thermal Conductivity

Stainless steel has low thermal conductivity, meaning heat does not transfer easily through it.

Difficult to Machine

Stainless steel is classified as a difficult-to-machine material. Its low thermal conductivity causes heat to build up in the cutting tool, shortening tool life. Austenitic grades also harden during machining, making them even more difficult to cut.

In addition, stainless steel has high adhesion to the cutting tool compared to other commonly machined metals, so chips often adhere to the tool edge, increasing wear and reducing precision.

Types of Stainless Steel

The crystal structure of stainless steel changes with alloying elements and heat treatment, which directly impacts performance.

Martensitic Stainless Steel

• ~13% chromium and high carbon content

• Lower corrosion resistance but excellent hardness and wear resistance

• Can be quenched and tempered to adjust properties

• Common uses: shafts, bearings

• Representative Grades: SUS329J1, SUS329J3L, SUS329J4L

Precipitation-Hardening Stainless Steel

• Strengthened through heat treatment (precipitation hardening)

• Combines high strength with corrosion resistance

• Used in aircraft, rockets, and engine components

• More expensive due to costly materials and complex processing

• Representative Grades: SUS630, SUS631, SUS660

Stainless Steel Processing Methods

CNC Machining: Performed with machining centers and NC lathes, machining stainless steel takes longer than softer metals but remains common for precision parts.

Cutting: Material is cut to the required size or shape.

Bending: Stainless steel is bent to specific angles, but tends to spring back, so adjustments are required to account for this deformation.

Welding: Welding joins separate parts into a single structure but requires careful control to maintain corrosion resistance. Weldability varies by stainless steel type.

Advantages of Stainless Steel Machining

Reduced Maintenance Frequency

Due to the chromium content in stainless steel, a passive film forms on its surface and reforms even if scratched, protecting against corrosion and strength deterioration of the base material.

Heat Resistance

Stainless steel maintains high tensile strength up to 500°C (932°F), so it can be used in high-temperature environments.

Suitable for High-Stress Applications

Its tensile strength, heat resistance, and rigidity make it ideal for structural parts used in building foundations and mechanical parts in machines.

Why Stainless Steel Processing Is Difficult

Machining

Low thermal conductivity prevents heat from dispersing into chips, keeping it in the tool and workpiece. Excessive heat can cause chipping or tool burn.

Work hardening further increases hardness during machining, making it necessary to select proper coolants and adjust machining conditions to prevent excessive temperature rise.

Welding

Welding can create discoloration or weld burn, which is not only an aesthetic issue, but damages the passive film and lowers corrosion resistance.

Weld burn can lead to reduced chromium content, which causes corrosion resistance to drop and deteriorates the protective property of the stainless steel. If left untreated, corrosion occurs. However, removing weld burn is difficult and requires post-processing to restore the protective layer. Skilled welders and proper treatment are essential to prevent these issues.

Surface Treatment of Stainless Steel

Key Takeaway

Stainless steel offers exceptional strength, durability, and corrosion resistance, making it a key material for CNC machining. However, its machining and welding challenges require skilled operators, proper tooling, and optimized processes to achieve the best results.

Step Up Your Project with Yumoto Electric

Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals using milling, turning, and specialized metal processing. We have experience with over 200 materials including aluminum alloys, stainless steel, molybdenum, and titanium. 

If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.