A Guide to Stainless Steel CNC Machining

Key Takeaways

• Stainless steel combines high strength, corrosion resistance, and durability, making it one of the world's most widely used engineering materials.

• Corrosion resistance comes from at least 10.5% chromium, which forms a self-healing passive oxide layer on the surface.

• Stainless steels are available in numerous grades and are primarily classified into martensitic, ferritic, austenitic, duplex (austenitic-ferritic), and precipitation hardening types.

• While highly durable, stainless steel is considered a difficult-to-machine material due to work hardening, low thermal conductivity, and high cutting tool wear.

• Selecting the appropriate grade for the intended application improves performance, service life, and manufacturing cost.

Overview

Stainless steel is one of the most versatile engineering materials available, offering an outstanding balance of corrosion resistance, strength, durability, and heat resistance. With numerous grades tailored for different environments and manufacturing methods, selecting the appropriate stainless steel is critical to maximizing performance and controlling costs.

Although stainless steel presents machining and welding challenges due to its low thermal conductivity, work hardening, and tool wear, these can be successfully managed through proper tooling, optimized machining conditions, and appropriate material selection.

What is Stainless Steel?

Stainless steel is an iron-based alloy containing at least 10.5% chromium, with many grades also containing nickel, molybdenum, carbon, and other alloying elements. Chromium reacts with oxygen to form an extremely thin passive oxide layer on the surface that protects the material from corrosion.

Although this protective layer is only a few nanometers thick, it continuously self-repairs whenever the surface is scratched or damaged, provided oxygen is present.

With more than 100 available grades, stainless steel can be optimized for applications ranging from precision-machined components and medical devices to structural frameworks and consumer products.

Characteristics of Stainless Steel

Excellent Corrosion Resistance

The defining feature of stainless steel is its resistance to corrosion. Chromium creates a passive protective film that greatly slows rust formation and extends service life.

While stainless steel is highly corrosion resistant, it is not completely rust-proof. In chloride-rich or marine environments, corrosion can still occur over time if an unsuitable grade is selected.

High Strength

Compared to ordinary carbon steel and iron, stainless steel offers excellent mechanical strength and durability.

Its high strength makes it ideal for:

• Structural components

• Machine parts

• Industrial equipment

• Building frameworks

• Civil engineering applications

Heat Resistance

Many stainless steels retain their mechanical strength at elevated temperatures.

Most grades maintain high tensile strength up to approximately 500°C (932°F), while specialized heat-resistant grades such as SUS310S can withstand temperatures approaching 1,000°C (1,832°F).

Low Thermal Conductivity

Stainless steel transfers heat relatively slowly compared to aluminum or copper.

While beneficial in some applications, this characteristic also contributes to machining challenges because heat remains concentrated in the cutting tool and workpiece.

Work Hardening

Many stainless steels, particularly austenitic grades, become harder as they are machined or cold-worked.

This phenomenon increases cutting forces, accelerates tool wear, and makes subsequent machining operations more difficult.

Difficult to Machine

Stainless steel is classified as a difficult-to-machine material because it combines several unfavorable machining characteristics:

• Low thermal conductivity causes heat buildup.

• High toughness increases cutting forces.

• Work hardening increases material hardness during machining.

• Chips tend to adhere to cutting tools, reducing tool life and machining accuracy.

As a result, machining stainless steel generally costs more than machining aluminum or brass.

Types of Stainless Steel

The properties of stainless steel depend largely on its crystal structure and alloy composition.

Martensitic Stainless Steel

Characteristics:

• High strength and hardness

• Excellent wear resistance

• Magnetic

• Lower corrosion resistance than other stainless steels

• Can be hardened through quenching and tempering

Common applications:

• Shafts

• Bearings

• Mechanical components

• Cutting tools

Representative grades:

• SUS403

• SUS410

• SUS420

• SUS440

Ferritic Stainless Steel

Characteristics:

• Good corrosion resistance

• Good formability and weldability

• Magnetic

• Lower strength than austenitic stainless steel

• Cannot be hardened by heat treatment

Ferritic grades also resist stress corrosion cracking because they contain little or no nickel.

Common grades:

• SUS430

• SUS444

Austenitic Stainless Steel

The most commonly used stainless steel family.

Characteristics:

• Best corrosion resistance

• Excellent ductility and toughness

• Good weldability

• Excellent formability

• Non-magnetic (may become slightly magnetic after cold working)

• Higher cost due to nickel content

One drawback is severe work hardening during machining, making these grades among the most difficult to machine.

Representative grades:

• SUS303

• SUS304

• SUS310S

• SUS316

Duplex (Austenitic-Ferritic) Stainless Steel

Duplex stainless steels combine the advantages of both austenitic and ferritic structures.

Characteristics:

• High strength

• Excellent corrosion resistance

• Good resistance to stress corrosion cracking

• Better strength than conventional austenitic grades

Representative grades:

• SUS329J1

• SUS329J3L

Precipitation Hardening Stainless Steel

These alloys achieve extremely high strength through precipitation hardening heat treatment.

Characteristics:

• Very high strength

• Good corrosion resistance

• Excellent strength-to-weight ratio

• Premium material cost

Typical applications include the following:

• Aircraft components

• Rocket components

• Engine parts

Representative grades:

• SUS630

• SUS631

• SUS660

Quick Reference Table of Stainless Steel Types

Advantages of Stainless Steel

Long Service Life

The self-healing passive oxide layer significantly reduces corrosion, minimizing maintenance and extending product life.

Excellent Heat Resistance

Many grades maintain strength under elevated temperatures, making them suitable for high-temperature applications.

High Strength

Stainless steel withstands heavy mechanical loads while maintaining dimensional stability.

Wide Material Selection

The large number of available grades allows engineers to optimize performance, manufacturability, and cost for nearly any application.

Why Stainless Steel Is Difficult to Machine

Machining Challenges

Several material properties contribute to difficult machining:

• Heat remains concentrated near the cutting edge.

• Work hardening increases cutting forces.

• Tough chips tend to adhere to cutting tools.

• Tool wear occurs more rapidly than with aluminum or mild steel.

Successful machining requires:

• Proper cutting parameters

• High-performance cutting tools

• Effective coolant application

• Optimized machining strategies

Welding Challenges

Welding stainless steel requires careful process control.

Improper welding may cause the following:

• Weld discoloration (heat tint)

• Chromium depletion near the weld

• Reduced corrosion resistance

Post-weld cleaning or passivation is often required to restore the protective passive layer.

Common Stainless Steel Processing Methods

CNC Machining

Produces highly accurate precision parts using machining centers and CNC lathes. Although slower than machining softer metals, it remains the preferred method for complex components.

Cutting

Cuts raw material into the required size or profile.

Bending

Stainless steel exhibits springback after bending, requiring compensation during forming operations.

Welding

Joins multiple components into a single structure while requiring careful control to preserve corrosion resistance.

Surface Treatment of Stainless Steel

Material Selection Considerations

When selecting stainless steel, engineers should consider more than corrosion resistance.

Important factors include:

• Required mechanical strength

• Operating temperature

• Corrosive environment

• Weldability

• Machinability

• Material cost

Certain grades also have additional considerations:

• Hydrogen embrittlement: Some stainless steels—particularly ferritic grades—can lose strength when hydrogen penetrates the material.

• Austenitic work hardening: Cold working increases hardness and machining difficulty.

• Marine environments: Grades containing molybdenum, such as SUS316, offer significantly better corrosion resistance.

• High-temperature applications: Heat-resistant grades like SUS310S perform better under prolonged exposure to elevated temperatures.

Stainless Steel CNC Machining at 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.