Stainless steel is an iron-based alloy with a chromium content of ≥10.5%. Thanks to its corrosion resistance, high strength, and dimensional stability, it has become a core raw material for CNC turning. Differences in the crystal structure of stainless steel directly determine machining difficulty, cutting tool selection, and cutting parameters. The mainstream stainless steels used in turning are classified into four categories: austenitic, martensitic, ferritic, and precipitation-hardening stainless steels.
Stainless Steel Material Properties Page.
Austenitic stainless steel is the most widely used and extensive type of CNC machining. Its crystal structure is face-centered cubic (FCC), it is non-magnetic, has excellent toughness and welding performance. However, it has characteristics such as rapid work hardening, poor thermal conductivity and easy sticking to the tool during processing, so the processing technology needs to be optimized accordingly.
| Grade | Category | Composition | Performance | Machining | Applicables |
|---|---|---|---|---|---|
| 304 / SUS304 | Austenitic | Cr 18%-20%, Ni 8%-12%, C ≤0.08%, Mo-free, high cost-performance ratio | Tensile strength ≥515MPa, hardness ≤HB201, non-magnetic, moderate corrosion resistance | Easy to work harden, prone to sticking to tools, moderate chip removal; mature technology, ideal for beginners | Most commonly used in conventional industrial structural parts, equipment housings, and appliance parts; highest order volume. |
| 304L / SUS304L | Austenitic | Low-carbon version of 304 (C≤0.03%), Mo-free, avoids intergranular corrosion during welding | Tensile strength ≥485MPa, hardness ≤HB201, non-magnetic, weldability superior to 304 | Similar to 304, slightly higher toughness, requires careful chip breaker design | Welded structural components, food processing equipment, low-temperature environment parts |
| 316 / SUS316 | Austenitic | Cr 16%-18%, Ni 10%-14%, Mo 2%-3%, C ≤0.08%, excellent corrosion resistance | Tensile strength ≥515MPa, hardness ≤HB217, non-magnetic, resistant to acids, alkalis, and seawater | High machining difficulty, high cutting force, requires coated tools, reduced depth of cut and increased speed | Parts for corrosive environments such as marine and chemical industries, ship fittings |
| 316L / SUS316L | Austenitic | Low-carbon version of 316 (C≤0.03%), balancing weldability and resistance to intergranular corrosion | Tensile strength ≥485MPa, hardness ≤HB217, non-magnetic, slightly better corrosion resistance than 316 | Similar to 316, but with higher toughness; requires optimization of chip breaking and cooling. | Applications include medical and food-grade precision parts, marine core components, and chemical corrosion-resistant equipment. |
| 303 / SUS303 | Austenitic (Free Machining) | Contains S/Se, C≤0.15%, Cr17%-19%, Ni8%-10% | Tensile strength ≥485MPa, hardness ≤HB201, non-magnetic, corrosion resistance slightly inferior to 304 | Optimal machinability, smooth chip removal, efficiency 20%-30% higher than 304 | Suitable for batch production of small precision parts, shafts, and gears; not suitable for highly corrosive environments. |
| 410 / SUS410 | Martensitic | Cr 11.5%-13.5%, C ≤0.15%, no Ni or Mo, simple composition, low cost | Annealed hardness ≤HB201, after quenching HRC40-45, magnetic, medium strength | Easy to machine after annealing, significantly more difficult after quenching, requires carbide tools | Suitable for cost-sensitive applications, valves, decorative parts, low-speed shafts |
| 420 / SUS420 | Martensitic | Cr 12%-14%, C 0.16%-0.25%, no Ni or Mo, carbon content higher than 410 | Annealed hardness ≤ HB223, quenched hardness HRC45-52, magnetic, excellent wear resistance | Prone to burring after annealing, can only be fine ground after quenching, requires rough machining followed by quenching | Applications: Cutting tools, mold parts, high-precision wear-resistant shafts, valve cores |
| 440C / SUS440C | Martensitic | Cr 16%-18%, C 0.95%-1.20%, Mo 0.40%-0.80%, highest hardness | Annealed hardness ≤HB269, quenched hardness HRC58-62, magnetic, excellent wear resistance | Extremely difficult to machine; annealing requires CBN tools; cannot be cut after quenching | Bearings, high-end cutting tools, mold inserts; CNC machining is only used for roughing after annealing. |
| 430 / SUS430 | Ferritic | Cr 16%-18%, C ≤0.12%, Ni and Mo free, low cost | Tensile strength ≥450MPa, hardness ≤HB201, magnetic, corrosion resistance slightly inferior to austenitic | Easily chipped during machining, requires sharp tools and optimized toolpaths; surface easily scratched | Suitable for simple machining of appliance decorative panels, kitchen utensils, outdoor decorative parts, etc. |
| 17-4PH / SUS630 | Precipitation hardening type | Cr 15%-17.5%, Ni 3%-5%, Cu 3%-5%, C ≤0.07%, age-hardening | Tensile strength after aging ≥1030MPa, hardness HRC38-45, corrosion resistance close to 304 | High machining difficulty, high cutting force, requires high-strength tools and multiple passes | Aerospace structural components, medical core parts, high-end precision molds |
Due to its significantly different physical properties compared with conventional carbon steel, stainless steel is one of the main causes of machining issues, accelerated tool wear, and dimensional deviations during CNC turning:
This standardized process is suitable for commonly used stainless steel grades such as 304, 316, and 410. For high-hardness martensitic stainless steels and precipitation-hardening stainless steels, machining parameters can be further optimized based on this workflow.
Select the appropriate stainless steel grade according to corrosion resistance, strength, and hardness requirements.
For martensitic and precipitation-hardening stainless steels, the raw material must undergo annealing treatment to relieve internal stress, preventing deformation during CNC turning.
Standard stainless steel grades generally use coated carbide inserts for CNC turning.
High-hardness materials such as 440C and 17-4PH stainless steel require advanced cutting tools such as CBN inserts or diamond-coated tools. Customized stainless steel cutting fluids are also recommended to improve tool life and machining stability.
CNC programming should follow the machining principles of:
Chip-breaking paths should be optimized, and sufficient machining allowance should be reserved for heat treatment and final finishing operations.
| Process Stage | Machining Method | Key Process Control |
|---|---|---|
| Workpiece Clamping | Use soft jaws or protective pads to hold the stainless steel workpiece | Balance clamping force to prevent deformation, vibration marks, and surface damage on thin-wall turned parts |
| Rough Turning | Remove most machining allowance and establish reference surfaces | Apply high-flow coolant continuously and remove tangled chips promptly to avoid tool damage |
| Semi-Finish Turning | Correct rough machining dimensional errors and improve surface condition | Remove rough tool marks and maintain a machining allowance of 0.2–0.5 mm per side |
| Heat Treatment (Optional) | Perform hardening and tempering for martensitic stainless steel semi-finished parts; aging treatment for 17-4PH | Austenitic stainless steel usually requires only stress-relief treatment |
| Finish Turning | Complete final machining of outer diameter, internal holes, and threads | Strictly control dimensional tolerances, reduce feed rate, and improve surface roughness |
| Process Stage | Process Description | Purpose |
|---|---|---|
| Deburring | Remove machining burrs using ultrasonic cleaning, brushing, or vibratory finishing | Prevent scratches on assembly surfaces and improve part safety |
| Surface Treatment | Apply polishing, brushing, or passivation processes | Passivation significantly improves corrosion resistance of stainless steel turned components |
| Precision Dimensional Inspection | Use calipers, micrometers, and CMM (Coordinate Measuring Machine) for tolerance verification; measure Ra values with a surface roughness tester | Ensure dimensional accuracy and surface finish meet drawing requirements |
| Cleaning and Rust Prevention | Remove cutting oil residue through ultrasonic cleaning, apply rust-preventive oil, dry, and package parts in controlled conditions | Maintain cleanliness and prevent corrosion during storage and transportation |
Note: Applicable to common stainless steel grades such as 304, 316, and 410. Parameters can be finely adjusted according to specific grades and precision requirements.
Stainless steel CNC turning parts are widely used across industries due to their excellent corrosion resistance, mechanical strength, dimensional stability, and surface performance. Different stainless steel grades are selected based on application requirements, including strength, wear resistance, corrosion resistance, and hygiene standards.
| Industry Application | Recommended Stainless Steel Grades | Typical CNC Turned Components | Key Performance Advantages |
|---|---|---|---|
| Medical Equipment | 316L, 17-4PH | Surgical instrument housings, implant components, precision medical fittings | Excellent corrosion resistance, biocompatibility, and hygienic passivation treatment capability |
| Marine & Chemical Industry | 316, 316L | Flanges, shafts, valve bodies, corrosion-resistant turned components | Superior resistance to seawater and chloride ion corrosion in harsh environments |
| Aerospace Industry | 17-4PH | High-strength structural turned parts, precision aerospace components | Combines high mechanical strength, fatigue resistance, and corrosion resistance |
| General Industrial Automation | 304, 303 | High-volume shafts, screws, bushings, and small precision turned parts | Excellent machinability, stable production efficiency, and cost-effective mass production |
| Kitchenware & Home Appliances | 430, 304 | Decorative turned components, appearance-focused hardware parts | Good surface appearance, corrosion resistance, and strong cost advantages |
Need a customized stainless steel CNC turning solution, material selection support, or precision turned parts manufacturing service?
Explore the complete stainless steel material properties and machining guide.
A: 303 free-machining stainless steel is the preferred choice for mass production. It contains sulfur and selenium additives that improve machinability, enabling smoother chip evacuation, lower tool wear, and 20%–30% higher machining efficiency compared with 304 stainless steel.
The main limitation is its slightly lower corrosion resistance, making it unsuitable for marine environments or strong acid/alkaline applications.
A: 316 stainless steel contains molybdenum, providing significantly better resistance to acids, alkalis, and seawater corrosion compared with 304. However, its higher cutting forces result in approximately 30% higher tool wear and increased machining costs.
For general indoor applications without severe corrosion exposure, 304 stainless steel is usually the economical choice. For chemical processing, marine equipment, and seawater environments, 316/316L stainless steel is recommended.
A: CNC turning can leave microscopic iron contamination, cutting residues, and stress layers on the machined surface, increasing the risk of oxidation and corrosion.
Passivation treatment creates a dense chromium oxide protective film, significantly improving long-term corrosion resistance while enhancing surface cleanliness and appearance.
For food-grade and medical-grade stainless steel turned parts, passivation is often a mandatory finishing process.
A: To prevent clamping deformation and surface damage during CNC turning:
Use copper soft jaws or protective pads on fixtures
Control clamping force accurately to avoid part distortion
Maintain continuous coolant supply during machining
Remove tangled chips promptly to prevent surface scratching
Use sharp, low-friction cutting inserts during finishing operations
Optimize tool paths to minimize unnecessary contact and friction
These measures ensure stable dimensions and high-quality surface finishes for precision stainless steel turned components.
A: For stainless steel CNC turning, it is recommended to use specialized stainless steel emulsified or semi-synthetic cutting fluids with excellent cooling, lubrication, and anti-adhesion properties.
Standard cutting fluids may cause:
Increased corrosion risk on workpieces
Faster cutting tool wear
Poor surface finish
Built-up edge formation
Therefore, stainless steel machining fluids should be selected according to material grade and machining conditions, and different cutting fluids should not be mixed.
Fiona Huang is a Sales Engineer with 16+ years of experience in the mechanical and precision manufacturing industry. She specializes in CNC machining components, custom fasteners, and engineering-driven manufacturing solutions. With strong expertise in international trade and technical project support, she focuses on transforming complex engineering requirements into high-precision, production-ready solutions for global clients.
