Stainless Steel CNC Machining | Machinability, Grades, Solutions

Stainless steel is popular in engineering for its high strength, toughness, and exceptional resistance to heat and corrosion. However, the very physical and mechanical properties that make it desirable also present significant challenges during the manufacturing process. From a machining perspective, stainless steel is often considered as a difficult material due to its severe work-hardening tendencies, high cutting forces, and low thermal conductivity.

Overview of Machinability

Machinability refers to the ease with which a metal material can be subjected to cutting processes. Once normal cutting is established, metal machinability is evaluated based on the surface roughness of the workpiece, cutting speed, and the degree of tool wear.

Machinability is a complex surface-layer phenomenon involving friction and high-speed elastic and plastic deformation. Therefore, the ease of cutting and the quality of the result are related to many factors:

1. Tool material and geometry.
2. Cutting performance of the workpiece material.
3. Presence and characteristics of cutting fluids during processing.
4. Cutting type and conditions.

Under other constant conditions, the easiest metal to cut should be the one that allows for the maximum cutting speed, minimum tool wear, and lowest energy consumption while obtaining the most satisfactory surface roughness. In most cases, the primary requirements for cutting are high speed and long tool life; however, in some situations, the requirements for surface smoothness are more stringent.

Methods for Determining Machinability

1. Comparing the life of the same tool when cutting different metals at a given cutting speed.
2. Comparing the depth drilled into different metals using the same pressure, rotational speed, and time.
3. Comparing machinability by the energy consumed or heat dissipated while cutting a certain volume of different metals.

These methods do not consider the surface roughness of the machined part; they only use the ease of cutting to relatively compare the machinability of metals.

Metal machinability is a very complex technological property related to other properties of the metal. First, we should understand the reasons why metals are difficult to cut:

1. When hardness or strength is too high, the energy consumed by cutting is large; when the speed increases, the generated heat can easily soften the tool edge, making cutting very difficult.
2. Soft metals with high plasticity are prone to producing built-up edge (BUE) and sticking to the tool edge during cutting, which makes cutting difficult and increases surface roughness.
3. Metals prone to work hardening are difficult to cut, such as high-carbon, high-manganese, wear-resistant steels, and austenitic stainless steels. Metals containing hard second phases are difficult to cut, such as carbides and oxides, which easily wear down the tool edge.

Secondly, we should know what kind of structure makes metal easy to cut. Second phases that are insoluble in the matrix, if they possess lubricity or increase the brittleness of the material, can improve machinability. For example, graphite, lead, and bismuth can increase the lubrication effect during cutting; brittle sulfides and phosphides (such as MnS and Fe3P in steel) can make chips easier to break.

Ways to Improve Machinability

1. Soften hard metals (tempering or annealing).
2. Harden metals that are too soft (cold working, grain refinement, normalizing).
3. Reduce hard second phases (improve smelting quality, reduce inclusions).
4. Improve the distribution of hard second phases (annealing or normalizing).
5. Add second-phase elements that improve cutting performance (Pb, Bi, Graphite, MnS, Fe3P, etc.).

The final method (5) can produce alloys with excellent machinability suitable for automatic machine tools, which are therefore called free-cutting steels and alloys.

Some physical and mechanical properties affecting machinability are insensitive to the microstructure and are thus difficult to change by improving the structure. These properties include the coefficient of thermal expansion, thermal conductivity, and the elastic modulus of the matrix metal.

Overview of Stainless Steel Cutting

Different types of stainless steel have different machining performances, and the differences are significant. Generally speaking, the machinability of stainless steel is worse than that of other steels. For example, compared to carbon steel, austenitic stainless steel has the worst machinability. This is caused by the severe work hardening and low thermal conductivity of austenitic stainless steel. For this reason, water-based cutting coolants must be used during the cutting process to reduce thermal deformation. Especially when post-weld heat treatment is poor, deformation is unavoidable regardless of how much cutting precision is improved. The machinability of other types of stainless steel, such as martensitic and ferritic stainless steels, is not significantly different from carbon steel as long as they are not cut after quenching. However, for both martensitic and ferritic types, the higher the carbon content, the worse the machinability. Precipitation-hardening stainless steels exhibit different machining performances due to differences in structure and treatment methods, but generally, in the annealed state, their machinability is basically the same as martensitic and austenitic stainless steels of the same series and strength.

Stainless steels are widely used because they possess special heat and corrosion resistance along with high strength and toughness. In many applications, the ability of one metal material to cut another is determined by their machinability. All stainless steels can be machined, just like all grades of stainless steel. The machinability of stainless steel, like its physical and mechanical properties, shows great variation.

Generally, stainless steel refers to iron-based alloys with a chromium content of over 11.5%. To achieve the required corrosion resistance and/or mechanical properties, elements such as carbon, nickel, manganese, silicon, aluminum, titanium, niobium, and other alloying elements can be added.

Types of Stainless Steel based on Machinability

Classifying based on microstructure and heat treatment conditions is the most common method. This divides all stainless steels into three categories: hardenable (martensitic), non-hardenable (ferritic), and austenitic. Each category includes some free-cutting grades.

Hardenable Stainless Steel

This group consists of 400-series steels that can be hardened by heat treatment. Free-cutting grades include: stainless steel 416, 420F, and 440F. Grades with general machinability include 403, 410, 420, 431, 440A, 440B, and 440C. All these are martensitic steels, magnetic in the annealed state, and can reach various strength levels through quenching and tempering. The easiest to machine is 416, while the most difficult is 440C because its high carbon content causes severe tool wear.

Non-hardenable Stainless Steel

This group includes 400-series steels that cannot be significantly hardened by heat treatment. They are ferritic and magnetic in the annealed state. Free-cutting grades include 430F; those with general machinability include 405, 430, and 446.

Austenitic Stainless Steel

These are the Cr-Ni 300-series steels, recently expanded to include some Cr-Ni-Mn 200-series steels. They are austenitic in the annealed state, basically non-magnetic, and cannot be hardened by heat treatment. However, most have high work-hardening rates—much higher than ferritic and martensitic steels. Free-cutting grades include 303 and 303Se. Standard grades include 201, 202, 301, 302, 304, 305, 308, 309, 310, 316, 317, 321, 347, etc. Austenitic steels are much more difficult to machine than most martensitic or ferritic steels. They are sticky in the annealed state and produce fibrous chips unless special techniques are used. High work-hardening rates also contribute to cutting difficulty.

Properties of Stainless Steel CNC Machining

The machinability of stainless steel is much worse than medium carbon steel. If 1045 steel is 100%, austenitic 321 is 40%; ferritic 10Cr28 is 48%; and martensitic 420 is 55%. Austenitic and austenitic+ferritic grades are the worst. Characteristics include:

Severe Work Hardening

Most prominent in austenitic and austenitic+ferritic steels. Strength after hardening can reach 1470–1960 MPa. The yield limit rises from 30%–45% in the annealed state to 85%–95% after hardening. The hardened layer depth can be 1/3 of the cutting depth or more, with hardness 1.4–2.2 times the original. This is due to high plasticity, lattice distortion, and the transformation of austenite to martensite under stress.

Large Cutting Forces

High plasticity especially in austenite, with elongation 1.5x that of 1045 steel increases force. Work hardening and high thermal strength further increase resistance and make chip breaking difficult. Unit cutting force for 06Cr18Ni11Ti is 2450 MPa, 25% higher than 1045 steel.

High Cutting Temperature

Large plastic deformation and friction generate significant heat. Combined with low thermal conductivity (1/4 to 1/2 of 1045 steel), heat concentrates at the cutting zone. Under the same conditions, the temperature for 321 is about 200°C higher than for 1045 steel.

Chips Difficult to Break

High plasticity and toughness lead to continuous chips that can scratch the surface. High affinity for other metals causes sticking and built-up edge, exacerbating tool wear and surface tearing.

Tool Wear

Affinity causes adhesion and diffusion wear, leading to cratering on the tool face and micro-chipping. Hard carbide particles in the steel also cause abrasive wear.

Large Linear Expansion Coefficient

About 1.5 times that of carbon steel. Under cutting heat, workpieces deform thermally, making dimensional accuracy hard to control.

Among various machining equipment, automatic lathes are highly efficient for processing stainless steel bars into countless components.

Considerations for stainless steel machining:

1. Tools should be rigid, advanced, and have high overload capacity. It is best to cut at below 75% of the machine’s rated capacity.
2. Workpieces and tools must be clamped securely. Tool overhang should be as short as possible; use extra support if needed.
3. Keep tools sharp at all times (HSS or Carbide). Grind them regularly; do not wait until absolutely necessary.
4. Use high-performance lubricants, such as chlorinated petroleum grease. This is effective for heavy cutting at slow feeds. For high-speed finishing, dilute with kerosene to keep temperatures low.
5. Pay attention to Cr-Ni austenitic steels. Use forced cutting, and avoid pauses to prevent work hardening and slipping.

Compared to carbon steel:

1. Annealed stainless strength is generally higher.
2. The gap between yield and tensile strength is larger.
3. Work-hardening rates are higher.
4. High-carbon grades (440A/B/C) contain free alloy carbides that harden the matrix and are abrasive, increasing wear.

Stainless Steel CNC Machining Processes

The manufacturing processes for stainless steel machining include CNC turning, milling, grinding, drilling, etc.

CNC Turning

Usually considered single-point cutting. Tools should have a positive rake angle. Martensitic types (420, 440) require a negative rake angle to reduce wear. Austenitic series (200/300) tend to work-harden; maintain forced feed and do not stop suddenly. Use high-power lathes with good structural rigidity. Carbide-tipped tools are recommended on turret lathes.

• Roughing feed: 0.229–0.406 mm.
• Finishing feed: 0.076–0.254 mm.
• Speed (Chrome types): 61–152 m/min.
• Speed (Cr-Ni types): 61–122 m/min.
• Use concentrated soluble oil mixtures.

CNC Milling

High adhesion and fusion tendencies mean chips stick to teeth. In up-milling, teeth slide on hardened surfaces, increasing hardening. Vibrations and shocks are high.

• Tools: Use HSS (W-Mo or high-vanadium) or Carbide (YG8, YW2, 813, 798, etc.).
• Geometry: Use large helix angles (20°–45°). While end mills are better at β 35 to avoid weakening teeth.
• Method: Use climb milling (down-milling) to ensure teeth exit the metal smoothly and chips are thrown off by centrifugal force.
• Cooling: Spray cooling is most effective; otherwise ensure full flow of 10% emulsion.
• Speed: Start at ~24 m/min for Carbide and ~9 m/min for HSS.

CNC Grinding

High toughness and heat strength mean grains with negative rake angles struggle to shear chips. Energy is high, temperatures reach 1000–1500°C, and chips clog the wheel. Low thermal conductivity can cause surface burns or annealing (depth 0.01–0.02 mm). Expansion leads to deformation, especially in thin-walled parts. Most stainless steels are non-magnetic, requiring mechanical clamping, which can cause vibration or distortion. Avoid uneven grinding or local overheating.

CNC Drilling

Typically use twist drills. For hardened steel, use carbide or super-hard HSS. Torques are high, and chips stick and harden. Grind chip-breaking grooves and thin the web to reduce axial force.

• Slips/Hardening: Sprinkle chalk powder on the edge or hole to help the cut start.
• Drill jigs: Keep bushings short; maintain a gap of one drill diameter for chip escape.
• Speed: 12–38 m/min depending on grade and depth.
• Feed: 0.051–0.508 mm/r. Minimize pauses to reduce hardening.

CNC Tapping

Free-cutting grades are similar to carbon steel. A 15° rake angle is better. Use thin-land taps for deep holes to reduce pressure. Use HSS taps with precision-ground threads and polished flutes. Spiral-fluted taps are better for chip control. Ensure threads do not exceed 75% depth (65% is often better) to prevent breakage. For high precision, drill slightly smaller (0.152–0.305 mm under size) and then ream before tapping. Avoid hand-grinding taps.

• Speed: 3–11 m/min.
• Lubrication: Sulfur-based oils for coarse threads; kerosene-diluted for fine; white lead powder for heavy tapping.

Cleaning and Passivation after Machining

To ensure corrosion resistance, remove all dirt and stains. A clean surface allows a passive film to form.

Degreasing: Must be complete and thorough to remove all lubricants and oils.

Nitric Acid Bath: Often 20% concentration at 49°C for at least 30 minutes. It dissolves iron particles and cleans corrosion sites.

Adjustment: For 300 series or high-chrome 400 series, 20%–40% concentration at 54–71°C for 30–60 minutes is used. Lower temperatures are used for low-chrome 400 series. Rinse with hot water and dry immediately.

Surface Finishes for Stainless Steel CNC Machining

Contrary to common belief, stainless steel is not entirely immune to rust, it is simply relatively resistant to it. A very thin, chromium-rich oxide film on the surface provides a protective barrier. However, this layer can be compromised by environmental factors and mechanical processing, such as surface scratches, improper cleaning, carbon steel contamination, or welding. These forms of surface pollution can significantly damage the protective layer.

Therefore, effective measures must be taken during the fabrication of stainless steel products. The best way to prevent corrosion is through proper surface treatment. Common processes include brushing, mirror polishing, sand blasting, and anti-fingerprint coatings, each offering unique aesthetic effects and functional benefits.

Mirror Polishing

Mirror treatment essentially involves polishing the stainless steel surface using either physical or chemical methods. Polishing can be applied to the entire surface or localized areas. The grades of mirror finish are categorized into standard polishing, 6K (standard mirror), 8K (fine grinding), and 10K (super-fine grinding). A mirror finish provides a high-end, minimalist, fashionable, and futuristic aesthetic.

Sand Blasting

This is a common surface treatment processes in stainless steel machining. It utilizes compressed air as power to project abrasive materials at high velocity onto the workpiece surface, resulting in a change to the surface texture. Sand blasting is primarily used for engineering and process optimization, such as increasing the adhesion of bonded parts, deburring machined surfaces, decontamination, and achieving a matte finish. This process is far superior to manual sanding, providing a uniform surface structure and a low-profile, durable appearance with high production efficiency. While manual sanding can create a rough surface, it is too slow, and chemical cleaning often leaves the surface too smooth for optimal coating adhesion.

Chemical Treatment

This process involves a combination of chemical and electrochemical methods to generate a stable compound layer on the stainless steel surface. Electroplating is a prime example of chemical treatment. This method primarily relies on individual or mixed acidic solutions and anodic solutions for descaling. Subsequently, protective films are generated through chromate treatment, phosphate or black oxide. This process is mainly used to create complex patterns or to meet specific vintage or contemporary design requirements.

Coloring

Surface coloring processes can provide various colors to stainless steel, making the metal more vibrant. Beyond the visual appeal, coloring can effectively improve the wear resistance and corrosion resistance of the product. Common coloring methods include chemical coloring, electrochemical oxidation coloring, ion deposition oxide coloring, high-temperature oxidation coloring, and gas-phase pyrolysis coloring.

Brushing

Metal brushing is a very common decorative method. It can produce several patterns, including straight lines, threaded patterns, wavy patterns, random patterns, and rotary patterns. Brushed surfaces are characterized by a pleasant tactile feel, delicate luster, and strong wear resistance. This treatment is widely applied in electronic devices, household appliances, and mechanical equipment.

Spray Coating

Stainless steel spray coating differs substantially from the coloring processes mentioned above. Depending on the materials used, some paints may actually damage the oxide layer of the stainless steel. However, certain spray coatings allow for different colors to be achieved through simple processes, and different types of coatings can be utilized to change the tactile feel or “hand” of the stainless steel surface.

Getzshape CNC Machining Tolerance and Capacities

Getzshape delivers high-quality custom CNC machining, sheet metal fabrication, electrical discharge machining, die casting and more. Our CNC machining capabilitites for stainless steel machining are listed as below.