304 stainless steel has excellent corrosion resistance, heat resistance, low-temperature strength, and comprehensive mechanical properties. It is widely used in food equipment, chemical equipment, and atomic energy industry equipment. This type of stainless steel exhibits good resistance to intergranular corrosion and shows superior corrosion resistance in many acids (such as HNO3), as well as strong corrosion resistance in alkaline solutions, most organic and inorganic acids, and in the atmosphere, water, and steam. However, 304 stainless steel is a typical difficult-to-machine material, with a relative machinability of approximately 0.4.
304 Stainless Steel Properties in Machining
304 stainless steel, its chemical composition is shown in Table 1, it has poor cutting performance. This is caused by its high cutting forces, severe work hardening, high local temperatures in the cutting zone, and susceptibility to adhesive tool wear.
Table 1: Chemical Composition of Stainless Steel 304
| C | Si | Mn | S | P | Cr | Ni |
| 0.08 | 1.00 | 2.00 | 0.030 | 0.045 | 18~20 | 8~10.5 |
High Cutting Forces
Although 304 stainless steel has relatively low hardness (≤187HBS), its high content of elements like Cr, Ni, and Mn results in good plasticity (elongation at break ≥40%, reduction of area ≥60%). During cutting, significant plastic deformation occurs. Furthermore, it maintains high strength even at elevated temperatures, unlike common steel, where the strength decreases notably as cutting temperature rises. This leads to the high cutting forces required for 304 stainless steel. Under conventional cutting conditions, the unit cutting force for 304 stainless steel reaches 2450MPa, which is over 25% higher than that of 1045 steel.
Severe Work Hardening
Cutting of 304 stainless steel is accompanied by pronounced plastic deformation, which causes severe distortion of the material’s crystal lattice. Due to the inherent instability of the austenitic structure, a small portion of the austenite transforms into martensite during this process. In addition, impurity compounds present in the austenite decompose upon heating during the cutting process. These diffusely distributed impurities create a hardened layer on the surface, making the work hardening phenomenon quite significant. The strength (σb) after hardening can exceed 1500 MPa, and the hardened layer depth is 0.1 to 0.3mm.
High Local Temperature in the Cutting Zone
The high cutting forces required for 304 stainless steel, coupled with the difficulty in chip separation, mean that a large amount of work is consumed in chip formation. Under conventional conditions, cutting 304 stainless steel consumes about 50% more energy than cutting low-carbon steel, generating more cutting heat.
Austenitic stainless steel has poor thermal conductivity; the thermal conductivity of 304 stainless steel is 16.3−21.5W/m⋅K, which is only about one-third of that of 1045 steel. Consequently, the cutting zone temperature is high. Although typically more than 70% of the cutting heat should be carried away by the chips, a large amount of heat concentrates in the cutting zone and the tool-chip contact surface. The heat transferred into the tool can reach 20%, compared to only 9% when cutting general carbon steel. As a result, under the same cutting conditions, the cutting temperature for 304 stainless steel is approximately 200−300 °C higher than for 1045 steel.
Susceptibility to Adhesive Tool Wear
Due to the high-temperature strength and significant work-hardening tendency of austenitic stainless steel, the cutting load is heavy. The affinity between the austenitic stainless steel, the tool, and the chip significantly increases during the cutting process, inevitably leading to phenomena such as adhesion and diffusion. This forms a “built-up edge” (BUE), causing adhesive tool wear. In particular, hard inclusions formed by a small amount of carbides accelerate tool wear and can even cause chipping, greatly reducing tool life and negatively affecting the surface quality of the machined part.
How to CNC Machine Stainless Steel 304
Due to the poor machinability of 304 stainless steel, a rational CNC turning process must be selected, including the appropriate choice of tool material, tool geometry parameters, cutting parameters, and cutting fluid, to achieve high productivity and machining quality.
Tool Material
Selecting the correct tool material is crucial for ensuring the efficient cutting of austenitic stainless steel. Analysis based on the difficult-to-machine characteristics of 304 stainless steel indicates that the selected tool material should possess high strength and toughness, along with good wear and heat resistance, while ensuring minimal affinity with stainless steel. Currently, the most commonly used cutting tool materials remain cemented carbide and high-speed steel.
(1) Cemented Carbide
Since difficult-to-machine materials require large cutting forces and the chip-to-rake face contact is short, the cutting force is mainly concentrated near the cutting edge, making chipping prone to occur. Therefore, YG-type cemented carbide tools can be used for machining. YG-type cemented carbide has good toughness, high wear resistance and hot hardness, and excellent thermal conductivity, making it suitable for machining austenitic stainless steel. Examples include YG3X, YG8, YW1, YW2A, and YW3. These materials possess high hardness (74−82 HRC) and high wear and heat resistance (850−1000°C). The YG8N tool, which includes Niobium (Nb), offers 1−2 times improved cutting performance compared to YG8 and performs well in rough and semi-finish machining.
New high-quality cemented carbides such as 813, 758, YM051, and YM052 can be selected based on actual needs. The 813 alloy is typical of these new materials, exhibiting excellent performance when cutting austenitic stainless steel. This is fundamentally due to its high hardness (≥91HRA) and strength (1570MPa), along with good performance in toughness, heat resistance, and anti-adhesion. Its dense structure also provides good wear resistance. When turning 304 stainless steel, the 813 cemented carbide tool is highly effective, offering a tool life 2−3 times longer than general cemented carbides.
(2) High-Speed Steel
High-speed steel tools can effectively prevent the damage that cemented carbide tools are prone to, which is often caused by the size, shape, and structure of stainless steel workpieces. Conventional high-speed steel tools (such as W18Cr4V) are no longer adequate in terms of durability for current machining requirements. However, new high-speed steel tools with superior cutting performance, such as aluminum-containing high-speed steel (e.g., W6Mo5Cr4V2Al) and nitrogen-containing high-speed steel (e.g., W12Mo3Cr4V3N), can be used.
Tool Geometry Parameters
The rational determination of selected tool geometry parameters is a key factor in effectively improving tool life and the machining results for 304 stainless steel. Generally, the tool is required to have a large rake angle and relief angle, as well as a sharp cutting edge.
(1) Rake Angle (γo)
The largest possible rake angle should be chosen, provided tool strength is fully considered. A larger rake angle can reduce cutting force and cutting temperature, while also effectively decreasing the depth of the hardened layer. When turning 304 stainless steel, the rake angle typically ranges from 12∘ to 20∘.
(2) Relief Angle (αo)
A larger relief angle should be selected, under the premise of ensuring tool strength, as it can effectively reduce friction between the flank face and the machined surface. However, this also slightly reduces the tool’s strength and heat dissipation capability. The selection of the relief angle is closely related to the cutting thickness; a larger relief angle is generally chosen for smaller cutting thicknesses. Practical experience suggests a relief angle of 10∘−20∘ for finish machining and 6∘−10∘ for rough machining. Furthermore, measures such as creating a negative land on the main cutting edge have a notable effect on strengthening the cutting edge. This disperses the heat generated during cutting to the rake and flank faces, reducing wear on the cutting edge and thereby improving tool durability.
(3) Setting Angle (κr), Auxiliary Setting Angle (κr′), and Tool-Nose Radius (rϵ)
The setting angle is generally chosen between 45∘ and 75∘, and the auxiliary setting angle is between 8∘ and 15∘. To effectively increase the strength of the tool nose, a tool-nose radius can be ground, typically 0.2−0.8mm. A larger tool-nose radius is generally selected for rough turning and large feed rates.
(4) Inclination Angle (λs)
In the cutting of 304 stainless steel, a negative inclination angle is typically chosen to enhance the strength of the tool nose. The inclination angle is generally set at −8∘ to −3∘, and can be set at −15∘ to −5∘ for interrupted cutting.
(5) Chip Breaker
304 stainless steel has good toughness and plasticity, making chip breaking difficult during machining. By optimizing the parameters of the chip-breaking groove on the rake face and the cutting parameters, forced chip deformation can be employed to facilitate chip breaking. Under conditions where cutting parameters are appropriately selected, a double inclination angle combined with an external oblique chip breaker can be used. This involves grinding a double inclination angle to give the chip cross-section a prismatic shape, and then grinding an external oblique arc-shaped chip breaker on the rake face. The chip-curling radius near the tool nose is large, while the radius near the outer edge is small. This guides the chip along the chip breaker to curl into a pagoda shape, forming short, tight helical chips. Simultaneously, the chip turns toward the surface being machined and breaks, resulting in ideal chip-breaking conditions.
(6) Tool Surface Roughness
Reducing the surface roughness of the tool’s rake face, flank face, and cutting edge can address the strong adhesion between the chip and the tool during the turning of AISI 304 stainless steel. It is best to meticulously grind the tool on a specialized tool grinder using a diamond wheel so that the tool surface roughness Ra≤0.4μm. This effectively reduces chip adhesion during processing and lowers the cutting resistance, thereby improving tool durability. If coated tools are selected, the coating material is mainly applied using the Physical Vapor Deposition (PVD) method to achieve a smoother tool cutting surface.
Cutting Parameters
304 stainless steel is a typical difficult-to-machine material, requiring careful selection of cutting parameters. Cutting parameters significantly affect work hardening, cutting force, heat generation, and machining efficiency. Cutting speed has the greatest impact on cutting temperature and tool life, followed by feed rate, while the depth of cut has the least impact.
(1) Cutting Speed (vc)
When turning 304 stainless steel, the cutting speed must be appropriately reduced to ensure reasonable tool life. The cutting speed can be selected as 40% to 60% of that used for turning ordinary carbon steel, typically ranging from 50−80m/min.
(2) Depth of Cut (ap)
For rough machining, a larger depth of cut can be used to avoid contact between the tool nose and the surface skin. This reduces the number of passes, thereby decreasing tool wear. The depth of cut for rough machining can be 2−5mm, but should not be excessively large to prevent cutting vibration. For finish machining, a smaller depth of cut should be used, generally 0.2−0.5mm, but it should not be too small to avoid cutting within the hardened layer.
(3) Feed Rate (f)
The feed rate has a large impact on machining quality. Increasing the feed rate increases the residual cutting height, significantly affecting the workpiece’s surface quality. A typical range is 0.1−0.8mm. For finish machining, a smaller feed rate should be used, generally 0.15−0.40mm/r. The value should not be too small to avoid cutting within the work-hardened layer.
Commonly used cutting parameters for 304 stainless steel are provided in Table 2 (using YG8 tool material). A higher spindle speed is preferred when the diameter is small, and vice versa.
Cutting Fluid
Because of the poor machinability of 304 stainless steel, the selected cutting fluid must possess superior cooling properties, lubricity, and penetrability (i.e., anti-adhesion properties). It is advisable to select emulsions or sulfurized oils that contain extreme pressure additives such as Sulfur (S) and Chlorine (Cl).
- Emulsions have good cooling properties and are primarily used for the rough turning of stainless steel.
- Sulfurized oils offer some cooling and lubrication, are lower in cost, and can be used for semi-finish or finish machining of stainless steel.
- Adding extreme pressure or oiliness additives to the cutting fluid significantly enhances its lubricating properties and is generally preferred for the finish turning of stainless steel.
- A cutting fluid mixture of carbon tetrachloride (CCl4), kerosene, and oleic acid significantly improves the penetrability of the cooling lubricant and is particularly suitable for finishing turning of 304 stainless steel.
Due to the large amount of cutting heat generated when machining 304 stainless steel, methods such as mist cooling and high-pressure cooling should be adopted whenever possible to enhance the cooling effect.
Conclusion
Practice has shown that machining 304 stainless steel should adhere to the following basic principles:
- Select cutting tools with good toughness and high strength, and rationally choose the tool’s relevant parameters based on machining requirements.
- Use an appropriate chip breaker to improve chip curling and breaking.
- Determine rational cutting parameters.
- The sufficient supply of cutting fluid also significantly impacts the machining process.
The selection of relevant cutting parameters should be considered holistically, and the optimal combination can be determined through comprehensive analysis using experimental methods.
