304 stainless steel is widely used for CNC machining due to its strength, ductility, and corrosion resistance. This guide covers its key properties, machinability, and material considerations to help you determine whether 304 stainless steel is suitable for your CNC parts and applications.
What Is 304 Stainless Steel
304 stainless steel is an austenitic stainless steel, usually denoted as AISI 304 or UNS S30400, and is also known as 18-8 stainless steel. It is primarily composed of iron and contains alloy elements such as chromium and nickel, which chromium and nickel are the key alloy elements that distinguish it from ordinary carbon steel.
The following values represent the chemical composition range of 304 stainless steel under common standards. When selecting materials and designing parts, you should refer to the actual purchasing material standards and material certificates.
| Element | Chemical Symbol | Content (%) |
| Iron | Fe | Balance |
| Chromium | Cr | 18.0–20.0 |
| Nickel | Ni | 8.0–10.5 |
| Manganese | Mn | ≤2.00 |
| Silicon | Si | ≤1.00 |
| Carbon | C | ≤0.08 |
| Phosphorus | P | ≤0.045 |
| Sulfur | S | ≤0.030 |
| Nitrogen | N | ≤0.10 |
Mechanical Properties of 304 Stainless Steel
Factors such as tensile strength, yield strength, and hardness are important parameters for evaluating the load-bearing capacity, deformation characteristics, and designing corresponding aspects of 304 stainless steel.
Tensile Strength
Tensile strength is the maximum stress that a material can withstand during a tensile test. The tensile strength is approximately 515 MPa after annealing or higher, and the specific value depends on the material standard and material condition you use.
When designing parts, you can use tensile strength as an important reference for evaluating the ultimate load-bearing capacity of 304 stainless steel. However, tensile strength does not equal the actual stress that the part can withstand.
During the design process, a comprehensive judgment needs to be made by considering the safety factor, load type, and structural dimensions of the part.
Yield Strength
The yield strength represents the stress at which a material begins to undergo significant plastic deformation. The yield strength of 304 stainless steel in the annealed state is typically around 205 MPa or higher.
For you, the yield strength usually has a more direct impact on the structural design of the part. If the part cannot undergo permanent deformation during operation, you need to pay close attention to whether the working stress is lower than the yield strength of the material, and set an appropriate safety margin based on the actual situation.
Hardness
Hardness reflects the material’s ability to resist indentation, scratching, and local plastic deformation, and it affects the wear resistance of the contact surface, local deformation, and the load on the cutting tool during processing. In the annealed state, the hardness of 304 stainless steel is usually below HRB 92, and the specific value may vary depending on the material condition and standard requirements.
It is important to note that 304 stainless steel is a CNC raw material that is prone to hardening during processing. After cold processing, its hardness and strength may significantly increase, so the actual hardness of the processed material may not be the same as that in the annealed state.
Elongation
304 stainless steel has good ductility, with elongation typically around 40% or higher in the annealed condition. This means that 304 stainless steel has good plasticity and can undergo a certain degree of plastic deformation when subjected to large loads, rather than suddenly breaking.
If the stainless steel part you design needs to withstand bending, forming, or local deformation, you can choose 304 stainless steel. However, specific selection still requires a comprehensive judgment based on the structure and load.
Elastic Modulus
The elastic modulus is used to describe the ability of a material to resist deformation during the elastic deformation stage. The elastic modulus of 304 stainless steel is typically around 193 GPa.
The higher the elastic modulus, the smaller the elastic deformation that the material usually generates under the same stress. Therefore, when designing 304 stainless steel CNC parts, you can use the elastic modulus to evaluate the deflection, elastic deformation, and structural stiffness of the part under working loads.
You cannot rely on a single mechanical performance parameter in the design. By combining these property parameters, you can more accurately determine whether 304 stainless steel meets your structural design requirements.
| Property | Typical Value |
| Tensile Strength | ≥515 MPa |
| Yield Strength | ≥205 MPa |
| Elongation | ≥40% |
| Elastic Modulus | ~193 GPa |
| Hardness | ≤HRB 92 |
| Density | ~7.93 g/cm³ |
| Thermal Conductivity | ~16 W/(m·K) |
| Thermal Expansion | ~17.2 × 10⁻⁶/K |
| Specific Heat | ~500 J/(kg·K) |
| Electrical Resistivity | ~0.72–0.75 μΩ·m |
Physical Properties of 304 Stainless Steel
The physical properties of 304 stainless steel mainly include density, melting point, resistivity, and magnetic properties, etc. These parameters do not directly determine the material’s load-bearing capacity, but they will affect your judgment regarding the weight, dimensional stability, electrical characteristics, and material state during the design of the part.
Density
The density of 304 stainless steel is typically around 7.93 g/cm³. For you, density is mainly used to calculate the weight of the parts. If you have determined the volume of the part, you can estimate its theoretical weight using density: Weight = Volume × Density.
Compared to aluminum alloys, the density of 304 stainless steel is significantly higher, so in the same volume, the weight of 304 stainless steel parts will also be greater. If your design has requirements for weight, density is one of the factors you need to consider when choosing your material.
Melting Temperature
304 stainless steel does not have a completely fixed single melting point; instead, it gradually melts within a certain temperature range. Its melting range is typically around 1400–1450°C.
It is important to note that the melting temperature does not equal the long-term operating temperature of 304 stainless steel. Even if the part does not reach the melting temperature, its strength, hardness, dimensional stability, and corrosion resistance may change at higher temperatures. Therefore, if the CNC part you design needs to be exposed to high-temperature conditions for a long time, it is necessary to conduct an assessment based on the material’s performance at specific temperatures.
Electrical Resistivity
The resistivity of 304 stainless steel is typically around 0.72–0.75 μΩ·m at 20°C. Resistivity indicates the degree of obstruction that a material presents to the flow of electric current. Although 304 stainless steel is a metallic conductor, it is not a highly conductive material. What you need to consider beforehand is that if your CNC parts are responsible for conducting electricity, grounding, or current transmission simultaneously, the resistivity will affect the resistance and heat generation in your design.
Magnetic Properties
304 stainless steel belongs to austenitic stainless steel. In the annealed state, it usually exhibits non-magnetic or weak magnetic properties. However, this does not mean that all 304 stainless steel is completely non-magnetic under any circumstances.
After 304 undergoes cold processing, bending, stamping, or other plastic deformation, some austenite may undergo a phase transformation, resulting in a certain magnetic response. Therefore, if the designed part needs to meet strict magnetic performance requirements, one cannot simply judge based on the concept that 304 is a non-magnetic stainless steel. It is necessary to understand the material state and the processing process.
Thermal Properties of 304 Stainless Steel
It mainly includes thermal conductivity, thermal expansion coefficient, specific heat capacity, and high-temperature performance. These parameters will directly affect your heat dissipation capability, dimensional stability, and thermal deformation when designing parts under high temperature or temperature-change conditions.
Thermal Conductivity
The thermal conductivity of 304 stainless steel at room temperature is typically around 16 W/(m·K), which is significantly lower than that of aluminum and copper. Therefore, its thermal conductivity is relatively low. If the part you are designing is required to have a certain heat dissipation function, then 304 stainless steel is not suitable for your design needs.
Coefficient of Thermal Expansion
The linear thermal expansion coefficient of 304 stainless steel is typically around 17.2 × 10⁻⁶/K within the normal temperature range. The thermal expansion coefficient indicates the degree of size change when the material’s temperature changes.
When the temperature rises, 304 stainless steel usually expands; when the temperature drops, it contracts. If your CNC parts need to operate within a wide temperature range, you need to consider this size change. Especially when it comes to positioning accuracy or the combination of multiple different materials, the size change caused by thermal expansion may affect the normal operation of the parts.
Specific Heat Capacity
The specific heat capacity of 304 stainless steel is typically around 500 J/(kg·K) near room temperature. You can use the specific heat capacity to assess the temperature changes of the part during heating or cooling processes.
However, the actual temperature rise is also related to factors such as the part’s mass, the power of the heat source, the heat dissipation conditions, the surface area, and the ambient temperature. Therefore, the actual temperature of the part cannot be determined only based on the specific heat capacity.
Corrosion Resistance of 304 Stainless Steel
304 stainless steel has excellent corrosion resistance, mainly due to the high content of chromium in the material. When chromium reacts with oxygen in the air, a very thin protective film forms on the surface of the material, preventing the corrosive medium from continuing to react with the base material.
When choosing 304 stainless steel, you need to consider both the material’s own corrosion resistance and the actual working environment.
- General atmospheric environment: 304 usually has good corrosion resistance
- Wet environment: Generally can meet requirements, but it needs to be determined whether it is a long-term exposure environment
- Freshwater environment: Has good corrosion resistance
- Chloride environment: Increased risk of pitting and crevice corrosion
- Surface scratches or contamination: May increase the risk of local corrosion.
Machinability of 304 stainless steel
304 stainless steel has good plasticity and toughness. However, from the perspective of CNC machining, it is not a particularly easy-to-machine material. Understanding its processing characteristics can help you better consider the structure and processing requirements during the design and machining of the parts.
During the CNC machining of 304 stainless steel, it is easy to undergo work hardening, which increases the hardness and strength of the processed area and makes subsequent processing more difficult.
Therefore, when designing 304 stainless steel parts, it is necessary to take into account their characteristic of being prone to hardening during processing, and confirm with the manufacturer the appropriate processing techniques to ensure that the parts can be processed stably.
FAQ
How long does 304 stainless steel last?
The service life of 304 stainless steel is not fixed. In a normal indoor environment, if the materials, processing, and maintenance are done properly, 304 stainless steel parts can be used for several decades or even longer.
How does 304 stainless steel perform in high temperatures?
304 stainless steel can still maintain a certain level of strength and oxidation resistance in high-temperature environments. However, when used at high temperatures for a long period of time, its mechanical properties and corrosion resistance will decline.
Is 304 Stainless Steel Food Grade?
Yes, 304 stainless steel can typically be used in food contact applications, and thus is often referred to as “food-grade stainless steel”. However, strictly speaking, “304” by itself does not automatically equate to “food grade”. Whether it is suitable for food contact also depends on specific material standards, surface treatment, manufacturing processes, as well as the food contact regulations in the country or region where it is used.
Which one is better, 304 or 316?
304 stainless steel has excellent corrosion resistance, processing performance, and high cost-effectiveness, making it suitable for general industries, food processing, kitchen equipment, and regular mechanical parts. 316 stainless steel contains molybdenum (Mo), so it usually has better resistance to pitting corrosion and corrosion in environments with chlorine, marine environments, and some highly corrosive conditions.
Final Thoughts
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