Choosing between 304 and 316 stainless steel is one of the most common material decisions in CNC machining. Both are austenitic stainless steels with excellent corrosion resistance, but they differ in composition, performance, and cost. Understanding these differences helps you specify the right material for your application without overpaying for properties you don't need.
This comparison covers the practical considerations we see when machining both alloys, including how material choice affects machining operations, lead times, and part cost.
Composition and Key Differences
The primary difference between 304 and 316 stainless steel is the addition of molybdenum in 316. This 2-3% molybdenum content significantly improves corrosion resistance, particularly against chlorides and acidic environments.
304 Stainless Steel composition:
- 18% chromium
- 8% nickel
- Maximum 0.08% carbon
- Iron and trace elements
316 Stainless Steel composition:
- 16-18% chromium
- 10-14% nickel
- 2-3% molybdenum
- Maximum 0.08% carbon
- Iron and trace elements
The molybdenum addition makes 316 more expensive than 304, typically 20-40% higher in raw material cost depending on market conditions. This cost difference carries through to the finished machined part.
Corrosion Resistance
Both alloys resist corrosion well in typical atmospheric conditions, but they perform differently in specific environments.
304 Stainless Steel
304 provides excellent corrosion resistance in most indoor environments and many outdoor applications. It performs well in:
- Food processing equipment
- Kitchen and restaurant equipment
- Architectural trim and fasteners
- General industrial equipment
- Mild chemical environments
304 can experience pitting and crevice corrosion when exposed to chlorides, salt water, or certain chemicals. It's not the best choice for marine environments or applications involving regular exposure to de-icing salts or pool chemicals.
316 Stainless Steel
The molybdenum content in 316 provides superior resistance to pitting and crevice corrosion, especially in chloride environments. Common applications include:
- Marine hardware and boat fittings
- Chemical processing equipment
- Pharmaceutical manufacturing equipment
- Medical devices and surgical instruments
- Coastal architectural applications
- Parts exposed to de-icing salts
If your parts will see regular exposure to salt water, chlorides, or aggressive chemicals, 316 is typically the safer choice. For most other applications, 304 offers adequate corrosion resistance at lower cost.
Machinability Considerations
From a machining perspective, both 304 and 316 are work-hardening austenitic stainless steels. They machine similarly but present some differences in the shop.
Cutting Characteristics
Both alloys work-harden rapidly during cutting, which means they become harder at the surface as you machine them. This requires sharp tooling, proper speeds and feeds, and adequate chip load to cut below the work-hardened layer.
304 is slightly easier to machine than 316. The molybdenum in 316 increases its toughness, which can lead to:
- Slightly longer cycle times
- Increased tool wear
- More stringy chips that require management
- Higher cutting forces
Both materials machine effectively with carbide tooling, typically uncoated or TiAlN-coated grades designed for stainless steel. Surface speeds generally run in the 200-400 SFM range depending on the operation and tooling, with feed rates adjusted to maintain sufficient chip load—critical for cutting beneath the work-hardened surface rather than burnishing it.
Chip control deserves attention with both alloys. The austenitic structure produces long, stringy chips that can tangle around tooling or workpieces. This becomes particularly challenging in deep hole drilling or grooving operations where chip evacuation is restricted. Through-spindle coolant or peck drilling cycles help break chips and clear them from the cut zone. The slightly greater toughness of 316 can make its chips even more problematic, sometimes requiring more aggressive chip-breaking strategies or tool geometries.
The machinability difference is noticeable but manageable with proper tooling and parameters. The material cost difference is usually more significant than any machining cost difference for most parts.
Surface Finish
Both alloys can achieve excellent surface finishes when machined properly. The work-hardening tendency means that dull tools or improper feeds can burnish the surface rather than cut it, leading to poor finishes and dimensional issues.
For parts requiring very fine surface finishes, the machining approach matters more than the choice between 304 and 316. Proper tooling, coolant, and cutting parameters produce good results with either material.
Mechanical Properties
The mechanical properties of 304 and 316 are similar in their annealed condition, though 316 shows higher hardness and slightly better strength in typical production material.
ASTM minimum properties for annealed condition:
304 Stainless Steel:
- Tensile strength: 75,000 psi minimum
- Yield strength: 30,000 psi minimum
- Elongation: 40% minimum
- Hardness: 201 HB maximum
316 Stainless Steel:
- Tensile strength: 75,000 psi minimum
- Yield strength: 30,000 psi minimum
- Elongation: 40% minimum
- Hardness: 217 HB maximum
While ASTM minimum specifications show identical tensile and yield strength values, the higher maximum hardness specification for 316 reflects its greater strength potential. In practice, typical mill-certified material often shows 316 with tensile strengths in the 80,000-85,000 psi range compared to 75,000-80,000 psi for 304, though both easily exceed the minimum requirements.
The molybdenum content in 316 also provides better creep resistance and strength retention at elevated temperatures, though for most room-temperature structural applications, the mechanical properties are close enough that the choice comes down to corrosion resistance requirements rather than strength. Neither alloy is typically chosen primarily for high strength—if that's the requirement, other stainless grades or materials may be more appropriate.
Cost Considerations
Material cost is often the deciding factor when corrosion resistance requirements don't clearly favor one alloy over the other.
316 stainless steel costs more than 304 due to its nickel and molybdenum content. The exact price difference fluctuates with commodity markets, but 316 typically runs 20-40% more than 304 for raw material.
For machined parts, this material cost difference flows through to the finished part price, though the impact varies based on the part's material-to-labor ratio. Consider these examples:
Low material content part: A simple bracket machined from small bar stock where material represents 20% of total cost. If 316 material costs 30% more than 304, the finished part price increases by about 6% (20% × 30% = 6%).
High material content part: A large manifold block where material represents 60% of total cost. With the same 30% material premium, the finished part price increases by about 18% (60% × 30% = 18%).
The cost difference becomes more significant in production quantities. A few prototype parts might see a negligible dollar difference, but production runs of hundreds or thousands of parts can show substantial cost savings with 304 if it meets the application requirements.
Specifying Material on Your RFQ
When requesting a quote for CNC machined stainless steel parts, clear material specification helps ensure accurate pricing and avoids delays.
Material Callout
Specify the exact grade on your drawing or in your RFQ notes. Common callouts include:
- 304 stainless steel
- 316 stainless steel
- ASTM A276 304 (bar stock)
- ASTM A479 316 (bar stock)
- 304L or 316L (lower carbon versions)
If your application has specific requirements—such as marine-grade 316 or food-grade 304—note that in your RFQ. Some customers require material certifications or test reports, which should be specified upfront as they affect pricing and lead time.
When the Choice Isn't Clear
If you're uncertain which alloy best suits your application, provide information about the operating environment:
- Indoor or outdoor use
- Exposure to chemicals, salt water, or chlorides
- Temperature range
- Required service life
- Budget constraints
A machine shop can often provide guidance based on similar parts they've produced, but the final material selection remains an engineering decision. The drawing typically serves as the controlling specification, so material choice should be confirmed before finalizing drawings.
Prototype vs Production Considerations
Material choice can affect both prototype and production runs differently.
Prototype Quantities
For prototype or low-quantity orders, the material cost difference between 304 and 316 is often small in absolute dollars. A prototype run might use 316 to test worst-case corrosion resistance, then switch to 304 for production if testing shows it's adequate.
Conversely, prototyping with 304 and switching to 316 for production is also common when the application environment becomes better defined during development.
Lead time for prototype quantities is usually similar for both alloys, as both are commonly stocked materials.
Production Quantities
Production runs make material cost differences more significant. The impact depends on the part's material content and the raw material price premium at the time of order.
For production, it's worth carefully evaluating whether 316's enhanced corrosion resistance is necessary, or whether 304 provides adequate performance at lower cost. Some customers run accelerated corrosion testing on prototype parts to validate material choice before committing to production quantities.
Making the Right Choice
Choosing between 304 and 316 stainless steel comes down to matching material properties to application requirements.
Choose 304 when:
- The environment is primarily indoor or mild outdoor
- Chloride exposure is minimal
- Cost is a significant factor
- Corrosion resistance requirements are moderate
Choose 316 when:
- Parts will be exposed to marine environments
- Chlorides or aggressive chemicals are present
- Enhanced corrosion resistance justifies the cost premium
- The application requires the highest reliability
Both materials machine well and are readily available. The choice rarely comes down to machinability—it's almost always about corrosion resistance versus cost.
Request a Quote for Your Stainless Steel Parts
Anco Precision machines both 304 and 316 stainless steel parts for prototype and production quantities. We work from your drawings or CAD files to produce precision parts to your specifications.
To request a quote, send us your engineering drawing or STEP file along with your material specification (304 or 316 stainless steel), quantity needed, critical tolerances or special requirements, and your desired delivery date. We'll review your requirements and provide a quote.
Contact Anco Precision to discuss your stainless steel machining project.