You are running a machine shop. Your customer needs precision parts made from stainless steel bar. You need to choose the right grade for machinability, corrosion resistance, and strength.
The most common stainless steel bars1 used in machining are 303 (free-machining)2, 304 (general purpose), 316 (marine grade), 410 (martensitic), and 17-4 PH (precipitation hardening). 303 is preferred for automatic screw machines because of its excellent machinability. 304 and 316 are used for parts requiring corrosion resistance. 410 is used for wear-resistant parts. 17-4 PH is used for high-strength applications.
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I have supplied stainless steel bars1 to machine shops for many years. A screw machine shop used 303 bar for high-volume fittings. A medical device manufacturer used 316 bar for surgical instruments. Let me walk you through the applications of stainless steel bars1 in the machining industry.
What are the most common stainless steel grades used in machining?
The most common stainless steel grades used in machining are 303, 304, 316, 410, and 17-4 PH. Each grade has different machinability, corrosion resistance, and strength.
303 is the best for machinability (free-machining grade). 304 is the standard grade for general parts. 316 is for marine and chemical parts. 410 is for wear-resistant parts that can be heat treated. 17-4 PH is for high-strength parts that need good corrosion resistance.
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Machining Grades Overview
Let me provide an overview of the most common machining grades.
| 303 Stainless Steel1 | Property | Value |
|---|---|---|
| Machinability | Excellent (best among austenitic) | |
| Corrosion resistance | Good (similar to 304) | |
| Strength | 205 MPa yield | |
| Best for | High-volume screw machine parts, fittings |
| 304 Stainless Steel | Property | Value |
|---|---|---|
| Machinability | Good | |
| Corrosion resistance | Excellent | |
| Strength | 205 MPa yield | |
| Best for | General machined parts, food equipment |
| 316 Stainless Steel2 | Property | Value |
|---|---|---|
| Machinability | Good (slightly lower than 304) | |
| Corrosion resistance | Excellent (marine) | |
| Strength | 205 MPa yield | |
| Best for | Marine parts, chemical, pharmaceutical |
| 410 Stainless Steel3 | Property | Value |
|---|---|---|
| Machinability | Good (in annealed condition) | |
| Corrosion resistance | Fair to good | |
| Strength | 205 MPa yield (heat treatable to higher) | |
| Best for | Wear-resistant parts, valves, shafts |
| 17-4 PH Stainless Steel4 | Property | Value |
|---|---|---|
| Machinability | Fair (in aged condition) | |
| Corrosion resistance | Good | |
| Strength | 1,000+ MPa yield (after heat treat) | |
| Best for | High-strength parts, aerospace, oil and gas |
Machinability Rating (Relative to 303 = 100%)
| Grade | Machinability Rating |
|---|---|
| 303 | 100% |
| 304 | 60-70% |
| 316 | 50-60% |
| 410 (annealed) | 60-70% |
| 17-4 PH (aged) | 30-40% |
My Experience
For a high-volume screw machine job, we supplied 303 bar. The customer reported 30% faster production than with 304.
Why is 303 stainless steel1 preferred for automatic screw machines2?
303 stainless steel1 is preferred for automatic screw machines2 because it contains added sulfur or selenium, which breaks chips into small pieces and reduces tool wear.
The free-machining additives (sulfur up to 0.15%) create chip breakers that prevent long, stringy chips from wrapping around the tool. This allows higher cutting speeds, longer tool life, and better surface finish. 303 machines at 150-200 surface feet per minute (SFM), while 304 machines at 80-120 SFM.
[^1] automatic screw machine parts](https://placehold.co/600x400 "303 Stainless Steel for Screw Machines")](https://cnsssheet.com/wp-content/uploads/2025/04/stainless-steel-bar-2.webp)
303 Stainless Steel Machining Advantages
Let me explain why 303 is the go-to grade for automatic screw machines2.
Machinability Comparison
| Grade | Cutting Speed (SFM) | Tool Life | Chip Formation |
|---|---|---|---|
| 303 | 150-200 | Excellent | Short, broken chips |
| 304 | 80-120 | Good | Long, stringy chips |
| 316 | 70-100 | Good | Long, stringy chips |
Typical Parts Made from 303 on Screw Machines
- Fittings and connectors
- Nuts and bolts
- Shafts and pins
- Valve components
- Hydraulic fittings
- Instrumentation components
Advantages of 303 for Screw Machines
- Higher cutting speeds (2× faster than 304)
- Longer tool life (less downtime for tool changes)
- Better surface finish
- No chip tangling (automatic operation)
- Lower production cost per part
Limitations of 303
- Lower corrosion resistance than 304 (due to sulfur inclusions)
- Not recommended for welding (sulfur causes hot cracking)
- Not suitable for marine or high-chloride environments
My Experience
A screw machine shop that switched from 304 to 303 for a fitting job reduced cycle time by 40% and tooling cost by 25%.
What are the key applications of 304 and 316 bars in machined parts?
304 and 316 stainless steel bars are used for machined parts that require corrosion resistance1. 304 is for general applications. 316 is for marine and chemical applications.
304 is used for food processing parts, pharmaceutical components, chemical fittings, and general industrial parts. 316 is used for marine hardware2, seawater system components, chemical plant parts, and pharmaceutical equipment3 where extra corrosion resistance1 is needed.
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Applications by Grade
Let me list the key applications for 304 and 316 machined parts.
304 Stainless Steel Machined Parts
| Application | Examples |
|---|---|
| Food processing | Pump components, valve bodies, fittings |
| Pharmaceutical | Equipment parts, connectors |
| Chemical | Fittings, flanges, adapters |
| General industrial | Shafts, bushings, pins, spacers |
| Water treatment | Pump shafts, valve stems |
316 Stainless Steel Machined Parts
| Application | Examples |
|---|---|
| Marine | Boat fittings, propeller shafts, seawater valves |
| Chemical (chlorides) | Pump housings, impellers, pipe fittings |
| Pharmaceutical (high purity) | Bioreactor components, sanitary fittings |
| Oil and gas | Instrumentation fittings, valve parts |
| Coastal infrastructure | Bridge components, handrail fittings |
Machinability Considerations
| Grade | Machinability | Recommended Cutting Speed (SFM) |
|---|---|---|
| 304 | Good | 80-120 |
| 316 | Good (slightly lower) | 70-100 |
Tooling Recommendations for 304/316
- Carbide tools with sharp edges
- Positive rake angles
- Copious coolant (flood or high-pressure)
- Reduce speed when work hardening occurs
My Experience
For a marine hardware2 manufacturer, we supplied 316 bar for propeller shaft components. The customer required the extra corrosion resistance1 for seawater service.
How to choose the right stainless steel bar for your machining project?
Choosing the right stainless steel bar for machining requires balancing machinability1, corrosion resistance2, strength, and cost.
Consider these factors: required machinability1 (303 is best, 304/316 are good, 17-4 is fair), corrosion resistance2 needed (304 for general, 316 for marine, 303 for dry environments), strength requirements3 (17-4 for high strength, 410 for wear resistance), and budget (303 and 304 are economical, 316 and 17-4 cost more).
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Selection Criteria
Let me provide a practical selection guide.
Step 1: Determine Corrosion Requirement
| Environment | Recommended Grade |
|---|---|
| Indoor, dry | 303, 304 |
| Outdoor, inland | 304 |
| Coastal, marine | 316 |
| Chemical (chlorides) | 316 |
| Food, pharmaceutical | 304, 316 |
Step 2: Determine Machinability Need
| Production Volume | Recommended Grade |
|---|---|
| High-volume automatic screw machine | 303 |
| Medium volume | 304 or 316 |
| Low volume, custom parts | 304 or 316 |
| Complex shapes, tight tolerances | 303 |
Step 3: Determine Strength Requirement
| Strength Need | Recommended Grade |
|---|---|
| Standard (200-300 MPa) | 303, 304, 316 |
| High (500-800 MPa) | 410 (heat treated) |
| Very high (800-1,200 MPa) | 17-4 PH |
Step 4: Consider Cost
| Grade | Relative Cost |
|---|---|
| 303 | 1.0× (base) |
| 304 | 1.0-1.1× |
| 410 | 0.9-1.0× |
| 316 | 1.4-1.6× |
| 17-4 PH | 2.0-2.5× |
Quick Selection Table
| If you need... | Choose... |
|---|---|
| Best machinability1 | 303 |
| General corrosion resistance2 + good machinability1 | 304 |
| Marine corrosion resistance2 | 316 |
| Wear resistance + heat treatable | 410 |
| High strength + corrosion resistance2 | 17-4 PH |
My Experience
For a customer making high-volume fittings for indoor use, we recommended 303 bar. For a customer making marine parts, we recommended 316 bar. The right choice saved them money and improved performance.
Conclusion
303 is best for automatic screw machines1 (high machinability). 304 and 316 are used for corrosion-resistant machined parts2. 410 is for wear-resistant parts. 17-4 PH is for high-strength parts. Choose based on machinability, corrosion resistance, and strength requirements.
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Discover the best materials for corrosion resistance to ensure durability and longevity in your projects. ↩ ↩ ↩ ↩ ↩ ↩ ↩ ↩ ↩ ↩ ↩ ↩ ↩
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Knowing the strength requirements is crucial for selecting the right stainless steel, ensuring your project meets performance standards. ↩ ↩ ↩
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Find out how 17-4 PH Stainless Steel offers high strength and corrosion resistance, making it ideal for aerospace. ↩
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