Introduction
If you’ve worked with 316L stainless steel, you know it’s the gold standard for corrosion resistance in many industries. But what happens when you need even more resistance to pitting and crevice corrosion, especially in highly aggressive chemical environments? Enter 317L. This low-carbon, molybdenum-rich austenitic stainless steel takes corrosion resistance to the next level. In this article, I’ll explain the key differences between 317L and 316L, including chemistry, mechanical properties, and how to machine it effectively.
Chemical Composition
The primary difference between 317L and 316L is the molybdenum content. 317L contains 3-4% molybdenum, compared to 2-3% in 316L. This extra molybdenum significantly enhances resistance to chloride-induced pitting and crevice corrosion. Here’s the typical composition per ASTM A240:
| Element | 317L (%) | 316L (%) |
|---|---|---|
| Carbon (C) | 0.030 max | 0.030 max |
| Manganese (Mn) | 2.00 max | 2.00 max |
| Phosphorus (P) | 0.045 max | 0.045 max |
| Sulfur (S) | 0.030 max | 0.030 max |
| Silicon (Si) | 0.75 max | 0.75 max |
| Chromium (Cr) | 18.0 – 20.0 | 16.0 – 18.0 |
| Nickel (Ni) | 11.0 – 15.0 | 10.0 – 14.0 |
| Molybdenum (Mo) | 3.0 – 4.0 | 2.0 – 3.0 |
| Nitrogen (N) | 0.10 max | 0.10 max |
The low carbon content (0.03% max) in both grades prevents sensitization during welding, reducing the risk of intergranular corrosion. However, 317L’s higher molybdenum and chromium give it a significantly higher Pitting Resistance Equivalent Number (PREN = %Cr + 3.3%Mo + 16%N). For 317L, PREN is typically 30-35, compared to 24-28 for 316L.
Mechanical Properties
317L offers slightly higher strength than 316L due to its alloy content. Typical mechanical properties in the annealed condition are:
| Property | 317L | 316L |
|---|---|---|
| Tensile Strength (ksi) | 85 – 110 | 75 – 95 |
| Yield Strength (0.2% offset, ksi) | 35 – 50 | 30 – 45 |
| Elongation in 2″ (%) | 40 – 55 | 40 – 50 |
| Hardness (Rockwell B) | 85 – 95 | 80 – 90 |
| Modulus of Elasticity (psi x 10^6) | 28 | 28 |
317L also has better creep resistance at elevated temperatures, making it suitable for service up to 1700°F (925°C) in oxidizing atmospheres.
Typical Applications
317L is specified where 316L just isn’t enough. Common applications include:
- Chemical processing equipment – Reactors, heat exchangers, and piping for sulfuric acid, phosphoric acid, and chloride-containing media.
- Pulp and paper industry – Bleaching equipment and digesters where chlorine dioxide is used.
- Pharmaceutical manufacturing – Vessels and piping for aggressive drug intermediates.
- Marine environments – Seawater-cooled heat exchangers and desalination plant components.
- Flue gas desulfurization (FGD) systems – Scrubbers and ductwork in power plants.
CNC Machining Guide
317L is more challenging to machine than 316L due to its higher strength and work-hardening rate. Here are my recommended parameters for CNC machining:
| Operation | Tool Material | Speed (SFM) | Feed (IPR) | Depth of Cut (in) |
|---|---|---|---|---|
| Turning (rough) | Carbide (C2 grade, TiAlN coated) | 200 – 350 | 0.008 – 0.015 | 0.080 – 0.150 |
| Turning (finish) | Carbide (C2 grade, TiAlN coated) | 300 – 450 | 0.003 – 0.007 | 0.010 – 0.040 |
| Milling (rough) | Carbide (TiAlN coated) | 180 – 300 | 0.003 – 0.006 (IPT) | 0.040 – 0.100 |
| Milling (finish) | Carbide (TiAlN coated) | 250 – 400 | 0.002 – 0.004 (IPT) | 0.010 – 0.030 |
| Drilling | Carbide (with coolant-through) | 100 – 180 | 0.002 – 0.005 | N/A |
Key tips: Use rigid setups to minimize vibration. High-pressure coolant (800+ psi) is highly recommended for chip evacuation. For tapping, consider thread milling instead of conventional tapping to reduce torque and breakage risk. Always use sharp tools—dull tools will work-harden the surface instantly.
Quality Control
At Dongguan Stirling Metal Products Co., Ltd., we ensure your 317L parts meet the highest standards. Our QC includes:
- PMI (Positive Material Identification) – XRF verification of molybdenum content (3.0-4.0%).
- Corrosion testing – ASTM G48 Method A (ferric chloride pitting test) to verify PREN.
- Dimensional inspection – CMM and laser scanning for complex geometries.
- Weld quality – Dye penetrant or X-ray inspection for welded assemblies.
Conclusion
317L stainless steel is the upgrade when 316L reaches its limits. Its higher molybdenum content provides superior resistance to pitting and crevice corrosion in aggressive environments. While it’s more challenging to machine, the performance gains in chemical, marine, and pharmaceutical applications are well worth the effort. If your project demands the best corrosion resistance in an austenitic stainless, 317L is the answer.
Dongguan Stirling Metal Products Co., Ltd. specializes in providing 317L material + CNC machining One-Stop Service:
- ✅ Material procurement: Genuine product guarantee, complete material certificates
- ✅ CNC machining: Precision ±0.01mm, smooth surface finish
- ✅ Sample production: Fast delivery in 3-5 days
- ✅ Batch production: Delivery time of 7-15 days
For a Free Quote, please feel free to contact us!
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