317L vs 316L Stainless Steel: Molybdenum Content, PREN and Grade Selection

Two Low-Carbon Austenitic Grades Compared

317L and 316L are both low-carbon, molybdenum-bearing austenitic stainless steels. The difference is one of degree: 317L carries higher minimum chromium, nickel and molybdenum contents than 316L, which raises its pitting resistance equivalent number and widens its tolerance for chlorides, low pH and elevated temperature. Mechanical behaviour, weldability and formability are otherwise closely matched, so the selection decision is almost always driven by the corrosivity of the service environment rather than by strength.

Chemical Composition

Composition limits below follow ASTM A240 for plate, sheet and strip. The European equivalents are EN 1.4404 for 316L and EN 1.4438 for 317L. Values are weight percent.

Element	316L	317L
Carbon (C)	0.030 max	0.030 max
Chromium (Cr)	16.0-18.0	18.0-20.0
Nickel (Ni)	10.0-14.0	11.0-15.0
Molybdenum (Mo)	2.00-3.00	3.00-4.00
Manganese (Mn)	2.00 max	2.00 max
Silicon (Si)	0.75 max	0.75 max
Nitrogen (N)	0.10 max	0.10 max

Applying PREN = Cr + 3.3 Mo + 16 N gives roughly 24 to 26 for 316L and 33 to 35 for 317L. That gap of about ten points is the quantitative basis for the upgrade, and it translates directly into a longer time to pitting initiation in chloride service.

Corrosion Resistance in Aggressive Service

317L shows a decisive advantage in environments that combine higher chloride concentration, lower pH and elevated temperature:

Chloride and sulfate process streams: 316L may pit or suffer crevice attack over time, while 317L keeps a larger safety margin.

Acidic condensates: Flue gas desulfurisation absorbers, ductwork and mist eliminators operate in low pH condensates where 317L is preferred.

Moderate phosphoric and sulfuric acid: The higher molybdenum content of 317L improves performance at intermediate concentration and temperature.

Pulp and paper digesters: White liquor and chlorine dioxide stages are typical 317L duties.

Not a substitute for nickel alloys: In hot concentrated acid or continuous seawater immersion, both grades reach their limits and a higher-alloy material is needed.

Mechanical Properties

Property	316L	317L
Tensile strength, min	485 MPa	515 MPa
Yield strength (0.2%), min	170 MPa	205 MPa
Elongation in 50 mm, min	40%	40%
Hardness, max	95 HRB	95 HRB

Both grades are austenitic, non-magnetic when annealed, and offer good toughness and formability. 317L work hardens slightly faster during cold forming, which can call for more power and intermediate annealing on deep-drawn parts.

Fabrication and Welding

Fabrication practice is nearly identical for the two grades. Both are welded with matching low-carbon filler metal, typically 316L or 317L wire, and neither requires post-weld heat treatment to retain corrosion resistance. Because 317L is more heavily alloyed, it needs closer attention to shielding gas coverage and interpass temperature control so that the weld pool does not oxidise or segregate alloying elements. Cleanliness before welding matters for both grades, since embedded iron, marking inks and chlorinated solvents cause premature pitting that is often mistaken for a material failure.

When the Upgrade Pays Back

Upgrading from 316L to 317L is justified when the cost of a foreseeable failure exceeds the material premium. Production downtime, environmental cleanup, product contamination and unplanned component replacement usually dwarf the difference in plate or pipe price. The clearest cases are new equipment in a known aggressive environment and existing 316L units that corrosion monitoring shows operating at the edge of their capability. For standard seawater systems with adequate flow, mild chemical service and food or pharmaceutical processing, 316L remains the economical and correct choice.

Frequently Asked Questions

Q: What is the main difference between 317L and 316L?
317L contains higher chromium, nickel and molybdenum than 316L, roughly 18-20% Cr and 3-4% Mo against 16-18% Cr and 2-3% Mo, which raises its pitting resistance equivalent to about 33-35.

Q: Is 317L stronger than 316L?
Slightly. ASTM A240 requires a minimum tensile strength of 515 MPa and yield strength of 205 MPa for 317L, against 485 MPa and 170 MPa for 316L.

Q: When should 317L be specified instead of 316L?
When the service combines elevated chlorides, low pH and higher temperature, or when corrosion monitoring shows 316L operating near its limit in an existing unit.

Q: Is 317L more difficult to weld?
Not fundamentally, but its higher alloy content demands better shielding gas coverage and tighter interpass temperature control to preserve corrosion resistance in the weld zone.

Q: Can 317L be used in seawater?
It suits brackish water and flowing cooling water circuits, but continuous seawater immersion with crevices exceeds its capability and requires a higher-alloy grade.

Q: Which grade is better value?
316L costs less and covers most general corrosion duties; 317L is worth the premium when downtime and replacement costs from premature pitting would exceed the material price difference.