317 vs 317L Stainless Steel: Carbon Limits, Composition and Welded Corrosion Performance

317 vs 317L: One Alloy Family, Two Carbon Limits

Type 317 and Type 317L are molybdenum-bearing austenitic stainless steels developed for service conditions that overwhelm 304 or 316. Both carry the same chromium, nickel and molybdenum ranges, and the only meaningful difference between them is the maximum carbon content. Type 317 permits up to 0.08% carbon, while Type 317L is capped at 0.030%. Carbon is the element that drives chromium carbide precipitation along grain boundaries in the heat-affected zone of a weld, so the low-carbon variant keeps its full corrosion resistance in the as-welded condition, while the standard grade is normally reserved for parts that are not welded.

Chemical Composition Compared

The ranges below follow ASTM A240 for plate, sheet and strip. The corresponding European designations are EN 1.4449 for 317 and EN 1.4438 for 317L. All values are weight percent.

Element	317	317L
Carbon (C)	0.08 max	0.030 max
Chromium (Cr)	18.0-20.0	18.0-20.0
Nickel (Ni)	11.0-15.0	11.0-15.0
Molybdenum (Mo)	3.0-4.0	3.0-4.0
Manganese (Mn)	2.00 max	2.00 max
Silicon (Si)	0.75 max	0.75 max
Phosphorus (P)	0.045 max	0.045 max
Sulfur (S)	0.030 max	0.030 max
Nitrogen (N)	0.10 max	0.10 max

The 3-4% molybdenum addition is what lifts pitting resistance well above that of 316L. Using the pitting resistance equivalent number, PREN = Cr + 3.3 Mo + 16 N, both grades fall in the range of roughly 33 to 35, against about 24 to 26 for 316L. That gap explains why these grades are selected for chloride-rich process streams rather than for general atmospheric exposure.

Corrosion Resistance and Welded Service

In the annealed condition the two grades behave almost identically in chloride and dilute acid service. The difference appears once a structure is welded:

Intergranular corrosion: Type 317L resists sensitisation because its carbon level stays below the point at which chromium carbides form at grain boundaries. Type 317 can be sensitised in the heat-affected zone unless the welded assembly receives a solution anneal.

Pitting and crevice corrosion: Both grades remain resistant in moderately concentrated chlorides at ambient to intermediate temperatures, and both can still pit under deposits or in tight crevices at elevated temperature.

Stress corrosion cracking: Neither grade is immune under hot concentrated chloride conditions; the higher nickel content of these grades only widens the safe operating window modestly.

Acid service: Molybdenum improves performance in sulfuric and phosphoric acid solutions at moderate concentration, but 317L is not a substitute for a nickel-chromium-molybdenum alloy in severe acid duty.

Mechanical Properties

Room-temperature requirements for both grades in ASTM A240 plate are the same, with Type 317 tending to sit slightly higher in tensile strength in practice because of its higher carbon content.

Property	317	317L
Tensile strength, min	515 MPa	515 MPa
Yield strength (0.2%), min	205 MPa	205 MPa
Elongation in 50 mm, min	40%	40%
Hardness, max	95 HRB	95 HRB
Typical carbon content	Up to 0.08%	Up to 0.030%

Both grades are austenitic, non-magnetic in the annealed state, and have comparable modulus of elasticity and thermal expansion, so design calculations rarely distinguish between them.

Fabrication and Welding Practice

Type 317L is the more forgiving grade to weld. No post-weld heat treatment is required to restore corrosion resistance, and matching low-carbon filler metal keeps the weld metal in step with the base metal. For Type 317, welded joints should be solution annealed where the service environment is aggressive. Both grades work harden quickly during cold forming, so bending and flanging schedules should use generous radii and intermediate anneals. Machining is easier in the annealed condition than after cold work, and heavy cuts should be avoided in favour of light passes with adequate coolant.

Applications and Grade Selection

Choose Type 317 for non-welded components that need slightly higher strength at elevated temperature, such as machined valve trim, pump internals and heat exchanger components produced without welding. Choose Type 317L for welded construction: storage and process tanks, flue gas desulfurisation absorber internals, pulp and paper digester components, offshore platform piping, and industrial waste treatment equipment where post-weld corrosion resistance determines service life. In short, if the part is welded and exposed to chlorides or dilute acids, specify the low-carbon grade.

Frequently Asked Questions

Q: Is 317L stronger than 317?
No. ASTM A240 sets the same minimum tensile and yield strength for both grades, and any small difference in a mill certificate reflects carbon content and processing rather than a design advantage.

Q: Can 317 be substituted for 317L?
Only for non-welded parts in mild service. In welded construction the higher carbon of 317 risks sensitisation of the heat-affected zone, which defeats the purpose of specifying a molybdenum-bearing grade.

Q: Do 317 and 317L need post-weld heat treatment?
317L does not normally need it to retain corrosion resistance. Type 317 welded joints benefit from a solution anneal in aggressive service because the treatment redissolves chromium carbides.

Q: What is the PREN of 317L?
Applying PREN = Cr + 3.3 Mo + 16 N to the specified composition gives approximately 33 to 35, which is roughly ten points above 316L and explains its better pitting performance.

Q: Can 317L handle seawater?
It is not recommended for continuous seawater immersion. Brackish water, cooling water and moderate chloride process streams are realistic applications; quiescent seawater with crevices still requires a higher-alloy grade.

Q: Are 317 and 317L magnetic?
Both are austenitic and essentially non-magnetic in the annealed condition. Cold working such as bending or thread rolling can introduce a slight magnetic response.