31603 vs 31703 Stainless Steel: Low-Carbon Molybdenum Boost

What the 31603 and 31703 Designations Mean

31603 and 31703 are the UNS numbers for two low-carbon austenitic stainless steels that the market usually calls 316L (UNS S31603, EN 1.4404) and 317L (UNS S31703, EN 1.4438). The "03" in each number signals the carbon limit of 0.030% maximum. That low carbon level is why both grades can be welded without a post-weld solution anneal and still resist intergranular corrosion in the heat-affected zone. The two alloys belong to the same austenitic 18-8 family, but 31703 carries roughly 50% more molybdenum and a higher chromium range, and that is where the performance gap starts.

Chemical Composition Compared

Element	31603 / 316L (UNS S31603)	31703 / 317L (UNS S31703)
Carbon (C), max	0.030%	0.030%
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), max	2.00%	2.00%
Silicon (Si), max	0.75%	0.75%
Nitrogen (N), max	0.10%	0.10%
Typical PRE = Cr + 3.3 Mo + 16 N	about 25	about 31

The pitting resistance equivalent number is the fastest way to compare the two grades. Using the common formula Cr + 3.3 x Mo + 16 x N, a mid-range 316L chemistry lands near 25, while a mid-range 317L chemistry reaches about 31. That six-point gap is what allows 31703 to survive chloride levels and acidic conditions that would eventually pit 31603.

Corrosion Resistance in Real Service

31603 handles coastal atmosphere, seawater splash zones, food and beverage processing, pharmaceutical clean-in-place chemistry and dilute industrial fluids. Its 2-3% molybdenum addition makes it far more tolerant of pitting and crevice corrosion than 304L, which is why it became the default alloy for wet, chloride-bearing duty.

31703 was developed for harder duties: sulfuric acid at moderate concentration and temperature, phosphoric acid, pulp and paper bleach liquors, high-chloride process streams and flue-gas desulfurisation systems. The higher chromium and molybdenum content gives more tolerance for reducing acids and heavy chloride loading, and it holds up where 31603 would show early pitting or crevice attack. Even so, 31703 is not a universal acid alloy. Actual performance still depends on acid concentration, temperature, aeration and flow velocity, so the final grade is confirmed against the real process medium rather than assumed from a data sheet.

Mechanical and Fabrication Properties

Tensile strength: minimum 515 MPa for both grades in the annealed condition, as required by ASTM A240 for plate and ASTM A312 for pipe.

Yield strength (0.2% offset): about 205 MPa for 31603, slightly higher for 31703 because the extra molybdenum and chromium raise the annealed yield point.

Elongation: 40% minimum in 50 mm, so both grades bend, roll, stretch-form and deep-draw without difficulty.

Weldability: both are welded with matching low-carbon filler metal using GTAW, GMAW, SMAW or SAW practice. 31703 needs no special technique and no post-weld heat treatment; heat input is simply controlled to avoid hot cracking.

Forming load: 31703 work-hardens a little faster and needs slightly higher forces, but tooling and springback compensation stay close to 31603.

Cost, Grade Selection and Applications

The molybdenum surcharge is the main cost driver. Because molybdenum is the most expensive of the major alloying elements, 31703 typically costs 15-20% more than 31603 for the same product form and size. A mill certificate should always be checked for the actual heat chemistry, since the permitted composition ranges of the two grades are adjacent and only the reported analysis shows what was really supplied.

Choose 31603 when the service environment is marine atmosphere, potable water, food, pharmaceutical or general chemical duty with moderate chloride levels. Move to 31703 when the process stream carries high chlorides or sulfuric and phosphoric acid, or when the design life makes a corrosion-related shutdown more expensive than the alloy premium. A practical rule: if a 31603 installation shows pitting or staining inside its expected service interval, uprating to 31703 is normally the cheapest remedy before considering any higher-alloy alternative.

Choosing between the two grades becomes concrete once the product forms are matched to the duty:

31603: marine hardware and fittings, desalination piping, pharmaceutical and dairy tanks, food processing lines, architectural and exterior components, laboratory and surgical equipment.

31703: sulfuric acid storage and handling, chemical reactor internals, pulp and paper digester and bleach plant equipment, offshore and coastal structures exposed to aggressive chloride attack, and flue-gas desulfurisation ductwork.

Frequently Asked Questions

Q: Is 31603 the same as 316L?
Yes. 31603 is the UNS number S31603 and 316L is the commercial name for the same low-carbon 16-18% Cr, 10-14% Ni, 2-3% Mo austenitic stainless steel supplied to ASTM A240, A276 or A312.

Q: Is 31703 the same as 317L?
Yes. UNS S31703 is the low-carbon version of 317 stainless steel and is sold as 317L, with nominally 18-20% Cr, 11-15% Ni and 3-4% Mo.

Q: Why is 31703 15-20% more expensive than 31603?
Both grades are priced from the same base plus alloy surcharges, and molybdenum carries the highest surcharge of the main alloying elements. 31703 contains about one percentage point more molybdenum plus extra chromium and nickel, and that difference flows straight into the selling price.

Q: Can 31603 and 31703 be welded to each other?
Yes. Dissimilar joints between the two grades are common and are made with 317L or 316L low-carbon filler metal; no post-weld heat treatment is required for either alloy.

Q: Which grade should be used for seawater piping?
31603 is widely accepted for splash zone, intermittent seawater and coastal atmosphere duty. Continuously flowing chlorinated seawater with high chloride concentration and low flow velocity favours 31703 because its higher PRE delays pitting and crevice initiation.

Q: Are mechanical properties the deciding factor between the two?
No. Tensile strength is 515 MPa minimum for both, and ductility is comparable. Corrosion resistance in the specific medium, not strength, is what normally decides between 31603 and 31703.