SUS316 vs SUS316L: Standard vs Low-Carbon Molybdenum Austenitic Stainless Steel

Chemical Composition per JIS G4303 and G4304

The two grades share the same alloy design except for the carbon limit, which is the entire basis of the distinction between them.

Equivalent designations: SUS316 = UNS S31600, EN 1.4401 (X5CrNiMo17-12-2), AISI 316; SUS316L = UNS S31603, EN 1.4404 (X2CrNiMo17-12-2), AISI 316L.

Mechanical Properties

Minimum annealed properties are shown below; the higher carbon of SUS316 gives it a small strength advantage.

The equivalent ASTM A240 minimums are 515/205 MPa for S31600 and 485/170 MPa for S31603, with 217 HBW maximum hardness for both. Both grades retain excellent toughness at cryogenic temperatures down to about -196°C, so the choice between them is rarely decided by low-temperature behavior.

Weldability and Intergranular Corrosion

When an unstabilized austenitic grade is heated in the 425-815°C range, carbon and chromium form chromium carbides at grain boundaries, depleting chromium and enabling intergranular corrosion. This happens in the heat-affected zone of welds and during service in that temperature range. SUS316, with 0.08% carbon, is susceptible to this sensitization, and for corrosive service its weldments must be solution annealed, which is impractical for large vessels and field joints. SUS316L, with 0.03% carbon, has so little carbon that chromium-carbide precipitation is effectively suppressed, so it resists intergranular corrosion in the as-welded condition and needs no post-weld heat treatment in most thicknesses. For welded construction in chloride, acidic or high-purity service, SUS316L is the standard choice; SUS316 suits non-welded parts such as forgings, valve bodies and machined components.

Corrosion Resistance and Service Temperatures

Both grades carry the same molybdenum, so their resistance to pitting, crevice corrosion and chloride stress corrosion in seawater, brine and chemical service is equivalent when the material is in the correctly heat-treated condition. The difference is confined to the welded state, where SUS316L keeps its resistance and SUS316 does not without annealing. Continuous service temperatures are commonly cited to about 870°C for lightly loaded parts in oxidizing atmospheres for both grades, with reduced strength at the top of the range; design above about 550°C should use creep data. In strongly reducing acids or very high chloride concentrations, neither grade is sufficient and a higher-alloy grade such as 904L or a 6% molybdenum super-austenitic should be considered.

Applications and Selection Guidance

SUS316 is preferred for non-welding or post-weld-annealable components in moderately corrosive service: forged valve bodies and stems, machined pump parts, shafting, fasteners, furnace components and architectural hardware. SUS316L is the default for welding-intensive fabrication in harsh environments: chemical reaction vessels, storage tanks and piping, seawater desalination plant, pharmaceutical and bioprocess equipment, marine engineering components, and food and beverage processing plant. The practical rule: if the part is welded and the environment is corrosive, use SUS316L; if the part is machined or forged, carries higher loads, and welding is absent, SUS316 offers a small strength and cost advantage.

Frequently Asked Questions

Q1: What is the only compositional difference between SUS316 and SUS316L? A1: Carbon content. SUS316 limits carbon to 0.08% maximum and SUS316L to 0.030% maximum; all other elements are identical.

Q2: Why is SUS316L preferred for welded equipment? A2: The low carbon content suppresses chromium-carbide precipitation in the weld heat-affected zone, so the welded joint resists intergranular corrosion without post-weld annealing. SUS316 weldments need solution annealing for severe corrosive service.

Q3: Is SUS316 stronger than SUS316L? A3: Slightly. Per JIS G4304, SUS316 has 520 MPa minimum tensile strength and 205 MPa proof stress versus 480 MPa and 175 MPa for SUS316L. The difference is small and rarely governs design.

Q4: Does SUS316L need post-weld heat treatment? A4: Not for corrosion resistance in most thicknesses. Very thick sections or service in the sensitization range may still warrant verification by intergranular corrosion testing per ASTM A262.

Q5: Which grade is more expensive? A5: SUS316L is typically 5-10% more expensive than SUS316 because of the tighter carbon control and more demanding refining. The premium is small relative to the welding and inspection costs it saves.

Q6: Can SUS316 replace SUS316L in welded service? A6: Only if the weldment can be solution annealed after welding. In large welded structures where annealing is impossible, the heat-affected zone of SUS316 risks intergranular corrosion and SUS316L must be used.