Grade Definition and Product Standards
Type 316 is an austenitic chromium-nickel stainless steel containing 2 to 3 percent molybdenum. The addition raises resistance to pitting and crevice attack compared with the unalloyed 304 type, particularly in chloride-bearing and mildly reducing environments. Plate, sheet and strip are specified under ASTM A240/A240M, bar and shape products under ASTM A276/A276M, and the EN equivalents under EN 10088-2 for flat products.
The low-carbon variant, 316L, is defined by the same standards with a maximum carbon content of 0.030 percent. Because carbon is the element that drives chromium carbide precipitation in the 425 to 815 degrees C range, 316L is the grade normally selected for weld-fabricated equipment where post-weld solution annealing is not practical.
Chemical Composition and Dual Certification
The compositional window is the same for 316 and 316L except for carbon, which makes dual certification straightforward: a heat that meets the 0.030 percent carbon limit of 316L and the higher strength requirements of 316 can be certified to both grades on one certificate.
| Element | 316 max % | 316L max % |
|---|---|---|
| Carbon | 0.08 | 0.030 |
| Manganese | 2.00 | 2.00 |
| Phosphorus | 0.045 | 0.045 |
| Sulfur | 0.030 | 0.030 |
| Silicon | 0.75 | 0.75 |
| Chromium | 16.0-18.0 | 16.0-18.0 |
| Nickel | 10.0-14.0 | 10.0-14.0 |
| Molybdenum | 2.00-3.00 | 2.00-3.00 |
Nickel should be held at the upper half of the range when the material will be heavily cold formed, since nickel stabilizes the austenite and reduces the risk of strain-induced martensite and the associated magnetic response.
Mechanical Properties in the Annealed Condition
ASTM A240/A240M requires a minimum tensile strength of 515 MPa and a minimum yield strength of 205 MPa for 316 plate, with 40 percent minimum elongation in 50 mm and a maximum hardness of 95 HRB. The 316L variant is slightly softer, at 485 MPa minimum tensile and 170 MPa minimum yield, with the same elongation requirement. Both grades are non-magnetic in the annealed condition, but severe cold working can introduce a slight magnetic response that should not be mistaken for a material mix-up.
Corrosion Performance and Service Limits
The molybdenum content gives a pitting resistance equivalent number, calculated as Cr plus 3.3 Mo plus 16 N, of roughly 24 to 26 for the grade, against about 18 for the plain 304 type. This is the basis for using 316 in marine atmosphere, process water with moderate chloride content, and dilute organic acid service. The grade is not immune to chloride stress corrosion cracking, and it should not be used for hot concentrated chloride solutions above about 60 degrees C without a duplex or higher-alloy alternative. Intergranular corrosion resistance of welded 316L is normally verified by the oxalic acid etch screening of ASTM A262 Practice A, with the Strauss test of Practice E used as the confirming test where required.
Welding, Filler Selection and Fabrication
Weld 316 and 316L with a matching low-carbon filler classification such as ER316L or E316L, keeping heat input moderate and interpass temperature below 150 degrees C to limit carbide precipitation and sigma-phase formation. Matching 316L filler is preferred even when the base metal is dual-certified, since dilution of higher-carbon base metal into the weld pool raises the deposited carbon level. Austenitic stainless steel has roughly 50 percent higher thermal expansion and lower thermal conductivity than carbon steel, so it requires more tack welds and better restraint to control distortion. Passivate welds and heat-tint after fabrication, and keep carbon steel tooling off the surface to avoid iron contamination that appears later as rust staining.
Ordering and Inspection Points
Purchase orders should state the product standard, grade or dual grade, condition, thickness tolerance and surface finish, and should require a certificate showing heat analysis and tensile results. Confirm that molybdenum, chromium and nickel are reported on the certificate rather than covered by a generic statement, and check the carbon level for the intended welding process.
Frequently Asked Questions
Q: What is the difference between 316 and 316L stainless steel?
A: Only the carbon limit, 0.08 percent maximum for 316 and 0.030 percent maximum for 316L, plus the associated minimum strengths of 515 MPa and 485 MPa tensile respectively.
Q: Can 316L be dual certified as 316?
A: Yes, when the heat analysis meets the 0.030 percent carbon limit of 316L and the mechanical test results reach the higher strength levels required for 316.
Q: Why is molybdenum added to type 316?
A: Molybdenum raises pitting and crevice corrosion resistance in chloride environments, giving a pitting resistance equivalent number of about 24 to 26 against roughly 18 for type 304.
Q: Is 316 stainless steel magnetic?
A: In the annealed condition it is essentially non-magnetic; severe cold working such as heavy bending or drawing can produce a small magnetic response in deformed areas.
Q: Which filler metal should be used to weld 316L plate?
A: A matching low-carbon austenitic filler such as ER316L or E316L, with moderate heat input and interpass temperature kept below 150 degrees C.
Q: Can 316 be used for low-temperature service?
A: Yes. The austenitic structure retains good toughness at cryogenic temperatures, which is one reason the grade is used for liquefied gas piping and tanks where the design code permits it.