AISI 316

What is AISI 316?

AISI 316 is a molybdenum-bearing austenitic stainless steel. It contains 16.0 to 18.0 percent chromium, 10.0 to 14.0 percent nickel and 2.0 to 3.0 percent molybdenum. The molybdenum stabilises the passive film against chloride ions, which can cause local film breakdown and initiate pitting. This addition is the basis for the common marine grade description.

AISI 316 contains slightly less chromium and slightly more nickel than AISI 304. The additional nickel offsets the ferrite-forming effect of the chromium and molybdenum changes and maintains a fully austenitic structure. Its forming, welding and cleaning characteristics resemble those of 304. It cannot be hardened by heat treatment and has a steep work-hardening curve.

Chloride tolerance is the principal difference from AISI 304. Atlas Steels rates 316 as resistant to pitting in potable water containing up to about 1,000 mg per litre of chlorides at ambient temperature. The corresponding figure for 304 is about 200 mg per litre. This is a factor of five for a molybdenum addition of two to three percent.

Laxcon Steels lists AISI 316 in its grades reference as an austenitic grade. The same steel is written 316, SS 316, SUS 316, 1.4401 and S31600.

What is the chemical composition of AISI 316?

Composition limits in weight percent are carbon 0.08 maximum, manganese 2.0 maximum, sulphur 0.03 maximum, phosphorus 0.045 maximum, silicon 0.75 maximum, chromium 16.0 to 18.0, nickel 10.0 to 14.0, molybdenum 2.0 to 3.0 and nitrogen 0.1 maximum.

What is AISI 316 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of AISI 316?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties of AISI 316

ASTM A276 places 316 and 304 in the same Condition A category. Hot-finished annealed bar must have a tensile strength of 515 MPa, a yield strength of 205 MPa at 0.2 percent offset, an elongation of 40 percent in 50 mm and a reduction of area of 50 percent. ASTM A240 specifies the same tensile and yield strengths for plate. It permits a hardness limit of 95 HRB or 217 HB, which is slightly higher than the limit for 304 and reflects solid-solution strengthening from molybdenum.

Cold-finished Condition A bar has specified tensile and yield strengths of 620 MPa and 310 MPa up to 12.70 mm. Above this size, the values return to 515 MPa and 205 MPa. ASTM A276 Condition B strain-hardened bar provides higher guaranteed strength without a grade change. It reaches 860 MPa tensile strength and 690 MPa yield strength up to 19.05 mm diameter. The requirements then decrease across five size bands to 655 MPa tensile strength and 310 MPa yield strength between 38.10 and 44.45 mm. The strength of 316 therefore depends on the amount of cold work and the section diameter.

The density is about 8,000 kg per cubic metre. The molybdenum makes 316 marginally denser than 304. Its elastic modulus is 193 GPa. It retains excellent toughness to cryogenic temperatures and has no transition.

Corrosion behaviour of AISI 316

The pitting resistance equivalent number of AISI 316 ranges from 22.6 at the specified composition minima to 29.5 at the maxima. The calculation uses chromium plus 3.3 times molybdenum plus 16 times nitrogen. Molybdenum accounts for about one-third of the minimum value.

Atlas Steels rates 316 as resistant to pitting in potable water containing up to about 1,000 mg per litre of chlorides at ambient temperature. The limit falls to about 500 mg per litre at 60 degrees Celsius. Seawater contains roughly 19,000 mg per litre of chloride, an order of magnitude more. AISI 316 is widely and successfully used in marine atmospheres and splash zones, but it is not fully resistant to seawater immersion. Many marine environments produce surface corrosion as brown staining, especially at crevices and on rough surface finishes.

Chloride stress corrosion cracking can occur above about 60 degrees Celsius when tensile stress is present. Molybdenum provides little improvement over 304 against this mechanism. Exposure between 425 and 860 degrees Celsius can cause sensitisation through chromium carbide precipitation, as in 304. Heavy welded sections of standard 316 require post-weld solution annealing to restore full corrosion resistance.

Processing of AISI 316

Solution annealing consists of heating to 1,010 to 1,120 degrees Celsius followed by rapid cooling. AISI 316 has no hardening heat treatment. All standard fusion welding processes can be used with matching 316 or 316L consumables. For joints intended for hot rather than wet service, 316Ti is often a better parent metal than a 316 fabrication that requires post-weld annealing.

Machining AISI 316 is slower than machining 304. Molybdenum increases hot hardness, and the chip remains tough. Cutting speeds are typically ten to twenty percent lower than for an equivalent 304 operation. Processing requires rigid work holding, sharp positive-rake tooling, sufficient feed to keep the cutting edge below the work-hardened skin and flood coolant to prevent the chip from welding to the rake face.

Applications of AISI 316

AISI 316 is used in chemical and pharmaceutical process plant exposed to aggressive media. Marine applications include hardware, boat fittings, deck fixtures and coastal architecture. Food-processing uses include equipment exposed to brine, curing salts or acidic cleaning where 304 would stain. Other applications include heat exchangers, screens for mining, quarrying and water filtration, pulp and paper equipment, and surgical and laboratory apparatus. ISO 3506 identifies it as fastener class A4. The A4-70 and A4-80 markings on a bolt head identify 316 with a stated proof strength.

Designations for AISI 316

The European designation is material number 1.4401, with the name X5CrNiMo17-12-2 under EN 10088-3. The UNS designation is S31600. The Japanese designation is SUS 316, commonly entered by purchasers as SUS316 or SS316. The older workshop name 18/10/3 refers to the nominal chromium, nickel and molybdenum contents. These designations are nearest counterparts rather than identical specifications. Material number 1.4401 permits a slightly different composition range from S31600, so the standard stated on the order governs.

In which product forms does Laxcon list AISI 316?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where AISI 316 is not the right choice

AISI 316 is unsuitable for full seawater immersion, hot chloride process streams and locations where evaporation concentrates chlorides in crevices. Its crevice corrosion temperature and critical pitting temperature are substantially lower than those of duplex or super-austenitic grades. Corrosion produces deep local pits rather than uniform thinning, so additional wall thickness does not provide protection. Duplex 2205 has a PREN of 34.1 to 37.8 and approximately twice the yield strength. Super duplex 2507 has a PREN of 37.7 to 47.6 and is used for warm seawater and desalination duty.

Stressed AISI 316 is susceptible to chloride stress corrosion cracking above about 60 degrees Celsius in chloride-bearing water. Molybdenum does not improve resistance to this mechanism. Duplex and ferritic grades provide a substantial advantage. AISI 316L limits carbon to 0.03 percent and avoids sensitisation in heavy welded sections that cannot be solution annealed. For service between about 600 and 900 degrees Celsius, titanium-stabilised AISI 316Ti retains carbon in stable carbides where plain 316 does not. AISI 316 is also unsuitable when machining time is more important than corrosion performance because a free-machining grade provides longer tool life.

Other austenitic grades