AISI 316L

What is AISI 316L?

AISI 316L is a low-carbon, 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. Its carbon content is limited to 0.03 percent, compared with 0.08 percent for standard AISI 316. Molybdenum provides resistance to chlorides. The low carbon limit prevents grain-boundary carbide precipitation during an ordinary welding cycle and reduces the risk of corrosion in bands parallel to welded joints.

AISI 316L is commonly specified for welded systems that must remain clean. Applications include wetted components in hygienic bioprocessing equipment, chemical plants, offshore topside pipework, and pharmaceutical and semiconductor fluid-handling systems.

AISI 316L has lower guaranteed room-temperature strength than AISI 316. Its minimum tensile and yield strengths are 485 MPa and 170 MPa, compared with 515 MPa and 205 MPa for AISI 316. The two grades have no measurable difference in corrosion resistance in the annealed condition. Their principal difference occurs after welding.

Laxcon Steels lists AISI 316L in its grades reference as an austenitic grade. The same steel is written 316L, SS 316L, SUS 316L, 1.4404 and S31603.

What is the chemical composition of AISI 316L?

Composition limits in weight percent are carbon 0.03 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 316L 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 316L?

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 316L

ASTM A276 specifies minimum properties for hot-finished annealed AISI 316L bar. These are 485 MPa tensile strength, 170 MPa yield strength at 0.2 percent offset, 40 percent elongation in 50 mm and 50 percent reduction of area. ASTM A240 specifies minimum tensile and yield strengths of 485 MPa and 170 MPa for plate. It limits hardness to 95 HRB or 217 HB. Cold-finished Condition A bar up to 12.70 mm in diameter has minimum tensile and yield strengths of 620 MPa and 310 MPa. Above that diameter, the minima return to 485 MPa and 170 MPa. Minimum elongation is 30 percent in both cases.

Some AISI 316L products carry dual 316/316L certification. Such a heat meets the 0.03 percent carbon limit and achieves the higher strength minima of 515 MPa tensile and 205 MPa yield. The same bar or plate can therefore be released under either specification. This reduces separate stock requirements. The mill certificate identifies the applicable certification. Dual-certified material may be unsuitable when a code case requires the carbon content of standard AISI 316 for elevated-temperature allowable stresses.

Use in hygienic and pharmaceutical systems

The ASME Bioprocessing Equipment standard requires AISI 316L for wetted components in hygienic systems. This requirement relates to fabrication. Product-contact tubes and fittings use autogenous orbital welding without added filler metal. The weld therefore retains the parent-metal chemistry. A parent metal with no more than 0.03 percent carbon prevents sensitisation at the joint.

This practice typically limits sulphur to 0.005 to 0.017 percent, a narrower range than the general specification. Sulphur affects movement of the molten weld pool. Insufficient sulphur can cause arc wandering and a wide, shallow weld. Excess sulphur can produce deep, narrow penetration and joint mismatch in a rotating head. Delta ferrite in the solidified weld is another controlled variable. The standard evaluates it through the weld-decay procedure designated S7. Retained ferrite can later undergo rouging and corrosion in high-purity water service. A general-purpose AISI 316L mill certificate does not show all these controls. Hygienic applications therefore use a tighter specification than the grade designation alone.

Corrosion behaviour of AISI 316L

AISI 316L has a pitting resistance equivalent number from 22.6 at the specified minimum composition to 29.5 at the specified maximum composition. This range is identical to that of standard AISI 316 because both grades have the same chromium, molybdenum and nitrogen limits. Their general corrosion resistance is effectively the same in the annealed condition. The difference concerns the welded condition.

AISI 316L resists pitting in potable water containing up to about 1,000 mg per litre of chlorides at ambient temperature. The approximate limit falls to 500 mg per litre at 60 degrees Celsius. The grade performs well in most organic acids, dilute sulphuric acid, dilute phosphoric service and the full range of clean in place chemistry. It remains vulnerable to chloride stress corrosion cracking above about 60 degrees Celsius when stressed. Passivation removes embedded iron and free-machining debris from the surface. Electropolishing provides the same function in high-purity systems. These treatments improve service performance beyond that indicated by the untreated values.

Welding and processing of AISI 316L

AISI 316L is compatible with all standard fusion-welding processes. Filler welding uses AISI 316L consumables because deposited weld metal also cools through the sensitising temperature band. Solution treatment, when applied, uses a temperature from 1,010 to 1,120 degrees Celsius followed by rapid cooling. The grade has no hardening heat treatment.

Hygienic pipe welding requires internal argon back-purging. Oxygen at the bore during welding causes heat tint, porosity and a surface oxide that is neither passive nor cleanable. External polishing cannot repair this internal condition. Cold forming follows the behaviour of the AISI 316 family. The grade has high springback and a high work-hardening rate. Heavy deformation produces a measurable increase in magnetic response.

Designations for AISI 316L

Under EN 10088-3, AISI 316L corresponds to material number 1.4404 and the name X2CrNiMo17-12-2. Its UNS number is S31603. Its Japanese designation is SUS 316L. Purchasers also use the unspaced forms SUS316L and SS316L, or the shortened designation 316L. EN 1.4435 is a related variant with higher molybdenum content. It does not have the same composition window as 1.4404. The standard and material number stated on the order determine the applicable grade.

In which product forms does Laxcon list AISI 316L?

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 316L is not the right choice

AISI 316L is unsuitable for sustained elevated-temperature service. Its reduced carbon content improves weldability but also lowers creep strength. Pressure design codes therefore assign it lower allowable stresses at elevated temperatures than standard AISI 316. It should not be specified above about 500 degrees Celsius. AISI 316Ti is stabilised with titanium. This stabilisation retains bound carbides through the range from 425 to 815 degrees Celsius and preserves their contribution to hot strength.

AISI 316L is also unsuitable for seawater service. Its PREN range of 22.6 to 29.5 is below the level normally used as the threshold for warm seawater. It can pit and stain during immersion and within crevices beneath marine biofouling. Duplex 2205 and super duplex 2507 are intended for this service and provide approximately double and triple the yield strength, respectively. Chloride stress corrosion cracking remains a risk above about 60 degrees Celsius in stressed components exposed to chloride-bearing water. Molybdenum does not eliminate this risk. The duplex grades provide resistance to this form of cracking. AISI 316L also offers no low-carbon benefit when a component requires substantially more machining than welding. Its gummy chip formation reduces tool life during machining.

Other austenitic grades