AISI 316Ti

What is AISI 316Ti?

AISI 316Ti is a titanium-stabilised austenitic stainless steel based on AISI 316. Under ASTM A240, 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 titanium content is at least five times the combined carbon and nitrogen content and is limited to 0.70 percent. Titanium has a greater affinity for carbon than chromium. It forms titanium carbide at high temperature before chromium carbides can form. Chromium therefore remains in solution and maintains the passive surface.

AISI 316Ti combines stabilisation with molybdenum content. AISI 316L limits sensitisation through its low carbon content, with lower strength at elevated temperature. AISI 321 uses titanium stabilisation but contains no molybdenum and lacks the associated chloride resistance. AISI 316Ti retains a carbon limit of 0.08 percent and the related elevated-temperature strength. It also resists intergranular attack after exposure from 425 to 860 degrees Celsius.

The grade is intended for equipment exposed to both elevated temperature and corrosive conditions. This service range is narrower than that of AISI 316 and requires AISI 316Ti to be specified directly.

Laxcon Steels lists AISI 316Ti in its grades reference as an austenitic grade. The same steel is written 316Ti, SUS 316Ti, 1.4571 and S31635.

Ti minimum 5x(C+N) per ASTM A240, maximum 0.70

What is the chemical composition of AISI 316Ti?

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, nitrogen 0.1 maximum and titanium 0.7 maximum.

What is AISI 316Ti 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 316Ti?

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 316Ti

ASTM A276 places type 316Ti in the same Condition A category as types 304 and 316. Hot-finished annealed bar has a minimum tensile strength of 515 MPa and a minimum 0.2 percent yield strength of 205 MPa. It also requires 40 percent elongation in 50 mm and 50 percent reduction of area. ASTM A240 gives plate the same minimum tensile and yield strengths of 515 MPa and 205 MPa. It requires 40 percent elongation and limits hardness to 95 HRB or 217 HB. Cold-finished Condition A bar up to 12.70 mm diameter has a tensile strength of 620 MPa and a yield strength of 310 MPa. Above that diameter, the values return to 515 MPa and 205 MPa.

The density is about 8,000 kg per cubic metre. The mean coefficient of thermal expansion is about 16.5 micrometres per metre per degree Celsius from 20 to 100 degrees Celsius. These values are essentially the same as those of AISI 316. The distinguishing property is the retention of strength and corrosion resistance after prolonged exposure to elevated temperature. Published creep data gives a rupture strength of about 100 MPa after 100,000 hours at 600 degrees Celsius. The grade also retains a substantial proportion of its room-temperature tensile strength at 815 degrees Celsius. Room-temperature test results do not distinguish AISI 316Ti from AISI 316 and therefore cannot verify the relevant elevated-temperature property.

Titanium stabilisation

Unstabilised austenitic steel held between about 425 and 860 degrees Celsius can precipitate chromium-rich M23C6 carbides at grain boundaries. Chromium diffuses more slowly than carbon at these temperatures. Carbide formation therefore removes chromium from a narrow region beside each boundary. This region can fall below the chromium level required for passivity and become susceptible to intergranular corrosion.

Titanium prevents this sequence by combining with carbon first. In AISI 316Ti, titanium carbide is distributed through the grains instead of being concentrated at grain boundaries. This occurs before the steel enters the critical temperature range. AISI 316L can tolerate the brief exposure associated with welding. AISI 316Ti can retain protection during years of service within the range.

Welding and processing of AISI 316Ti

AISI 316Ti can be welded by standard fusion processes without post-weld annealing. This property supports its use in heavy welded sections operating at elevated temperature. Titanium does not transfer reliably across a welding arc because it is highly oxidising. Titanium-stabilised covered electrodes are therefore unavailable for manual welding. Niobium-stabilised consumables of the 318 class are used instead. Niobium performs the same carbide-binding function and remains stable during arc transfer.

A stabilising anneal may be used for severe service. This treatment holds the material near 870 to 890 degrees Celsius after solution treatment. It converts residual dissolved carbon into titanium carbide before service. Machining behaviour resembles that of AISI 316. The material is slow to machine, produces tough chips and work-hardens. Titanium carbide and titanium nitride particles are abrasive and further reduce tool life.

Applications of AISI 316Ti

AISI 316Ti is used for expansion joints and bellows in hot chemical plants. Other applications include flue and exhaust ductwork carrying condensable acids, heat exchangers operating above the sensitisation threshold, jacketed reactors, pulp and paper digester components, and welded pressure equipment. Such equipment may operate from 500 to 800 degrees Celsius, where AISI 316L is not permitted and unstabilised AISI 316 would sensitise. European process plants also commonly specify the grade because 1.4571 has historically served as the standard molybdenum grade in that market, as AISI 316L has elsewhere.

Designations for AISI 316Ti

The European material number is 1.4571. Its name is X6CrNiMoTi17-12-2 under EN 10088-3. Drawings in much of Continental Europe commonly specify 1.4571 where British or American drawings specify 316Ti. The UNS designation is S31635. The Japanese designation is SUS 316Ti, also written without spaces as SUS316Ti or SS316Ti. ASTM and EN requirements differ slightly in their titanium and carbon limits. The standard stated on the order therefore governs the material requirements.

In which product forms does Laxcon list AISI 316Ti?

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 316Ti is not the right choice

AISI 316Ti is unsuitable for high-purity and hygienic surfaces. Its titanium carbides and nitrides are hard, angular inclusions. They can drag during polishing and produce streaks in mirror finishes. In electropolished bioprocessing equipment, they can prevent uniform surface passivation. AISI 316L is specified for these applications.

AISI 316Ti is also susceptible to knife-line attack. This form of intergranular corrosion occurs in a narrow band beside a weld. The high temperature at the fusion line dissolves titanium carbides. If the joint then cools through the sensitising range without stabilising treatment, the affected strip behaves as unstabilised steel. The risk occurs in multi-pass welds exposed to strong oxidising acids. A post-weld stabilising anneal prevents this condition.

AISI 316Ti provides no additional resistance over AISI 316 in chloride environments because both have a PREN range of 22.6 to 29.5. Seawater, warm brines and hot chloride process streams require duplex 2205 or super duplex 2507 instead. Stabilisation also provides no benefit below about 500 degrees Celsius. AISI 316L is therefore the appropriate grade when the component will not use the elevated-temperature property.

Other austenitic grades