What Is UNS S31254 Stainless Steel? 6% Mo Super-Austenitic Grade Guide

What Is UNS S31254?

UNS S31254 is a super-austenitic stainless steel with a nominal composition of 20 % chromium, 18 % nickel and 6 % molybdenum, further strengthened by nitrogen and copper. The 6 % molybdenum level is the defining feature: it raises the pitting resistance equivalent number (PREn) to roughly 42, well above the values of about 24-26 typical for 316/316L, and it gives the grade the ability to withstand seawater and other high-chloride media where standard austenitic grades fail by pitting or crevice corrosion.

The grade is produced to ASTM A240 for plate, sheet and strip, ASTM A312 for seamless and welded pipe, and ASTM A269 for tubing. Its fully austenitic structure keeps it essentially non-magnetic in the annealed condition and gives excellent toughness, including at low temperatures. Because the alloy content is high, the grade costs more than 316, and it is selected only where service conditions justify the premium.

Chemical Composition per ASTM A240

ASTM A240 specifies the following composition limits for UNS S31254. Single values are maximums unless shown as a range.

Element	UNS S31254
Carbon	0.020 max
Manganese	1.00 max
Phosphorus	0.030 max
Sulfur	0.010 max
Silicon	0.80 max
Chromium	19.5-20.5
Nickel	17.5-18.5
Molybdenum	6.0-6.5
Nitrogen	0.18-0.22
Copper	0.50-1.00

Nitrogen is a deliberate addition: it dissolves interstitially in the austenite, raises strength and markedly improves resistance to localised corrosion. Copper further improves resistance in sulphuric-acid media. Together the molybdenum and nitrogen give UNS S31254 a critical pitting temperature well above that of 316L in standard chloride test solutions.

Mechanical Properties

Per ASTM A240, annealed product must meet the following minimums.

Property	UNS S31254 (minimum)
Tensile strength	650 MPa (94 ksi)
Yield strength (0.2 % offset)	300 MPa (44 ksi)
Elongation in 50 mm	35 %

The yield strength is roughly 40-50 % higher than that of 316L, which allows thinner sections in pressure equipment. The grade keeps useful toughness at sub-zero temperatures, making it suitable for cryogenic services as well as hot chloride media.

Corrosion Resistance and Where the Grade Is Used

UNS S31254 resists pitting and crevice corrosion in seawater, brackish water and chloride brines, and it performs well in oxidising acids such as sulphuric acid containing chlorides. It is a common choice for seawater cooling systems, desalination plants, flue-gas desulphurisation equipment, pulp bleaching circuits, chemical processing, offshore topside piping and pharmaceutical and food equipment requiring aggressive cleaning cycles. For even higher resistance to crevice corrosion in warm seawater, the nitrogen-enriched 6 % molybdenum family with higher nickel content (such as UNS N08367) may be considered; the two are frequently compared in marine design guides.

Welding and Fabrication

The grade is weldable by standard processes including gas tungsten arc welding. Because of the high alloy content, the recommended practice is to control heat input to about 1.5 kJ/mm maximum and keep interlayer temperature at or below 100 °C to avoid segregation and precipitation of intermetallic phases. Matching or over-alloyed austenitic filler is used, and shielding gas should be free of moisture. After welding, solution annealing at approximately 1150-1200 °C followed by rapid water quenching restores optimum corrosion resistance. Cold forming is possible, but the high yield strength requires more forming force than 316, and springback is greater.

FAQ

Q1. Why is 6 % molybdenum important? Molybdenum is the main driver of resistance to pitting and crevice corrosion in chloride environments. At 6 %, combined with nitrogen, UNS S31254 resists seawater and strong chloride brines that pit 316L rapidly.

Q2. Is UNS S31254 the same as 316L? No. 316L contains 2-3 % molybdenum and no deliberate nitrogen addition; UNS S31254 contains 6-6.5 % molybdenum plus nitrogen and copper, giving substantially higher strength and localised-corrosion resistance.

Q3. What is the PREn of UNS S31254? Using the formula PREn = Cr + 3.3 x Mo + 16 x N, the value is approximately 42, compared with about 24-26 for 316L.

Q4. Can UNS S31254 be welded without post-weld heat treatment? Yes, when welding is carried out with the correct filler and controlled heat input; PWHT is not required. Solution annealing after heavy cold work or complex welds can restore maximum corrosion resistance.

Q5. What service temperatures are suitable? The grade serves from cryogenic temperatures up to about 300-350 °C in many aqueous services. Above that, chloride stress-corrosion cracking risk and loss of mechanical strength must be evaluated for the specific environment.