What is 1.4547 Stainless Steel?

What Is 1.4547 Stainless Steel?

1.4547 is the EN 10088 designation for a 6% molybdenum super austenitic stainless steel, known in North America as UNS S31254 and in EN nomenclature as X1NiCrMoCuN25-20-7. The steel combines roughly 20% chromium, 18% nickel, 6.5% molybdenum, 0.2% nitrogen and about 0.7% copper. The result is a fully austenitic, non-magnetic material with a pitting resistance equivalent number above 42, which puts its resistance to chloride pitting and crevice corrosion well beyond 316L and 317L, and comparable to many nickel-base alloys in seawater service. It is produced as plate, sheet, strip, bar, tube and pipe under ASTM A240, A312, A269 and related product standards.

Chemical Composition

The guaranteed ranges below follow EN 10088-2 for sheet and plate and ASTM A240 for UNS S31254; both specifications set essentially the same limits.

Element	1.4547 (EN 10088-2)	UNS S31254 (ASTM A240)
Carbon (C)	≤ 0.020%	≤ 0.020%
Silicon (Si)	≤ 0.70%	≤ 0.80%
Manganese (Mn)	≤ 1.00%	≤ 1.00%
Phosphorus (P)	≤ 0.030%	≤ 0.030%
Sulfur (S)	≤ 0.010%	≤ 0.010%
Chromium (Cr)	19.50-20.50%	19.50-20.50%
Nickel (Ni)	17.50-18.50%	17.50-18.50%
Molybdenum (Mo)	6.00-6.50%	6.00-6.50%
Nitrogen (N)	0.18-0.22%	0.18-0.22%
Copper (Cu)	0.50-1.00%	0.50-1.00%

Silicon varies slightly between the two systems, but the chromium, nickel, molybdenum, nitrogen and copper ranges are identical, which is why 1.4547 and UNS S31254 are treated as interchangeable designations for the same alloy.

Mechanical Properties

The table lists the guaranteed values for annealed sheet and plate per EN 10088-2; ASTM A240 sets equivalent minimums for UNS S31254.

Property (annealed)	Value
0.2% proof stress Rp0.2	≥ 300 MPa
Tensile strength Rm	650-850 MPa
Elongation	≥ 35%
Hardness	≤ 223 HBW (typical maximum)

Corrosion Resistance and PREN

The pitting resistance equivalent number is calculated as PREN = Cr + 3.3 × Mo + 16 × N. Using the mid-range composition of 1.4547 gives roughly 20 + 3.3 × 6.25 + 16 × 0.20, which is above 42. In practical terms the alloy resists pitting and crevice corrosion in seawater, brackish water and chloride-bearing process streams where 316L fails within months. It also shows good resistance to stress corrosion cracking in chloride environments because of its high nickel content, and it is accepted for sour-service components under ISO 15156-3 (NACE MR0175) within the specified limits.

Applications and Fabrication

Typical applications are seawater-cooled heat exchangers and condensers, offshore fire-water and seawater piping, desalination plants, flue-gas desulphurisation scrubbers, pulp mill bleach plants, chemical tankers, and pump and valve components for aggressive chloride service. Fabrication is straightforward but discipline is required: the alloy should be solution annealed at 1150-1200 °C and rapidly quenched, and welding should use over-alloyed filler such as a high-molybdenum austenitic consumable to keep the weld metal at least as resistant as the base material. Because of the high molybdenum content, hot forming must start above 1150 °C and finish above 1000 °C to avoid cracking.

FAQ

What does 6% Mo mean?

It refers to the molybdenum content of about 6.0-6.5%, which is roughly double that of 316L. Molybdenum is the key element for resistance to pitting and crevice corrosion in chloride environments.

How does 1.4547 differ from 316L?

1.4547 has much higher molybdenum and nitrogen, higher chromium and nickel, and a PREN above 42 compared with about 24 for 316L. It resists seawater and chloride pitting where 316L cannot, but it is significantly more expensive.

What is the PREN of 1.4547?

Using the mid-range composition, the PREN is approximately 42-45, calculated as Cr + 3.3 × Mo + 16 × N. Specifications commonly cite a minimum of 42.

Can 1.4547 be welded?

Yes. Use over-alloyed austenitic filler wire or electrodes with high molybdenum content, keep interpass temperature below 150 °C, and avoid excessive heat input. Post-weld solution annealing is not normally required if correct welding practice is followed.

What standards cover 1.4547?

EN 10088-2 covers sheet and plate, ASTM A240 covers plate and sheet, ASTM A312 covers seamless and welded pipe, ASTM A269 and A249 cover tubing, and ISO 15156-3 lists the alloy for sour oil and gas service. Bar is covered by EN 10088-3 and ASTM A276.

Is 1.4547 suitable for seawater service?

Yes, it is one of the most widely used stainless steels for seawater piping, heat exchangers and pumps, and it performs well in both flowing and stagnant seawater conditions where 316L would pit rapidly.