UNS S31254 (F44): The Premium 6% Molybdenum Super-Austenitic Stainless Steel

Grade Overview and Designations

UNS S31254 is the most widely specified 6% molybdenum super-austenitic stainless steel in the world. It combines about 20% chromium, 18% nickel, 6.1% molybdenum, 0.7% copper and 0.20% nitrogen, and it is designated F44 in ASTM A182 for forgings. Because of the high nickel, molybdenum and nitrogen content, the alloy is fully austenitic and non-magnetic in the annealed condition, and its nitrogen strengthening raises yield strength well above that of conventional austenitic grades. The principal designations are listed below.

Designation system	Grade name	Reference standard
UNS	S31254	ASTM A240, A312, A269, A276
ASTM forged products	F44	ASTM A182
EN	1.4547 (X1CrNiMoCuN20-18-7)	EN 10088
JIS	SUS312L	JIS G4304

Chemical Composition per ASTM A240

The following limits for UNS S31254 are taken from ASTM A240 for plate, sheet and strip.

Element	C	Mn	P	S	Si	Cr	Ni	Mo	Cu	N
Limit, %	0.020 max	1.00 max	0.030 max	0.010 max	0.80 max	19.5-20.5	17.5-18.5	6.0-6.5	0.5-1.0	0.18-0.25

Mechanical Properties

Minimum room-temperature properties for plate per ASTM A240 and for pipe per ASTM A312 are shown below.

Property	Value
Tensile strength, MPa	650 min
0.2% yield strength, MPa	310 min
Elongation, %	35 min
Brinell hardness, HBW	223 max

PREN and Localized Corrosion Resistance

Why PREN Matters

The Pitting Resistance Equivalent Number is calculated as %Cr + 3.3 x %Mo + 16 x %N. For UNS S31254 the result is above 43, compared with about 24-26 for 316L and about 34-36 for duplex 2205. In practice this means the alloy resists pitting and crevice corrosion in hot, concentrated chloride solutions where 316L fails rapidly, and it also resists chloride stress corrosion cracking in most service conditions because of its fully austenitic, high-nickel structure.

Resistance to Aggressive Media

The alloy is resistant to oxidizing acids, seawater, brine, hypochlorite solutions and chloride-bearing process streams, and it also performs well in many reducing acid mixtures where its copper content is beneficial. This combination of resistance to localized attack and to general acid corrosion makes it a versatile upgrade material.

Comparison with 316L and Duplex 2205

UNS S31254 provides a dramatic improvement in corrosion performance over 316L, particularly in hot chloride solutions where 316L would pit rapidly. Compared with duplex 2205, UNS S31254 has significantly better resistance to crevice corrosion and to oxidizing acids such as hot sulfuric acid; 2205 has higher yield strength, but the superior corrosion margin of UNS S31254 in severe environments often makes it the safer choice for critical, unmaintained equipment. It effectively fills the gap where 2205 is at its limits and where nickel-based alloys become cost-prohibitive.

Grade	PREN	0.2% yield strength, MPa	Typical role
316L	24-26	170	General corrosive service
Duplex 2205	34-36	450	High strength, moderate chloride service
UNS S31254	43+	310	Severe chloride and acid service

Fabrication and Welding

Because of its high alloy content, UNS S31254 has a higher work-hardening rate than standard austenitic grades, requiring more power for forming and machining. Welding should use matching or over-alloyed filler metals such as ERNiCrMo-3 (UNS N06625), with strict control of heat input and interpass temperature to preserve corrosion resistance. Proper shielding gas and back purging are essential to prevent oxidation, and post-weld cleaning is critical to remove oxides that could initiate corrosion in service.

Lifecycle Cost Justification

UNS S31254 costs more per kilogram than 316L or 2205, but the decision should be based on lifecycle cost and risk. If a lower-grade alloy would require frequent inspection, chemical inhibitors, early replacement, or carries a high risk of catastrophic failure, then UNS S31254 is justified. Its higher initial cost is offset by extended service life, reduced maintenance and operational reliability. For new projects in aggressive environments, specifying the grade from the outset is usually more economical than retrofitting or repairing equipment made from inadequate materials.

Frequently Asked Questions

1. What is the composition, key advantage, and primary application of UNS S31254?

The composition is about 20% chromium, 18% nickel, 6.1% molybdenum, 0.7% copper and 0.20% nitrogen, giving a Pitting Resistance Equivalent Number above 43. The key advantage is exceptional resistance to localized corrosion and chloride stress corrosion cracking with good strength and ductility. It is used in seawater cooling systems, offshore oil and gas equipment, chemical plants handling chlorides and acids, and flue gas desulfurization units where conditions are too severe for 316L or duplex steels.

2. How does its corrosion resistance compare to 316L and duplex 2205?

It is dramatically better than 316L in hot, concentrated chloride solutions. Compared with duplex 2205, it has significantly better crevice corrosion resistance and better resistance to oxidizing acids such as hot sulfuric acid. While 2205 has higher strength, the corrosion margin of UNS S31254 often makes it the safer choice for critical equipment in severe environments.

3. In which environments is UNS S31254 the minimum recommended material?

It is the minimum recommended material for untreated seawater cooling systems, especially above about 40°C, and for highly concentrated chlorides, bromides, or acidic streams with oxidizing agents in chemical plants. Environments containing wet chlorine, hypochlorite, or hot acidic chloride solutions are typical applications where equipment failure would carry severe consequences.

4. What are the important fabrication and welding guidelines?

The high alloy content gives a higher work-hardening rate, so more power is needed for forming and machining. Weld with matching or over-alloyed filler such as ERNiCrMo-3, control heat input and interpass temperature, use proper shielding gas and back purging, and clean the weld area after welding to preserve pitting resistance.

5. What factors justify the investment in UNS S31254 over lower-grade alloys?

Base the decision on lifecycle cost and risk assessment. If a lower-grade alloy would require frequent inspections, chemical inhibitors, early replacement, or poses a high risk of catastrophic failure, UNS S31254 is justified. The higher initial cost is offset by extended service life, reduced maintenance and operational reliability, especially when specified from the outset of a new project.