A Comprehensive Guide to Super Duplex Stainless Steel: F61 (UNS S32550/1.4507)

What is F61 (UNS S32550/1.4507)?

Super Duplex Stainless Steel: F61 (UNS S32550/1.4507/Ferrinox 255) is a duplex stainless steel containing 25.5% chromium, 6% nickel with significant molybdenum, copper and nitrogen additions and a PREN (pitting resistance equivalent) of=40.0. The copper content of 1.6% is particularly beneficial in promoting optimum resistance to corrosion in acidic environments. The chemical composition and mechanical properties conform to the requirements of UNS S32550, 1.4507 and several oil and gas industry specifications. The microstructure is controlled with a phase balance close to 50:50 austenite and ferrite with complete freedom from grain boundary carbides, sigma phase and other deleterious precipitates.

EURONORM/DIN	WERKSTOFF NR.	AISI	UNS
X2 CrNiMoCuN 25-6-3	1.4507	F61	S 32550

Chemical Composition of F61 (UNS S32550/1.4507)

Standard	Steel Grade
	Chemical Composition %
	C	Mn	Si	P	S	Cr	Mo	Ni	Cu	N
EN	1.4507 – X2CrNiMoCuN25-6-3
	<0.03	<2.0	<0.7	<0.035	<0.015	24.0 – 26.0	3.0 – 4.0	6.0 – 8.0	1.0 – 2.5	0.20 – 0.30
ASTM	UNS S32550 – A182 grade F61 – Alloy 255
	<0.04	<1.5	<1.0	<0.040	<0.030	24.0 – 27.0	2.9 – 3.9	4.5 – 6.5	1.5 – 2.5	0.10 – 0.25
ASTM	UNS S32520 – A182 grade F59
	<0.03	<1.5	<0.8	<0.035	<0.020	24.0 – 26.0	3.0 – 5.0	5.5 – 8.0	0.5 – 3.0	0.20 – 0.35
JIS	SUS329J4L – SUS 329J4L
	<0.03	<1.5	<1.0	<0.040	<0.030	24.0 – 26.0	2.5 – 3.5	5.5 – 7.5	–	0.08 – 0.30

Mechanical Properties of F61 (UNS S32550/1.4507)

Yield Strength 0.2% N/mm²	Tensile Strength N/mm²	% Elongation	Hardness
			(Hv)	(HB)
≧500	700 – 900	≧25	–	≦270

Physical Properties of F61 (UNS S32550/1.4507)

Physical Property	Density (Kg/m3)	Elastic Modulus ( Gpa )	Co-eff of Expansion (µm/m/°C)	Thermal Condc. (W/m.K)	Electric Resistivity (nΩ.m)
Value	7850	200	14.5	19	850

Resistance to Corrosion of F61 (UNS S32550/1.4507)

Compared to typical austenitic alloys such as 304, 316 and 317 stainless steels, alloy F255 displays superior corrosion resistance in most media. Also, alloy F255 typically outperforms duplex alloy 2205. Alloy F255 performs well in sulfuric, phosphoric and nitric acids. It also shows excellent resistance to organic acids such as acetic and formic acid. Highly reducing media should be avoided. Pitting resistance equivalent numbers (PREN) are typically used to compare the pitting resistance of various alloys. This number can be calculated from several different equations, which are based on specific alloying elements that contribute to the alloy’s pitting resistance. For duplex stainless steels, the equation typically used to calculate the PREN is as shown below. A word of caution: the PREN is not a guarantee of the corrosion performance of any alloy and should only be used as a guide to help the user select potential alloys for specific use.

PREN = %Cr + (3.3 x %Mo) + (16 x N)

Fabrication and Heat Treatment of F255 (UNS S32550/1.4507)

Hot and cold forming can be performed on alloy F255 using traditional fabrication methods. However, the user must keep in mind that alloy F255 has higher strength properties when compared to 316 stainless steel. Hot working the alloy should be between the temperatures of 1800°F and 2100°F, followed by an annealing heat treatment at 1950°F and water quench. Cold-working the alloy to induce more than 10% deformation will require a similar heat treatment. For forming above 20% deformation, intermediate heat treatments should be performed. Machining alloy F255 can be performed using the same methods as traditional stainless steels. Carbide-tipped tools are preferred. Stress relieving by heating to 675°F briefly, followed by rapid cooling, can be performed on heavily machined components. Welding can be performed by TIG, MIG or SMAW and should be performed on material in the annealed condition. Preheat treatment is not necessary, but it is important to clean the surfaces being welded carefully. If pickling is the desired method for cleaning the surface, a solution with the following composition can be used: 15% HNO₃ + 2% HF (volume by volume) at a minimum temperature of 55°F Post weld heat treatment is not necessary but preferred when welding heavy sections to optimize corrosion resistance.

Applications of F61 (UNS S32550/1.4507)

Grade F61 steel is widely used in the following applications:

• Process industry components in nitric acid, dioxide, polypropylene & PVC production, caustic evaporators;
• Equipment handling organic and fatty acids;
• Marine industry and shipbuilding propellers, shafts, rudders, shaft seals, pumps, bolts, fasteners, valves and instrumentation;
• Oil and chemical tankers;
• Oil and gas industry pumps, valves, pipe fittings, steel pipes, flanges, forgings, vessels, wellhead equipment and subsea equipment;
• Pollution control systems, fans and pumps, wet scrubbers, incinerators, flue gas desulphurization (FGD);
• Pulp and paper industry components such as black liquor heater tubes, digester blow valves, rotary feed valves, brown stock washers, precipitators and bleaching components;
• Food Industry applications, including sugar cane centrifuges, corn and vegetable processing plants;
• Agrochemicals production such as fertilizers (based on wet phosphoric acid);
• Sewage treatment, seawater cooling, desalination plants.