Alloy F255 / UNS S32550 / W.Nr. 1.4507
Alloy F255 is a highly alloyed, super duplex (ferrite-austenite) solid solution strengthened alloy, which provides high strength and wear resistance while oﬀering superior corrosion resistance compared to 316 stainless steel. This alloy also oﬀers better resistance to stress corrosion cracking as well as crevice and pitting corrosion than lower alloyed materials. Operating temperatures should be considered when selecting alloy F255 as secondary phases could form within higher temperature ranges. If the alloy is heat treated and allowed to soak between the temperatures of 1000°F and 1800°F, carbides (M23C6), nitrides (M2N) and sigma phase can form. At temperatures approaching 900°F, alpha prime can form. The maximum continuous operating temperature according to ASME Boiler and Pressure Vessel Code, Section VIII is 500°F.
Industries & Applications
The combination of strength and wear resistance makes alloy F255 ideal for demanding applications in several diﬀerent industries, including oﬀshore oil and gas, pulp and paper, nuclear, marine, chemical processing and ﬂue gas desulfurization. Alloy F255 can be a cost eﬀective option for many applications, including oil & gas industry sub-sea equipment, chemical process industry equipment, pollution control scrubbers, marine seals, pumps and valves, bolts and fasteners, grain and vegetable processing equipment, pulp and paper bleaching components, water and sewage treatment and desalination equipment.
Resistance to Corrosion
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 out performs 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 alloys 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
Hot and cold forming can be performed on alloy F255 using traditional fabrication methods, although 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 carefully clean the surfaces being welded. If pickling is the desired method for cleaning the surface, a solution with the following composition can be used:
15% HNO˜ + 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.
Common Trade Names
Ferralium® is a registered trademark of Meighs Ltd, Langley Alloys Division