Super duplex 2507

What is Super duplex 2507?

Super duplex 2507 is a highly alloyed ferritic-austenitic stainless steel. Its composition contains 24.0 to 26.0 percent chromium, 6.0 to 8.0 percent nickel, 3.0 to 5.0 percent molybdenum and 0.24 to 0.32 percent nitrogen. Carbon is limited to 0.03 percent, and copper is permitted up to 0.50 percent. These ranges exceed those of standard duplex grades. Its nitrogen content is approximately twice that of duplex 2205.

Super duplex is a classification for duplex stainless steels with a pitting resistance equivalent number of at least 40. Atlas Steels associates this threshold with good resistance to pitting and crevice corrosion in warm seawater and other high-chloride environments. It rates 2507 above grade 904L and near the six percent molybdenum super austenitic grades.

Super duplex 2507 differs from duplex 2205 in strength, corrosion resistance and processing requirements. It has a minimum yield strength of 550 MPa and a PREN range of 37.7 to 47.6. Its high alloy content also accelerates intermetallic precipitation. Welding therefore requires tighter control than for 2205.

Laxcon Steels lists Super duplex 2507 in its grades reference as a duplex grade. The same steel is written 2507, S32750, F53 and 1.4410.

Super duplex, UNS S32750, ASTM A240 and ASTM A182 F53.

What is the chemical composition of Super duplex 2507?

Composition limits in weight percent are carbon 0.03 maximum, manganese 1.2 maximum, sulphur 0.02 maximum, phosphorus 0.035 maximum, silicon 0.8 maximum, chromium 24.0 to 26.0, nickel 6.0 to 8.0, molybdenum 3.0 to 5.0, nitrogen 0.24 to 0.32 and copper 0.5 maximum.

What is Super duplex 2507 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of Super duplex 2507?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical properties

ASTM A240 for plate and ASTM A790 for pipe specify UNS S32750 with a minimum tensile strength of 795 MPa, a minimum 0.2 percent yield strength of 550 MPa and a minimum elongation of 15 percent in 50 mm. Hardness is limited to 32 HRC or 310 HB. AISI 316L has specified tensile and yield strengths of 485 MPa and 170 MPa. The guaranteed yield strength of 2507 is therefore more than three times that value. Duplex 2205 has corresponding values of 655 MPa and 450 MPa, giving 2507 a further 20 percent in yield strength. The designation for forgings under ASTM A182 is F53.

The minimum elongation of 2507 is 15 percent, compared with 25 percent for 2205 and 40 percent for austenitic grades. The grade combines high strength with adequate ductility but does not provide austenitic formability. Its density is about 7,800 kg per cubic metre, and its elastic modulus is about 200 GPa. The mean coefficient of thermal expansion is approximately 13.0 micrometres per metre per degree Celsius from 0 to 100 degrees Celsius. This value is well below those of austenitic grades.

Corrosion behaviour

The pitting resistance equivalent number for 2507 ranges from 37.7 at the specified composition minima to 47.6 at the maxima. It is calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. Material at the lean end of the composition range does not reach the PREN 40 threshold for super duplex steel. Material at the rich end exceeds it. Demanding seawater specifications therefore use a minimum measured PREN or a critical pitting temperature test rather than the grade name alone.

Atlas Steels describes the general corrosion resistance of 2507 as superior to virtually all other stainless steels. The grade has high resistance to intergranular corrosion. It also has very high resistance to stress corrosion cracking in chloride and sulphide environments. Sulphide stress cracking in sour wells differs from chloride stress corrosion cracking, so qualification for one mechanism does not establish qualification for the other. Applications also include strong acid solutions and severe high-temperature marine environments.

Heat treatment, welding and forming

Solution treatment consists of heating to 1,040 to 1,120 degrees Celsius, followed by rapid air cooling or water quenching. Heat treatment cannot harden the grade.

Standard welding methods can join 2507 without preheating. Consumables are over-alloyed with nitrogen and nickel to obtain the required phase balance and avoid excess ferrite in the weld deposit. A typical ISO designation is 25 9 4 L N. Heat input should remain between 0.5 and 1.5 kJ per millimetre. Lower heat input can produce cooling that is too rapid for austenite to re-form. Higher heat input can retain the weld in the intermetallic temperature range long enough for sigma and chi phases to precipitate. Autogenous welding requires post-weld annealing. Other welding methods do not require it. Nickel-based consumables such as Alloy C22 may be used when the weld requires greater corrosion resistance than the parent metal.

Hot forging takes place between 1,200 and 1,025 degrees Celsius and requires subsequent solution treatment. The low hot strength of duplex grades can require physical support during heat treatment or forging to prevent sagging. Cold work above about 20 percent requires an intermediate solution anneal. Heavy deformation such as cold forging is not possible. Forming also requires high forces and produces substantial springback.

Applications and designations

Applications for super duplex 2507 include oil and gas exploration, processing and support systems. Other uses include flue gas desulphurisation, pollution control plant, desalination plant, marine and high-chloride environments, and chemical processing, transport and storage. The grade is also used in pulp and paper processing, seawater cooling systems, subsea manifolds, umbilical tubing and pressure vessels that require both strength and chloride resistance.

The European material number is 1.4410, and the European name is X2CrNiMoN25-7-4. The UNS designation is S32750, and the forging designation is F53. Purchaser designations include 2507, SAF 2507, S32750, SS2507 and superduplex2507.

In which product forms does Laxcon list Super duplex 2507?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where Super duplex 2507 is not the right choice

Super duplex 2507 is unsuitable outside an approximate service range of minus 50 to plus 300 degrees Celsius. Its practical range can be narrower because it embrittles faster than leaner duplex grades. Above 300 degrees Celsius, ferrite decomposes into chromium-rich and iron-rich regions, reducing toughness. Between approximately 600 and 1,000 degrees Celsius, sigma and chi phases precipitate rapidly. A full solution anneal is required to reverse either mechanism. Austenitic 316L or a nickel alloy is the appropriate choice at either temperature extreme.

Super duplex 2507 is unsuitable where welding heat input cannot remain between 0.5 and 1.5 kJ per millimetre. Heat input outside this range can either reduce austenite or produce intermetallic phases. Both conditions reduce corrosion resistance even when the joint appears acceptable. Autogenous welding without post-weld solution annealing is also unsuitable.

Super duplex 2507 is unsuitable for deep drawing, cold heading and other severe cold-forming operations. It is also unsuitable for parts in which machining time is the dominant consideration. Its high strength causes slower cutting and shorter tool life. Where corrosion conditions are less severe, duplex 2205 has a PREN range of 34.1 to 37.8 and is more available and easier to process. Where ductility is more important than strength, a six percent molybdenum super austenitic grade provides comparable corrosion resistance with substantially higher elongation.

Other duplex grades