The typical chemical composition for this grade is given in the table below, together with composition limits given for the product according to different standards. The required standard will be fully met as specified on the order.

The chemical composition is given as % by mass.

The duplex stainless steels have much higher mechanical strength compared to standard stainless steels. If the high strength of the duplex grades can be utilized, downgauging can be done in many applications, leading to cost-efficient solutions. The allowable design values may vary between product forms. The appropriate values are given in the relevant specifications.

¹⁾Elongation according to EN standard:
A₈₀ for thickness below 3 mm.
A for thickness = 3 mm.
Elongation according to ASTM standard A_2” or A₅₀.

Uniform corrosion

The alloying elements chromium and molybdenum give Forta SDX 2507 excellent resistance to uniform corrosion. This type of corrosion occurs in acids or in hot alkaline solutions.  For guidance on material selection in a large number of environments capable of causing uniform corrosion, consult the tables and iso-corrosion diagrams in the Outokumpu Corrosion Handbook.

Pitting and crevice corrosion

Chloride ions in a neutral or acidic environment facilitate local breakdown of the passive layer. As a result, pitting and crevice corrosion can propagate at a high rate, causing corrosion failure in a short time. Since the attack is small and may be covered by corrosion products or hidden in a crevice, it often remains undiscovered until perforation or leakage occurs. Resistance to pitting corrosion is determined mainly by the content of chromium, molybdenum, and nitrogen in the stainless steel. Forta SDX 2507 has good resistance to pitting and crevice corrosion due to the chromium and nitrogen content.

Stress corrosion cracking

Forta SDX 2507 has excellent resistance to stress corrosion cracking, a form of corrosion that can occur in chloride-containing environments at elevated temperatures.

Another type of stress corrosion cracking can be induced by hydrogen sulfide. Good resistance to this type of stress corrosion cracking is important in the oil and gas industry. Forta SDX 2507 is approved according to NACE MR0175/ISO 1515 for use in H2S-containing environments in oil and gas production.

PRE Pitting Resistant Equivalent calculated using the formula: PRE = %Cr + 3.3 x %Mo + 16 x %N
CPT Corrosion Pitting Temperature as measured in the Avesta Cell (ASTM G 150), in a 1M NaCl solution (35,000 ppm or mg/l chloride ions).
CCT Critical Crevice Corrosion Temperature is the critical crevice corrosion temperature which is obtained by laboratory tests according to ASTM G 48 Method F

The physical properties at room temperature are shown in the table below. Data according to EN 10088 or EN 10095.

Duplex stainless steel applicable to all forming processes available for stainless steel. The high proof strength compared to austenitic and ferritics stainless steel can impose some differences in forming behavior depending on the chosen forming technique, such as an increased tendency to springback. This point is particularly relevant to forming of any high strength steel. If the forming process is not already decided, it is possible to choose the most suitable one for duplex grades. Moreover, an excellent interplay between high proof strength, work hardening rate, and elongation mean that the duplex grades are particularly well suited to lightweight and cost-efficient applications with complex shapes. The impact of the high strength varies for different forming techniques. Common to all is that the estimated forming forces will be higher than for the corresponding austenitic and ferritic stainless steel grades. This effect will usually be lower than expected from just the increase in strength since the choice of duplex stainless steel is often associated with downgauging. It is important to consider that duplex stainless steel may also place higher demands on the for the tool materials and lubricants. Downgauging should also be considered in this case.

New test line

Machining

Duplex steels are generally more demanding to machine than conventional austenitic stainless steel such as Supra 316L/4404, due to the higher hardness. More information can be found in the machining guidelines which are available for each duplex grade.
Machining guidelines Forta SDX 2507.

Welding

Forta SDX 2507 has good weldability and can be welded using most of the common methods used for stainless steel:

• Shielded metal arc welding (SMAW)
• Gas tungsten arc welding  TIG(GTAW)
• Gas metal arc welding MIG (GMAW)
• Flux-cored arc welding (FCW)
• Plasma arc welding (PAW)
• Submerged arc welding (SAW)
• Laser welding
• Resistance welding
• High frequence welding

The following general instructions should be followed:

• The material should be welded without preheating.
• The material should be allowed to cool between passes, and the interpass temperature should not exceed 100 °C
• To obtain good weld metal properties in the as-welded condition, filler material should be used.
• The recommended arc energy should be kept within certain limits to achieve a good balance between ferrite and austenite in the weld. The heat input should  be below 1.5 KJ/mm 
• Post-weld annealing after welding with filler is not necessary.
• To ensure optimum pitting resistance when using GTAW and PAW methods, the addition of nitrogen to the shielding/purging gas is recommended.

Forta SDX 2507 is more prone to inter-metallic precipitation in the weld metal. If welding is done from only one side and the root side will be exposed to corrosive media, it is important to make the root thick and following beads thin with low heat input. This minimizes the amount of detrimental sigma phase in the root. For SAW, the wire should not exceed Ø2.4 mm to facilitate low heat input welding.

For more information see Outokumpu Welding Handbook.

More detailed information concerning welding procedures can be obtained from the Outokumpu Welding Handbook, available from our sales offices.

Outokumpu produces and certifies materials to most international and national standards. Work is continuously on-going to get the different grades approved for relevant standards.