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DUPLEX 2507 STAINLESS STEEL

• Daxun Alloy Stainless Steel inventory currently contains Duplex 2507 Stainless Steel Pipe, which is a super duplex stainless steel containing 25% chromium, 4% molybdenum and 7% nickel. The alloy is suitable for applications requiring high strength and corrosion resistance, such as chemical processing, petrochemical and seawater equipment. The steel has strong resistance to chloride stress corrosion cracking, good thermal conductivity and a low coefficient of thermal expansion. The high chromium, molybdenum and nickel content provides excellent resistance to pitting, crevice and general corrosion.

GENERAL PROPERTIES

• Duplex 2507 (UNS S32750) is a super duplex stainless steel pipe containing 25% chromium, 4% molybdenum and 7% nickel. It is designed for demanding applications requiring high strength and corrosion resistance, such as chemical processing, petrochemical and seawater equipment. The steel has strong resistance to chloride stress corrosion cracking, high thermal conductivity and low coefficient of thermal expansion. The high chromium, molybdenum and nickel content provides excellent resistance to pitting, crevice and general corrosion.

Applications

• Oil and gas industry equipment
• Offshore platforms, heat exchangers, process and service water systems, fire-fighting systems, injection and ballast water systems
• Chemical process industries, heat exchangers, vessels, and piping
• Desalination plants, high pressure RO-plant and seawater piping
• Mechanical and structural components, high strength, corrosion-resistant parts
• Power industry FGD systems, utility and industrial scrubber systems, absorber towers, ducting, and piping

Standards

• ASTM/ASME ………. A240 – UNS S32750
  EURONORM………… 1.4410 – X2 Cr Ni MoN 25.7.4
  AFNOR……………….. Z3 CN 25.06 Az

CORROSION RESISTANCE

General corrosion

• SAF 2507™ is highly resistant to corrosion by organic acids, e.g. experience less than 0.05 mm/year in 10% formic and 50% acetic acid where ASTM 316L has corrosion rate higher than 0.2 mm/year. Pure formic acid see Figure 4. Also in contaminated acid SAF 2507™ remains resistant.
• Figure 5 and Figure 6 show results from tests of SAF™ 2507 and various stainless steels and nickel alloys in acetic acid contaminated with chlorides which in practice are frequently present in processes.
• 

Figure 4. Isocorrosion diagram in formic acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy) in stagnant test solution.

Figure 5. Corrosion rate of various alloys in 80% acetic acid with 2000 ppm chloride ions at 90°C.

Figure 6. Corrosion rate of various alloys in concentrated acetic acid with 200 ppm chloride ions.

Practical experience with SAF™ 2507 in organic acids, e.g. in terephthalic acid plants, has shown that this alloy is highly resistant to this type of environment. The alloy is therefore a competitive alternative to high alloyed austenitics and nickel alloys in applications where standard austenitic stainless steels corrode at a high rate.

Resistance to inorganic acids is comparable to, or even better than that of high alloy austenitic stainless steels in certain concentration ranges. Figures 7 to 9 show isocorrosion diagrams for sulfuric acid, sulfuric acid contaminated with 2000 ppm chloride ions, and hydrochloric acid, respectively.

Figure 7. Isocorrosion diagram in naturally aerated sulfuric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy) in a stagnant test solution.

Figure 8. Isocorrosion diagram, 0.1 mm/year (4 mpy) in a naturally aerated sulfuric acid containing 2000 ppm chloride ions.

Figure 9. Isocorrosion diagram in hydrochloric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy) in stagnant test solution.

Pitting and crevice corrosion

The pitting and crevice corrosion resistance of stainless steel is primarily determined by the content of chromium, molybdenum and nitrogen. The manufacturing and fabrication practices, e.g. welding, are also of vital importance for the actual performance in service.

A parameter for comparing the resistance to pitting in chloride environments is the PRE number (Pitting Resistance Equivalent).
The PRE is defined as, in weight-%
PRE = %Cr + 3.3 x %Mo + 16 x %N

For duplex stainless steels the pitting corrosion resistance is dependent on the PRE value in both the ferrite phase and the austenite phase, so that the phase with the lowest PRE value will be limiting for the actual pitting corrosion resistance. In SAF™ 2507 the PRE value is equal in both phases, which has been achieved by a careful balance of the elements.

The minimum PRE value for SAF™ 2507 seamless tubes is 42.5. This is significantly higher than e.g. the PRE values for other duplex stainless steels of the 25Cr type which are not super-duplex. As an example, UNS S31260 25Cr3Mo0.2N has a minimum PRE-value of 33.

One of the most severe pitting and crevice corrosion tests applied to stainless steel is ASTM G48, i.e. exposure to 6% FeCI₃ with and without crevices (method A and B respectively). In a modified version of the ASTM G48 A test, the sample is exposed for periods of 24 hours. When pits are detected together with a substantial weight loss (>5 mg), the test is interrupted. Otherwise, the temperature is increased by 5 °C (9 °F) and the test is continued with the same sample. Figure 11 shows critical pitting and crevice temperatures (CPT and CCT) from the test.

Potentiostatic tests in solutions with different chloride contents are presented in Figure 11. Figure 12 shows the effect of increased acidity. In both cases the applied potential is 600 mV vs SCE, a very high value compared with that normally associated with natural unchlorinated seawater, thus resulting in lower critical temperatures compared with most practical service conditions.

Figure 10. Critical pitting and crevice temperatures in 6% FeCl₃, 24h (similar to ASTM G48).

The scatter band for SAF™ 2507 and 6Mo+N illustrates the fact that both alloys have similar resistance to pitting, and CPT-values are within the range shown in the figure.

Tests were performed in natural seawater to determine the critical crevice corrosion temperature of samples with an applied potential of 150 mV vs SCE. The temperature was raised by 4°C (7^oF) steps every 24 hours until crevice corrosion occurred. The results are shown in the table below.

In these tests the propagation rates of initiated crevice corrosion attacks, at 15-50°C (59-122°F) and an applied potential of 150 mV vs SCE were also determined. These were found to be around ten times lower for SAF™ 2507 than for the 6Mo+N alloy.

Figure 11. Critical pitting temperatures (CPT) at varying concentrations of sodium chloride, from 3 to 25% (potentiostatic determination at +600 mV SCE with surface ground with 600 grit paper).

Figure 12. Critical pitting temperatures (CPT) in 3% NaCl with varying pH (potentiostatic determination at +600 mV SCE with surface ground with 600 grit paper).

The corrosion resistance of SAF™ 2507 in oxidizing chloride solutions is illustrated by critical pitting temperatures (CPT) determined in a ‘Green death’ -solution (1% FeCI₃ + 1% CuCl₂ +11% H₂SO₄ + 1.2% HCI) and in a ‘Yellow death’ -solution (0.1 % Fe₂(SO₄)₃ + 4% NaCl + 0.01 M HCI). The table below shows CPT-values for different alloys in these solutions. It is clear that the values for SAF™ 2507 are on the same level as those for the nickel alloy UNS N06625. The tests demonstrate a good correlation with the ranking of alloys for use as reheater tubes in flue gas desulfurization systems.

Critical pitting temperature (CPT) determined in different test solutions.

Stress corrosion cracking

SAF™ 2507 has excellent resistance to chloride induced stress corrosion cracking (SCC).

The SCC resistance of SAF™ 2507 in chloride solutions at high temperatures is illustrated in Figure 13. There were no signs of SCC up to 1000 ppm Cl^–/300°C and 10000 ppm Cl^–/250°C.

SAF™ 2507 U-bend specimens exposed for 1000 hours in hot brine (108°C, 226°F, 25% NaCl) showed no cracking.
The threshold stress for SAF 2507^® in 40% CaCl₂ at 100 °C (210 °F) and pH = 6.5 is above 90% of the tensile strength for both parent metal and welded joints (TIG-welded with Alleima® 25.10.4.L or MMA-welded with Alleima® 25.10.4.LR).

Figure 14 shows the result of testing in 40% CaCl₂ at 100 °C (210 °F) acidified to pH = 1.5. Acidifying of the standard test solution to pH = 1.5 lowers the threshold stress for SAF™ 2205, but not for SAF™ 2507. This applies to both parent metal and welded joints.

The threshold stress for both parent metal and welded joints of SAF™ 2507 in boiling 45% MgCl₂ , 155°C (311°F) (ASTM G36), is approximately 50% of the proof strength.

Figure 13. SCC resistance in oxygen-bearing (abt. 8 ppm) neutral chloride solutions. Testing time 1000 hours. Applied stress equal to proof strength at testing temperature.

Figure 14. Results of SCC tests with constant load in 40% CaCl₂, pH=1.5, at 100 °C (210°F) with aerated test solution.

Figure 15. Constant load SCC tests in NACE solution at room temperature (NACE TM 0177).

Figure 15 shows the results of SCC tests at room temperature in NACE TM0177 Test solution A (5% sodium chloride and 0.5% acetic acid saturated with hydrogen sulfide). No cracking occurred on SAF™ 2507, irrespective of the applied stress.

In aqueous solutions containing hydrogen sulfide and chlorides, stress corrosion cracking can also occur on stainless steels at temperatures below 60 °C (140 °F). The corrosivity of such solutions is affected by acidity and chloride content. In direct contrast to the case with ordinary chloride-induced stress corrosion cracking, ferritic stainless steels are more sensitive to this type of stress corrosion cracking than austenitic steels.

In accordance with ISO 15156/NACE MR 0175 solution annealed and rapid cooled wrought SAF™ 2507 is suitable for use at temperatures up to 450 °F (232 °C) in sour environments in oil and gas production, if the partial pressure of hydrogen sulphide does not exceed 3 psi (0.20 bar).

SAF™ 2507, with a maximum hardness of 32 HRC, solution annealed and rapidly cooled, according to NACE MR0103, is suitable for use in sour petroleum refining.

Hydrogen Induced Stress Cracking (HISC)

Hydrogen Induced Stress Cracking (HISC) is an embrittlement phenomenon that may occur in cathodically protected subsea steel constructions in the presence of high tensile stresses. When connected to cathodically protected carbon steels, super duplex stainless steels will also be cathodically protected even though this is not necessary. At the prevalent low electrochemical potentials, atomic hydrogen will be generated on the steel surfaces by the reduction of sea water. Embrittlement due to HISC may occur when hydrogen diffuses into the metal.

Hydrogen diffuses much faster in the ferrite phase than in the austenite phase. Therefore, ferritic steels and ferrite containing steels, e.g. super duplex stainless steels, are more susceptible to HISC than austenitic stainless steels. A high mechanical stress increases the risk of HISC by increasing the hydrogen diffusion rate, crack initiation and propagation in the material.

In super duplex stainless steels, cracks tend to propagate in the embrittled ferrite phase and arrest at ferrite-austenite phase boundaries. Susceptibility to HISC significantly increases with increasing austenite spacing. Coarse-grained microstructures are therefore more susceptible. A testing program performed at Alleima Materials Technology has confirmed that tendency to HISC is reduced when austenite spacing is less than 30 μm, as recommended by DNV RP-F112. Cold pilgered and solution annealed tubes with austenite spacing between 5-15 μm have shown very good resistance to HISC under applied stress up to 130% of the yield strength without cracking.

The acceptable stress without HISC occurring for products with different austenite spacing is illustrated in figure 16.

Figure 16. Tolerable stress as a percentage of the actual yield strength at 4°C without HISC occurring is schematically shown for tube and bar products with different austenite spacing. SAF™ 2507 (UNS S32750) has been tested on hydrogen pre-charged tensile specimens, using constant load with a dead weight, with an applied potential of -1050 mV/SCE in a electrochemical cell with 3% NaCl solution at 4°C, 500 hours. [NACE paper no. 07498]

Intergranular corrosion

SAF™ 2507 is a member of the family of modern duplex stainless steels whose chemical composition is balanced to give quick reformation of austenite in the high temperature heat affected zone of a weld. This results in a microstructure that provides the material with good resistance to intergranular corrosion. SAF™ 2507 passes testing to ASTM A262 Practice E (Strauss test) without reservation.

Erosion corrosion

The mechanical properties combined with corrosion resistance give SAF™ 2507 a good resistance to erosion corrosion. Testing in sand containing media has shown that SAF™ 2507 has an erosion corrosion resistance better than corresponding austenitic stainless steels. Figure 17 below shows the relative mass loss rate of the duplex SAF™ 2507, SAF™ 2205 and an austenitic 6Mo+N type steel after exposure to synthetic seawater (ASTM D-1141) containing 0.025-0.25% silica sand at a velocity of 8.9-29.3 m/s (average of all tests is shown).

Figure 17. Relative mass loss rate after testing of the resistance against erosion corrosion.

Corrosion fatigue

Duplex stainless steels which have a high tensile strength usually have a high fatigue limit and high resistance to both fatigue and corrosion fatigue.

The high fatigue strength of SAF™ 2507 can be explained by its good mechanical properties, while its high resistance to corrosion fatigue has been proven by fatigue testing in corrosive media.

PROCESSING

Hot Forming
2507 can be hot formed at temperatures ranging from 1875°F to 2250°F. This should be followed by a solution anneal at a minimum temperature of 1925°F, followed by a rapid air or water quench.

Cold Forming
Cold working 2507 can be accomplished using most typical stainless steel forming processes. Because the alloy has a higher yield strength than austenitic steels, but lower ductility, fabricators may need to use higher forming forces, larger bend radii, and springback allowances. Deep drawing, stretching, and other similar processes are more difficult for 2507 than for austenitic stainless steels. Solution annealing and quenching are recommended when the forming process requires more than 10% cold deformation.

Heat Treatment
Heat treatment for 2507 consists of solution annealing and quenching after hot or cold forming. Solution annealing should be performed at a minimum temperature of 1925°F. Annealing should be followed immediately by a rapid air or water quench. Heat treated items that are pickled and rinsed have excellent corrosion resistance.

Welding
2507 is weldable and can be joined to other materials using SMAW, GTAW, PAW, FCW, or SAW. When welding 2507, a 2507/P100 filler metal is recommended to ensure proper duplex weld construction.

2507 does not require preheating except to prevent condensation on cold metal. Interpass welding temperatures should not exceed 300°F or weld integrity will be compromised. To maximize corrosion resistance, root protection should be applied using either argon or 90% N2/10% H2 purge gas. The latter provides higher corrosion resistance.

2507/S32750 grade stainless steel pipe

Wide choice of dimensions and free cutting

• Whatever your needs, Daxun Alloys has the perfect stainless steel tube for you. We offer 3m and 6m lengths, with diameters from 3mm to 2000mm (Grade 2507) and ¾” to 20″ (Grade S32750).
• If you require a specific length, we offer a free unlimited cutting service with every purchase. Using state-of-the-art bandsaws, our precision cutting service can cut the tube to the exact length you require. Just let us know when ordering. It’s another reason why we are a leading stainless steel tube supplier to our domestic and international customers.

Contact your stainless steel tube suppliers

• You can easily order stainless steel pipes online, in any size, in any quantity.
• Alternatively, our friendly sales team will be happy to take your order. Simply call +86 13382898899 or email Daxunhejin@gmail and we will get back to you as soon as possible.