Sandvik SAF 2205™ Cold Worked

Due to its excellent corrosion properties, Sandvik SAF 2205™ is a highly suitable material for service in environments containing chlorides and hydrogen sulphide. The material is suitable for use in production tubing and flowlines for the extraction of oil and gas from sour wells, in refineries and in process solutions contaminated with chlorides. Sandvik SAF 2205™ is particularly suitable for heat exchangers where chloride-bearing water or brackish water is used as a cooling medium. The steel is also suitable for use in dilute sulphuric acid solutions and for handling, organic acids, e.g. acetic acid and mixtures.

The high strength of Sandvik SAF 2205™ makes the material an attractive alternative to the austenitic steels in structures subjected to heavy loads.

The good mechanical and corrosion properties make Sandvik SAF 2205™ an economical choice in many applications by reducing the life cycle cost of equipment.

Approvals:

If Sandvik SAF 2205™ is exposed to temperatures exceeding 280°C (540°F), for prolonged periods, the microstructure changes, which results in a reduction in impact strength. This does not necessarily affect the behavior of the material at the operating temperature. For example, heat exchanger tubes can be used at higher temperatures without any problems. Please contact Sandvik for more information. For pressure vessel applications, 280°C (540°F) is required as a maximum according to VdTÜV-Wb 418 and NGS 1606.

The starting force needed for bending is slightly higher for Sandvik SAF 2205™ than for standard austenitic grades (ASTM TP304L and TP316L). Sandvik SAF 2205™ can be cold-bent to 25% deformation without requiring subsequent heat treatment. For pressure vessel applications in Germany and the Nordic countries, heat treatment may be required after cold deformation in accordance with VdTÜV-Wb 418 and NGS 1606.

Under service conditions where the risk of stress corrosion cracking starts to increase, heat treatment is recommended even after moderate cold bending, for example, where the material temperature is nearly 150°C (300°F) in an oxygen-bearing, environment with around 100 ppm Cl⁻.

General corrosion: In most media, Sandvik SAF 2205™ possesses better resistance to general corrosion than steel of type ASTM TP316L and TP317L. The improved resistance of Sandvik SAF 2205 is illustrated by the isocorrosion diagram for corrosion in sulphuric acid, Figure 3, and the diagram showing the corrosion rates in mixtures of acetic and formic acid, Figure 4. Figure 5 shows the isocorrosion diagram for Sandvik SAF 2205 in hydrochloric acid.

Impurities that increase corrosivity are often present in process solutions of acids. If there is a risk of active corrosion, higher alloyed stainless steels should be chosen, e.g. the austenitic grades Sandvik 2RK65™ or Sanicro® 28, or the super-duplex grade Sandvik SAF 2507®.

Pitting corrosion: The pitting resistance of a steel is determined primarily by its chromium and molybdenum contents, but also by its nitrogen content and its slag composition and content. The manufacturing and fabrication practices, e.g. welding, are also of vital importance for actual performance in service.

A parameter for comparing the resistance of different steels to pitting, is the PRE number (Pitting Resistance Equivalent).

The PRE is defined as, in weight-%

PRE = % Cr + 3.3 x % Mo + 16 x % N

The PRE number for Sandvik SAF 2205™ is compared with other materials in the following table:

The ranking given by the PRE number has been confirmed in laboratory tests. This ranking can generally be used to predict the performance of an alloy in chloride containing environments. Because of the high Mo and N contents, the PRE number for Sandvik SAF 2205™ is significantly higher than what would be the case with lower Mo and N contents which are still within the limits of UNS S31803.

The results of laboratory tests, to determine the critical temperature for the initiation of pitting (CPT) at different chloride contents are shown in Figure 6. The chosen testing conditions have yielded results that match well with practical experience. Thus, Sandvik SAF 2205™ can be used at considerably higher temperatures and chloride contents than ASTM TP304 and ASTM TP316 without pitting. Sandvik SAF 2205™ is, therefore, far more serviceable in chloride-bearing environments than standard austenitic steels.

Stress corrosion cracking: The standard austenitic steels ASTM TP304L and ASTM TP316L are prone to stress corrosion cracking (SCC) in chloride-bearing solutions at temperatures above 60°C (140°F).

Duplex stainless steels are far less prone to this type of corrosion. Laboratory tests reveal good resistance to stress corrosion cracking of Sandvik SAF 2205™. Results from the tests are presented in Figure 7. The diagram indicates the temperature-chloride range within which Sandvik SAF 2205 and the standard steels ASTM TP304L and ASTM TP316L have low susceptibility to stress corrosion cracking.

Results of laboratory tests carried out in calcium chloride are shown in Figure 8. The tests have been continued to failure or a max. test time of 500 h.

The diagram shows that Sandvik SAF 2205™ has a much higher resistance to SCC than the standard austenitic steels ASTM TP304L and ASTM TP316L.

In aqueous solutions containing hydrogen sulphide 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 ordinary chloride-induced stress corrosion cracking, ferritic stainless steels are more sensitive to this type of stress corrosion cracking, than austenitic steels.

Laboratory tests have shown that Sandvik SAF 2205™ possesses good resistance to stress corrosion cracking in environments containing hydrogen sulphide. This has also been confirmed by available operating experience.

In accordance with NACE MR0175/ISO 15156 solution annealed and cold-worked Sandvik SAF 2205™ is acceptable for use at any temperature up to 450°F (232°C) in sour environments, if the partial pressure of hydrogen sulphide does not exceed 0.3 psi (0.02 bar) and its hardness is not greater than HRC 36. In the solution annealed and rapidly cooled condition Sandvik SAF 2205™ is acceptable for use at any temperature up to 450°F (232°C) in sour environments, if the partial pressure of hydrogen sulphide does not exceed 1.5 psi (0.1 bar).

According to NACE MR0103 solution annealed and rapidly cooled Sandvik SAF 2205™, with hardness maximum HRC 28 is acceptable in sour petroleum refining.

Figure 9 shows the results of stress corrosion cracking tests at room temperature in NACE TM 01777 test solution A with hydrogen sulphide. The high resistance of Sandvik SAF 2205™ is shown in the figure by the fact that very high stresses, about 1.1 times the 0.2% proof strength, are required to induce stress corrosion cracking. The resistance of welded joints is slightly lower. The ferritic chromium steel ASTM 410 fails at considerably lower stress.

Intergranular corrosion: Sandvik SAF 2205™ is a member of the family of modern duplex stainless steels whose chemical composition is balanced in such a way that the reformation of austenite in the heat-affected zone, adjacent to the weld, takes place quickly. This results in a microstructure that gives corrosion properties and toughness roughly equal to that of the parent metal. Testing according to ASTM A262 PRE (Strauss´ test) presents no problems for welded joints in Sandvik SAF 2205™, which pass without reservations.

Crevice corrosion: In the same way as the resistance to pitting can be related to the chromium, molybdenum and nitrogen contents of the steel, so can the resistance to crevice corrosion. Sandvik SAF 2205™ possesses better resistance to crevice corrosion than steels of the ASTM 316L type.

Erosion corrosion: Steels of the ASTM 316 type are attacked by erosion corrosion if exposed to flowing media containing highly abrasive solid particles, e.g. sand, or to media with very high flow velocities. Under such conditions Sandvik SAF 2205™ displays very good resistance because of its combination of high hardness and good corrosion resistance.

Corrosion fatigue: Sandvik SAF 2205™ possesses higher strength and better corrosion resistance than ordinary austenitic stainless steels. Consequently, Sandvik SAF 2205, has considerably better fatigue strength under corrosive conditions than such steels.

In rotary bending, fatigue tests in a 3% NaCl solution (pH = 7; 40°C (104°F); 6000 rpm), the following results were obtained. The values shown indicate the stress required to bring about rupture after 2x10⁷ cycles.

Compared with austenitic stainless steels, Sandvik SAF 2205™ has higher proof and a tensile strengths. This must be borne in mind when expanding tubes into tube-sheets. Normal expanding methods can be used, but the expansion requires higher initial force and should be undertaken in one operation.

Tubes are normally delivered in the heat-treated condition. If additional heat treatment is needed due to further processing, the following is recommended.

Solution annealing: 1020 - 1100°C (1870-2010°F), rapid cooling in air or water.

Hot bending is carried out at 1100-950°C (2010-1740°F) and should be followed by solution annealing.

Heat treatment is carried out in the form of solution annealing (see under Heat treatment) or resistance annealing.

Being a two-phase (austenitic-ferritic) material, Sandvik SAF 2205™ will present a different tool wear profile from that of single phase steels of types ASTM TP304/304L and TP316/316L. The cutting speed must, therefore, be lower than that recommended for ASTM 304/304L and 316/316L. Built-up edges and chipping are to be expected. It is recommended that a tougher insert grade is used than when machining austenitic stainless steel, e.g. ASTM TP304L.

A version with improved machinability, Sandvik Sanmac 2205™, is available as bar and hollow bar.

Forms of supply:

Seamless tube and pipe in Sandvik SAF 2205™ is supplied in dimensions up to 260 mm (10.2 in.) outside diameter. They are delivered in the solution annealed condition and either white pickled or bright annealed. They can also be delivered cold-worked without subsequent heat treatment.

Other product forms

The weldability of Sandvik SAF 2205™ is good. Welding must be carried out without preheating and subsequent heat treatment is normally not necessary. Suitable methods of fusion welding are manual metal-arc welding (MMA/SMAW) and gas-shielded arc welding, with the TIG/GTAW method as first choice.

For Sandvik SAF 2205™, heat input of 0.5-2.5 kJ/mm and interpass temperature of <150°C (300°F) are recommended.

Recommended filler metals:

• TIG/GTAW or MIG/GMAW welding
  • ISO 14343 S 23 7 N L (e.g. Exaton 23.7.L)
  • ISO 14343 S 22 9 3 N L / AWS A5.9 ER2209 (e.g. Exaton 22.8.3.L)
• MMA/SMAW welding
  • ISO 3581 E 22 9 3 N L R / AWS A5.4 E2209-17 (e.g. Exaton 22.9.3.LR)
  • ISO 3581 E 22 9 3 N L B / AWS A5.4 E2209-15 (e.g. Exaton 22.9.3.LB)
  • ISO 14343 S 22 9 3 N L / AWS A5.9 ER2209 (e.g. Exaton 22.8.3.L) wire or strip electrodes are recommended for overlay welding of tube sheets and high-pressure vessels in cases where corrosion resistance, equal to that of Sandvik SAF 2205™, is required.