Sandvik SAF 2304®

Sandvik SAF 2304^® possesses good mechanical and physical properties, excellent resistance to stress corrosion cracking and other forms of corrosion as well as good weldability. These properties make it both a technically and economically superior alternative to stainless steels such as the austenitic steels ASTM 304, 304L, 316, 316L, 321 and 347, the ferritic chromium steels ASTM 430 and 444 and the martensitic chromium steels of the ASTM 410 and 420 type. Sandvik SAF 2304^® also has advantages over low-alloy steels. Examples where Sandvik SAF 2304^® offers advantages over other materials are given in the following table.

The good mechanical and corrosion properties make Sandvik SAF 2304^® an economical choice in many applications by reducing the life cycle cost of equipment.

General corrosion

Due to the high chromium content and its well-balanced composition Sandvik SAF 2304^® possesses excellent corrosion resistance in acidic environments. The table below gives the corrosion rate in different acids and Figures 4-6 show isocorrosion diagrams for Sandvik SAF 2304^® in sulphuric, formic and nitric acid, respectively.

Corrosion rates in acid-water mixtures, mm/year (mpy). Activated specimens, 1+3+3 days, average of the last two periods for two specimens.

B = Boiling

Figure 4. Isocorrosion diagram for Sandvik SAF 2304^®, ASTM 304L and ASTM 316L in a naturally aerated stagnant sulphuric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy).

Figure 5. Isocorrosion diagram for Sandvik SAF 2304^®. ASTM 304L and ASTM 316L in formic acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy).

Figure 6. Isocorrosion diagram for Sandvik SAF 2304^® and ASTM 304L in nitric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy)

Pitting

The pitting resistance of a steel is determined primarily by its chromium and molybdenum contents, but the nitrogen content also has an effect. The manufacturing and fabrication practice, e.g. welding, are also of vital importance for the actual performance in service.

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

The minimum PRE numbers for Sandvik SAF 2304^® and the austenitic standard stainless steels ASTM 316L and ASTM 304L are given in the following table.

As the table shows, the PRE number for Sandvik SAF 2304^® is considerably higher than the number for ASTM 304L and comparable to the number for ASTM 316L. Potentiostatic tests in solutions with different chloride contents (pH = 6) are reported in Figure 7. Each curve is based on at least 4 measuring points, each of which is derived from a minimum of three separate measurements. Variation in the measurement results is within the range +/-5°C (+/-9°F). The test results for ASTM 316L have been compared with results obtained under practical conditions of service, and the correlation is good. As is evident from Figure 7, the critical temperature for pitting for Sandvik SAF 2304^® is higher than that for ASTM 304L and comparable to that for ASTM 316L. The test results correspond with the PRE ranking.

Figure 7. Critical pitting temperature (CPT) for Sandvik SAF 2304^®, ASTM 304L and ASTM 316L in neutral chloride solutions (potentiostatic determination at +300 mV/SCE).

Stress corrosion and cracking

Because of its two-phase structure and its well balanced composition, Sandvik SAF 2304^® possesses very good resistance to stress corrosion cracking (SCC). This is evident from the results of tests in both concentrated chloride solutions and oxygen-containing dilute chloride environments.

Figure 8 shows the results of tests carried out in 40% CaCl₂ solution at 100°C (210°F). In this environment, the stress that is required to cause fracture due to SCC (known as the threshold stress) is much higher for Sandvik SAF 2304^® than for ASTM 304/304L and ASTM 316/316L. This demonstrates the superior resistance of Sandvik SAF 2304^®.

Welding of Sandvik SAF 2304^® does not appreciably reduce the resistance to SCC, provided that the welding recommendations are followed (see Welding). The threshold stress of material that has been TIG-welded with Sandvik 22.8.3.L or MMA-welded with Sandvik 22.9.3.LR is on a level with that of the parent metal.

The results of SCC testing of Sandvik SAF 2304 in chloride solutions at high temperatures and pressures are presented in Figure 9. The curve for Sandvik SAF 2304^® is based on tests performed in various chloride contents and at different temperatures. Usually, six separate measurements were taken at each measuring point. The testing method involved spring-loaded specimens stressed to the proof strength (R_p0.2) at the testing temperature or U-bends in which the stress at the apex is at least equal to the proof strength of the solution annealed material. The oxygen content of the inlet water (refreshed autoclaves were used) was 4.6 to 10 ppm and the pH 4.5 to 7. The curve for ASTM 304/304L and ASTM 316/316L is based on published test results and on practical experience.

Figure 8. Results of SCC tests with constant load on Sandvik SAF 2304^®, ASTM 304L and ASTM 316L in 40% CaCl₂, pH = 6.5, at 100°C (210°F) with aerated test solution.

The results in Figure 9 indicate that Sandvik SAF 2304^® can be used in dilute oxygen-bearing chloride solutions up to about 125°C (260°F) without any risk of SCC. This is much higher than for ASTM 304/ 304L and ASTM 316/316L, which should not be used above 60°C (140°F) in such environments. At temperatures above 125°C (260°F), Sandvik SAF 2304^® should not be used in oxygen-bearing solutions in environments with more than about 10 ppm Cl^-. For such conditions, we recommend Sandvik SAF 2205™, Sandvik SAF 2507^® or Sanicro^® 28. At low oxygen contents, which are common in the process and power industries, considerably higher chloride contents and temperatures can be tolerated by Sandvik SAF 2304^® without any risk of SCC.

Figure 9. SCC resistance in oxygen-bearing (approximately 8 ppm) neutral chloride solutions. Testing time 1000 hours. Load > yield strength at testing temperature.

Intergranular corrosion

Sandvik SAF 2304^® is a member of the family of modern duplex stainless steels whose chemical manner 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 similar to that of the parent metal. Welded joints in Sandvik SAF 2304^® easily pass Strauss' intergranular corrosion testing according to SS-EN ISO 3651-2 Method A.

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 2304^® possesses better resistance to crevice corrosion than steels of the ASTM 304/304L type and is comparable with ASTM 316/316L.

Erosion corrosion

Steels of the ASTM 304/304L and ASTM 316/316L type can be attacked by erosion corrosion if exposed to flowing media containing highly abrasive solid particles, e.g. sand, or media with very high flow rates. Under such conditions, Sandvik SAF 2304^® displays very good resistance because of its combination of high mechanical strength and good corrosion resistance.

Corrosion fatigue

Sandvik SAF 2304^® possesses higher strength and better corrosion resistance than ordinary austenitic stainless steels. Consequently, Sandvik SAF 2304^® has considerably better fatigue strength under corrosive conditions than such steels.

Bending

The starting force needed for bending is slightly higher for Sandvik SAF 2304^® than for ASTM 304/316. Sandvik SAF 2304^® can be cold bent to 25% deformation without requiring subsequent heat treatment. Under service conditions where the risk of stress corrosion cracking starts to increase, for example where the material temperature is nearly 125°C (255°F) in a neutral oxygen-bearing environment with around 100 ppm Cl^- , heat treatment is recommended even after moderate cold bending. For pressure vessel applications in Germany and the Nordic countries heat treatment may be required after cold deformation in accordance with VdTÜV-Wb 496 and NGS 1607. Heat treatment is carried out in the form of solution annealing (see under Heat treatment) or resistance annealing. Hot bending is carried out at 1100-950°C (2010-1740°F) and should be followed by solution annealing.

Expanding

In comparison with austenitic stainless steels, Sandvik SAF 2304^® has a higher proof strength. 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.

Contact us for detailed recommendations on expanding duplex stainless steels.

Machining

Being a two-phase material (austenitic-ferritic) Sandvik SAF 2304^® will present a different tool wear profile from that of single-phase steels of type ASTM 304L. The cutting speed must, therefore, be slightly lower than that recommended for ASTM 304L and as a first recommendation 25 % lower cutting speeds should be used compared to the speed for machining Sanmac 304/304L. It is, however, recommended that a tougher insert grade is used for semi-roughing operations, Sanmac^® 304/304L.

Seamless tube and pipe- finishes and dimensions

Seamless tube and pipe in Sandvik SAF 2304^® is supplied in dimensions up to 260mm outside diameter. The delivery condition is solution annealed and either white pickled or bright annealed.

For hydraulic tubing applications, Sandvik SAF 2304^® is also marketed under the name PW 400.

Other forms of supply:

• Welded tube and pipe
• Fittings
• Strip, annealed or cold-rolled to different hardness
• Bar steel
• Plate, sheet and wide strip
• Forged products
• Cast products

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

Solution annealing

930 -1050°C (1710 -1920°F), rapid cooling in air or water.

The following figures apply to the material in the solution annealed condition. Tube and pipe with wall thickness above 20 mm (0.787 in.) may have slightly lower values. For seamless tubes with a wall thickness below 4 mm we guarantee proof strength (R_p0.2) values that are 50 MPa higher than those listed below at 20°C (68°F) as well as those listed at higher temperatures. More detailed information can be supplied on request.

At 20 °C (68°F)

Tube and pipe with a wall thickness max. 20 mm (0.79 in.)

1 MPa = 1 N/mm²
a) R_p0.2 and R_p1.0 correspond to 0.2% offset and 1.0% offset yield strength, respectively.
b) Based on L₀ = 5.65 √S₀ where L₀ is the original gauge length and S₀ the original cross-section area.
c) For tubes with outside diameter max. 25.4 mm we guarantee 0.2% proof strength 450 MPa (65 ksi) and tensile strength min. 690 MPa (100 ksi) (ASTM A789).

Impact strength

Sandvik SAF 2304^® has good impact strength both at room temperature and at low temperatures, as shown by Figure 2.

Figure 2. Impact strength of Sandvik SAF 2304^® compared with carbon steel. Lowest value for a TIG-joint is also reported for SAF 2304^®. Charpy test specimens (10 x 10 mm, 0.394 x 0.0394 in.) taken from tube 260 x 20 mm (10.2 x 0.29 in.) in size.

At high temperatures

If Sandvik SAF 2304^® 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 496.

Density: 7.8 g/cm³, 0.28 lb/in³

Thermal conductivity

Thermal expansion, mean values in temperature ranges (x10^-6) SAF 2304^® has a far lower coefficient of thermal expansion than austenitic stainless steels and therefore possesses certain design advantages.

Figure 3. Thermal expansion, per °C (30-100°C, 86-210°F).

The weldability of Sandvik SAF 2304^® 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 2304^®, 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 23 7 N L R

ISO 3581 E 22 9 3 N L R / AWS A5.4 E2209-17 (e.g. Exaton 22.9.3.LR)