Sandvik SAF 2304 (Plate and sheet)

Chemical composition (nominal) %

Forms of supply finishes and dimensions

Plate and sheet are delivered in the solution annealed and pickled condition.

The size ranges available are given below

Plate sheet and coil

Sandvik SAF 2304 Plate, sheet are stocked in a wide range of sizes according to ASTM.

Details of our stock programme are given below:

Sandvik Australia's Stock Program Plate (HR-No1 Finish)

Thickness Range - 5mm - 40mm

Widths - 2000mm

Sheet (CR-2B Finish)

Thickness Range - 2mm

Widths - 1500mm,

Lengths - 3000mm

Mechanical properties

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.

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.

Impact strength

SAF 2304 has good impact strength both at room temperature and at low temperatures, as shown by figure 2.

Figure 2. Impact strength of SAF 2304 compaed with carbon steel. Lowest value for a TIG-joint is also repoted 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 SAF 2304 is exposed for prolonged periods to temperatures exceeding 280 ^oC (540 ^oF), the microstructure changes which results in a reduction in impact strength. This effect does not necessarily affect the behaviour of the material at the operating temperature. For example, heat exchanger tubes may be used at higher temperatures without any problems. Contact Sandvik for advice.

For pressure vessel applications, 280 °C (540 °F) is required as maximum according to VdTÜV-Wb 496.

For constructions according to RN78 (NGS 1607) 5-15% lower design valuesare approved.

Physical properties

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).

Corrosion resistance

General corrosion

Due to the high chromium content and its well-balanced composition SAF 2304 possesses excellent corrosion resistance in acidic environments. The table below shows the corrosion rate in different acids and figures 4-6 show isocorrosion diagrams.

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 SAF 2304, AISI 403L and AISI 316L in stagnant sulphuric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy).

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

Figure 6. Isocorrosion diagram for SAF 2304 and AISI 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. 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 % Mo + 16 x % N

The minimum PRE-number for SAF 2304 and the austenitic standard stainless steels AISI 316L and AISI 304L are given in the following table.

As the table shows, the PRE-number for SAF 2304 is considerably higher than the number for AISI 304L and comparable to the I number for AISI 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, and the variation in the measurement results lies within the range +/-5°C (+/-9°F). The test results for AISI 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 SAF 2304 is higher than that for AISI 304L and comparable to that for AISI 316L. The experimental results thereby agree with the PRE ranking.

Figure 7. Critical pitting temperature (CPT) for SAF 2304, AISI 304L and AISI 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, 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 SAF 2304 than for AISI 304/304L and AISI 316/316L. This indicates the superior resistance of SAF 2304.

Welding of 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 SAF 2304 in chloride solutions at high temperatures and pressures are presented in figure 9. The curve for SAF 2304 is based on tests performed at various chloride contents and different temperatures. Usually six separate measurements were performed 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. I. The curve for AISI 304/304L and AISI 316/316L is based on published test results and on practical experience.

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

The results in figure 9 indicate that SAF 2304 can be used in oxygen-bearing chloride solutions up to about 125°C (260°F) without any risk of SCC.

This is much higher than for AISI 304/ 304L and AISI 316/316L, which should not be used above 60°C (140°F) in such environments. At temperatures above 125°C (260°F), 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, 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 SAF 2304 without any risk of SCC.

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

Intergranular corrosion

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 SAF 2304 easily pass Strauss' intergranular corrosion testing according to DIN 50914 and SS 117105.

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. SAF 2304 has better resistance to crevice corrosion than steels of the AISI 304/304L type and is comparable with AISI 316/316L.

Erosion corrosion

Steels of the AISI 304/304L and AISI 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. Due to its combination of high hardness and good corrosion resistance, SAF 2304 displays very good resistance under such conditions.

Corrosion fatigue

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

Heat treatment

Plate,sheet and coil are normally delivered in heat treated condition. If additional heat treatment is needed after further processing the following is recommended.

Solution annealing

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

Welding

The weldability of SAF 2304 is good. Suitable welding methods are manual metal-arc welding with covered electrodes or gasshielded arc welding.

Welding should be undertaken within the heat input range 0.5-2.5 kJ/mm. Preheating or post-weld heat treatment is not normally necessary.

Filler metals that give an austenitic-ferritic weld metal should be used in order to obtain a weld metal with corrosion resistance and mechanical properties close to the parent metal. For gas-shielded arc welding, we recommend Sandvik 22.8.3.L, and for manual metal-arc welding the covered electrode Sandvik 22.9.3.LR. These filler metals can also be used for welding SAF 2304 to carbon steels, stainless steels and nickel alloys. The covered electrode Sandvik 23.12.2.LR and the wire electrode Sandvik 24.13.L can also be used for this purpose. When welding components for use in high concentrated nitric acid the choice of filler metal should be discussed with Sandvik.

Fabrication

Bending

The starting force needed for bending is slightly higher for SAF 2304 than for AISI 304/316. 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 this heading) 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, 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. Detailed recommendations on expanding duplex stainless steels will be supplied on request.

Machining

Being a two-phase material (austenitic-ferritic) SAF 2304 will present a different wear picture from that of single-phase steels of type AISI 304L. The cutting speed must therefore be slightly lower than that recommended for AISI 304L. It is recommended that a tougher insert grade is used than when machining austenitic stainless steels, e.g. AISI 304L.

Applications

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 AISI 304, 304L, 316, 316L, 321 and 347, the ferritic chromium steels AISI 430 and 444 and the martensitic chromium steels of the AISI 410 and 420 type. SAF 2304 also has advantages in many cases over low-alloy steels. Examples of areas where SAF 2304 offers advantages over other materials are given in the following table.

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