SAF 3207 HD™

SAF 3207 HD™ hyper-duplex steel has been developed for deepwater umbilicals; an application with extreme requirements on pitting and crevice corrosion resistance, mechanical strength and fatigue properties.

Offering greater yield strength, greater corrosion resistance and superior fatigue properties compared with super-duplex steels, SAF 3207 HD™ hyper-duplex steel is also an excellent material choice for a range of other oil and gas applications. For example, when resistance to hydrogen sulfide (H₂S) is a requirement, such as in raw seawater injection, control lines and chemical injection lines, SAF 3207 HD™ offers high reliability and operational safety leading to lower operating costs.

General corrosion

SAF 3207 HD™ is highly resistant to corrosion by organic acids, e.g. formic acid and acetic acid, see Figure 4.

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

Resistance to inorganic acids is comparable to, or even better than that of high alloy austenitic stainless steels in certain concentration ranges. Figures 5 and 6 show isocorrosion diagrams for sulfuric acid and hydrochloric acid, respectively.

Figure 5. Isocorrosion diagram in sulphuric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy) in a stagnant test solution.

Figure 6. Isocorrosion diagram in hydrochloric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy) in a 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 practice, 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
The minimum PRE-value for SAF 3207 HD is 50.

One of the most severe pitting corrosion tests, applied to stainless steels, is ASTM G48 i.e. exposure to 6% FeCl3. In a modified version of the ASTM G48A 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 continued with the same sample.

The crevice corrosion test was performed in 6% FeCl3, with a crevice specified in the MTI-2 procedure, where an artificial crevice is mounted on the sample with a torque of 0.28 Nm. The values obtained and comparisons with SAF 2507^® are given in Figure 7. SAF 3207 HD™ performs better than SAF 2507^®.

Figure 7. Critical pitting temperature measured in modified G48A, starting temperature 55°C. Critical crevice corrosion temperature obtained in testing with a crevice specified in the MTI-2 testing.

Figure 8: Design curve for butt welded Sandvik SAF 3207 HD and Sandvik SAF 2507 showing initiation of crevice corrosion in seawater versus potential and temperature.

Machining

Being a dual phase material (austenitic-ferritic), SAF 3207 HD™ will present a different tool wear profile from that of austenitic stainless steels.

The cutting data (speed and feed) must, therefore, be lower than that recommended for austenitic grades.

Seamless tube

Seamless tubes in SAF 3207 HD™ can be supplied in typical umbilical tube dimensions. The delivery condition is either solution annealed and either white pickled or polished.

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

1040-1140°C (1905-2085°F), rapid cooling in air, protective atmosphere or water.

At 20°C (68°F)

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.

At high temperatures

If SAF 3207 HD™ is exposed to temperatures exceeding 250 °C (480 °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. Please contact Sandvik for more information.

Fatigue strength

Figure2: S-N curves for Sandvik SAF 3207 HD, SAF 2507 and DNV design curves

Impact strength

SAF 3207 HD™ possesses good impact strength. The ductile-brittle transition temperature is below -50°C (-58°F). Figure 3 shows the impact strength for SAF 3207 HD™.

Figure 3. Impact energy curve for Sandvik SAF 3207 HD, using half size specimens (5x10x55mm) average of 3 at each temperature. Parent metal samples are taken in the longitudinal direction from 114,3x6,35 cold pilgered and annealed (1120°C, 2048°F) tube.

Density: 7.7 g/cm³, 0.28 lb/in³

Specific heat capacity

Thermal conductivity

Thermal expansion

SAF 3207HD™ has a coefficient of thermal expansion close to that of carbon steel. This gives SAF 3207HD™ definite design advantages over austenitic stainless steels in equipment comprising of both carbon steel and stainless steel. The values given below are average values in the temperature ranges.

Modulus of elasticity, (x10³)

The weldability of SAF 3207 HD™ is good. Welding must be carried out without preheating and subsequent heat treatment is normally not necessary. A suitable method of fusion welding is gas tungsten arc welding GTAW/TIG with shielding gas of Ar+2% N₂. For tube to tubesheet welding, it is recommended to use Ar+3% N₂ as shielding gas to have proper weld metal structure.

For SAF 3207 HD™, heat input of 0.2-1.0 kJ/mm and interpass temperature of <100°C (210°F) are recommended.

Recommended filler metals

GTAW/TIG welding

Exaton 27.7.5.L. For information on filler metal and welding method, please contact Sandvik.