Safurex^® high-alloy duplex steel is suitable for the following applications in urea plants:

Urea carbamate found in the high-pressure urea process is very aggressive and corrosion problems in form of intergranular or/and general corrosion are found when an inferior material is applied.

Variations in corrosion properties have been found especially on ASTM 316L type materials for urea applications (e.g. Sandvik 3R60 Urea Grade). The Huey test has been included as material test for Sandvik 3R60 U.G. and Sandvik 2RE69 to verify a low general corrosion rate and a low intergranular attack. Past experience has showed that a high corrosion rate in service corresponds to a high corrosion rate in the Huey test.

Under normal service conditions in the Stamicarbon urea process the chloride concentration in the boiler water is extremely low and no chloride induced corrosion will appear. If the chloride concentration is unintentionally increased, stress corrosion cracking (SCC) can occur on austenitic stainless steels like Sandvik 3R60 U.G. and Sandvik 2RE69. The risk for SCC increases with increasing chloride concentration, stresses and temperature.

At the prevailing conditions even small concentrations in the range 10–50 ppm may be sufficient to cause SCC. Austenitic-ferritic (duplex) grades have in general a much improved resistance to SCC (see Figure 1). Pitting and crevice corrosion are formed at much higher chloride concentrations, which are unlikely to appear in service. Safurex has superior resistance to pitting and crevice corrosion compared to Sandvik 3R60 U.G. and Sandvik 2RE69.

Stress corrosion cracking (SCC)

Safurex^® high-alloy duplex steel has excellent resistance to chloride induced stress corrosion cracking (SCC). The resistance of various alloys to stress corrosion cracking (SCC) determined by constant load testing in aerated 40% CaCl₂ , pH 1.5, at 100°C (210°F), (modified ASTM G36 method) is shown in Figure 1.

Figure 1. Constant load testing of Safurex and some other alloys in 40% CaCl₂, pH 1.5 at 100°C (210°F). Time to failure vs. applied stress in percentage of tensile strength is shown.

Intergranular corrosion

Safurex^® has, owing to its high content of important alloying elements, excellent resistance to intergranular corrosion.

The resistance to intergranular corrosion is often studied by means of either Huey testing (ASTM A262 practice C, 5x48 h in boiling HNO₃) or Streicher testing (ASTM A262 practice B, 120 h in boiling H₂SO₄ + FeSO₄). For Safurex^®, the Streicher test is a more relevant test method. In the table below, some results from this test are presented. Regarding Huey testing, Safurex^® meets the same requirements as Sandvik 2RE69 according to specifications from Stamicarbon.

Pitting and crevice corrosion

Safurex^® high-alloy duplex steel has a carefully balanced composition with a high content of chromium and nitrogen and a moderate amount of molybdenum. This enables a very high resistance to localized corrosion, such as pitting and crevice corrosion.

One of the most severe pitting and crevice corrosion tests applied to stainless steel is ASTM G48, i.e. immersion in 6% FeCl₃. When pits are detected following a 24 hours exposure, together with a substantial weight loss (>5 mg), the test is interrupted. Otherwise the temperature is increased 5°C (9°F) and the test is continued with the same sample.

In the table below, Safurex^® is compared with some other grades used in the urea process with regard to pitting resistance equivalent (PRE) and the critical pitting temperature (CPT) determined by ASTM G48A.

Critical pitting temperature (CPT) acc. to ASTM
G48A for Safurex^® and some other urea grades

Potentiostatic tests in solutions with different chloride contents are reported in Figure 2. The applied high potential, 600 mV SCE, corresponds to very harsh conditions, thus resulting in conservative data with a lower critical temperature compared with most practical situations.

Critical crevice corrosion temperature (CCT) was determined with the MTI-2 procedure which is immersion in 6% FeCl₃, 12 crevices on each side of the test coupon and a torque of 0.28 Nm. Duplicate samples were immersed for 24 hours. The CCT for Safurex^® was approximately 40°C (100°F).

Figure 2. Critical pitting temperature (CPT) at varying concentrations of sodium chloride (NaCl), from 3 to 25% (potentiostatic determination at +600 mV SCE with surface ground to 600 grit paper).

Bending

The force needed for bending Safurex^® high-alloy duplex steel is higher than that for standard austenitic stainless steels which is a natural consequence of the grade's higher proof strength.

Expanding

Compared with austenitic stainless steels, Safurex has a higher proof and tensile strength. This must be kept in mind when expanding tubes into tubesheets. Normal methods can be used, but the expansion requires higher initial force and should be undertaken in a one-step operation. As a general rule, tube to tubesheet joints should be welded to ensure a leak free joint.

Machining

Being a dual phase material (austenitic-ferritic) Safurex^® will present a different wear picture from that of a single phase material like Sandvik 2RE69. The cutting speed must therefore be lower than that recommended for austenitic grades. Contact us for further information.

Seamless tube and pipe

Safurex^® can be supplied as straight or U-bent tubing. The size range is up to OD 457 mm (18.1 in.).

The tubes are supplied in the solution annealed and white pickled condition or in the bright-annealed condition.

Other forms of supply:

• Fittings
• Flanges
• Plate
• Round bar
• Forgings

The mechanical properties are determined for heat exchanger tubes. If Safurex^® is exposed for prolonged periods to temperature ranges exceeding 280°C (540°F), the microstructure changes, which results in a reduction in toughness. This does not necessarily affect the behavior of the material at the operating temperature. The listed values are valid for tube and pipe.

At 20°C (68°F)

1 MPa = 1 N/mm²
a) R_p0.2 corresponds to 0.2% offset offset yield strength.
b) Based on L₀ = 5.65 √S₀ where L₀ is the original gauge length and S₀ the original cross-section area.

At high temperatures

The weldability of Sandvik Safurex^® is good. Welding must be carried out without preheating and subsequent heat treatment is normally not necessary. 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 Sandvik Safurex^®, heat input of 0.2-1.5 kJ/mm and interpass temperature of <150°C (300°F) are recommended.

Recommended filler metals

GTAW/TIG welding

Sandvik Safurex^® / Exaton 29.8.2.L

Sandvik Safurex^® / Exaton 29.8.2.L wire or strip electrodes are recommended for overlay welding of tubesheets and high-pressure vessels in cases where corrosion resistance, equal to that of Sandvik Safurex^®, is required.