Sandvik 2RE69

Sandvik 2RE69 is used successfully in all critical high-pressure units of modern urea processes, such as:

Sandvik 2RE69 has also found extensive use in other corrosive environments in fertilizer plants, such as:

Urea specifications issued by: Stamicarbon, Snamprogetti, Urea Casale and Kellogg

Approvals:

The excellent formability of Sandvik 2RE69 permits cold bending to small bending radii. Annealing is not normally necessary after cold bending. Heat treatment, if any, should take the form of solution annealing, see under 'Heat treatment'.

General corrosion: Sandvik 2RE69 was originally developed for stripper tubes used in the production of urea. Practical experience has confirmed its excellent corrosion resistance in urea/carbamate solutions at high pressures and temperatures.

Sandvik 2RE69 is also highly resistant to inorganic acids, as illustrated by the iso-corrosion diagram for nitric acid, Figure 1.

Intergranular corrosion: Sandvik 2RE69 is highly resistant to intergranular corrosion after welding. Huey test values for TIG welded specimens are typically below 0.12 mm/year for unsensitized material (4.8 mpy, 0.66 μm/48 h).

A statistical evaluation of Huey tests, as specified in the urea specifications, is shown in Figure 2. A maximum value of 0.12 mm/year (4.8 mpy, 0.66 μm/48 h) can be guaranteed for straight Sandvik 2RE69 tubes in the 'as delivered' condition. Max. 20 μm (8 mils) selective attack is similarly guaranteed.

Pitting and crevice corrosion: Sandvik 2RE69 has very good resistance to pitting, as illustrated in Figure 3, and is also far more resistant to crevice corrosion than ASTM 316L.

Stress corrosion cracking (SCC): Conventional austenitic stainless steels of type ASTM 304 and 316 are susceptible to stress corrosion cracking (SCC) in chloride-bearing solutions at temperatures exceeding about 60°C (140°F). The higher nickel content makes Sandvik 2RE69 slightly more resistant, as assessed in laboratory tests, see Figure 4.

Erosion corrosion: The good mechanical strength of Sandvik 2RE69 makes it resistant to erosion-corrosion. Ferrules for urea strippers are one application where this property is utilized.

If heat treatment is needed after further processing the following is recommended.

Solution annealing: 1070–1120°C (1960–2050°F), 5–15 minutes, depending on size, followed by rapid cooling in water or air.

Hot bending is carried out at 1100–850°C (2010–1560°F), usually without subsequent heat treatment.

The chip removing machining of stainless steels always requires an adjustment of cutting data and machining method to give satisfactory results. When turning is undertaken with carbide tipped tools, the cutting speed should be reduced by 30% for finish machining and 60% for rough machining compared with the cutting speeds applied for ASTM 316L. Much the same applies to other operations. If high speed steel tools are used, the cutting speed should be reduced by 20 % compared with the cutting speeds applied for ASTM 316L.

Sandvik can provide detailed recommendations for the choice of tools and cutting data. Data to be selected as for grade Sanmac 316/316L, taking into account the above comments.

Forms of supply:

Sandvik 2RE69 seamless tube and pipe is supplied in dimensions up to 355 mm (14.0 in.) outside diameter in the solution annealed and white pickled condition, or solution annealed by a bright annealing process. U-tubes can be supplied on request.

Other forms of supply:

The weldability of Sandvik 2RE69 is good. Welding must be carried out without preheating, and normally there is no need for any subsequent heat treatment. 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.

In common with all fully austenitic stainless steels, Sandvik 2RE69 has low thermal conductivity and high thermal expansion. Welding plans should therefore be carefully selected in advance, so that distortions of the welded joint are minimized. If residual stresses are a concern, solution annealing can be performed after welding.

For Sandvik 2RE69, heat-input of <1.0 kJ/mm and interpass temperature of <100°C (210°F) are recommended. A string bead welding technique should be used.

Recommended filler metals:

• TIG/GTAW or MIG/GMAW welding
  • ISO 14343 S 25 22 2 N L (e.g. Exaton 25.22.2.LMn)
• MMA/SMAW welding
  • ISO 3581 E 25 22 2 N L B (e.g. Exaton 25.22.2.LMnB)
  • ISO 14343 S 25 22 2 N 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 2RE69, is required.