Sandvik SAF 2906™

Typical applications for Sandvik SAF 2906™ are:

Approvals:

The force needed for bending Sandvik SAF 2906™ is higher than that for standard austenitic stainless steels which is a natural consequence of the higher yield strength.

Caustic environment: Sandvik SAF 2906™ has, owing to its high chromium content, a very good resistance in caustic environments. For pure caustic soda, the performance of Sandvik SAF 2906™ can be observed in the iso-corrosion diagram (0.1 mm/year), see Figure 1. In the production of caustic soda, however, impurities in the form of chlorides and chlorates are present. Usually in the caustic soda manufacturing process Ni200 or high nickel alloys are used, especially in critical equipment such as the evaporators.

Results of corrosion tests in sodium hydroxide (NaOH) containing different amounts of sodium chloride (NaCl) and sodium chlorate (NaClO₃ ), similar to the content found in the evaporation plants of diaphragm and membrane processes, are presented in Table 1, 2 and Figure 2. The results clearly indicate the good performance of Sandvik SAF 2906™ in caustic environments.

Table 1. Performance in caustic environments - Diaphragm process

Table 2. Performance in caustic environments - Membrane process

Pitting and crevice corrosion: Sandvik SAF 2906™ has a carefully balanced composition with a high content of chromium and nitrogen and a moderate amount of molybdenum. This gives the material a high resistance to localized corrosion caused by chlorides. An index for comparing pitting corrosion resistance is the PRE number (Pitting Resistance Equivalent).

The PRE is defined as, in weight-PRE = %Cr + 3.3 x %Mo + 16 x %N A very severe test for pitting or crevice corrosion is the ASTM G48 test, i.e. immersion in 6% FeCl₃. Results for pitting test according to modified method A, as well as crevice corrosion testing according to method B, are presented in Table 3. The crevice corrosion test was performed 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 critical pitting temperature (CPT) and the critical crevice temperature (CCT) are the temperatures where pitting or crevice corrosion starts to develop on the material.

Table 3. PRE, CPT and CCT values

Potentiostatic tests in solutions with different chloride contents are reported in Figure 3. Figure 4 shows the effect of increased acidity. In both cases 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.

Stress corrosion cracking (SCC): Sandvik SAF 2906™ 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 5.

Nitric acid: Due to its balance of chromium and molybdenum, Sandvik SAF 2906™ presents good resistance to nitric acid. The iso- corrosion diagram, see Figure 6, shows the performance of the material compared to Sandvik 2RE10, a high purity austenitic grade widely used for nitric acid.

Compared with austenitic stainless steels, Sandvik SAF 2906™ 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.

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

Solution annealing: 1040-1080°C (1900-1980°F), rapid cooling in air or water.

Being a dual phase material (austenitic-ferritic) Sandvik SAF 2906™ 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. Further information is available on request.

Forms of supply:

Sandvik SAF 2906™ can be supplied as seamless tube and pipe. Tubes can be supplied straight or U-bent.

Other forms of supply:

The weldability of Sandvik SAF 2906™ 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 SAF 2906™, heat input of 0.2-1.5 kJ/mm and interpass temperature of <150°C (300°F) are recommended.

Recommended filler metals: