Sandvik 3RE60

3RE60 is an excellent material for use in chloride-bearing environments where pitting and stress corrosion cracking are potential problems. In such environments, 3RE60 is far superior to the standard austenitic steels. The material is therefore particularly well suited for use in heat exchangers that work with cooling water with moderate chloride contents.

Typical application examples are found in oil refineries, chemical and petrochemical plants and within the pulp industry.

The high strength and hardness of 3RE60 make the material an attractive alternative to the austenitic steels in structures that are subjected to heavy loads or wear.

Approvals:

If 3RE60 is exposed for prolonged periods to temperatures exceeding 300 °C (570 °F), the microstructure changes which results in a reduction in impact strength. This effect does not necessarily affect the behavior 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, 300 °C (570 °F) is required as maximum according to VdTÜV-Wb 385 and NGS 1604.

Bending

The force requirement for bending is about twice as high for 3RE60 as for AISI 304L/316L, but when the proof strength is exceeded, the plastic deformation takes place just as easily in 3RE60 as in austenitic stainless steels. 3RE60 can be cold-bent to 25% deformation without requiring subsequent heat treatment. However, under service conditions where the risk of stress corrosion cracking starts to increase for example where the material temperature is nearly 150°C (300°F) in an oxygen-bearing environment with around 100 ppm Cl– , heat treatment is recommended even after moderate cold bending.

General corrosion

In terms of resistance to general corrosion, 3RE60 is comparable or superior to AISI 316L in most media. The table below contains examples of corrosion data obtained from laboratory tests in formic and hydrochloric acid.

Resistance to sulphuric acid is shown by the isocorrosion diagram in figure 3. The diagram shows that 3RE60 compares well with AISI 316L in this respect.

For use under strongly oxidizing conditions, e.g. in nitric acid, 3RE60 is not recommended. In such environments, an austenitic steel should be chosen, e.g. Sandvik 3R12 (AISI 304L) or the special steel Sandvik 2RE10 (AISI 310L).

Pitting: The pitting resistance of a steel is improved by increasing the contents of chromium and molybdenum, for example. Compared to steel of the AISI 316 type, 3RE60 has a higher chromium content and therefore better resistance to pitting. Compared to steel of the AISI 304 type, which does not contain molybdenum, 3RE60 is clearly far superior. This is confirmed by potentiostatic measurements of the critical temperature for the initiation of pitting corrosion (CPT) performed in chloride-bearing aqueous solutions, figure 4.

Stress corrosion cracking: The standard austenitic steels AISI 304L and AISI 316L are prone to stress corrosion cracking (SCC) in chloride-bearing solutions at temperatures exceeding about 60°C (140°F).

Duplex stainless steels are much less sensitive to this type of corrosion. The good resistance of 3RE60 to SCC has been proved by laboratory tests, but more significantly by extensive operating experience during the past 20 years. Operating experience and laboratory results have been compiled in figure 5.

The diagram indicates the temperature-chloride range within which 3RE60 and the standard steels AISI 304L and AISI 316L can be used without any risk of stress corrosion cracking. At high chloride contents, resistance to pitting is often a limiting factor. In such cases, we recommend using the duplex stainless steel Sandvik SAF 2205, a steel with 22% Cr, 5.5% Ni and 3% Mo.

Results of laboratory tests carried out in calcium chloride are shown in figure 6. The tests have been continued to failure or a maximum of 500 h. The diagram shows the much greater SCC resistance of 3RE60 compared to the austenitic standard steels AISI 304L and AISI 316L.

Intergranular corrosion: 3RE60 is a member of the family of modern duplex stainless steels whose composition has been balanced in such a manner that, when welded, the reformation of austenite in the heat-affected zone adjacent to the weld takes place quickly. This results in a micro-structure that gives corrosion properties and toughness roughly equal to that of the parent metal. The long experience with 3RE60 in welded structures has confirmed the steels high resistance to intergranular corrosion.

Crevice corrosion: In the same way as the resistance to pitting can be related to the chromium and molybdenum contents of the steel, so can the resistance to crevice corrosion. 3RE60 therefore possesses better resistance to crevice corrosion than steels of the AISI 316L type.

Erosion corrosion

Steels of the AISI 316L type are attacked by erosion corrosion if exposed to flowing media containing highly abrasive solid particles. Due to its high hardness, 3RE60 displays very good resistance under such conditions.

Corrosion fatigue: In certain applications, e.g. suction rolls in paper mills, the resistance of the material to corrosion fatigue has a crucial bearing on its service life. Laboratory tests have shown that 3RE60 has far better fatigue strength under corrosive conditions than the AISI 316L type of steels. This applies to both quench-annealed and welded material. The difference between the two steel grades is explained by the superior mechanical strength of 3RE60.

Rotary bending fatigue tests have been carried out on 3RE60 and AISI 316L at room temperature in a solution containing 400 ppm Cl– and 250 ppm SO42– , pH = 3.5. The test results are shown in figure 7.

In comparison with austenitic stainless steels, 3RE60 has a higher 0.2% proof strength and a higher tensile 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.

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

Solution annealing: 990-1130°C (1815-2065°F), rapid cooling in air or water.

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.

The mechanical 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 20% for finish machining and 60% for rough machining compared to the cutting speeds applied for AISI 316. Much the same applies to other operations. If high-speed steel tools are used, approximately the same cutting speed can be used as for AISI 316.

Detailed recommendations for the choice of tools and cutting data are provided in brochure S-1,462-ENG. Select data as for grade 5R60 (AISI 316), taking into account the above comments.

Forms of supply:

Seamless tube and pipe in Sandvik 4C54 is supplied in dimensions up to 125 mm outside diameter in the solution annealed and white pickled condition or in the bright annealed condition.

Other forms of supply

Sandvik 3RE60 can also be supplied in forms of:

Seamless tube and pipe: Seamless tube for heat exchangers according to ASTM A789 is stocked in the solution-annealed and white-pickled condition in size (average wall): 32 x 1.5 mm.

Filler metal for welding is stocked in the following diameters:

The weldability of Sandvik 3RE60 is good. Welding must be carried out without preheating and subsequent heat treatment is normally not necessary. 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.

For Sandvik 3RE60, heat input of 0.5-2.5 kJ/mm and interpass temperature of <150°C (300°F) are recommended.

Recommended filler metals:

• TIG/GTAW or MIG/GMAW welding
  • ISO 14343 S 22 9 3 N L / AWS A5.9 ER2209 (e.g. Exaton 22.8.3.L)
• MMA/SMAW welding
  • ISO 3581 E 22 9 3 N L R / AWS A5.4 E2209-17(e.g. Exaton 22.9.3.LR)