Sandvik 6R44 – High-Performance Austenitic Stainless Steel for Extreme Temperatures

Sandvik 6R44 is used for superheater tubes in steam power plants.

It is also frequently used for cooling tubes in ammonia converters, because of its good resistance to nitrogen absorption and good corrosion resistance. With its good hot-strength, and good resistance to hydrogen sulfide and intergranular corrosion, Sandvik 6R44 is a suitable material for furnace tubes in refineries. Furnace tubing used in vinyl chloride production is another example of applications in which this steel is often used.

Annealing after cold bending is not normally necessary, but this point must be decided with regard to the degree of bending and the operating conditions. Heat treatment, if any, should take the form of stress relieving or solution annealing, see under "Heat treatment".

Wet corrosion: Sandvik 6R44 is generally used at temperatures above 500°C (930°F), where wet corrosion is not relevant. The stabilization with niobium gives Sandvik 6R44 good resistance to intergranular corrosion. The steel may be sensitive to pitting and crevice corrosion even in solutions of relatively low chloride content. Austenitic steels are susceptible to stress corrosion cracking. This may occur at temperatures above about 60°C (140°F), if the steel is subject to tensile stresses and at the same time comes into contact with certain solutions, particularly those containing chlorides. Such service conditions should therefore be avoided. Conditions when plants are shut down must also be considered as the condensates which are then formed can develop a chloride content that leads to both stress corrosion cracking and pitting.

Gas corrosion: Sandvik 6R44 can be used in air up to 850°C (1560°F) and in steam up to 750°C (1380°F). It is suitable for use in mild synthesis gas up to about 550°C (1020°F).

Creep behavior should also be taken into account when using the steel in the creep range. In flue gases containing sulfur, the corrosion resistance is reduced. In such environments this steel can be used at temperatures up to 600-750°C (1110-1380°F) depending on service conditions. Factors to consider are whether the atmosphere is oxidizing or reducing, i.e. the oxygen content, and whether impurities such as sodium and vanadium are present.

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

Stress relieving: 850-950°C (1560-1740°F), 10- 15 minutes, cooling in air.

Solution annealing: 1000-1100°C (1830-2010°F), 5-20 minutes, rapid cooling in air or water.

Hot bending is carried out at 1100-850°C (2010-1560°F) and should be followed by solution annealing.

Forms of supply:

Seamless tube and pipe in Sandvik 6R44 is supplied in dimensions up to 260 mm outside diameter in the solution-annealed and white-pickled condition or in the bright annealed condition.

The weldability of Sandvik 6R44 is good. Welding must be carried out without preheating and subsequent heat treatment is normally not required. 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 6R44, heat input of <1.5 kJ/mm and interpass temperature of <150°C (300°F) are recommended.

Recommended filler metals:

• TIG/GTAW or MIG/GMAW welding
  • ISO 14343 S 19 9 Nb / AWS A5.9 ER347 (e.g. Exaton 19.9.Nb)
  • ISO 14343 S 19 9 Nb Si / AWS A5.9 ER347Si (e.g. Exaton 19.9.NbSi)
• MMA/SMAW welding
  • ISO 3581 E 19 9 Nb R / AWS A5.4 E347-17 (e.g. Exaton 19.9.NbR)
  • ISO 14343 S 19 9 Nb / AWS A5.9 ER347 (e.g. Exaton 19.9.LNb) 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 6R44, is required.