Sandvik 6R35 – Titanium‑Stabilized Austenitic Stainless Steel for Corrosive Environments & High‑Temperature Applications

Good resistance to hydrogen sulfide gas and intergranular corrosion coupled with good high temperature strength make Sandvik 6R35 a suitable material for applications as tubes in heating furnaces and heat exchangers in sulfurization and hydro-treating plants. In the petrochemical industry the steel is used in cracking furnaces for the production of ethylene and vinyl chloride. It is also frequently used for heat exchangers and piping in the chemical and petrochemical industries.

Approvals: PED (Pressure Equipment Directive) 2014/68/EU and AD2000

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.

General corrosion: Sandvik 6R35 has with some limitations (nitric acid) the same resistance as the unstabilized steel ASTM 304. Consequently, the grades have good resistance to:

Intergranular corrosion: The stabilization with titanium gives Sandvik 6R35 improved resistance to intergranular corrosion.

Pitting and crevice corrosion: Pitting and crevice corrosion may occur even in solutions of relatively low chloride content. However, the stabilization with titanium results in a somewhat better resistance than that of ASTM 304.

Stress corrosion cracking: Austenitic steels, like Sandvik 6R35 are susceptible to stress corrosion cracking. This may occur at temperatures above about 60°C (140°F), if the material is subjected 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.

In applications demanding high resistance to stress corrosion cracking we recommend the austenitic-ferritic steel Sandvik SAF 2304.

Gas corrosion, Sandvik 6R35 can be used in:

Creep 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 normally delivered as described above. If additional heat treatment is needed after further processing the following is recommended.

Solution annealing:

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

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

Sandvik 6R35 has good machining properties. Please contact Sandvik for detailed recommendations on the choice of tools and cutting data.

Forms of supply:

Seamless tube and pipe in Sandvik 6R35 is supplied in dimensions up to 260 mm outside diameter in the solution-annealed and white-pickled condition or solution-annealed in a bright-annealing process.

Hollow bars are supplied solution-annealed and white-pickled.

Sizes in stock:

The weldability of Sandvik 6R35 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 6R35, 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)
• 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 6R35, is required.