VDM® Alloy 20

Nicrofer 3620 Nb is used in a wide variety of applications up to temperatures of approximately 500°C. Typical applications are: Equipment for the manufacture of sulphuric acid and for processes based on sulphuric acid, extraction columns in the production of amines and the processing of pharmaceuticals, production of plastics and synthetic fibres, equipment for food processing to protect against contamination.

Cold working should be carried out on annealed material. Nicrofer 3620 Nb has a work-hardening rate similar to that of austenitic stainless steel and the forming equipment must be adapted accordingly. When cold working is performed, interstage annealing may become necessary. After cold reductions of more than 15 %, a final stabilising anneal is required before use.

Nicrofer 3620 Nb has excellent corrosion resistance to sulphuric, phosphoric and organic acids and to aqueous solutions of their salts. Resistance to nitric acid is also good. Due to the controlled chemical composition, the alloy also has excellent resistance to such forms of corrosion as intergranular corrosion and stress corrosion. The molybdenum content ensures good resistance to pitting and crevice corrosion. Optimum corrosion resistance can only be obtained if the material is in the correct metallurgical condition and clean.

Nicrofer 3620 Nb should be machined in annealed condition. The alloy’s high work-hardening rate should be considered, i.e. only low surface cutting speeds are possible compared with low-alloy standard austenitic stainless steel. Tools should be engaged at all times. Heavy feeds are important in getting below the work-hardened ‘skin’.

Soft or stabilising annealing should be carried out in the temperature range 920 to 960°C (1690 to 1760°F), preferably at about 950°C (1740°F). Water quenching or rapid air cooling is recommended for thicknesses above about 3 mm (1/8 in.) and is essential for maximum corrosion resistance. Stress-relief annealing may be performed at temperatures up to 540°C (1000 °F). During any heating operation, the precautions outlined earlier regarding cleanliness must be observed.

Nicrofer 3620 Nb may be hot-worked in the range 1150 to 900°C (2100 to 1650°F). The final hot-working temperature must not exceed 950°C (1740°F). Cooling should be by water quenching or as fast as possible. Annealing after hot working is required to ensure maximum corrosion resistance and an optimum microstructure. For hot working, the material may be charged into the furnace at maximum working temperature. After soaking for the required time the material should be withdrawn immediately and worked within the specifed range. If the metal temperature falls below the minimum working temperature, it must be reheated.

Nicrofer 3620 Nb has a face-centred cubic structure. The balanced chemical composition and optimum annealing temperature promote the formation of niobium carbides and ensure that the corrosion resistance is not impaired by sensitisation.

Nicrofer 3620 Nb can be welded by all the conventional methods. Proven welding processes are: GTAW (TIG), GMAW (MIG), Plasma, PHW, SMAW. Pulsed arc welding is the preferred technique. Prior to welding, material should be in annealed condition, clean and free from scale, grease, marking paints, etc. A zone approximately 25 mm (1 in.) wide on each side of the joint should be ground to bright metal. Sometimes tarnishing can be removed by brushing the joint in the warm condition. Low heat input is necessary. The interpass temperature should not exceed 120 °C (250 °F). Neither pre- nor post-weld heat treatment is required.