VDM® Alloy 33

Sulphuric acid production plant, sulphuric acid heat recovery and distribution systems, nitric-hydrofluoric acid pickling plant, seawater systems, plate or tubular heat exchangers using brackish or seawater as coolant, bleaching plant for chemical pulp

The workpieces should be in the annealed condition for cold forming. VDM® Alloy 33 has a higher work hardening than other austenitic stainless steels. This must be taken into account in the design and selection of forming tools and equipment, and in the planning of forming processes. Intermediate annealing is necessary for heavy-duty cold forming work. For cold forming of >15%, a final solution annealing must be conducted.

The austenitic material VDM® Alloy 33 has an extremely high chrome concentration of 33% and therefore offers the basis for the excellent corrosion resistance in oxidizing media. The low addition of molybdenum and copper improves resistance in phosphorus acid and simplifies passivation in sulfuric acid. Besides the excellent resistance against nitric acid/hydrofluoric acid mixture, the material also has great corrosion resistance against all alkali media. Furthermore, the resistance against pitting corrosion in media containing chloride is also excellent. Optimal corrosion resistance can only be assured if the material is used in clean, metallic bright condition.

Machining of VDM® Alloy 33 should take place in an annealed condition. Since the alloy is prone to strong work hardening, a low cutting speed should be selected with a feed speed that is not too high and the cutting tool should stay engaged at all times. An adequate chip depth is important in order to cut below the previously formed strain-hardened zone. Optimum heat dissipation through the use of large quantities of suitable, preferably aqueous, lubricants has considerable influence on a stable machining process.

Solution annealing should take place at temperatures between 1,100 and 1,150°C (2,012-2,102°F), preferably at 1,120 °C (2,048°F). The retention time commences with the temperature equalization of the workpiece; longer times are generally considerably less critical than retention times that are too short. Cooling down should be accelerated with water to achieve optimum properties. Fast air cooling can also be carried out at thicknesses below approx. 3 mm. For strips as the product form, the heat treatment can be performed in a continuous furnace at a speed and temperature that is adapted to the strip thickness. The cleanliness requirements listed under "Heating" must be observed.

VDM® Alloy 33 should be hot-formed in a temperature range between 1,200 and 1,000°C (2,192-1,832°F) with subsequent rapid cooling down in water or air. For heating up, workpieces should be placed in a furnace that is already heated up to the target value. Heat treatment after hot forming is urgently recommended for achieving optimal corrosion behavior.

VDM® Alloy 33 has a cubic face-centered crystal structure. The content of approx. 0.4% nitrogen together with 31% nickel stabilizes the austenitic crystal structure and reduces the dispersion speed of intermetallic phases.

For welding, VDM® Alloy 33 must be in a solution-annealed condition and free of scale, grease and markings. VDM® Alloy 33 is weldable in the same way as the filler material tested for suitability for welding, VDM® FM 33 TIG, (TÜV engineering specifications no. 07528). When welding the root, care should be taken to achieve best quality root protection (min. Ar 4.6) and the welding edge must be largely free of oxides after welding. Any heat tints must be removed. As shielding gas, argon with max. 5% hydrogen should be used.