VDM Alloy 31 Plus® – Advanced Nickel‑Iron‑Chromium‑Molybdenum Alloy with Superior Corrosion Resistance

Chemical processes with sulfuric acid; Treatment of sulfuric acids from waste; Components for flue gas desulfurization plants; Clad tanks; Plants for the production of phosphoric acid via the wet digestion process; Ocean water and brackish water applications; Evaporation and crystallization of salts; Pickling plants for sulfuric acid and for nitric-hydrofluoric acid; Hydrometallurgy, e.g. digestion of laterite ores in the HPAL process; Fine chemicals, special chemicals and organic acids; Components for the cellulose and paper industry

The workpieces should be in the annealed condition for cold forming. VDM Alloy 31 Plus® has a significantly higher work hardening rate than other widely used austenitic stainless steels. This must be taken into account during the design and selection of forming tools and equipment and during the planning of forming processes. Intermediate annealing is necessary for major cold forming work. For cold forming of > 15%, a final solution annealing must be conducted.

The material is resistant to inter-crystalline corrosion in the delivery condition and when welded according to the test procedure according to ASTM-G 28, Method A. The corrosion rate determined via the mass loss according to ASTM-G 28, Method A (test period 24 hours), is maximum 0.5 mm/a (0.020 mpy) in the delivery condition and when welded. A very good resistance is also provided against crevice corrosion and pitting. The corrosion resistance is comparable with the material VDM® Alloy 31.

VDM Alloy 31 Plus® should be machined in the heat-treated condition. Because of the considerably elevated tendency toward work hardening in comparison with low-alloy austenitic stainless steels, a low cutting speed and a feed level that is not too high should be selected and the cutting tool should be engaged at all times. An adequate depth of cut 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 1,140 and 1,170°C (2,084 and 2,138°F). The retention time commences with material temperature equalization; longer times are generally considerably less critical than retention times that are too short. For maximum corrosion resistance, the workpieces must be quickly cooled from the annealing temperature particularly through the range of 1,100 to 500°C (2,012 to 932°F) with a cooling rate of >150 °C/min (>302 °F/min). The material must be placed in a furnace that has been heated up to the maximum annealing temperature before any heat treatment. The cleanliness requirements listed under "Heating" must be observed. For strip products, the heat treatment can be performed in a continuous furnace at a speed and temperature that is adapted to the strip thickness.

VDM Alloy 31 Plus® should be hot-formed in a temperature range of 1,200 to 1,050°C (2,192 to 1,922°F) with subsequent rapid cooling in water or in air. For heating up, workpieces should be placed in a furnace that has been heated up to the maximum hot-forming temperature (solution annealing temperature). Once the furnace has reached its temperature again, the workpieces should remain in the furnace for around 60 minutes per 100 mm (3.94 in) of thickness. After this, they should be removed from the furnace immediately and formed within the temperature range stated above, with reheating necessary once the temperature reaches 1,050 °C (1,922 °F). Heat treatment after hot forming is recommended in order to achieve optimal properties.

VDM Alloy 31 Plus® has a face-centered cubic structure. The nitrogen and nickel content reduces the tendency for precipitation of intermetallic phases and stabilizes the austenitic microstructure.

VDM Alloy 31 Plus® can be welded in most applications with VDM® FM 59 using conventional processes. This includes TIG and MAG welding. Pulsed arc welding is preferred for gas-shielded welding processes. For welding, VDM Alloy 31 Plus® should be in a solution-annealed condition and free of scale, grease and markings. When welding the root, care should be taken to achieve best quality root protection using pure argon, purity 99.99% or better so that the welding edge is free of oxides after welding the root. Root protection is also recommended for the first and, in certain cases depending on the welded construction, also for the second intermediate layer weld after root welding. Any tempering colors must be removed while the welding edge is still hot, preferably using a stainless steel brush.