VDM® Alloy 31

Components for flue gas desulfurization plants, plants for the production of phosphoric acid, pipes and heat exchangers for sulfuric acids contaminated with chlorides, pipes containing ocean water and brackish water, condensers and chillers, pickling plants for sulfuric acid as well as nitric-hydrofluoric acid, treatment of sulfuric acids from waste, evaporation and crystallization of salts, components for the cellulose and paper industry, digestion of ores (e.g. in HIPAL plants), mineral oil production and ester synthesis, fine chemicals, strain-hardened transport and feed pipes as well as slicklines, wirelines and flowlines in the transport of oil and gas.

The workpieces should be in the annealed condition for cold forming. VDM® Alloy 31 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.

Optimal corrosion resistance is only ensured if the material is used in a solution annealed, clean and metallic bright condition. The chemical composition of VDM® Alloy 31 is designed to enable the achievement of a high corrosion resistance in halide-containing media. At the same time, the material has excellent resistance in pure and contaminated sulfuric acid over a broad range of concentrations and temperatures up to 80 °C.

VDM® Alloy 31 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 1,150 and 1,180°C (2,102 and 2,156°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 of at least 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 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 has a face-centered cubic lattice. The 0.2% nitrogen content stabilizes the austentite and reduces the precipitation rate of intermetallic sigma phases.

VDM® Alloy 31 can be welded using conventional processes with metals of the same type as well as many other metals. This includes GTAW (TIG), GMAW (MIG/MAG) and plasma welding. Pulsed arc welding is preferred for gas-shielded welding processes. The use of a multi-component shielding gas (Ar + He + H2 + CO2) is recommended for the MAG processes. For welding, VDM® Alloy 31 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 (argon 4.6) 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.