VDM® Alloy 690

Thanks to its excellent resistance to wet and high-temperature corrosion, and its good mechanical properties, VDM® Alloy 690 is suitable for a wide range of applications. Typical applications are: Treatment of radioactive waste, components in boilers and steam generators in pressurised water reactors, production of alkali metal sulphates using Mannheim furnaces, fittings in combustion units for crude oil (oil ash corrosion) and, glass and silicate production.

Cold working should be carried out on annealed material. VDM® Alloy 690 has a higher work hardening rate than austenitic stainless steels. This must be taken into account during design and selection of forming tools and equipment and during the planning of forming processes. Intermediate annealing may be necessary for high degrees of cold working deformation. Before use, heat treatment is required after cold working with more than 10% deformation.

VDM® Alloy 690 is resistant to a wide range of corrosive media and atmospheres. The high chromium content makes the material particularly suitable for strongly oxidising conditions. The high chromium content also confers resistance to high-temperature corrosion in gases having an oxidising and sulphidising effect. Due to its high nickel content, VDM® Alloy 690 is exceptionally resistant to stress corrosion cracking which can occur in the primary water loops of nuclear power stations. The material also shows good resistance in mixtures of nitric and hydrofluoric acid. It demonstrates remarkable behaviour in concentrated (98.5%) sulphuric acid at temperatures of up to 150°C.

VDM® Alloy 690 should preferably be machined in the annealed condition. Since the material exhibits a considerable work hardening rate, low cutting speeds should be used and the tool should remain continuously in contact. An adequate cutting depth is important in order to cut below the previously formed work-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 be carried out at temperatures between 1,020 and 1,070°C (1,870 to 1,960°F). If use in a high-temperature range with increased creep resistance is intended, the solution annealing temperature should be raised to between 1,080 and 1,150°C (1,980 to 2,100°F). Water quenching should be carried out on workpiece thicknesses over 1.5 mm so that the optimum corrosion resistance of the material can be reached. Workpieces of less than 1.5 mm thickness can be cooled using air nozzles. If use in pressurised water reactors is intended, a subsequent heat treatment of around 10 hours at between 700 and 740°C is necessary in order to ensure that carbides are segregated specifically at grain boundaries.

VDM® Alloy 690 can be hot worked at a temperature range of between 1,230 and 900°C (2,250 and 1,650°F) with subsequent rapid cooling down in water or by using air nozzles. The workpieces should be placed in the furnace heated to hot working temperature in order to heat up. Once the temperature has equalised, a retention time of 60 minutes for each 100 mm of workpiece thickness is recommended. After this, the workpieces should be removed immediately and formed during the stated temperature window. If the material temperature falls below the minimum hot working temperature, the workpiece must be reheated. Heat treatment after hot working is recommended in order to achieve optimum properties and corrosion resistance.

VDM® Alloy 690 has a cubic face-centred structure.

VDM® Alloy 690 should be in solution-annealed condition for welding, and should be free of scale, grease and markings. VDM® Alloy 690 can be welded using TIG and plasma welding processes. When welding roots, sufficient protection of the root needs to be ensured with pure argon (Ar 4.6) so that the welding seam is free of oxides after welding. Root backing is also recommended for the first and, in certain cases depending on the welded construction, also for the second intermediate root pass. Any discolouration/heat tint should be removed, preferably using a stainless steel brush, while the welding seam is still hot from the welding heat.