VDM® Alloy 602 CA

VDM® Alloy 602 CA has a wide range of application in the high-temperature field of thermotechnical and chemical processes, in power plants and in the automotive industry. Typical applications are: Radiant tubes, furnace muffles, rotary and shaft furnaces, kiln rollers and other furnace installations, glass pot for melting radioactive waste, methanol and ammonia synthesis, hydrogen production, reformers in the chemical and petrochemical industries, components in automotive exhaust systems, glow plugs for diesel engines.

The workpieces should be in the solution-annealed condition for cold working. VDM® Alloy 602 CA has a higher work hardening than austenitic stainless steels. This must be taken into consideration when choosing and designing forming equipment and forming processes. Intermediate annealing is necessary for major cold forming work. Solution annealing is to be recommended if deformation degrees over 7 % were reached. When bending and trimming metal sheets, an inner radius should be maintained that is more than three-times the thickness of the metal sheet in order to avoid damage.

VDM® Alloy 602 CA is especially characterized by very good oxidation resistance, which is better across the entire application range up to 1200 °C than with VDM® Alloy 601. Even under extreme conditions, such as cyclic heating and cooling, VDM® Alloy 602 CA retains this property, which is caused by a tight and adhering alumina layer, which is very resistant to chipping. High temperature oxidation tests show that, compared to other high temperature materials, this material has the lowest mass loss under cyclic stress. Caused by its chromium and aluminum content, VDM® Alloy 602 CA is also highly resistant in oxidizing sulfur-containing atmospheres at elevated temperatures. The good resistance of VDM® Alloy 601 against carburizing can be increased even further by VDM® Alloy 602 CA. This also applies to the metal dusting resistance of the material.

VDM® Alloy 602 CA is preferably processed in a solution-annealed condition. Since the alloy is prone to work-hardening, a low cutting speed should be selected with a feed speed that is not too high. 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. The strong heat development during machining should be countered by a sufficient amount of cooling lubricant. Water-based emulsions such as those used for structural and stainless steels are very suitable for this purpose. Suitable cutter geometries, suitable cutter materials and cut values can be taken from the VDM metals publication N 579 entitled "Processing information for austenitic stainless steels and nickel-based alloys".

During each heat treatment, the material is to be inserted into the furnace already heated up to the annealing temperature. The cleanliness requirements listed under the "Heating" section must be observed. VDM® Alloy 602 CA is usually used in the solution-annealed state, where optimal creep strength is a given. VDM® Alloy 602 CA is solution-annealed at 1,220°C (2,228°F) for maximum creep strength in order to specifically achieve a particle size of ≥70 μm. The retention time commences with material temperature equalization; longer times are generally considerably less critical than retention times that are too short. If additional processes steps take place after the solution annealing, the cooling from the solution annealing temperature should take place at an accelerated speed, e.g. with water or with compressed air (for metal sheets less than 3 mm (0.12 in) thickness). If solution annealing is the last processing step before use, cooling can be carried out more slowly in order to avoid warpage. Solution-annealed VDM® Alloy 602 CA is sensitive to stress relaxation cracks in a range between 600 and 750°C (1,112 and 1,382°F) and should therefore be stabilization-annealed if permanent deployment (>100 h) is to take place in the aforementioned temperature range. Stabilization-annealing should be carried out before or after welding work. In addition, VDM® Alloy 602 CA should be stabilization-annealed before repair welding. Stabilization-annealing should be carried out at 950°C (1,742°F) for at least three hours. Heating-up and cooling-down speeds are not critical in this case and should not be too high in order to avoid warping.

VDM® Alloy 602 CA can be hot-worked at a temperature range of between 1,200 and 900°C (2,192 and 1,652°F) with subsequent rapid cooling down in water or using air nozzles. The workpieces should be placed in the furnace heated up to hot-working temperature in order to heat up. Once the temperature has equalized the workpieces can be removed and worked within the stated temperature window. If the lower temperature limit is fallen below, the workpiece must be reheated. Heat treatment after hot forming is recommended in order to achieve optimal properties.

Alloy 602 CA has a face-centered cubic lattice. The good mechanical properties, especially at temperatures above 1000 °C, are generated by primarily precipitated M₂₃C₆ or M₇C₃ carbides. Additional gamma precipitations can form below 800°C.

It must be ensured that work is carried out using targeted heat application and low heat input. The stringer bead technique is recommended. The interpass temperature should not exceed 120°C (248°F). In principle, it is necessary to check the welding parameters.