VDM® Alloy 926

Cronifer 1925 hMo is a polyvalent material which finds application in a wide range of industries: fire fighting systems, seawater filtration systems and process, hydraulic and reinjection piping systems in the offshore industry, bleaching plants in cellulose pulp production, polished rods for corrosive oil wells, flexible pipe systems for the offshore industry, tubing and couplings, wire lines and flowline systems in sour gas production. In sour gas environment the alloy is listed in NACE Standard MR0175 (Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment) as acceptable up to Level V in the annealed or cold-worked condition at a hardness level of 35 HRC max., components for flue-gas desulphurization plants, such as dampers and stacks, evaporators, heat exchangers, filters and mixers used in the manufacture of phosphoric acid, sulphuric acid distribution systems and coolers, concentration and crystallization of salts by evaporation, condensers and piping systems in power stations using polluted cooling waters, containers for transportation of aggressive chemicals, production of organic derivatives with acid chloride catalysts, reverse-osmosis desalination plant. Cronifer 1925 hMo is approved for pressure vessels by VdTÜV from -196 to 400 °C, and by ASME.

The workpieces should be in the annealed condition for cold forming. Due to its high molybdenum concentration, VDM® Alloy 926 has a higher work hardening rate than other austenitic stainless steels. This must be taken into account for 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 above 15%, a final solution annealing must be conducted.

VDM® Alloy 926 is an austenitic, special stainless steel with largely the same chemical composition as VDM® Alloy 904 L, while the nitrogen content was raised to 0.2 % and the molybdenum concentration to about 6.5 %. At the same time, the resistance against stress crack corrosion induced by pitting corrosion is improved. The nitrogen concentration raised to rounded 0.2 % improve the austenite stability and lowers the propensity to form inter-metallic phases. VDM® Alloy 926 is a good pick for media such as diluted sulfuric and phosphorus acids that are contaminated with chlorides, and also for salt concentrations and crystallizations against which the material proves a higher resistance against wear and tear. Due to its good corrosion resistance in seawater, VDM® Alloy 926 is also used for facilities on offshore platforms. The optimal corrosion behavior is only given if the material is used in clean, passivated condition.

VDM® Alloy 926 is preferably processed in 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 and the cutting tool should stay engaged at all times. An adequate chip depth is important in order to cut below the previously formed work-hardened zone. An optimal heat dissipation by using large quantities of suitable, preferably aqueous, cold forming lubricants has considerable influence on a stable machining process.

Solution annealing should take place at temperatures between 1,150 to -1,200°C (2,102 to -2,192°F) and preferably at 1,180°C (2,156°F). The retention time during annealing depends on the semi-finished product thickness. The retention time starts with material temperature equalization; 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 of less than approx. 1.5 mm. The material must be placed in a furnace that has been heated up to the maximum annealing temperature before any heat treatment. 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.

VDM® Alloy 926 can be hot-formed in a temperature range between 1,200 and 900°C (2,192 and 1,652°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 1,200 °C (2,192°F). Heat treatment after hot forming is recommended for achieving optimal corrosion behavior.

VDM® Alloy 926 has a cubic, face-centered crystal structure.

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. In principle, checking of welding parameters is necessary and it is therefore required to observe the section energy that is suitable for the material.