VDM® Alloy K-500

VDM® Alloy K-500 is used in sea water, offshore engineering, in the chemical process and petrochemicals industry and shipbuilding. Typical applications are: Valve seals, pump sleeves and wearing rings in sea water, pump shafts and propeller shafts, mounting elements and fasteners, e.g. bolts in sea air and splash water zones, tug rope armoring, springs, components of drilling equipment in the oil industry, aircraft instrument components.

Cold forming should be conducted on annealed material. VDM® Alloy K-500 has similar forming and strain hardening properties as 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. To achieve a high strength, a combination of cold forming with subsequent age hardening is an option.

The corrosion resistance of VDM® Alloy K-500 is generally equivalent to that of VDM® Alloy 400. The alloy proves excellent resistance against many media, from pure water to highly concentrated salt solutions and alkalis. VDM® Alloy K-500 is virtually immune against stress corrosion cracking induced by chloric ions. In the age hardened condition, the material can be sensitive to stress corrosion cracking in hot hydrofluoric acid vapor under tensions near the yield strength. In fast-flowing seawater and in sea air, good resistance is proven but in hardly moving or standing sea water, pitting can occur. VDM® Alloy K-500 is also very resistant against acid gas media.

VDM® Alloy K-500 is preferably processed in the annealed condition. The best results in terms of the surface quality of the finished product are achieved, however, by pre-treatment before age hardening and finishing after age hardening. Age hardened material can undergo a heat treatment for the purpose of tension compensation after the finishing. Since the alloy has a tendency to strain hardening, a low cutting speed should be selected and the cutting tool should stay engaged at all times. An adequate chip depth is important in order to cut below a previously formed strain-hardened zone. Optimum heat dissipation through the use of large quantities of suitable, preferably aqueous, lubricants has con-siderable influence on a stable machining process.

Annealing is conducted in the temperature range from 850 to 1,000°C (1,562 to 1,832°F) with retention times between 3 and 5 minutes per mm thickness and preferably in 980°C (1,796°F). Temperatures above 1,000°C (1,832°F) are not recommendable because strong grain growth can occur. Cooling down in water or accelerated cooling down in the air with thicknesses of less than 3 mm (0.12 in) or diameters of less than 12.5 mm (0.49 in) is recommendable and important so to avoid precipitations. The tension compensation achieved by retention times of 1 to 2 hours at 300 to 350°C (572 to 662°F) with subsequent cooling down in the air is necessary to remove the tension concentration from material that has been machined before the age hardening. The tension compensation is also recommendable for material that contains tensions from the cold forming after the last heat treatment, e.g. for straightening.

VDM® K-500 can be hot-formed in a temperature range from 900 to 1,150°C (1,652 to 2,102°F) with subsequent rapid cooling down in water. Cooling down in the air can cause age hardening and then lead to cracks when the material is heated up again. After the hot forming, annealing between 850 and 900°C (1,562 and 1,652°F) with subsequent water quenching is recommendable for compensating tensions and any mixed microstructure. The subsequent deformation should be at least 25 % and be implemented below 1,050°C (1,922°F) to achieve an optimal toughness.

VDM® Alloy K-500 has a face-centered cubic lattice. In the age hardened condition, the γ’ phase is precipitated.

The material can be welded using all conventional methods: GTAW (TIG), GTAW (TIG) hot wire, plasma, MIG and submerged arc welding. The material should be in its solution-annealed condition for welding, and should be free of scale, grease and markings. Application of the impulse technique is preferable in gas-shielded metal welding processes. 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 from 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 heat tint must be removed, preferably using a stainless steel brush, while the welding edge is still hot. The interpass temperature should be max. 120°C (248°F). After the welding and before the age hardening, it is recommended to anneal the components between 850°C and 900°C (1,562 to 1,652°F) with subsequent cooling down as quickly as possible.