VDM® Alloy 718

Based on its high-temperature resistance up to 700 °C, its excellent oxidation and corrosion resistance, and its good workability, VDM® Alloy 718 is used in many demanding applications. Originally, it was developed and used for static and rotating components in aircraft turbines such as housings, mounting elements and turbine disks, where tough requirements apply for creep resistance and fatigue behavior, in particular for the rotating applications. Due to its properties, its good workability and efficiency, the material is additionally widely used for static and rotating components in stationary gas turbines, rocket drives and spacecraft, motor vehicle turbo chargers, high-strength screws, springs and mounting elements, and for heat-resistant tools in forgeries, extruders and separating shearers. The variant VDM® Alloy 718 CTP, which is laid out specifically for the requirements of the oil and gas industry, finds increasing uses in drilling equipment and pump shafts. The components used must ensure an efficient and safe oil and gas extraction with increasing drilling hole depth and pressures and temperatures becoming more critical, and withstand the acid gas environment (H₂S, CO₂, chloride) that is prevalent there (see special data sheet VDM® Alloy 718 CTP).

Cold working should take place in the solution-annealed condition. The material 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 the forming processes.

Based on the high chrome and molybdenum concentrations, VDM® Alloy 718 has a good resistance in many media against abrasive and local corrosion such as pitting. By virtue of its high nickel content, VDM® Alloy 718 also has good resistance against stress corrosion cracking.

While VDM® Alloy 718 in the solution annealed condition is easier to process and the strain on tools is less, better surface quality is achieved in the hardened condition. The best results in terms of the surface quality of the finished product are achieved by pre-treatment before hardening and by finishing in the hardened condition. As the alloy is more prone to work-hardening than other low alloyed materials, low cutting speeds and appropriate feed rates should be used and the tool should stay engaged at all times. Sufficient chip depths are important to get below the work-hardened surface layer.

Through various heat treatments, the mechanical properties of VDM® Alloy 718 can be adjusted specifically. In the solution-annealed condition, the material can be processed and worked more easily. In the solution-annealed and age hardened condition, VDM® Alloy 718 has a high mechanical strength. The solution-annealed condition is obtained through a heat treatment in the temperature range from 940 to 1,065°C (1,724 to 1,949°F). Here, for example, annealing by inserting into a pre-heated furnace at a temperature of 980°C (1,796°F) for 1 hour is common. Cooling can be done by placing the workpiece in water or oil or also in open air. The hardening takes place by annealing in the temperature range from 620 to 790°C (1,148 to 1,454°F). A two-stage heat treatment is common here with insertion in a pre-heated furnace at 720°C (1,328°F) for 8 hours, followed by a furnace cool down to 620°C (1,148°F) and repeated holding for 8 hours. Cooling is usually done in open air.

The hot working should generally be conducted after the homogenization with subsequent cooling in air. It should be done evenly in order to receive a homogeneous structure and to prevent the formation of a duplex grain structure.

VDM® Alloy 718 has an austenitic structure; multiple phases can be precipitated. By means of different heat treatments, graduated mechanical properties of the material can be reached. The excellent mechanical properties of VDM® Alloy 718 result from the γ’’-formation during the precipitation hardening.

It must be ensured that work is carried out using targeted heat application and low heat input. The interpass temperature should not exceed 100°C (212°F). The stringer bead technique is recommended. In principle, checking of welding parameters is necessary.