The Siemens Industrial Turbomachinery factory in Lincoln, England, has seven Carbolite chamber furnaces and a specially designed thermal cycling rig (TCR) that has expanded the facility’s research and development on thermal barrier coatings.
The factory manufactures small industrial gas turbines for power generation and mechanical drive applications with outputs from 3 to 13 MW. Siemens uses the Carbolite equipment to evaluate coating materials such as yttria-stabilized zirconia bonded to cast and wrought nickel-based super alloys used in the production of hot-gas-path components.
While the furnaces provide isothermal exposure for the work, enabling samples to be heated to temperature, held and then cooled, the TCR allows samples to be subjected to long cycles of heating and cooling that accurately simulate thermal conditions within a turbine. The equipment has a maximum operating temperature of 2,732°F (1,500°C), but the maximum temperatures used by Siemens generally are between 1,832 and 2,372°F (1,000 and 1,300°C).
The TCR consists of a heating zone above a loading and quench zone, with an actuator that raises and lowers samples between the two. In a typical long-term test, a sample is lifted into the heating zone, heated to a preset temperature, held at that temperature and then lowered into the quench area, where it is air-cooled before being raised again into the heating zone for another cycle.
Time and temperature are controlled through a cascade system with overtemperature protection for the heating chamber. A probe thermocouple in the lifting rod’s center provides temperature readings for the control system and also links to a six-channel digital-display chart recorder.
Rather than the silicon carbide elements usually used for this type of application, Carbolite, headquartered in Hope Valley, U.K., has installed wire wound elements, which are less expensive and last longer. The elements are mounted horizontally above the hearth and controlled in three zones by three-term digital instruments with overtemperature protection. Because the elements remain on when the furnace door is opened, an internal guard has been fitted so they cannot be touched by operators during loading and unloading. Low thermal mass insulation enables the furnace to heat up quickly when it is switched on.
7 Furnaces Boost Thermal Barrier Coatings R&D
August 9, 2010