CFC. The perfect material for new heat treatment fixtures
When steel drills, crankshafts, or gear wheels are hardened, the frames which hold the workpieces are subject to high thermal stress. They have to withstand temperatures of up to 1300 deg C and changes in temperature of up to 300 degrees per second. Even special highquality heat-resistant steels age rapidly under those conditions. But now a material used in aerospace applications could help bring about a drastic reduction in the high process costs of heat treatment. Carbon-fibre-reinforced carbon (CFRC) is twenty times stronger than steel at 1000 deg C. Unlike metals, its strength increases at higher temperatures. However, the great advantage of the new material is its low weight. "CFRC frames are at least five times lighter than conventional steel frames. As a result, the net capacity of the workpiece holders and batch loaders can often be increased by up to 100 per cent" explains Jörg Demmel, an engineer at the Fraunhofer Technology Development Group (TEG) in Stuttgart. That reduces the costs per part considerably, because better use is made of the heat-treatment furnaces and the energy consumption is then lower. Another advantage is that CFRC hardly expands on heating. Whereas a steel part changes its length by about 15 mm per metre when heated from 20 degree to 1000 degree, CFRC does not change its size or shape noticable. " The frames remain distortion-free, even under large changes in temperature" says Demmel; "That makes it much easier to automate the workpiece handling of the heat treatment." Altogether, the process costs can be reduced by up to 50 per cent by using CFRC workpiece holders. In collaboration with the Fraunhofer Institute for the Mechanics of Materials (IWM) the TEG has now optimized the CFRC constructions and successfully employed them in several industrial applications. The scientists are now engaged in automating the process.