CFC composite materials continually force steel into the background today when it comes to heat treatment of metals in inert or reduced atmospheres (e.g. for hardening, soldering or sintering). Frequent temperature changes cause steel frames to suffer from severe material warping which makes realigning (often with a hammer) necessary after only a few hardening operations. Contrastingly, CFC frames do not exhibit any signs of warping after numerous operations, thanks to their low thermal coefficient of expansion (more than one order of magnitude less than steel). This also facilitates automatic positioning of metal parts. Steel frames also tend to become brittle in the carbonic atmospheres in hardening furnaces, due to carburization (penetration of carbon into the grid structure). The large thermal mass of steel frames is another disadvantage (which increases heating energy costs). Frames made of CFC material clearly have the advantage here, due to their extremely low density (density of CFC: approx.1.6 g/cm³; density of steel: 7.9 g/cm³). However, use is limited to non-oxidic furnace atmospheres.