The instability of invar

Abstract Almost since its invention in 1896, invar has been known to be a dimensionally unstable pendulum rod material. A few articles have been published over the years, trying to address the dimensional instability and eliminate it. Data published in 1927 showed a dimensional growth of 50 ppm over a 27-year interval. The growth was exponential, gradually slowing down with time. Invar's growth today still follows the same exponential pattern, although shrinkage is occasionally observed. In 1950, invar's instability was tied to the presence of impurities, especially carbon. The lower the level of impurities, the more stable the invar is. Invar's impurity level has been reduced over the years, so that today's invar, using the traditional furnace melt process, is more stable than it was 20 years ago. Today's regular invar has a dimensional stability of 2-27 ppm per year, at room temperature. There are three types of invar available today: regular invar, free machining invar, and super invar. Super invar's thermal expansion coefficient is three times smaller than that of regular invar.