CREEP AND RECOVERY IN GRAPHITES AT HIGH TEMPERATURES

Abstract

Flexural creep tests were made on molded carbons and several types of graphites over the temperature range from 2200 to 3000°C. The creep of a carbon decreased as the temperature of graphitization and the time at graphitization temperature increased. The activation energy for creep increased as the graphitization temperature was raised from 2800 to 3000°C. Activation energies for various types of graphites ranged from 23 to 96 kcal/mole; for some graphites, no activation energy could be determined. Creep in flexure depends on the grain orientation of the specimen, the density, and the type and size of particles in the material. Analysis of creep data for ATJ graphite indicates that the creep curves can be fitted to several equations. The most promising equation for characterizing the behavior of the graphite is one based on a four-parameter model for a viscoelastic material. The model was used to obtain an elastic modulus, which decreased linearly with temperature, and a modulus for the anelastic component, which also decreased with temperature. The steady state creep rate was found to be proportional to the external force for low forces and to vary as an exponential function of the force for intermediate values of the force. The activation energy for steady state creep was found to be between 70 and 76 kcal/mole, and varied with the force. Recovery data for ATJ graphite indicate that the model used to describe creep can also be used to describe recovery after creep. Good qualitative agreement was obtained between recovery data and model predictions. The simple anelastic term in the model does not accurately describe the anelastic part of the creep and recovery, and some suggestions are made for modifying the model to better describe the data.