Stress and temperature dependence of time to rupture of heat resisting steels

A new relation among the time to creep rupture, the stress and the temperat ure has been developed. Assuming the velocity of dislocations is controlled by a thermally activated process, the lime to rupture, t(r) is expressed a s log tr = Q-V sigma/2.3RT -C, where Q, V, sigma, R, T, C are respectively the activation energy, the acti vation volume, the applied stress, the gas constant, the absolute temperatu re and the Larson-Miller constant, i.e. about 20. The new equation reads that the applied stress in a linear scale is a linea r function of the Larson-Miller parameter, P=T(log t(r)+C). The new relatio n has been applied to the long term rupture data of several heat resisting steels obtained by National Research Institute for Metals. According to the equation the rupture data for each material can be classified into 2-4 gro ups where the deformation mechanism differs with each other. It has been fo und that for each group the data are excellently fitted to the equation. A set of the data from several hours to longer than 100 000 h are roughly w ell fitted to another expression of a hyperbolic tangent type. The expressi on is the approximate equation of the above theoretical expression.