In Situ Observation of High Temperature Creep Behavior During Annealing of Steel
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HIGH-TEMPERATURE creep of materials involving time-dependent deformation occurs as a result of long-term exposure to high levels of stress below the yield strength of such materials. Creep properties of materials are usually determined by the dependence of minimum creep rate e_ m on stress r, temperature T, and grain diameter d using power-law equations of the form given by Eq. [1].[1–3] m AGb b r n D0 expðQ=RTÞ ½1 e_ m ¼ kT d G where A is a dimensionless constant, D0 is the frequency factor, G is the shear modulus, b is the Burgers vector, k is the Boltzmann’s constant, R is the gas constant, Q is the activation energy of creep mechanism, and m and n are constants indicating inverse grain size exponent and the stress exponent, respectively, whose magnitudes depend on creep mechanism. The fact that n, m, and Q vary depending on the imposed test conditions is explained by assuming different mechanisms. Typically, intragranular dislocation creep (i.e., power-law creep) processes involve m = 0 and n ‡ 3,[4,5] whereas diffusion creep (i.e., Nabarro–Herring, Coble, and Harper– Dorn creep) processes involve m ‡ 1 and n = 1.[4,6] Usually, creep rates under diffusion creep processes are strongly influenced by grain size, whereas creep rates under dislocation creep are independent of grain size.[2,7]
X. F. ZHANG, Research Fellow, H. TERASAKI, Associate Professor, and Y. KOMIZO, Professor, are with the Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: xfzhang@jwri. osaka-u.ac.jp Y. MURAKIMI and K. YASUDA, Researchers, are with the Steel Research Laboratory, JFE Steel Corporation, 1 Kawasakicho, Chuo-ku, Chiba 260-0835, Japan. Manuscript submitted September 9, 2011. Article published online July 31, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
For Harper–Dorn creep to make a significant contribution, the grain size of the material should be large (>100 lm); otherwise, Nabarro–Herring and Coble creep dominate.[2,7] Although it is usually assumed that power-law creep is independent of grain size, in practice this is not the case.[1,7–9] Moreover, except for grain size, changes in substructure during high-temperature creep also influences the creep rate. Coarse slip bands have been observed for various materials mainly at an early stage of creep strain and their average spacing has often been correlated with the reciprocal of applied stress.[3,10] Another important feature is the formation of deformation bands, kink bands, and folds.[3,11,12] Deformation bands or kink bands begin to appear as strain increases and parallel tilt boundaries form around them, having a spacing which increases away from the kink bands.[3,13] Similarly, different types of substructures have been observed in different localities at an early stage of transient creep. Small subgrains elongating in the direction of deformation band have been found in some regions, while cells contained within large subgrains have been observed in other regions.[14] These results suggest a c
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