The effect of rapid thermal cycling on the creep properties of alpha iron
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MosTcreep data and theories
are based on experiments c a r r i e d out under constant s t r e s s and t e m p e r a ture conditions. However, stable conditions are r a r e in most engineering systems. In most systems parts are subjected to rapid temperature and load changes, and it is questionable whether the existing creep data are directly applicable for such non-stable conditions. Extensive r e s e a r c h has been carried out in the field of thermal fatigue which has shown a combination of creep and fatigue damage,* but, surprisingly, no systematic investigation on the creep properties of materials under thermal cycling conditions has yet beeu done. Some creep experiments at varying temperatures were made by Webster et al. 2,s and by Vladimtrova et al., 4,%e but the thermal cycles were in the order of hours. Based on the limited experimental data, it is now believed that rapid thermal cycling a c c e l e r a t e s creep rate when compared to the creep rate at a constant temperature corresponding to the average cycle temperature. %s The temperature dependence of creep rate can be expressed as:
= A exp[-Qc/kT
]
[1]
where T is the absolute temperature, Qc is the activation energy for creep, k is the Boltzmann's constant and A is a constant depending on s t r e s s . Due to the exponential relationship, the creep rate during cycling above the average temperature is much higher than the creep rate during cycling below the average temperature (Fig. 1). For this reason, the expected mean creep rate 4~ while cycling over and under a certain average temperature Tav , should be higher than the creep rate at constant temperature equal to Tav. From Fig. 1 it can be seen that the " h i g h e r " creep rate under D. EYLON, formerly with the Department of Materials Engineering, Technion, are now Aerospace Research Labs., Wright-Patterson AFB, Ohio, D. G. BRANDON on leave from the Department of Materials Engineering, Technion, now at the Department of Metallurgy and Material Science, University of Cambridge, England. A. ROSEN is with the Department of Materials Engineering, Technion, Haifa, Israel. Manuscript submitted November 20, 1972. METALLURGICAL TRANSACTIONS
thermal cycling conditions is a direct consequence of Eq. [1] and is not an accelerating effect due specifically to thermal cycling. To determine the real effect of thermal cycling on the creep properties, it is not only n e c e s s a r y to build a special creep testing equipment capable of rapid and controlled temperature cycles, but a new method must be developed for evaluating creep results under nonuniform temperature conditions. By using specially designed equipment ~ and a new testing method it has already been reported m that the creep rate of Inconel 718 under thermal cycling conditions is lower than the creep rate calculated from results of isothermal experiments. The aim of the present work was to study i) the effect of thermal cycling on the creep properties of alpha iron as well as ii) the transient creep behavior associated with an abrupt increase
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