Some aspects of thermomechanical fatigue of AISI 304L stainless steel: Part I. creep- fatigue damage
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I.
INTRODUCTION
THE mechanical behavior of the austenitic stainless steels AISI 304 and AISI 304L at elevated temperatures has been studied quite often. In the literature, work is reported dealing with the creep behavior, I~ 71 the behavior under monotonic loading (e.g., Reference 8), under low-cycle fatigue (LCF) conditions, t8 ~3j and in thermomechanical tests, l~4-~8j These investigations are of great technical interest, since austenitic stainless steels are widely used for applications at intermediate temperatures. Nevertheless, the knowledge regarding the temperature dependence of the mechanical behavior of these steels is relatively poor, since in most studies, the tests have been carried out in a narrow temperature range or at few temperature values. Furthermore, in the case of the thermomechanical fatigue (TMF) behavior, there is a lack of information about the correlation between the mechanical response and the microstructural processes. Recently, the authors of this article have reported the results obtained for AISI 304L in an extensive study on the temperature dependence of the cyclic deformation behavior and the related dislocation arrangements under isothermal and thermomechanical conditions. It could be shown in accordance with other studies (e.g., References 8, 9, 13, 20, and 21) that during cyclic loading at elevated temperatures, dynamic strain aging (DSA) occurs, giving rise to a maximum of the stress amplitude at about 450 ~ in plastic strain-controlled tests at a constant plastic strain rate, ~,t, of 5.0 9 10 -4 S -1.
The dislocation glide mode changes drastically with temperature. At temperatures below the onset of DSA, a wavy glide character exists, whereas in the temperature R. Z A U T E R , D e v e l o p m e n t E n g i n e e r , is with B a b c o c k Rohrleitungsbau GmbH, Oberhausen, Germany. H.-J. CHRIST, formerly with the University of Erlangen, is Professor, Institute for Materials Technology, University of Siegen, Siegen, Germany. H. MUGHRABI, Professor, is with the Institute for Materials Science, Lehrstuhl I, University of Erlangen-Nfirnberg, Erlangen, Germany. Manuscript submitted March 26, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
range of DSA, a more planar appearance of the dislocation arrangement could be observed 9 At temperatures above the region in which the cyclic deformation behavior is strongly affected by DSA, the glide mode becomes wavy again. The DSA has a strong influence on the TMF behavior if the temperature range of the TMF cycle lies partially or completely in the region of DSA. The results obtained under TMF conditions could be interpreted and understood on the basis of the isothermal tests (the reader is referred to Reference 19 for more details)9 The present article, Part I in a series of two, focuses on the damage which cumulates during TMF of AISI 304L. A simple equation will be deduced which allows the prediction of the failure mode as a function of the test conditions9 In order to verify this equation, experiments with different values of the plas
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