Influence of grain size on the low-cycle fatigue lives of austenitic stainless steels at high temperatures
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1. I N T R O D U C T I O N
2. E X P E R I M E N T A L P R O C E D U R E
9A U S T E N I T I C stainless steels are of interest as a structural material for elevated temperature applications in power generation and chemical industries. For these applications, components of the structure are often subject to repeated thermal stresses as a result of temperature gradients which occur on heating and cooling during start-up and shutdown. Therefore, the problem of high temperature low-cycle fatigue failure is of great importance to the strength design of the components for improvement of reliability.' In recent years it has become apparent that the fatigue lives of austenitic stainless steels depend not only on testing temperature and strain range but also on strain rate and strain wave shape. 2-9 However, heatto-heat or type-to-type variations in the fatigue lives of austenitic stainless steels have been scarcely examined l~ though they have much engineering significance. These variations in the fatigue lives are considered to be caused by the variations in chemical composition, heat-treatment, grain size, grain boundary structure and so on. In this paper we paid attention to grain size in discussing heat-to-heat and type-to-type variations in high temperature low-cycle fatigue lives for eight kinds of solution annealed austenitic stainless steels. The authors had examined the influence of grain boundary structure on high temperature low-cycle fatigue lives for solution annealed and aged austenitic stainless steels. ~3
2-1 Materials
KOJ1 YAMAGUCHI and KENJI KANAZAWA are Researcher and Senior Researcher, respectively, Fatigue Testing Division, National Research Institute for Metals, Nakameguro, Meguro-ku, Tokyo 153, Japan. Manuscript submitted September 11, 1979.
The materials used in this study were eight kinds of solution annealed austenitic stainless steels such as Type 304, 316, 321,347 and 310. The chemical composition, heat-treatment and grain size number are listed in Table I. The grain size number was measured in accordance with the Japan Industrial Standards (JIS G 0551). In the case of mixed grains, mean grain size number was used. Photographs of microstructure of the steels are shown in Fig. 1. Mechanical properties of the steels are listed in Table II. There seems to be a good correlation between the hardness and the grain size, that is, the hardness tends to decrease with decreasing the grain size number. There is no good correlation between the tensile properties at high temperature and the grain size. 2-2 Fatigue Tests Fatigue specimens were machined from the bar stock to the shape shown in Fig. 2. Fatigue tests were carried out in air in push-pull type, electrohydraulic testing machines under total axial strain control. Strain wave shape was triangular and mean strain was zero. The strain rates were 6.7 x 10-3/s and 6.7 x 10-5/s. Testing temperatures were 600 and 700 ~ For each testing condition the fatigue tests were performed at about five ranges of the total strain to obtain an S-N curve. In additio
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