Fatigue fracture mechanism maps for a type 304 stainless steel
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Fatigue Fracture Mechanism Maps for a Type 304 Stainless Steel M. KIMURA, K. YAMAGUCHI, M. HAYAKAWA, K. KOBAYASHI, S. MATSUOKA, and E. TAKEUCHI Fatigue fracture mechanism maps at room temperature and 573 K for a type 304 stainless steel were constructed by correlating the crack propagation rate with information obtained on the fracture surface. Depending on the crack propagation rate, ranging from 1 106 to 1 1011 m/cycle, three types of fracture surfaces were observed. One was a striation region; the second was a “featureless” fracture region, which appeared rough under scanning electron microscope (SEM) observation; and the third was crystallographic fracture region, which appeared smooth under SEM observation. The area fractions and the indexes of the fracture surfaces were quantified and identified by the etch-pit method. From the results, crack initiation and propagation mechanisms were cleared and fatigue fracture mechanism maps were constructed. The maps may be useful for investigating the cause of the fatigue failure accident of structures made of type 304 steels. I. INTRODUCTION
OBSERVATION of fracture surfaces is extremely helpful in investigating the causes of the failure accident of machines or structures.[1,2,3] Fatigue fracture mechanism maps are being constructed for fatigue failure of several metallic materials.[1–4] These maps correlate the fatigue crack propagation rate with information on the fracture surface.[4] For austenitic stainless steels, however, few data have been reported that correlate crack propagation behavior at low rates and high temperatures with information on fracture surfaces. Still less, no fatigue fracture mechanism maps have yet been established, in spite of austenitic stainless steel currently being in wide use as a heat- and corrosion-resistant material. The purpose of this study is to construct a fatigue fracture mechanism map for a typical austenitic type 304 stainless steel at room temperature and at 573 K. The crystallographic orientation on the fracture surfaces is identified by the etchpit method to investigate the fracture mechanisms. An X-ray diffraction analysis is also carried out to confirm whether plasticity-induced martensitic transformation occurs. Few studies have been performed on the fracture mechanisms of structural materials that make use of experimental results such as the etch-pit method on fracture surfaces. II. EXPERIMENTAL PROCEDURES The material used in this study is a typical austenitic type 304 stainless steel plate of 12-mm thickness. Its chemical composition is shown in Table I. Figure 1 shows the microstructure of the material. The average austenitic grain size is 10 to 30 m in diameter. The solution-treatment temperature is 1353 K. The fatigue specimen is a compact-tension type with a width of 50 mm and a thickness of 8 mm. To measure the
M. KIMURA, Researcher, K. YAMAGUCHI, Group Leader, M. HAYAKAWA, K. KOBAYASHI, and E. TAKEUCHI, Senior Researchers, and S. MATSUOKA, Deputy-director-general, are with the F
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