Effect of nodularity on resonant vibration fracture behavior in upper bainitic and ferritic cast irons
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I. INTRODUCTION
SPHEROIDAL graphite (SG) cast irons have excellent castability, machinability, and damping capacity that allows them to be employed in many structural applications.[1] For example, SG cast irons are part of cam shafts, crank shafts, or disk brake calipers.[2] The possibility of vibration fracture is an important problem in transportation application materials due to the occurrence of some abnormal vibration conditions. Notably, failure rapidly occurs when the applied vibration frequency reaches the resonant frequency of SG cast iron. Our previous report[3] investigated how the matrix structure affects resonant vibration fracture characteristics of ductile iron. However, the effect of nodularity change on resonant vibration fracture behavior resulting from matrix variables has not yet been clarified. Although the tensile strength and fatigue limit of ferritic cast iron extended with increasing nodularity, the damping capacity decreased with increasing nodularity according to previous reports.[4,5] Bainitic cast iron exhibits different dependence since the possibility of flow stress increases with decreasing nodularity due to a significantly higher volume fraction of retained austenite.[6,7] However, this work attempts to clarify the resonant vibration behavior of ductile ferritic and bainitic cast iron with various nodularity. II. EXPERIMENTAL PROCEDURES A. Material Preparation and Vibration Test Six kinds of testing materials were prepared by altering the spheroidized and heat-treatment process. The chemical composition of all testing materials was controlled in a constant range within 3.6 wt pct C and 2.9 wt pct Si, to examine the resonant vibration fracture behavior and the crack propagation behavior under resonant vibration frequency. The SG cast iron samples were melted in a 100 kg capacity induction furnace at a pouring temperature of 1450 8C. The
S.C. LIN, Graduate Student, and T.S. LUI and L.H. CHEN, Professors, are with the Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, Republic of China. Manuscript submitted June 4, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fe-75 wt pct Si alloy was adopted for inoculation and an Fe-45 wt pct Si-5 wt pct Mg alloy was employed for spheroidization, and the alloy was cast into Y-shaped CO2 sand molds. The addition of spheroidizer was varied with 1.5, 1, and 0.5 mass pct in order to vary the nodularity. Thus, three cast materials with distinct nodularity (from 85%, 60% to 45%) were obtained, namely, SG, compacted graphite (CG1), and CG2. A flaky graphite (FG) cast iron was also utilized for comparison. Table 1 lists the compositions. Heat-treated conditions were selected for obtaining ferritic and bainitic structure to comprehend the effect of the matrix structure on vibration fracture behavior. The materials were soaked at 925 8C for 3 hours and 750 8C for 5 hours; then, the furnace was cooled for ferritization and austempered for 30 minutes in a salt bath at 350 8C to obtain a bainitic matrix. The
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