High-temperature tensile deformation and thermal cracking of ferritic spheroidal graphite cast iron
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I.
INTRODUCTION
SPHEROIDAL graphite (SG) cast irons are important engineering materials because of their low cost, good castability, excellent damping capacity, and fair mechanical properties. Many high-temperature structural components, such as ingot mold, brake drum, exhaust manifold, engine cylinder head, and hot mill rollers, are often made of SG cast irons.[1–4] These components are frequently operated at temperatures as high as 500 7C to 970 7C and under repeating thermal cycles.[1] When a component is subjected to thermal cycling that has steep temperature gradients to induce sufficiently large thermal stress, crack initiation can occur even without external load. Repetitive thermal cycles enable the thermal cracks to propagate until the component fails, a phenomenon that has been described as thermal fatigue.[1,5] The thermal fatigue of SG cast irons is affected by the material properties, including thermal conductivity, the thermal-expansion coefficient, and Young’s modulus.[1,6–8] Oxidation resistance and structural stability are also important factors.[1] Furthermore, when high-temperature structural applications are concerned, the high-temperature tensile properties must be considered.[9,10] According to Chijiiwa and Hayashi,[11] who studied the tensile properties of the SG cast iron with 3.6 wt pct C and 2.4 wt pct Si, the tensile strength increases and elongation decreases at the tensile temperatures around the eutectoid temperature at which the ferrite phase transforms to austenite. Among various SG cast irons, ferritic SG cast iron is C.P. CHENG, 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, Taiwan 70101, Republic of China. S.M. CHEN, formerly Graduate Student, Department of Industrial Education, National Taiwan Normal University, Taipei, Taiwan 10612, Republic of China, has graduated. Manuscript submitted June 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
more suitable for high-temperature application because of its better oxidation resistance and structural stability.[1,12] Many aspects of the effect of silicon content on high-temperature applications of SG cast irons have been explored.[12,13] However, the influence of silicon content on the high-temperature tensile properties and thermal-cracking behavior has not been studied yet. To report this influence is therefore the main purpose of the current article. In addition, the effect of different heating media on the thermal-cracking resistance and the role of microstructural size will also be clarified. II.
EXPERIMENTAL PROCEDURE
As shown in Table I, the SG cast irons used in this study have about the same carbon concentration (3.48 to 3.65 wt pct), and six different silicon concentrations, from 1.1 to 3.9 wt pct. These test materials are designated in the table and, hereafter, according to their silicon concentrations. They were prepared by melting pig iron, ferrosilicon, and scrap in an induction furnace. After the sphero
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