| Summary: | 碩士 === 國立臺灣師範大學 === 工業教育研究所 === 88 === ABSTRACT
The mechanical properties of two different carbon equivalent （4.0、3.7）ductile cast irons after austempered treatment in 360℃ and 390℃ were investigated. Added Ni in the CE4.0 ductile cast irons were compared with no Ni ones. Used image analysis system to analysis graphite parameter. X-ray diffraction was used to evaluate retain austenite fraction. SEM was applied for micro-graph observation of etched specimens of ADIs.
Experimental results showed that Ni additions had not remarkable effect on nodularity, nodule count and mechanical properties. Effect of elongation by austempered time in 390℃ was more remarkable than in 360℃. Under same nodule count and treatment, the tested elongations increased by the nodular graphite diameter decreased.
Ultra tensile strength （UTS） was associated with nodularity increased, but UTS increased little. Increasing nodule count, the tested elongations had a increased trend. Increasing graphite area fraction reduced both UTS and tested elongations. Increasing the diameter of nodular graphite decreased tested elongations.
The mechanical properties of this study applied for ASTM A-897-90 standard, the heat treatment processes had two methods：（1）specimens were preheated at 550℃ for 15 min and asutenitized at 900℃ for 90 min, after which they were quenched in a salt bath at 360℃ for 120 min. After the austempered treatment, specimens were then air-cooled to room temperature. After this method the mechanical properties can reach ASTM A-897-90 1200/850/4 grade standard.（2）specimens were preheated at 550℃ for 15 min and asutenitized at 900℃ for 90 min, after which they were quenched in a salt bath at 360℃ for 60 min. After the austempered treatment, specimens were then air-cooled to room temperature. After this method the mechanical properties can reach ASTM A-897-90 1050/700/7 grade standard.
Keyword：processing variables , low carbon equivalent, graphite, ADI.
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