| Summary: | 碩士 === 遠東科技大學 === 機械工程研究所 === 103 === This study used SCM440 Cr-Mo steel, a common element in wearable materials, as substrates, which were subjected to electrical discharge alloying (EDA) for steel surface modification. Two types of cast iron with lubricating graphite phase and different graphite patterns, i.e. flake graphite (FG) (FC250) and spheroidal graphite (SG) (FCD450), were selected as the electrodes in alloying. The approach with fixed discharge current and duty factor and varying pulse durations (100~1000ms) was used to find the machining parameters that lead to the optimal pulse durations for both the FG and SG cast iron electrodes in an attempt to obtain the alloy layers that contain graphite phase and have adequate hardness, corrosion resistance and surface roughness. On the other hand, assessment was made on whether it is feasible to increase the amounts of alloy elements diffused into the alloy layers, the number of eutectic graphite nodules and the thickness of these layers while maintaining better corrosion resistance and surface roughness. As such, two-stage variation of discharge current was considered. The discharge current was set at the high value of 50A for a machining duration of 5 minutes in the first stage and at the low value of 20 A for a machining duration of 25 minutes in the second stage. This was done to examine how variation of machining parameters (two-stage variation of discharge current) affects the microstructural characteristics, corrosion behavior and surface roughness on the alloy layers of the FG and SG cast iron electrodes.
The experimental results show that the FC250 electrodes have produced thicker EDA alloy layers in the pulse durations of 500 ms and 1000 uns compared to the FCD450 electrodes and have produced alloy layers with maximum hardness up to Hv 1172 in the pulse duration of 100 ms. The alloy layers produced by the FC250 electrodes exhibit lower corrosion current (Icorr.=1.255 A • cm-2) compared to those produced by the FCD450 electrodes (Ecorr.=-0.4619 V). Both types of electrodes have produced alloy layers of which the surface roughness is directly proportional to the pulse duration. In addition, comparison of surface properties between EDA alloy layers produced respectively with one stage of discharge current, 20A, and two stages of discharge current, 50A and 20A, reveals that those in the latter case exhibit poorer thickness, hardness and surface roughness than those in the former case, with the exception that the FCD450 electrodes exhibit better corrosion resistance in the pulse duration of 1000ms. Moreover, X-ray diffraction analysis of the 20A-FC250, 20A-FCD450 and 50A+20A-FC250 specimens reveals that all the alloy layers have Cr3C2, Fe3C, C0.17Fe0.81Si0.02 and MoFe phases and only the 50A+20A-FCD450 specimens have all the phases except Fe3C.
Keywords: Cr-Mo steel, electrical discharge alloying, cast iron, machining parameters
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