片狀石墨與球狀石墨鑄鐵的高溫氧化

High Temperature Oxidation of Flake and Spheroidal Graphite Cast Irons

選用片狀石墨鑄鐵（FC200）與球狀石墨鑄鐵（FCD400）為試驗材料，分析400 ~ 750℃靜滯空氣氧化動力學，探討石墨型態對鑄鐵抗高溫氧化性能的影響。低碳鋼（SB450）為對照組材料，比較兩種鑄鐵材料與低碳鋼之高溫氧化。實驗結果顯示石墨型態對於鑄鐵材料抗高溫氧化性有明顯的影響。片狀石墨鑄鐵之抗高溫氧化性低於球狀石墨鑄鐵。觀察片狀石墨鑄鐵高溫氧化層的連續成長行為顯示，由於氧氣持續由片狀石墨晶界滲入，促使鐵氧化物沿片狀石墨晶界持續快速生長，氧化層往內側生長增厚，因此片狀石墨鑄鐵氧化速率常數高出球狀石墨鑄鐵氧化速率常數5倍，而球狀石墨鑄鐵高溫氧化速率常數與低碳鋼相當接近。根據以上高溫氧化動力學與氧化層成長行為分析結果，片狀石墨對鑄鐵材料抗高溫氧化能力有明顯的負面影響，而球狀石墨對鑄鐵之抗高溫氧化性則無明顯的影響。

Flake and spheroidal graphite cast irons with a similar composition were subjected to high temperature oxidation, and the effects of graphite morphology and distribution on the oxidation behavior of the cast irons were investigated. High temperature oxidation tests were performed at temperatures between 400 and 750 ℃ in air. For comparative purposes, low carbon steel was also tested. Graphite morphology significantly affected the high-temperature oxidation resistance of the cast irons. The flake graphite cast iron exhibited a worse high-temperature oxidation resistance than the spheroidal graphite cast iron. Since the graphite flakes acted as suitable sites for the growth of iron oxide and were almost interconnected, the iron oxide grew faster and penetrated along the boundaries of graphite flakes, resulting in subsurface oxidation. Due to the severe subsurface oxidation the parabolic oxidation rate constant of the flake graphite cast iron was 5 times higher than that of the spheroidal graphite cast iron. In the meantime, the parabolic oxidation rate constant and the oxide layer thickness of the spheroidal graphite cast iron were similar to those of the low carbon steel. Therefore, graphite flakes could induce a negative effect on the high temperature oxidation resistance of the cast iron.