灰口(FC)鑄鐵 about grey cast iron－leometwu的部落格

下:片狀石墨的2D影像; 3D 影像在最後面---試片先以強酸蝕去鐵基地然後以電子顯微鏡拍攝.

Above: Photo A, B, 上:100x,200x. FC120; 數量龐大又粗大的石墨片, 其實就是肥粒鐵基地上的微細裂口. 鑄鐵強度偏低, 或加工面毛細孔太粗是拜這些石墨片所賜. 鋼鐵強度(從全波來鐵到全肥粒鐵基地)約800~400MPa, 有石墨片出現後則降到350~100 MPa (視石墨片數量, 大小, 分佈與基地上波來鐵%). 但石墨可降低鐵水熔點(到1120~1130C), 使熔解成本大幅降低. 又有吸收振動的功能, 故被廣用於工具機結構上. 生產高強度鑄鐵的要點在於: 1.使基地上的石墨片變少(靠加低碳鋼稀釋銑鐵的4.0%C), 變小 (靠接種處理). 2. 使基地上的波來鐵%高一點(肉太厚時靠添加合金:Cr,Mo,Cu...). 下: FC ~250 . 與上面比較, 石墨數量明顯變少變小. 基地也有許多波來鐵.

Photo C, D. 100x, 100x; 上左:拋光後; 白色部位為以樹枝狀(dendrite)方式成長的初晶, 黑色部位為D-形, E-形石墨 . 上右: Nital 浸蝕後才能看出初晶部位肥粒鐵與波來鐵的面積率%. 下左:200x;下右:500x. 分解後剩下的波來鐵, 較亮處為支幹中心(arm center), 是最先凝固處, Fe純度較高, 合金較少, 所以這裡的Pearlite先分解而剩Ferrite. 較暗處為支幹邊界(arm boundary), 是最後凝固處, 合金與 C 都排擠到此, 故出現石墨, 此處Pearlite 合金濃度較支幹中心高, 較不易分解. 這種從枝幹中心到邊界上, 微小尺度的 "偏析", 稱為微偏析(micro-segregation). *冶金術語: 偏析= 口語: 成份不均勻. 鐵水凝固時支幹邊界有時未填滿而形成空孔(voids, pores), 稱為interdendritic pores 微縮孔

Below, Photo E, F. 200x, 500x. etched.

Pig iron has aprox. 4% C and 1.6 ~ 2.2% Si. Machine parts made directly from pig iron will have too low a tensile strength (T.S. 100~120 MPa) because of its large numbers of graphite flakes(=microcracks) in the matrix of ferrite (low in strength). To obtain higher T.S., the number of microcracks has to be reduced. So, low carbon steel scrap is added to dilute the C% (in pig iron) from 4% to 3.50~3.00% while maintaining 0.60~0.9%Mn to obtain T.S. of 200 ~350 MPa . For higher strength irons, pearlite is absolutely needed for strengthening matrix of cast iron. Lower C.E. and higher alloying elements (Mn, Cr, Cu...) promote formation of pearlite. But too much alloying elements will result in the formation of eutectic carbides which is very harmful to cutting tools.

Photo A, B above: Iron with high CE%. The amount of graphite flakes is very high and matrix is fully ferritic. Mixture like this will have very good damping capacity, but very poor mechanical properties. Photo C ~ F. Iron with lower CE%. The amount of graphite flakes is reduced and its mechanical properties are thus improved. Since D- and E type graphite are present, Pearlite % is not very high. Below: 1. FC 300 ; 2. fully pearlitic matrix; 3. 3-D image of graphite flakes. Iron matrix was etched away by strong acid.