Metallographic analysis and kinetic equation of the graphitization process of graphitized steel
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摘要: 在650、680和710 ℃不同溫度條件下對碳質量分數為0.66%的淬火高碳鋼進行了石墨化處理，并利用場發射掃描電子顯微鏡、電子探針、X-射線衍射儀和透射電子顯微鏡對其石墨化過程的組織進行金相分析，以及利用組織轉變動力學理論，繪制了其石墨化過程的動力學曲線，并建立了相應的動力學方程。研究結果顯示：在石墨化過程中，淬火馬氏體首先向析出碳化物的穩定狀態轉變，且在碳化物為滲碳體Fe3C時，石墨粒子析出速度開始明顯增加；基體組織中針葉狀α-Fe發生再結晶，由等軸狀鐵素體逐步代替針葉狀的α-Fe；鐵素體中的碳含量隨著石墨化時間的延長而逐步降低，即由過飽和狀態轉變為穩定態，碳含量在石墨粒子中突變增為峰值，而鐵含量則突變降為谷值，由此表明，滲碳體分解的碳向石墨核心擴散，鐵自石墨核心處擴散出來，而形成石墨粒子；石墨粒子面積分數隨時間變化的曲線呈S形狀，即該動力學過程符合動力學模型JMAK（Johnson-Mehl-Avrami-Kolmogorov）方程，且該方程中的n值為1.5～1.7。
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關鍵詞:
- 石墨化鋼 /
- 石墨粒子 /
- 鐵素體 /
- 金相分析 /
- JMAK(Johnson-Mehl-Avrami-Kolmogorov)方程
Abstract: Graphitized steel can have good machinability and formability, or high strength through controlling microstructure. The graphitization process is formation of graphite particles in graphitized steel, which is key to control the microstructure and properties of the steel. In this paper, the quenched high carbon steel with 0.66% carbon (mass fraction) was graphitized at 650, 680, and 710℃, respectively. The microstructure formed during the graphitization process was analyzed by a field emission scanning electron microscope, electron probe microanalysis, X-ray diffraction, and a transmission electron microscope. According to the dynamic theory of phase transformation, the kinetic curve of the graphitization process was drawn, and the corresponding kinetic equation was established. The results show that in the graphitization process, the quenched martensite is first transformed to the stable state of precipitation carbide. When the carbide is cementite Fe3C, the precipitation rate of graphite particles increases significantly. The acicular α-Fe in the matrix recrystallizes, and is gradually replaced by equiaxed ferrite. With prolonged graphitization time, the carbon content in ferrite decreases gradually; that is, it changes from a supersaturated state to a stable state. The carbon content increases to the peak value in graphite particles, whereas that of Fe decreases to the valley value. These changes show that the decomposed carbon of cementite, Fe3C, diffuses into the graphite core, whereas Fe diffuses from the graphite core, and then graphite particles are formed. Additionally, when steel is graphitized, the curve of graphite particle area fraction with time is an S shape; that is, the dynamic process of the tested steel is in accordance with the JMAK (Johnson-Mehl-Avrami-Kolmogorov) equation, and the value of n in the equation is between 1.5 and 1.7. -
圖 1 用于石墨化處理試樣的原始金相組織及其元素分布曲線。（a）金相組織；（b）C、Fe元素分布
Figure 1. Original structure and element distribution curve of the graphitized sample: (a) metallographic structure; (b) distribution of C and iron Fe
圖 2 石墨化處理5 min時試樣的微觀組織及其元素分布曲線。（a）金相組織；（b）C、Fe元素分布
Figure 2. Microstructure and element distribution curve of sample graphitized for 5 min: (a) metallographic structure; (b) distribution of C and Fe
圖 3 石墨化處理30 min時試樣的微觀組織及其元素分布曲線。（a）金相組織；（b）C、Fe元素分布
Figure 3. Microstructure and element distribution curve of a graphitized sample for 30 min: (a) metallographic structure; (b) distribution of C and Fe
圖 4 石墨化處理30 min時試樣中的α-Fe[111]晶帶軸與Fe3C[012]晶帶軸復合電子衍射圖。（a）TEM圖像；（b）電子衍射圖
Figure 4. Composite electron diffraction pattern of α-Fe [111] and Fe3C [012] crystal belt axes in the sample graphitized for 30 min: (a) TEM image; (b) electron diffraction pattern
圖 5 石墨化處理30 min時試樣電解萃取產物的XRD衍射曲線
Figure 5. XRD diffraction curve of electrolytic extraction products from the samples graphitized for 30 min
圖 6 石墨化處理50 min時試樣的微觀組織及其元素分布曲線。（a）金相組織；（b）石墨粒子的形態；（c）C、Fe元素分布（基體）；（d）C、Fe元素分布（鐵素體微區）
Figure 6. Microstructure and element distribution curve of samples graphitized for 50 min: (a) metallographic structure; (b) morphology of a graphite particle; (c) distribution of C and Fe in the matrix; (d) distribution of C and Fe in a ferrite region
圖 7 石墨化處理1 h（a）和3 h（b）試樣的微觀組織
Figure 7. Microstructures of the samples graphitized for 1 h (a) and 3 h (b)
圖 8 石墨化處理5 h時試樣的微觀組織及其元素分布曲線。（a）金相組織；（b）C、Fe元素分布
Figure 8. Microstructure and element distribution curve of the samples graphitized for 5 h: (a) metallographic structure; (b) distribution of C and Fe
圖 9 石墨化處理16 h時試樣的微觀組織及其C元素分布曲線。（a）金相組織；（b）C、Fe元素分布
Figure 9. Microstructure and carbon element distribution curve of the graphitized samples for 16 h: (a) metallographic structure; (b) distribution of carbon
圖 10 石墨粒子的TEM圖像及其電子衍射圖。（a）石墨粒子的TEM圖像；（b）石墨粒子的電子衍射圖
Figure 10. TEM images and electron diffraction patterns of graphite particles: (a) TEM image of a graphite particle; (b) electron diffraction pattern of a graphite particle
圖 11 實驗用鋼石墨化過程的動力學曲線
Figure 11. Kinetics curve of the graphitization process of tested steel
圖 12 ln t與ln[ln(1?y)?1]之間的線性回歸
Figure 12. The linear regression between ln t and ln[ln(1?y)?1]
表 1 不同溫度下的n值和k值
Table 1. Values of n and k at various temperatures
Temperature / ℃ n k 650 1.68 0.0403 680 1.61 0.0707 710 1.56 0.1279 
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