不同石墨型態低熱膨脹鑄鐵之尺寸熱穩定性分析

Thermal Dimensional Stability of Different Low Thermal Expansion Graphite Cast Irons

陳翰霖(Hanlin Chen)

指導教授 ： 潘永寧

國立臺灣大學/工學院/機械工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU201600327

摘要

本研究之內容包含三部分，第一部分係針對具不同石墨型態之低熱膨脹鑄鐵，探討均質化熱處理對於合金元素Ni之偏析程度及基地固溶C量之影響，並進一步探討其對於熱膨脹係數(α值)的影響；第二部分係以拘束型熱循環試驗來分析比較具不同石墨型態之低熱膨脹鑄鐵的尺寸穩定性；第三部分係依據相關力學及熱傳理論，以三維實體模型針對具不同石墨形態之低熱膨脹鑄鐵進行試片經拘束型熱循環試驗後之溫度分佈、熱應力值以及尺寸變化量之模擬分析，並進一步探討α值與熱應力及合金尺寸穩定性之關聯性。
本研究為探討固溶C量與Ni偏析程度對於α值之影響，分別進行了三項迴歸分析；固溶C量對α值、Ni偏析程度對α值以及兩者對於α值之複合影響，分析結果如下:
固溶C量對α值:α=1.06%C+4.6; R2=0.33
Ni偏析程度對α值:α=1.01%Nid+2.81; R2=0.92
固溶C量與Ni偏析程度對α值:α=0.68%C+1.05%Nid+2.41; R2=0.96
由分析結果可知，固溶C量與Ni偏析程度兩個因子會同步影響α值，故欲降低α值，必須同時降低合金之Ni偏析程度及固溶C量，但Ni偏析程度之影響較固溶C量高。
此外，由本研結果究得知，球墨鑄鐵、片墨鑄鐵、縮墨鑄鐵於鑄態時，α值由大至小為球墨>縮墨>片墨；而在施以同樣熱處理條件T1(1150oC/4hr/FC/750oC/4hr/WQ)下，α值為片墨>縮墨>球墨。
另外，針對三爐次(片墨、縮墨、球墨)在經過均質化熱處理後，進行拘束型熱循環疲勞試驗，並量測試片之形狀變化量，並與一般球墨鑄鐵及304不銹鋼進行比較。實驗結果顯示，三爐次其形狀變化量皆低於一般球墨鑄鐵以及304不鏽鋼，且合金之尺寸穩定性與α值有明確的相關性，當α值愈小時，所造成之變形量愈小，故尺寸穩定性愈佳，而尺寸穩定性為球墨>縮墨>片墨。

關鍵字

低熱膨脹鑄鐵 ； 熱膨脹係數 ； 均質化熱處理 ； 鎳偏析 ； 固溶碳量 ； 尺寸穩定性 ； 形狀變化量 ； 有限元素法

並列摘要

The primary purposes of this research are three fold: (1) to investigate the effect of a specific heat treatment (TI: 1150oC-4h/FC/750oC-4h/WQ) on the Ni segregation, C content dissolved in the matrix, and α value in three different graphitic cast irons, (2) to conduct the constrained thermal cyclic tests to evaluate the dimensional stability of the alloys studied, and (3) to employ the finite element method (ANSYS) to simulate the temperature field, thermal stress and shape change of specimens after the thermal cyclic tests, and further to assess the correlation among α value, thermal stress and dimensional stability.
Regression analyses were performed to correlate the carbon content dissolved in the matrix and/or degree of Ni segregation with α value, with the results being shown below:
(1) α value vs. C content dissolved in the matrix: α = 1.06%C + 4.6; R2 = 0.03
(2) α value vs. Degree of Ni segregation: α = 1.01Nid + 2.81; R2 = 0.92
(3) α value vs. both C content dissolved in the matrix and Degree of Ni segregation: α = 0.68%C + 1.05Nid + 2.41; R2 = 0.96
Based upon the regression analysis results, α value can be decreased by reducing both the carbon content dissolved in the matrix and degree of Ni segregation, with the latter being the dominant factor.
Regarding the effect of the graphite type on α value, α value decreases according to the following order: SG > CG > FG, in the as-cast condition. On the other hand, α value decreases according to the following order: CG > FG > SG, in the T1 heat treatment condition.
Shape change (△PV) of the specimens after constrained thermal cyclic tests (500 cycles) were measured for low thermal expansion cast irons with different graphite shape (in T1 heat treatment condition) and two other alloys, SUS 304 and regular ductile cast iron. The results indicate that the shape changes in low thermal expansion cast irons regardless of graphite shape are substantially lower than both SUS 304 and regular ductile cast iron. Furthermore, among the low thermal expansion cast irons the shape change or dimensional stability is closely related with α value, that is, the lower the α value, the less the shape change or the better the dimensional stability. Therefore, the order of dimensional stability is SG > CG > FG.

並列關鍵字

Thermal expansion coefficient ； Cast iron ； Homogenization heat treatment ； Ni segregation ； Carbon content in the matrix ； Dimensional stability ； Finite element method ； ANSYS

參考文獻

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