合金組成與熱處理條件對低熱膨脹鑄鐵之熱膨脹係數的影響

Effects of Chemical Composition and Heat Treatment on Thermal Expansion Coefficient of Low Thermal Expansion Cast Irons

陳力豪(Li-Hao 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(2013年)

https://doi.org/10.6342/NTU.2013.00107

摘要

本研究之內容包含兩部分，第一部分係探討以各種不同合金元素取代Ni之可行性。在合金設計上，選定(1.5~2.0)%C、(1.5~2.0)%Si及~25%Ni為基本合金，並添加各種不同合金元素以探討其對於α值之影響，所設定之目標熱膨脹係數值為α100 ≦ 8 × 10-6/oC。此部分之實驗結果顯示，在所分析之各合金元素中(Mn, Ti, Zr, Cu, V, Al, Sn, Cr, W)，除Zr外，其餘均會造成α值之增加。又，添加Zr雖然不會增加α值，但在顯微組織中會出現大量之富Zr析出相，使得組織呈現不均勻的現象，因此，合金之韌性期會降低。綜上，擬以較低價格之合金元素來取代Ni之構想，並不具實際功效。
第二部分係進一步探討均質化熱處理對於α值的影響，由於合金在鑄造狀態(as-cast)時，一方面，合金元素Ni會有(逆)偏析現象，另一方面，固溶於基地中的C含量係處於非平衡狀態，這些現象均會影響(增高) α值。進行均質化熱處理預期將會有助於Ni之均勻分布，且會改變基地中之C含量，而改變α值。本研究之目的主要係建立最佳之熱處理條件以降低α值，並進一步探究均質化熱處理如何影響α值之機理。均質化溫度及時間均是影響固溶C含量及Ni偏析的重要因素。當均質化熱處理溫度為750oC時，雖然可以得到較低的固溶C含量，但卻無法有效解決Ni偏析問題；而當熱處理溫度提高到1200oC時，雖能夠有效解決Ni偏析問題，但卻會造成固溶C含量的增高。因此，欲降低熱膨脹係數應同步降低Ni之偏析程度以及降低基地中的固溶C量。本研究得知，最佳熱處理條件為T6 (1200oC/4hr/FC/750oC/2hr/WQ)，其中1200oC/4hr之均質化熱處理溫度幾可完全消除Ni偏析，而在經過750oC/2hr之恆溫保持可降低基地中固溶C含量，兩者均有利於α值之降低，使得T6之α30~200oC約在1~2×10-6/oC之間。此外，α值與固溶C含量及無因次Ni偏析程度之複合迴歸方程式為：α =3.1132 -2.0167%C-0.3466Ni +5.9870 (%C)2+0.1824(Ni)2， R2=0.68。另外，針對B1爐次在經過不同均質化熱處理後，進行拘束型熱循環試驗，並量測尺寸變化量以及形狀變化量，以評估均質化熱處理對於尺寸穩定性之影響，並與一般球墨鑄鐵及304不銹鋼進行比較。實驗結果顯示，針對B1爐次，其形狀變化量皆低於一般球墨鑄鐵以及304不鏽鋼，且與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。

關鍵字

低熱膨脹鑄鐵 ； 熱膨脹係數 ； 均質化熱處理 ； 鎳偏析 ； 固溶碳含量 ； 尺寸穩定性 ； 形狀變化量

並列摘要

The primary purposes of this research are twofold: (1) to study the feasibility of alloying substitutions for Ni at different additions to achieve low α values, (2) to establish the optimal heat treatment conditions for attaining the lowest possible α values, and further to analyze the mechanisms related to the change in α values with microscopic features.
The experimental results regarding the alloying substitutions of Mn, Ti, Cu, V, Al, Sn Cr, W or Zr for Ni indicate that, except for Zr, all other alloying elements cause an increase in α value, and also fail to achieve the target value of α100oC≦8×10-6/C. Although a partial substitution of 5% Zr for equal amount of Ni did not increase the α value, it did cause the formation of Zr-rich phase in the matrix, which embrittled the material. Therefore, the substitution of a certain portion of Ni in the low thermal expansion cast irons with other alloying elements with an intention to reduce the material cost and at the same time to reduce the α value has been found infeasible.
In the second phase of this research, the alloys with chemical compositions of (1.5-2.0)%C, (1.5-2.0)%Si and 35%Ni had been selected to investigate the effect of homogenization heat treatment on the degree of Ni segregation, carbon content dissolved in the matrix and also the α value. Phenomena of Ni (inverse) segregation and the C concentration departing from the equilibrium, which is normally higher than the equilibrium value, are present in the alloys in the as-cast state. As a result, the α value was adversely affected. When the alloys were homogenized at a relatively low temperature, e.g., 750oC, the elimination of Ni segregation was not very effective, but the C concentration in the matrix was moderately reduced. On the other hand, as the alloys were homogenized at a relatively high temperature, e.g., 1000oC, reverse results were obtained. Consequently, not much improvement (reduction) in α value can be achieved. This can be observed in Heat B1, where the α30-100oC values of 5.87×10-6/ oC, 5.74×10-6/ oC, 4.67×10-6/ oC, 4.69×10-6/ oC, and 5.19×10-6/ oC were registered for T0 (as-cast), T1 (750oC/3hr), T2 (750oC/20hr), T3 (1000oC/3hr), and T4 (1000oC/20hr), respectively. Similar results were also obtained in Heats B2 and B3. In order to lower the α value, a compound homogenization heat treatment procedures was performed, namely, 1200oC-4hr/750oC-2hr, in which a higher homogenization heat treatment temperature (1200oC) can effectively eliminate the degree of Ni segregation on the one hand, while the C concentration in the matrix can be reduced to a low level during the holding stage at 750oC. Both effects favor the reduction in α value. As a result, very low α values of around (1-2)×10-6/oC were obtained. Furthermore, Heat B1 with different chemical compositions and homogenization heat treatment conditions, together with a conventional ductile cast iron and a 304 stainless steel, were selected for constrained thermal cyclic tests (30~200oC) to evaluate the dimensional stability of afore-mentioned the alloys based upon the changes in the shape of the test specimens. This can be observed in Heat B1, where the degree of shape change of the low thermal expansion ductile cast irons are well below those of the conventional ductile cast iron (634.01μm) and the 304 stainless steel (428.93μm). Furthermore, correlation between the amount of shape change after thermal cyclic tests and α30~200oC value shows a similar trend. The degree of shape change follows the following sequence: T0(156.93μm) – T1(63.25μm) – T4(27.94μm) – T2(25.77μm) – T3(15.56μm)–T6(7.41μm) verse the α30~200oC value of A0(5.87x10-6/oC)– A1 (5.74x10-6/oC) – A4(5.19x10-6/oC) – A2(4.67x10-6/oC) – A3(4.69x10-6/oC) – A6(1.72x10-6/oC). Accordingly, the homogenization heat treatment will alter (reduce) the α value and the dimensional stability of the alloys will be affected. The present results indicate that the alloy with the homogenization heat treatment of 1200oC-4hr/750oC-2hr can obtain the lowest α value, and hence, possesses the best dimensional stability.

Keywords: low thermal expansion ductile cast iron, α value, homogenization heat treatment, Ni segregation, carbon content dissolved in the matrix, dimensional stability, the changes in the shape

並列關鍵字

low thermal expansion ductile cast iron ； α value ； homogenization heat treatment ； Ni segregation ； carbon content dissolved in the matrix ； dimensional stability ； the changes in the shape

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