均質化熱處理對低熱膨脹鑄鐵之熱膨脹係數及尺寸穩定性的影響

The Effect of Homogenization Heat Treatment on Thermal Expansion Coefficient and Dimensional Stability of Low Thermal Expansion Cast Irons

陳力豪(L. H. Chen) ； 卓炳勳(B. S. Juo) ； 劉宗霈(Z. P. Liu) ； 潘永寧(Y. N. Pan)

《鑄造工程學刊》 40卷2期 (2014/06) Pp. 17-25

摘要

由於合金在鑄造狀態（as-cast）時，合金元素Ni會有（逆）偏析現象，且固溶於基地中的C量係處於非平衡狀態，這些現象均會影響（增高）α值。均質化熱處理之溫度及保持時間均是影響固溶C量及Ni偏析的重要因素，本研究之目的主要係建立最佳之均質化熱處理條件以降低α值，並進一步探究均質化熱處理影響α值之機理。研究結果顯示，當均質化溫度為750℃時，雖然可以得到較低的固溶C量，但卻無法有效解決Ni偏析問題；而當均質化溫度提高到1200℃時，雖能夠有效解決Ni偏析問題，但卻會造成固溶C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，本研究之最佳均質化熱處理條件為I-T3℃（1200℃／4hr／FC／750℃／℃2hr／WQ），其中1200℃/4hr之均質化處理幾可完全消除Ni之偏析現象，而在後續之750℃／2hr之恆溫保持階段可降低基地中之固溶C量，兩者均有利於α值之降低，使得T3之α_(30-200)℃值降至約1~2×10^(-6)／℃。又，本研究亦建立了α值與固溶C量及Ni℃偏析程度之複合迴歸關係式：α=3.1132-2.0167%C-0.3466Ni+5.9870(%C)^2+0.1824(Ni)^2，R^2=0.68。另一方面，本研究進行拘束型熱循環試驗，以評估不同熱處理條件之低熱膨脹鑄鐵之尺寸穩定性，並與304不鏽鋼及一般球墨鑄鐵進行比較。實驗結果顯示，低熱膨脹鑄鐵之長度變化率及寬度變化率均遠低於304不鏽鋼及一般球墨鑄鐵，但厚度變化率則差異不大。由於低熱膨脹鑄鐵在熱循環試驗前後之尺寸變化量極微，因此再以試片之形狀變化量進一步探討合金之尺寸安定性。結果顯示，所有低熱膨脹鑄鐵之形狀變化量均顯著低於304不鏽鋼及一般球墨鑄鐵。且形狀變化量與α_(30-200)℃值呈正相關，即，α_(30-200)℃值愈低，則形狀變化量亦愈低。例如，爐次I-T3（1200℃-4hr／750℃-2hr）的α_(30-200)℃℃值最低（1.72x10^(-6)／℃），故其形狀變化量亦最低（7.41μm）。因此，欲獲致較佳之尺寸穩定性，可實施均質化熱處理，且以（1200℃-4hr／750℃-2hr）之熱處理條件為最佳。

關鍵字

低熱膨脹鑄鐵 ； 均質化熱處理 ； Ni偏析 ； 固溶碳含量 ； 尺寸穩定性

並列摘要

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, α value was adversely affected. The primary purposes of this research are to investigate the effect of homogenization heat treatment on the carbon content dissolved in the matrix, the degree of Ni segregation, and also the α value, with an intention to establish the optimal heat treatment conditions for attaining the lowest possible α value, and further to analyze the mechanisms related to the change in α values. When the alloys were homogenized at a relatively low temperature, e.g., 750℃, 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., 1200℃, reverse results were obtained. Consequently, not much improvement (reduction) in α value can be achieved. In order to lower the α value, a compound homogenization heat treatment pr℃edures was performed, namely, 1200℃-4hr/750℃-2hr, in which a higher homogenization heat treatment temperature (1200℃) 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 750℃. Both effects favor the reduction in α value. As a result, very low α values of around (1-2)×10^(-6)/℃ were obtained. In addition, low thermal expansion ductile cast irons, together with a regular ductile cast iron and a 304 stainless steel, were subjected to constrained thermal cyclic tests (30~200℃) to evaluate the dimensional stability of the alloys in terms of the changes in dimensions and shape of the test specimens. The experimental results indicate that the change rates along the longitudinal axis (length) and the transverse axis (width)for low thermal expansion cast irons with different homogenization heat treatment conditions are substantially lower than those of a regular ductile cast iron and the 304 stainless steel. Furthermore, the degree of distortion or shape change of the test specimens was chosen as a criterion to evaluate the dimensional stability of the alloys investigated. Positive correlation between theα30-200℃ value and the amount of shape change after thermal cyclic tests was obtained. Among the alloys investigated, Heat I-T3 (1200℃-4hr/750℃-2hr) exhibits the lowest α30-200℃ value (1.72x10^(-6)/℃), and hence achieves the least shape change (7.41μm) or the best dimensional stability.

並列關鍵字

Low thermal expansion ductile cast iron ； Homogenization heat treatment ； Ni segregation ； Dissolved carbon content ； Dimensional stability