低熱膨脹球墨鑄鐵之尺寸熱穩定性分析

Thermal Dimensional Stability of Low Thermal Expansion Ductile Cast Irons

劉宗霈(Zong-Pei Liu)

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(2014年)

https://doi.org/10.6342/NTU.2014.02871

摘要

本研究之內容包含三部分，第一部分係探討均質化熱處理對於低熱膨脹球墨鑄鐵之Ni偏析程度及基地固溶C量之影響，並進一步探討上述合金組成變化對於α值的影響，最後，深入分析均質化熱處理如何影響α值之機理；第二部分係以拘束型熱循環試驗，針對不同化學組成(35%Ni、30%Ni+5%Co)，但相同Ni當量之合金成份下，探討不同熱處理條件對於低熱膨脹球墨鑄鐵之尺寸(長度、寬度、厚度)變化率及形狀變形量(翹曲度)之影響，並進一步分析α值對於工件尺寸安定性的影響；第三部分係以理論模擬試片經拘束型熱循環試驗後之尺寸變化情況，方法上分別以一維軸向、二維平面及三維實體來探討不同合金: 不鏽鋼(SUS304)、一般球墨鑄鐵(SG)和低熱膨脹球墨鑄鐵(D-5)，之溫度場及熱應力分布，並比較三種模擬結果之差異。最後，深入分析α值對於熱應力之影響，並分析其與試片尺寸變化量之關係。
本研究結果顯示，Ni偏析程度隨著均質化熱處理溫度及持溫時間之增加而降低，而有利於α值之降低。另一方面，固溶C量理論上應隨均質化熱處理溫度及持溫時間增加而增高，但本研究之固溶C量隨不同熱處理條件之變化並不大。研究結果得知，T4(1150℃/4hr/FC/750℃/4hr/WQ)熱處理幾乎可以完全消除Ni偏析現象，且可獲致最小之α值:2.85x10-6(1/℃)。此外，為進一步瞭解固溶C量及Ni偏析程度對於α值之影響，以迴歸分析得出下列迴歸方程式:
A爐次(35%Ni): ；
B爐次(30%Ni+5%Co): ；
綜合A、B爐次:
由上述迴歸方程式可知，欲獲致較低之α值，必須同時降低固溶C量及Ni偏析程度。
另一方面，試片經拘束型熱循環試驗後，不同均質化熱處理條件與試片之形狀變形量( )之關係如下:
A爐次(35%Ni):
T0(338.51μm)>T1(301.82μm)>T2(237.95μm)>T3(190.70μm)>T4(61.24μm)；
B爐次(30%Ni+5%Co):
T0(286.24μm)>T1(252.64μm)>T2(189.44μm)>T3(125.46μm)>T4(48.23μm)。
分析結果顯示，均質化熱處理可有效降低試片之形狀變形量，但以T4(1150℃/4hr/FC/750℃/4hr/WQ)之熱處理具有最佳之效果。同時，α值越低，ΔPV值亦越小。
最後，以三種维度進行溫度場、熱應力場、變形量之模擬結果可知，三種不同維度所模擬得到之溫度分布差異並不大，溫差大小之排序為: D-5>SUS304>SG，與合金之熱傳導係數(k值)呈正相關。另一方面，熱應力大小之排序為:SUS304>SG>D-5，分析結果顯示，合金之楊氏係數(E)與α值對於熱應力之影響較溫度梯度顯著。另， 值與熱應力之關係可由下式表示: ，呈高度線性相關，亦即試片所承受之熱應力越大，其 值亦越大。此外，經適當熱處理，即
(1150℃/4hr/ FC/750℃/4hr/WQ)，之低熱膨脹球墨鑄鐵，可獲致最小的α值與 值。

關鍵字

低熱膨脹球墨鑄鐵 ； 均質化熱處理 ； α值 ； 熱應力 ； 尺寸穩定性

並列摘要

The objectives of this study are threefold: (1) To investigate the effect of homogenization heat treatment on both the degree of Ni segregation and the content of dissolved carbon in the matrix of the low thermal expansion ductile cast irons, and then to analyze the influence of compositional factor on α value; (2) To study the effects of alloy composition and homogenization heat treatment on the dimensional and shape changes of the test specimens by means of constrained thermal cyclic fatigue tests (30~200℃); (3) The temperature distribution and the thermal stress field in the test specimens after the constrained thermal cyclic fatigue tests were analyzed first by both calculation and simulation (ANSYS), and then the dimensional and shape changes of the alloys studied (Alloy D-5, Regular ductile iron and 304 stainless steel) were calculated and compared with the measured data. In addition, the correlations among α value, thermal stress and dimensional change were evaluated.
The experimental results indicate that the degree of Ni segregation can be reduced by increasing the homogenization heat treatment temperature and/or time, rendering a decrease in α value. On the other hand, the dissolved C content in the matrix showed little affected by homogenization heat treatment, regardless of the fact that increasing both heat treatment temperature and time will increase the dissolved C content. Among the various heat treatment procedures employed, heat treatment T4(1150℃/4hr/FC/750℃/4hr) not only can effectively eliminate the Ni segregation, but also can reduce the C concentration in the matrix, resulting in a very low α value of around (2-3)×10-6/℃. Furthermore, regression equations were derived to correlate the degree of Ni segregation and dissolved C content with α value, as expressed below:
Heat A (35%Ni):
Heat B (30%Ni + 5%Co):
Heats A & B:
It is clear from the above equations that in order to achieve a low α value, both the degree of Ni segregation and the dissolved C content should decrease.
Constrained thermal cyclic fatigue tests (30~200℃) were performed to compare the dimensional and shape changes among the low thermal expansion ductile cast irons with different homogenization heat treatment procedures, and also among three different alloys studied herein, namely, Alloy D-5, Regular ductile iron and 304 stainless steel. The extent of distortion or shape change of the test specimens (∆PV) was used as a criterion to evaluate the dimensional stability of the alloys investigated. The effect of homogenization heat treatment on ∆PV can be expressed by the following order:
Heat A (35%Ni): T0(338.51μm)>T1(301.82μm)>T2(237.95μm)>T3(190.70μm)>T4(61.24μm)
Heat B (30%Ni + 5%Co):
T0(286.24μm)>T1(252.64μm)>T2(189.44μm)>T3(125.46μm)>T4(48.23μm)
It is clear from the above results that Alloy A with heat treatment T4 (1150℃/4hr/FC/750℃/4hr) exhibits the lowest shape change (48.23μm), implying that an alloy with a lower α value can achieve a better dimensional stability.
Finally, numerical simulation by finite element method (FEM) was employed to obtain the temperature distribution and thermal stress field for different alloys (D-5, regular SG and SUS304) after thermal cyclic fatigue tests. The results show that the values of temperature gradient follow the following order: SG>SUS304>D-5, while the order of the thermal stress is: SUS304>SG>D-5. Furthermore, regression analysis was performed to obtain the correlation between thermal stress and ∆PV, with the results shown as follows: . It is obvious that the lower the thermal stress, the lower the ∆PV value. In conclusion, low thermal expansion ductile iron with T4 heat treatment (1150℃/4hr/ FC/750℃/4hr/WQ) exhibits the best dimensional stability due to its lowest α value (1.72x10-6/℃).

並列關鍵字

Low thermal expansion ductile cast iron ； Homogenization heat treatment, α value ； Dimensional stability ； Finite element analysis

參考文獻

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