薄壁球狀石墨鑄鐵之高溫熱疲勞性質研究

Study on the High Temperature Thermal Fatigue Property of Thin-Section Spheroidal Graphite Cast Iron

張宏源(H. Y. Chang) ； 范振佶(C. C. Fan) ； 鄭進華(J. H. Jheng) ； 潘永寧(Y. N. Pan)

《鑄造工程學刊》 34卷4期 (2008/12) Pp. 33-46

https://doi.org/10.29809/JTFS.200812.0005

摘要

本研究的主要目的是建立耐高溫薄壁球墨鑄鐵(2-3mm)之鑄造技術，研究上探討一些相關製程及冶金參數對於顯微組織及熱疲勞性質之影響，以期獲致耐高溫薄壁球墨鑄鐵之最佳製程條件。
實驗結果顯示，在固定3.0%C之條件下，球墨數目及球化率均隨Si含量之增加而增加，在約3.8-3.9%Si處達到最高值，然後再下降。肥粒鐵量則隨Si含量之增加而逐漸增加，在約4.7-4.8%Si時達到最高值，並維持一定值或稍降。又，在相同的C、Si含量下，2mm試片之球墨數目高於3mm試片，而球化率則差異不大。改變造模材料（化學自硬性模、濕砂模），對於球墨數目與球化率之影響不大。對於基地組織而言，4.77% Si含量可得到約85%之肥粒鐵量，且，試片厚度較大、化學自硬性模鑄造及澆鑄溫度較高之試片其肥粒鐵量亦較高。在相同Si含量下，影響肥粒鐵量以試片厚度最大，造模材料次之，澆鑄溫度則最小。
分析C、Si或C. E.對於熱疲勞性質之影響可得知，C. E.值介於4.5-4.6%之間，並配合高Si低C組合（如：3.0%C+4.8%Si），可以得到最佳之熱疲勞性質。且，最佳熱疲勞性質係對應於最高肥粒鐵量以及適當之球墨數目。此外，添加約0.5%Mo可以顯著提升熱疲勞壽命。綜合本研究結果，對於薄壁(2-3mm)球墨鑄鐵而言，最佳之合金設計為C：~3.0%、Si：4.7-4.8%、C.E.：4.5-4.6%、Mo：0.5%。此外，本研究對於試片在熱循環過程中，顯微組織之變態情形及裂紋之引發及破裂機制亦有詳細的探討。

關鍵字

薄壁球墨鑄鐵 ； 球墨數目 ； 肥粒鐵 ； 熱疲勞性質

並列摘要

The primary purpose of this research is to establish the optimal casting conditions for producing thin-section (2-3mm) spheroidal graphitic cast irons for high temperature applications (up to 800℃). Experimentally, the microstructures and thermal fatigue property were evaluated and correlated with alloy design and casting parameters, such as molding material and pouring temperature.
The results show that, for a fixed C content of about 3%, both the nodule count and nodularity increase first with increasing Si content, reach maxima at around 3.8-3.9%Si, and then decrease with further increase in Si content. On the other hand, the percent ferrite increases gradually with increasing Si content, reaches the maximum at around 4.7-4.8%Si, and then remains more or less constant or increases slightly. Regarding the effects of casting parameters on microstructure, the results show that higher nodule counts can be obtained in castings with a thinner section or with a lower pouring temperature. However, the effect of molding material (chemically-bonded sand and green sand) on nodule count is not significant. On the other hand, all the above three casting parameters exert little influence on graphite nodularity. Furthermore, the ferrite percentage is higher in castings with a thicker section, a higher pouring temperature and molded with chemically-bonded sand.
The optimal alloy design for attaining the best thermal fatigue property has been found to be a slightly hypereutectic composition, i.e., 4.5-4.6%CE, with a combination of relatively low C content and high Si content, e.g., 3.0%C+4.8%Si. In addition, the best thermal fatigue property corresponds to a microstructure with the highest ferrite content and moderate nodule count. Furthermore, adding some 0.5%Mo to the iron significantly increases the thermal fatigue life. Based upon the results obtained herein, the optimal alloy design is: C: ~3%, Si: 4.7-4.8% (CE: 4.5-4.6%), Mo: 0.5%.
In addition, the phenomenon of microstructure transformation and the failure mechanism during the cyclic thermal process were also analyzed in this study.

並列關鍵字

Thin-section spheroidal graphitic cast iron ； Nodule count ； Ferrite ； Thermal fatigue Property