臺灣博碩士論文加值系統

　　本研究欲評估波來鐵組織於球墨鑄鐵中生成，對抑制沿晶脆性破壞的可行性，因此將對球墨鑄鐵之中溫脆性及熱循環誘發脆性進行探討，後者由於波來鐵高溫時的不穩定性，故以熱循環之最高加熱溫度效應，及其微觀組織變化為首要探討目標。實驗材料為經添加波來鐵促進元素(銅、錳及鉻)的球墨鑄鐵，鑄造狀態下基地為牛眼組織，並利用肥粒鐵化熱處理，得到較低波來鐵含量之球墨鑄鐵為比對材。
　　實驗結果顯示，裂紋的成核位置不因波來鐵組織的存在而改變，由共晶胞界區域起始的裂紋，其傳播受波來鐵的阻礙亦不明顯，不同波來鐵含量的球墨鑄鐵，其中溫脆性破壞行為仍有一致性存在，即沿晶脆性破壞僅發生於肥粒鐵區域。藉由不同波來鐵含量的球墨鑄鐵，其RT~550℃拉伸強度與破斷面形貌，可知波來鐵與肥粒鐵之晶粒強度有差異性存在，晶界強度亦然，造成波來鐵臨界脆性破壞強度大於肥粒鐵臨界破壞強度，且波來鐵之晶粒-晶界等強溫度高於肥粒鐵晶粒-晶界等強溫度，此可由球墨鑄鐵特定溫度下沿晶脆性破壞只發生在肥粒鐵區域證實。
另一方面，熱循環溫度800℃時，有明顯的麻田散鐵相變態發生，導致拉伸性質產生明顯變化。熱循環溫度750℃時，波來鐵分解為造成室溫拉伸性質變化之主要原因。值得注意的是，低波來鐵分率之球墨鑄鐵，分解現象較不明顯。最後利用750℃熱循環後中溫拉伸，證實波來鐵臨界脆性破壞強度大於肥粒鐵臨界脆性破壞強度。

　This study is intended to evaluate how feasible it is to suppress the intergranular embrittlement fracture behavior of spheroidal graphite (SG) cast iron with the formation of pearlite in the cast iron. Intermediate-temperature embrittlement and cyclic heating induced embrittlement are investigated in this study. In the latter case, the primary objective is to examine the effect of cyclic heating at the maximum temperature and the resultant microstructural change due to the instability of pearlite at high temperatures. Two types of SG cast iron have been chosen. One is as-cast SG cast iron that has been added with pearlite promoting elements (Cu, Mn and Cr) and has a bull’s eye structure. The other is SG cast iron which has undergone a ferritization heat treatment to achieve a matrix with lower pearlite content and is used as a contrast material.
　Experimental results indicate that the location of crack nucleation remains the same despite the existence of pearlite. The hindrance of pearlite to the propagation of cracks that emerge from eutectic cell boundary regions is also unapparent. Consistence is found in the intermediate-temperature embrittlement fracture behaviors between SG cast irons with different pearlite contents, namely that in-tergranular fractures only occur in the ferritic region. The RT~550℃ tensile strength and crack cross sections of SG cast irons with different pearlite contents reveal the differences in grain strength and grain boundary strength between pearlite and ferrite. This also leads to the higher critical fracture stress of pearlite compared to ferrite and the higher equicohesive temperature(ECT) in pearlite compared to ferrite. These are verified by the result that intergranular fractures only occur in the ferritic region of SG cast iron at specific tem-peratures.
　On the other hands, when the cyclic heating temperature reaches 800℃, significant martensitic phase transformation occurs and results in anmarked change in the tensile property. When cyclic heating temperature reaches 750℃, the decomposition of pearlite is mainly responsible for the change in the tensile property at the room temperature. More notably, such decomposition is less significant in the SG cast with lower pearlite percentage. Finally, an intermediate temperature tensile test following cyclic heating confirms that pearlite has a higher critical fracture stress than ferrite.

中文摘要 I
Abstract II
致謝 IV
總目錄 V
表目錄 VII
圖目錄 VIII
第一章 前言 1
第二章 文獻回顧 2
2-1 肥粒鐵基球墨鑄鐵之中溫脆性破壞 2
2-2 動態應變時效及其對球墨鑄鐵機械性質之影響 2
2-3 球墨鑄鐵之熱循環誘發脆性破壞 3
2-4 石墨所導引之不均勻應力場 4
2-4-1 彈性變形時石墨導引之應力集中 4
2-4-2 塑性變形時石墨導引之應力集中 5
2-5 本研究之探討目標 6
第三章 實驗步驟與方法 13
3-1 實驗材料 13
3-2 微觀組織觀察 13
3-3 拉伸試驗 14
3-4熱循環試驗 14
第四章 實驗結果 18
4-1 微觀組織與元素分佈解析 18
4-1-1 OM微觀組織觀察 18
4-1-2 SEM微觀組織觀察 18
4-1-3 EPMA元素分佈解析 18
4-2 RT~550℃溫度區間之拉伸試驗 18
4-2-1 真應力-應變曲線 19
4-2-2 拉伸破斷面觀察 19
4-3 熱循環試驗 19
4-3-1 基地組織觀察 19
4-3-2 熱循環後之拉伸性質 20
4-3-2-1 各溫度熱循環後之室溫拉伸性質 20
4-3-2-2 750℃熱循環後之400℃拉伸性質 20
4-3-3 熱循環後之拉伸破斷面觀察 20
第五章 討論 47
5-1 RT~550℃溫區拉伸性質之波來鐵組織效應探討 47
5-1-1 波來鐵組織對球墨鑄鐵動態應變時效之影響 47
5-1-2 不同波來鐵含量球墨鑄鐵之中溫脆性 48
5-2 鑄態球墨鑄鐵熱循環之最高加熱溫度效應 49
5-3 共晶胞界區域波來鐵組織對拉伸性質之影響 50
第六章 結論 57
參考文獻 58

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