厚壁球墨鑄鐵之機械性質控制研究

Abstract

本研究旨在探討厚壁球墨鑄鐵之鑄造技術並提升其機械性質以達到實務上之應用。實驗共可分為兩大部分，第一部分係探討鎳(Ni)元素對於球墨鑄鐵機械性質(抗拉強度、降伏強度、伸長率、硬度、低溫衝擊值)及顯微組織(球化率、球墨數目、肥粒鐵含量、波來鐵含量)之影響，並以迴歸分析方法建立化學成分、顯微組織與機械性質之相關性迴歸方程式；第二部分為開發及建立貯存用過核子燃料之球墨鑄鐵芯最佳鑄造技術，藉由合金設計及控制製程參數使得鑄鐵芯實體鑄件之機械性質可達到規範EN-GJS-400-15U之要求(抗拉強度>370MPa、降伏強度>240MPa及伸長率>7%)，針對鑄造過程中各種可能造成瑕疵的原因加以探討，並尋求解決策略。 第一部分研究結果顯示，鎳元素的添加有助於提高球墨鑄鐵的抗拉強度以及降伏強度，同時鎳亦具有強化及硬化肥粒鐵相之作用，使球墨鑄鐵的硬度也獲得提升。然而，鎳雖具有降低轉脆溫度之功效，但容易受其他因素如：矽、錳含量、基地波來鐵比例、球墨數目之影響，使添加鎳的效果不顯著。本部分研究亦建立鎳添加量、基地波來鐵比例及球墨數目等參數與衝擊值之關係，迴歸分析方程式顯示欲生產耐低溫衝擊之球墨鑄鐵，除了添加鎳元素之外，還需降低基地波來鐵相之比例及降低球墨數目。 第二部分研究結果顯示，縮尺小型球墨鑄鐵芯第一次測試結果不合格的主因為實體試樣之顯微組織中出現過多的不規則石墨以及塊狀石墨，會有此現象發生是因為稀土元素鈰(Ce)在厚壁鑄件較長的凝固冷卻時間下晶出不良的塊狀石墨組織，進而導致機械性質大幅下降。藉由添加適量的銻(Sb)可中和鈰(Ce)之負面影響，採用此改善對策後進行縮尺小型球墨鑄鐵芯第二次測試，其石墨組織中之塊狀石墨大幅減少，且機械性質測試結果可符合規範要求。接著以此兩次澆鑄及測試作業之經驗，進一步加大鑄件之尺寸，進行縮尺型球墨鑄鐵芯之澆鑄測試作業，測試結果亦可符合規範要求。本部分研究亦計算各爐次試樣之品質指標並與規範值比較，作為另一種檢定鑄件品質優劣的方法。

The primary purposes of this research are two folds: (1) To investigate the effect of Ni on the mechanical properties (tensile strength, yield strength, elongation, hardness, and low-temperature impact value) and microstructures (nodularity, nodule counts and percent pearlite) of heavy-section ductile iron. Based on the experimental results, multiple regression analyses were performed to correlate the mechanical properties with chemical composition and microstructure. (2) To establish the optimal conditions for the production of ductile cast iron inserts for the storage of the spent nuclear fuels through alloy design and manufacturing process control. Furthermore, in this study, the microstructure that occurred in the castings were analyzed and schemes were proposed to eliminate those abnormities, aiming to conform with the specification of EN-GJS-400-15U (T.S.>370MPa, Y.S.>240MPa and El.>7%) for the ductile cast iron inserts. The results of the first part indicate that the addition of Ni to the ductile cast irons can enhance the mechanical properties, i.e., as tensile strength, yield strength, microhardnesses of both ferrite and pearlite phases. However, the effect is affected by other factors, such as the Si and Mn contents, the pearlite (or ferrite) percentage, and the nodule count. The multiple regression analyses were performed to correlate the impact value at various temperatures with the selected metallurgical parameters (percent Ni, percent pearlite, and nodule counts). The results show that the impact value increase with increasing Ni content, and decreasing the percent pearlite and nodule counts. Regarding the trial tests of the reduced-length small scale ductile cast iron inserts, the results of the first attempt fail to conform with the specification due to the presence of chunky graphite in the microstructure, which causes a significant drop in tensile properties. The presence of chunky graphite can be attributed to the excessive amount of Cerium (Ce), especially in heavy section castings. To counteract the adverse effect of Ce, an appropriate amount of antimony (Sb) was added in the second trial. The results of the second trial of the reduced-length small scale ductile cast iron insert can meet the requirement of the specification. Finally, the reduced-length ductile cast iron insert was poured based upon the optimal casting conditions obtained from the first two trials for the reduced-length small scale ductile cast iron inserts. Again, the results fulfill the specification. In addition, the quality indices of all the specimens obtained from the three ductile cast iron inserts, together with the standard grades ductile cast iron, were calculated and compared. The comparisons in quality index can serve as basis for the evaluation of casting performance.