化學組成及熱處理條件對於低熱膨脹鑄鐵尺寸穩定性之影響

Abstract

本研究是以B1系列(1.8%C, 1.8%Si, 30%Ni, 5%Co)、B2系列(2.0%C, 1.8%Si, 35%Ni)及B3系列(2.0%C, 2.0%Si, 30%Ni, 5%Co)三種不同化學組成之合金經過不同均質化熱處理後，進行熱循環試驗，再量測尺寸變化量以及形狀變化量，以評估不同合金之尺寸穩定性，此外，並與一般球墨鑄鐵及304不銹鋼進行比較。 實驗結果顯示，不同合金成分及均質化熱處理條件之熱膨脹鑄鐵合金其長度變化率(0% ~ 0.044%)均遠低於一般球墨鑄鐵(0.4853%)以及304不鏽鋼(0.6817%)；就寬度變化率而言，低熱膨脹鑄鐵合金(0.0055% ~ 0.0268%)亦遠低於一般球墨鑄鐵(0.0863%)以及304不鏽鋼(0.1877%)，但厚度變化率則差異不大，在熱循環試驗前後低熱膨脹鑄鐵合金與一般球墨鑄鐵及不鏽鋼均在(0%~0.0995%)之間。另一方面，就不同低熱膨脹鑄鐵合金而言，熱循環試驗前後之尺寸變化量均非常微量，其中最大長度變化量為0.030 mm，最大寬度之變化量則為0.005 mm，而厚度變化量則僅有0.002 mm，因此以試片之形狀變化來進一步討論合金之尺寸安定性。 低熱膨脹鑄鐵合金之最大形狀變化量為B2系列，其鑄態(219.71μm)小於一般球墨鑄鐵(634.01μm) 及304不鏽鋼 (428.93μm)。而比較相同合金成分下施以不同均質化熱處理條件之結果為，在B1系列(1.8%C, 1.8%Si, 30%Ni, 5%Co)中，熱循環試驗前後之形狀變化量依下列順序遞減：B1-T0 (156.93μm) > B1-T1 (63.25μm) > B1-T4 (27.94μm) > B1-T2 (25.77μm) > B1-T3 (15.56μm) > B1-T6 (7.41μm) ，而熱膨脹係數(α30~200)值係依下列順序遞減：B1-T0 (5.87x10-6) > B1-T1 (5.74x10-6/oC) > B1-T4 (5.19x10-6/oC) > B1-T2 (4.67x10-6/oC) ≈ B1-T3 (4.69x10-6/oC) > B1-T6 (1.72 10-6/oC) ，結果顯示兩者之變化趨勢相同。在B2系列中，形狀變化量依下列順序遞減：B2-T0 (219.71μm) > B2-T1 (86.14μm) > B2-T4 (56.96μm) > B2-T2 (33.25μm) > B2-T3 (24.13μm) ，而熱膨脹係數(α30~200)值係依下列順序遞減：B2-T0 (7.51 x10-6/oC) ≈ B2-T1 (7.87 x10-6/oC) > B2-T4 (7.64 x10-6/oC) > B2-T2 (6.09 x10-6/oC) > B2-T3 (5.83 x10-6/oC)，兩者之變化趨勢亦相同；在B3系列中，形狀變化量依下列順序遞減：B3-T0 (178.15μm) > B3-T1 (61.40μm) > B3-T4 (46.48μm) > B3-T3 (30.26μm) > B3-T2 (28.20μm)，而熱膨脹係數(α30~200)值係依下列順序遞減：B3-T0 (8.28x10-6/oC) > B3-T1 (7.93x10-6/oC) > B3-T2 (7.64x10-6/oC) > B3-T4 (6.62x10-6/oC) > B3-T3 (6.44x10-6/oC)，其中除了B3-T2之變化量較預期為低或熱膨脹係數(α30~200)值較預期為高外，兩者之趨勢亦大致相同，推斷B3-T2之相關數據可能為量測誤差。 其結果顯示經過均質化熱處理可以改善Ni的偏析狀況及固溶C當量，所以會改變(降低)熱膨脹係數(α)值，因此會改善尺寸穩定性。其中，若熱膨脹係數(α值)下降，則由熱循環試驗中所形成之熱應力(σth)愈小，因此由熱循環應力所造成的形狀變化量亦愈小。因此欲獲致較佳之尺寸穩定性，鑄件可實施均質化熱處理，且以(1200oC-4hr/750oC-2hr)之熱處理條件為最佳。

In this study, three low thermal expansion ductile cast iron series, B1(1.8%C, 1.8%Si, 30%Ni, 5%Co), B2(2.0%C, 1.8%Si, 35%Ni) and B3(2.0%C, 2.0%Si, 30%Ni, 5%Co) with different chemical compositions and homogenization heat treatment conditions, together with a regular ductile cast iron and a 304 stainless steel, were selected for constrained thermal cyclic tests (30~200oC) to evaluate the dimensional stability of the alloys studied by comparing the changes in dimensions and shape of the test specimens. The experimental results indicate that the change rates along the longitudinal axis (length) for low thermal expansion cast irons with different chemical compositions and homogenization heat treatment conditions (nil~0.044%) are substantially lower than those of a regular ductile cast iron (0.4853%) and the 304 stainless steel (0.6817%). Also, the change rates along the transverse axis (width) for low thermal expansion cast irons (0.0055~0.0268%) are substantially lower than those of a regular ductile cast iron (0.0863%) and the 304 stainless steel (0.1877%). On the other hand, no clear difference in the change rate in thickness were obtained for all the alloys investigated, all within the range of nil~0.0995%. Due to the fact that the dimensional changes of low thermal expansion cast irons are quite minute, with the maximum values of 0.03mm, 0.005mm and 0.002mm for length, width and thickness, respectively, the degree of distortion or shape change of the test specimens was used as a criterion to evaluate the dimensional stability of the alloys investigated. Among the three series of low thermal expansion ductile cast irons, series B2 (219.71μm) exhibited the largest shape change, which was followed by series B3 (178.15μm) and then series B1 (156.93μm). Nevertheless, all three series of low thermal expansion ductile cast irons are still well below those of the 304 stainless steel (634.01μm) and the regular ductile cast iron (428.93μm). Furthermore, correlation between the amount of shape change after thermal cyclic tests and α30-200oC value shows a similar trend. Take series A as an example, the degree of shape change follows the following sequence: B1-T0(156.93μm) – B1-T1(63.25μm) – B1-T4(27.94μm) – B1-T2(25.77μm) – B1-T3(15.56μm) –B1-T6(7.41μm) verse the α30-200oC value of B1-T0 (5.87x10-6/oC) –B1-T1(5.74x10-6/oC) –B1-T4(5.19x10-6/oC) –B1-T2(4.67x10-6/oC) –B1-T3(4.69x10-6/oC) –B1-T6(1.72x10-6/oC). However, it has to be noticed that alloy B1-T6 (1200oC-4hr/750oC-2hr) exhibits a very low α30-200oC value of 1.72x10-6/oC, as a result, the degree of shape change is also very small, 7.41μm. Similar results were also obtained for series B2 and B3. In conclusion, the homogenization heat treatment may improve the degree of Ni segregation and also change the amount of C content dissolved in the matrix, which in turn will alter (reduce) the α value. As a result, the dimensional stability of the alloys will be affected. The present results indicate that the alloy with the homogenization heat treatment of 1200oC-4hr/750oC-2hr can obtain the lowest α value, and hence, is the alloy having the best dimensional stability.