| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 101 === The primary purposes of this research are twofold: (1) to study the feasibility of alloying substitutions for Ni at different additions to achieve low α values, (2) to establish the optimal heat treatment conditions for attaining the lowest possible α values, and further to analyze the mechanisms related to the change in α values with microscopic features.
The experimental results regarding the alloying substitutions of Mn, Ti, Cu, V, Al, Sn Cr, W or Zr for Ni indicate that, except for Zr, all other alloying elements cause an increase in α value, and also fail to achieve the target value of α100oC≦8×10-6/C. Although a partial substitution of 5% Zr for equal amount of Ni did not increase the α value, it did cause the formation of Zr-rich phase in the matrix, which embrittled the material. Therefore, the substitution of a certain portion of Ni in the low thermal expansion cast irons with other alloying elements with an intention to reduce the material cost and at the same time to reduce the α value has been found infeasible.
In the second phase of this research, the alloys with chemical compositions of (1.5-2.0)%C, (1.5-2.0)%Si and 35%Ni had been selected to investigate the effect of homogenization heat treatment on the degree of Ni segregation, carbon content dissolved in the matrix and also the α value. Phenomena of Ni (inverse) segregation and the C concentration departing from the equilibrium, which is normally higher than the equilibrium value, are present in the alloys in the as-cast state. As a result, the α value was adversely affected. When the alloys were homogenized at a relatively low temperature, e.g., 750oC, the elimination of Ni segregation was not very effective, but the C concentration in the matrix was moderately reduced. On the other hand, as the alloys were homogenized at a relatively high temperature, e.g., 1000oC, reverse results were obtained. Consequently, not much improvement (reduction) in α value can be achieved. This can be observed in Heat B1, where the α30-100oC values of 5.87×10-6/ oC, 5.74×10-6/ oC, 4.67×10-6/ oC, 4.69×10-6/ oC, and 5.19×10-6/ oC were registered for T0 (as-cast), T1 (750oC/3hr), T2 (750oC/20hr), T3 (1000oC/3hr), and T4 (1000oC/20hr), respectively. Similar results were also obtained in Heats B2 and B3. In order to lower the α value, a compound homogenization heat treatment procedures was performed, namely, 1200oC-4hr/750oC-2hr, in which a higher homogenization heat treatment temperature (1200oC) can effectively eliminate the degree of Ni segregation on the one hand, while the C concentration in the matrix can be reduced to a low level during the holding stage at 750oC. Both effects favor the reduction in α value. As a result, very low α values of around (1-2)×10-6/oC were obtained. Furthermore, Heat B1 with different chemical compositions and homogenization heat treatment conditions, together with a conventional ductile cast iron and a 304 stainless steel, were selected for constrained thermal cyclic tests (30~200oC) to evaluate the dimensional stability of afore-mentioned the alloys based upon the changes in the shape of the test specimens. This can be observed in Heat B1, where the degree of shape change of the low thermal expansion ductile cast irons are well below those of the conventional ductile cast iron (634.01μm) and the 304 stainless steel (428.93μm). Furthermore, correlation between the amount of shape change after thermal cyclic tests and α30~200oC value shows a similar trend. The degree of shape change follows the following sequence: T0(156.93μm) – T1(63.25μm) – T4(27.94μm) – T2(25.77μm) – T3(15.56μm)–T6(7.41μm) verse the α30~200oC value of A0(5.87x10-6/oC)– A1 (5.74x10-6/oC) – A4(5.19x10-6/oC) – A2(4.67x10-6/oC) – A3(4.69x10-6/oC) – A6(1.72x10-6/oC). Accordingly, the homogenization heat treatment will alter (reduce) the α value and 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, possesses the best dimensional stability.
Keywords: low thermal expansion ductile cast iron, α value, homogenization heat treatment, Ni segregation, carbon content dissolved in the matrix, dimensional stability, the changes in the shape
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