A Study on the Metallurgical Characteristics of Low Thermal Expansion Cast Irons
Date Issued
2005
Date
2005
Author(s)
Lin, Ming-shan
DOI
zh-TW
Abstract
Abstract
The thermal expansion coefficients of Invar are quite low, normally at (1~2)x10-6/oC(or(0.56~1.1)x10-6/oF). However, due to the low C and Si contents in the alloys (C<0.2%，Si<0.4%), and the austenitic phase in the microstructure, Invar experiences relatively poor machinability and castability. The high-Ni (34~37%) austenitic cast irons, due to their relatively high C and Si contents in the alloys (2%<C<4%，2%<Si< 4%), have thermal expansion coefficients of (5~6) x 10-6/oC ( or (2.8~3.3) x 10-6/oF), while exhibit much better machinability and castability than Invar. The objective of this research was fourfold：(1) to develop low thermal expansion cast irons with the coefficients less than 2x10-6/oC, (2) to obtain the data of the solidification temperature ranges and the solidification shrinkage and solid contraction of the alloys, (3) to derive the riser design rules for the low thermal expansion cast irons, and (4) to understand the machinability of low thermal expansion cast irons, including cutting force, tool wear, surface roughness and chip characteristic.
The contents of this research include: (1) Investigate the effects of C and Si contents (both in the range of 1.0~2.0%), Ni and Co contents (36%Ni, 33%Ni-3%Co and 30%Ni-6%Co), homogenizing heat treatment and graphite morphology (both flaky and spheroidal) on the thermal expansion coefficient. (2) Measure the solidification temperatures (liquidus and eutectic) and the percentages of solidification shrinkage and solid contraction, together with the design of risers. (3) Study the effects of alloy composition and graphite morphology (both flaky and spheroidal) on the machinability, including cutting force, tool wear, surface roughness and chip characteristic, of low thermal expansion cast irons.
Microstructure observations indicate that very small amounts of steadite were observed in the microstructures of FG irons. The presence of steadite is due to a somewhat higher P content in the charge materials. Carbides are also observed in heats whose CE values are relatively low. For Heats that contain somewhat lower Ni content (about 30%), sporadic martensite can be found adjacent to the eutectic cell boundaries. The formation of martensite in the otherwise austenitic matrix may be due to the negative segregation of Ni, resulting in a depletion of Ni near the cell boundaries. Furthermore, the amount of martensite in the microstructure is slightly higher in SG irons than in FG irons, implying that the degree of Ni segregation is more severe in the former than in the latter. For SG irons, chunky graphite forms in 1-in thick specimens when SN (%C+ 0.2%Si+0.06%Ni) value exceeds 3.7 and 4.56 for alloys treated with a nodularizer that contains rare earths and without rare earths, respectively. On the other hand, chunky graphite appears in 0.5-in thick specimens when SN value exceeds 5.04 for alloys treated with a nodularizer without rare earths.
The results of thermal expansion coefficient measurements indicate that α value decreases with decreasing C and/or Si contents (or CE). For cast irons with nominal compositions of 1.0%C-1.0%Si-36%Ni, the α values in the temperature range of 50 to 150oC are (2.3 ~ 3.3) x 10-6/oC and (3.5 ~ 3.8) x 10-6/oC for FG and SG irons, respectively. The coefficient value can be further lowered by replacing a fraction of Ni with Co at a constant NiE of 36%. As an example, α values of (1.2 ~ 2.5) x 10-6/oC were achieved for alloys containing nominal 1.0%C-1.0%Si- 30%Ni-6%Co. In addition, the results show that the α value was reduced further when the alloy was homogenized. For cast irons containing 1.0%C-1.0%Si-36%Ni, the α value of 0.83 x 10-6/oC (at 90oC) when the alloy was homogenized at 950oC for 20 hours and then quenched in water.
Thermal analysis results conclude that the alloy liquidus temperatures can be expressed in terms of CE value for both flake and spheroidal graphite cast irons (FG：TL, oC=－7.93(CE)2－43(CE)+1535；SG：TL, oC=－21.42(CE)2－6(CE)+1535). The CE here is defined as follows : CE = %C + 0.33%Si + 0.047%Ni－0.0055%Ni x %Si. It has been well recognized that in the Fe-C-Si system the eutectic temperatures of both stable and metastable reactions are influenced mainly by the Si content or SiE(SiE=%Si+0.141%Ni-0.0165%Ni x %Si). Whereas, the coefficients of determination are not high enough to assume satisfactory correlation.
The measurement results of alloy solidification shrinkage and solid contraction indicate that the percent solidification shrinkage for FG irons that contain 1.0%C-1.0%Si-35%Ni is about 5.2%, while an additional 1.5% should be added for SG irons of similar compositions. For FG irons that contain 2.0%C-2.0%Si-35%Ni, the percent solidification shrinkage is about 1.63% ~ 2.36%, while an additional 1.2% should be added for SG irons of similar compositions. Due to the contents of C and Si, the riser design concept used in this study is different --- “geometric method” is for 1.0%C-1.0%Si alloys, and “directly applied riser design method” is for 2.0%C-2.0%Si alloys. The experimental results indicate that the suitable ratio of the riser modulus(MR) to casting modulus(MC) is about 1.27 for FG irons, whereas, 1.43 for SG irons. In addition, when riser sleeve was used, the riser volume can be reduced by some 78% for LTE cast irons that contain 1.0%C-1.0%Si-35%Ni. Furthermore, when both riser sleeve and exothermal compounds were used, the riser volume can be reduced by 82%. When riser sleeve was used, the riser volume can be reduced by some 65% for LTE cast irons that contain 2.0%C- 2.0%Si-35%Ni. Furthermore, when both riser sleeve and exothermal compounds were used, the riser volume can be reduced by 80%.
Regarding the machinability of low thermal expansion cast irons, the results indicate that as the C and Si contents decrease, both the cutting force and the shear stress increase; the length of the chips increases; the extent of crater wear on the tool rake face increases, and the surface finish of the workpieces improves. Replacing 6% Ni with an equivalent amount of Co only slightly affects machinability. With respect to graphite morphology, the results reveal that cast iron with flake graphite has shorter chips than that with spheroidal graphite, because of a better chip breaking effect of the flake graphite. Moreover, the long, flaky shape and interconnectedness of flake graphite in a eutectic cell accounts for the observed cracking and tearing on the machined surface of LTE FG cast irons. Therefore, LTE FG cast irons exhibit a poorer surface finish than LTE SG cast irons. Finally, the machinability of LTE cast irons with lacy or chunky graphite are between that of LTE FG and SG cast irons.
Subjects
切削性
冒口設計
熱分析
均質化熱處理
膨脹係數
鑄鐵
Riser design
Machinability
Cast iron
Expansion coefficient
Thermal analysis
Homogenizing heat treatment
Type
thesis
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