Thermal Dimensional Stability of Different Low Thermal Expansion Graphite Cast Irons

The primary purposes of this research are three fold: (1) to investigate the effect of a specific heat treatment (TI: 1150oC-4h/FC/750oC-4h/WQ) on the Ni segregation, C content dissolved in the matrix, and α value in three different graphitic cast irons, (2) to conduct the constrained thermal cyclic tests to evaluate the dimensional stability of the alloys studied, and (3) to employ the finite element method (ANSYS) to simulate the temperature field, thermal stress and shape change of specimens after the thermal cyclic tests, and further to assess the correlation among α value, thermal stress and dimensional stability. Regression analyses were performed to correlate the carbon content dissolved in the matrix and/or degree of Ni segregation with α value, with the results being shown below: (1) α value vs. C content dissolved in the matrix: α = 1.06%C + 4.6; R2 = 0.03 (2) α value vs. Degree of Ni segregation: α = 1.01Nid + 2.81; R2 = 0.92 (3) α value vs. both C content dissolved in the matrix and Degree of Ni segregation: α = 0.68%C + 1.05Nid + 2.41; R2 = 0.96 Based upon the regression analysis results, α value can be decreased by reducing both the carbon content dissolved in the matrix and degree of Ni segregation, with the latter being the dominant factor. Regarding the effect of the graphite type on α value, α value decreases according to the following order: SG > CG > FG, in the as-cast condition. On the other hand, α value decreases according to the following order: CG > FG > SG, in the T1 heat treatment condition. Shape change (△PV) of the specimens after constrained thermal cyclic tests (500 cycles) were measured for low thermal expansion cast irons with different graphite shape (in T1 heat treatment condition) and two other alloys, SUS 304 and regular ductile cast iron. The results indicate that the shape changes in low thermal expansion cast irons regardless of graphite shape are substantially lower than both SUS 304 and regular ductile cast iron. Furthermore, among the low thermal expansion cast irons the shape change or dimensional stability is closely related with α value, that is, the lower the α value, the less the shape change or the better the dimensional stability. Therefore, the order of dimensional stability is SG > CG > FG.