A Fe-Mn-Cr-Ni-Si seismic damper alloy with extremely long plastic fatigue life has been developed at NIMS. This alloy has not been suitable for welding, but subsequent improvements have led to the development of a damper alloy with a composition that is resistant to weld cracking. This newly developed alloy is characterized by increased Cr and decreased Ni, which allows the solidification mode to be FA maintaining the phase stability of γ and ε. The solidification mode changes with Cr and Ni content, but the effects of Cr and Ni on the solidification mode and casting microstructure remain unclear. In this study, permanent mold castings were prepared by changing the composition of Cr and Ni in the damper alloys, and the effects of composition on microstructure and solidification mode were investigated.
EPMA observations revealed that Mn, Ni, and Si were enriched in the interdendritc region. In the case of solidification in the FA mode, regions of high Cr and Si were observed in the center of the dendrites. In the vicinity of the composition of the developed damper alloys, it was found that it is possible to determine whether the solidification mode is A-mode or FA-mode by examining the compositional correlation between Cr and Ni or Cr and Si. In the case of A-mode solidification in this alloy system, the partition coefficient can be determined from the compositional analysis. By using this value, the microsegregation of the cast microstructure of the permanent mold castings can be roughly predicted by the Scheil equation. The Thermo-Calc prediction of the solidification mode agrees with the experimental results, and new equations for Cr and Ni equivalents were proposed to predict the primary crystallization applicable to this alloy system.
