Structures of Unidirectionally Solidified Hypoeutectic White Cast Iron

Abstract

  Solidification of hypoeutectic white cast iron starts by the freezing of primary austenite into a form of cellular dendrite and ends in the precipitation of eutectic ledeburite of a cellular form. The structures of the primary dendrites and the eutectic cells are markedly affected by the chemical composition and the conditions of solidification. An investigation was conducted to clarify the influences of the cooling rate and the carbon content on the structures of hypoeutectic white cast iron solidified unidirectionally in a mold with a water-cooled copper chill plate and exothermic materials. The carbon content of these alloys were within the range of 1.8% to 4.2%,
  The experimental results were as follows:
  (1) The average distance between the stalkes of the primary dendrite was inversely proportional to the square root of the cooling rate at the early stage of solidification of primary austenite and was independent of the carben content.
  (2) The second-arm-spacing (s) of the primary dendrite could be expressed by a function of the cooling rate (V_p) at the early stage of solidification of primary austenite and the carbon content (c) ;
               S = 884/V_p^0.31·C^1.48
where,      S in μ, V_p in °C/min, C in %
  (3) The primary austenite is consisted of groups of several dendritic cells, each group having a constant crystallographic orientation. The size of a group of dendritic cells was inversely proportional to about the two thirds power of the cooling rate at the early stage of solidification of primary austenite and the size became larger with the decrease of the carbon content when it was less than 3.9%.
  (4) The size of a colony of ledeburite was inversely proportional to the square root of the cooling rate immediately after the precipitation of ledeburite and was independent of carhon content.
  (5) The eutectic structure consisted of groups of several colonies lining up in parallel. The size of the group of colonies was inversely proportional to about the two thirds power of the cooling rate immediately after the precipitation of ledeburite and was influenced also by the carbon content.