鋳鉄に及ぼす熔解雰囲気の影響

抄録

  The structure of grey iron as well as the graphitization of white iron effected by the melting atmospheres were investigated. Synthetic irons of Fe-C-Si-P alloy system with hypo, hyper and just the eutectic compositions of carbon were melted in vacua, then they kept melting in the flow of the mixed gasses such as nitrogen, hydrogen and carbon monoxide with argon respectively. The surrounding atmospheres of the molten irons were controlled by changing the mixing ratios of the gasses to argon. The graphite structure melted in vacuum was undercooled but it coarsened progressively to flaky when the atmosphere contained nitrogen or hydrogen. The coarsening of graphite was remarkable in the irons of hypo or hyper eutectic compositions depends upon the atmosphere containing nitrogen or hydrogen respectively.
  Generally, in hypo eutectic alloy, the graphites once coarsened and then refined by increasing the partial pressures of nitrogen and hydrogen of the melting atmosphere, which seemed to be attributed by the segregation of gasses in the eutectic liquid owing to the separation of primary dendrites of austenite.
  It is suggested that the segregation of gasses has responsible to the eutectic solidification as the flake graphite was formed at the boundaries of the eutectic cells preferably but proceeded inwardly in the cells by increasing the gas contents.
  When the atmosphere contained carbon monoxide, the oxygen in the molten iron increased proportionally by the partial pressure of the gas but not so much change in the graphite structures, while ledeburite tended to form with refined dendrites of primary austenite in hypo eutectic alloy.
  Small addition of sulphur coarsened remarkably the structure of graphite in grey iron and promoted the graphitization of white irons.
  Then, the synthetic irons of Fe-C-Si system were melted in various atmospheres as before and sucked up in a silica tube to freeze as white, and the graphitization of which was investigated by dilatometric method annealed in argon atmosphere. It was found that both nitrogen and oxygen retard the first stage graphitization and the former was effective at higher partial pressure but the latter at lower one.