白銑の黒鉛化に及ぼす窒素の影響

抄録

  An agreement in opinions of various investigaters about on the influence of nitrogen on the first stage graphitization of white cast iron has not yet been thoroughly reached. The authors carried out the present investigation in order to add further knowledges about this problem. White cast iron specimens, 5mmφ×80mm, were prepared from cast samples of much the same size as them.
  Each cast sample was made by casting into the shell mould the molten iron prepared using high purity special pig iron, electrolytic iron and ferro-silicon and adding nitrogen with potassium ferro-cyanide.
  Each specimen was heated in argon atmosphere and the time required for each of the first graphitization was determined by means of the dilatometer method.
  The time interval between the beginning time of holding at 950°C and the time which the dilatation curve became horizontal was defined as the time for the first graphitization. The time interval between the beginning time of holding at the temperature 2°C below the temperature at which the abnormal expansion accuring on passing through Ar₁ transfomation stopped and the time which the dilatation curve became horizontal was defined as the time for the second stage graphitization. The nitrogen contents were analysed by the distillation method, and hydrochloric acid-soluble nitrogen (N_S) and hydrochloric acid-insoluble nitrogen (N_I) were fractionally determined. The chemical composition of each specimen is in the following ranges : 2.38∼2.52%C, 0.90∼1.15%Si, 0.23∼0.25%Mn, 0.006∼0.007%P, 0.006∼0.007%S, 0.0042∼0.0197%N_S, 0.0034∼0.0085%N_I, 0.0077∼0.0253%N_T.
  The results of the investigation are as follows:
  (1) The time for the first stage graphitization increases proportionally with third power of Ns in the range from 0.042% to 0.0197%N_S. The time for the second stage graphitization also increases proportionally with third power of N_S in the range below about 0.007% N_S, but seems to increase under participation of other unknown factors than N_S.
  (2) The effect of N_S for the retardation of the graphitization of white cast iron is much more remarkable on the second stage graphitization rather than on the first stage graphitization.
  (3) The increase of N_S decreases the number of temper carbon nodules and increases their size.
  (4) A few carbide which very much time is required for vanishing its existence at 950°C remains in the ferritic matrix of specimens after graphitization annealing by the dilatation method with the increase of N_S beyond about 0.007%N_S. The time for the disappearance of this residual carbide at 950°C increases remarkably with the increase of N_S.
  (5) If we assume that N_S is the index of nitrogen soluble in solid iron on the basis of the fact that samples used in the present investigation have much the same chemical compositions and their contents of impurity elements are very low, it may be concluded that nitrogen soluble in solid iron retards remarkably both the graphitization of the first and the second stages of white cast iron.