鋳鉄の高周波燒入について

抄録

  The induction surface hardening of cast iron has been adopted to the industrial use recently, but yet the number of investigations on this subject is very few.
  The authors studied the induction surface hardening of several cast irons such as flaky graphite, finest graphite cast iron inoculated by Ca-Si and two kinds of nodular graphite cast irons ; the one was treated by pure Mg, the other by Ni-Mg alloy.
  The results obtained are as follows : (1) Hardenability and quenched strain of inoculated cast iron are equal to those of nodular graphite cast iron, and the treatment is no difficult. By these reasons, this type of cast iron is very favorable as a heat treatable cast iron in future. (2) The minimum heating time needed for surface hardening of these irons at 15 kw input are respectively 8.5, 5 and 7 sec., inoculated cast iron is most hardenable. (3) As the heating time becomes longer, the surface hardness becomes lower increasing the amount of residual austenite and finally, melting begins at austenite grain boundaries, and cracks are apt to grow tn the melted zone. These phenomena are also occured equally to the case of nodular graphite cast iron. (4) In case of suitable quenching, the matrix of the surface structure is fine martensite and at the inner portion, it becomes finer and it is nearly called hardenite. At the transition layer, incomplete diffused quenching structure or troostite appears mainly around graphite. (5) On heating the nodular graphite cast irons consisted of Bull's eye structure, the diffusion to austenite begins to occur around ferrite structure, and as the temperature reaches higher, it develops inner to inner to the part where ferrite contacts with graphite and the diffusion continues along the boundaries of ferrite grain. (6) It is not the expansion of graphite itself which seems such as the expansion of graphite in the hardening part of flaky graphite cast iron but this seems due to the expansion of shrinkage cavities on heating, and very scarecely, it dues to the expanded shrinkage cavities by quenched stress or micro-crack along graphite.