QB' treatment has been developed to produce a fine microstructure with good fracture properties in austempered ductile iron (ADI). Prequenched ductile cast irons are reaustenitized at (α + γ) range for various time followed by isothermal treatment at bainitic transformation temperature range. The toughness increases with the heating time at (α + γ) range. This is mainly caused by the increase in ductility as the number of carbide decreased with the holding time. It is clarified that the carbide precipitates during reheating process of prequenched martensitic matrix to (α + γ) range. The carbide are identified as Fe₃C by TEM microscopy. It is found that there is little change in the toughness with the heating time at (α + γ) range in the case of ferritic prior-structure because the holding time influenced only on coarsening of the matrix. While in the case of pearlitic prior-struture, the toughness increases with the holding time because long heating time is necessary to decompose the relatively stable carbide around the eutectic cell boundary.

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  Toughness of austempered ductile iron (ADI) tempered at 673, 723 and 773 K for various time periods are examined. The toughness increases gradually with increasing tempering time at 673 K while that decreases drastically even in short time tempering at 773 K. The toughness shows a peak value, and then decreases by tempering at 723 K. The highest toughness value is obtained by tempering at 723 K for 10 min in this study. The tempering process of ADI is found to be composed of two stages by SEM, TEM and X-ray analyses. In the first stage, the toughness increase is attributed to the increase in the ductility caused by the decrease in dislocation density mainly in the ferrite phase that has been introduced during the austempering process. The decrease in toughness occurs with the precipitation of carbides corresponding to the decomposition of retained austenite in the second stage. The carbides are identified to be Fe₃C by TEM.

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  Erosive wear tests were performed on austempered ductile iron (ADI), and ferritic ductile iron (FDI) and pearlitic ductile iron (PDI), using a shot blast machine. Erosion damage was measured by the removed material volume at impact angles between 10 and 90 deg. The mechanism of erosive wear, the effect of impact angles, and differnces in wear features of the specimens were discussed. The erosion rate of ADI is about 1/10-1/25 of FDI and PDI, showing that ADI has superior erosion resistance. The surface hardness of eroded ADI specimens increased up to HV 700 from the initial HV 350 after 600 s of blasting. The amount of retained austenite was measured as about 40 % before the test, but it was decreased by the transformation of austenite to martensite, giving the hardening effect on the surface and as a result, lowering the erosion rate. It was shown that ADI has excellent erosion resistance and it is expected to find wide application as a wear resistant material.

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  It is well known that the graphite morphology of cast iron which contains lead changes into Widmannstäten graphite (W-G). We made some experiments to make clear the mechanism by following experiments. (1) Unidirectional solidification of pure Fe-C-Si-Pb alloys. (2) Interaction of Pb and S. (3) Influence of inoculation (Fe-Si and Ca). (4) Influence of pure Ca inoculation for pure Fe-C-Si-Pb alloy. The results were as follows, (1) W-G was not formed with the single-addition of Pb. (2) W-G was formed with the coexistence of Pb and Ca. (3) W-G was generated from γ-phase after solidification. (4) Ca should be adhered on the surface of graphite during solidification, and Pb attached to the Ca. It meant that the excess-C in γ could not precipitated on the graphite, which was covered with Pb, then the graphite morphology changed from flake to W-G by means of the super-saturation of C.

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  Recently, ceramics sand (spherical mullite beads) has become to be widely used as foundry sand because of its excellent reclamation performance. In this paper, the durability (rupture resistance) of ceramics sand, silica sand, chromite sand and zircon sand is studied by the S-6 method and the N. R. R. method. It is found that the rupture resistance is in a order of ceramics sand > zircon sand > chromite sand > silica sand. Particularly, the ceramics sand is hardly broken down even by heating and cooling it repeatedly, therefore little fine particles are formed in the process showing its good reclamation performance. However, silica sand and chromite sand are broken down without heating them. It is concluded that the breaking down in the later case is caused by not the thermal shock but the lower mechanical strength of the particles.

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