This study aims to investigate the influence of tempering temperature on strength, hardness, and wear characteristics of alloy tool steel casting. Test samples added with 0.5mass%Ti with lower C and Cr contents than those in SKD11 (JIS G4404) were manufactured by the investment casting process, and tempered at temperatures 453K, 573K, 673K, 773K, and 873K after quenching from 1293K. The amount of M₇C₃ type Cr carbides decreased, and continuity of the carbides was greatly reduced with the decrease of C and Cr contents. In addition MC type Ti carbides were observed in the matrix microstructure. Carbide structure did not change with varying tempering temperature. Although hardness tended to decrease with increasing tempering temperature, the hardness increased by secondary hardening at temperature of 773K and significantly decreased at 873K. Bending strength did not change largely with tempering temperature and showed maximum value at 453K. Fraction of wear loss did not change during tempering temperature from 473K to 773K but it increased significantly at 873K. The optimum tempering temperature for this alloy steel was found to be 453K from experimental results of strength, hardness, and wear characteristics.

  The machinability of austempered spheroidal graphite cast iron made by continuous casting (A-FCD600) was investigated. In this study, spheroidal graphite cast iron made by continuous casting (FCD600) was used to examine the influence of the austempering on machinability. In addition, austempered gray cast iron made by continuous casting (FC250) was used to examine the influence of the morphology of graphite on machinability. From the results of a tool wear test using continuous turning, the machinability decreased in the order of FC600, A-FC250, and A-FCD600. When relative machinability ratings between each material were calculated using tool life equations decided by the result of the tool wear test, the machinability of A-FCD600 was approximately 1.9 times inferior to FCD600, and approximately 1.3 times inferior to A-FC250. One characteristic of A-FCD600 was that its mechanical properties were relatively near steel. Therefore, a similar tool wear test was carried out with a P10 cemented carbide tool for steel. In this case, the tool life extended 25% compared to the K10 cemented carbide tool for cast iron. These results suggest that tool life can be improved in A-FCD600 cutting when tools for steel are used.

  The effects of nitrocarburizing or nitriding treatments on rotating-bending fatigue properties were investigated on four kinds of pearlitic FCD700 (JIS G5502) -class ductile cast iron samples with V (0.1%), Al (0.1%), Al (0.1%) & Cr (0.1%) and Al (0.1%) &V (0.1%). Tensile and hardness characteristics of the nitrocarburized samples were compared to those of the nitrided ones and as-cast FCD700 without alloying element. Fe₄N nitride formed on the surfaces of the nitrided samples, while Fe₄N and Fe_2-3N nitrides formed on the surfaces of the nitrocarburized ones. The practical nitrided depth and micro-Vickers hardness at 0.03mm below the surface in the nitride layer of the nitrided samples were larger and higher than those of nitrocarburized ones, respectively. The addition of alloying elements to the nitrided and nitrocarburized samples increased the practical nitrided depth and hardness in the vicinity of the surface, compared to the samples without alloying element. The fatigue existing in the higher stress range from 500 to 650MPa was found to be longer in the order of as-cast FCD700, nitrocarburized and nitrided samples. However, the fatigue limit at 10⁷ cycles in the lower stress range ranged from 410 to 450MPa and no significant difference was seen among the nitrocarburized and nitrided samples. The improvement of fatigue characteristics by nitrocarburing and nitriding treatments is considered to be efficient only in higher stress ranges. The fatigue strength in high stress ranges is considered to be related to the difference in the initiation time of the fatigue crack. This suggests that the larger the nitrided depth and/or the higher the hardness in the vicinity of surface promoted by the addition of alloying elements, the more delayed will the crack initiation be.

  In this study, the machinability of austempered spheroidal graphite cast iron made by different casting methods was investigated. Spheroidal graphite cast iron samples made by sand mold casting and continuous casting, respectively ADI-S and ADI-C, were used. From the results of cutting tests, the machinability of ADI-C was always excellent compared with that of ADI-S at cutting speeds from 100 to 365m/min. The feed and thrust forces of ADI-S were higher than those of ADI-C at high cutting speeds, although their cutting resistance was almost the same at low cutting speeds. In addition, the microstructure of ADI-S chips was found to be greatly deformed near the chip-tool interface for ADI-S compared with ADI-C. It has been reported that there always exists retained austenite in austempered spheroidal graphite cast iron, and that the retained austenite transforms to deformation-induced martensite on the machined surface when the austempered spheroidal graphite cast iron is machined. From the results of the comparative analysis of ADI-S and ADI-C, the average relative volume ratio of retained austenite increased with increasing cutting speed for both ADI-S and ADI-C, and was about double in the case of ADI-S at high cutting speeds such as 365m/min. From these results, it is clear that the retained austenite in both ADI-S and ADI-C does not transform to deformation-induced martensite at high cutting speeds, and that ADI-C can be machined at cutting temperatures and with cutting resistances lower than those necessary for ADI-S, suggesting that the machinability of ADI-C is better than ADI-S.