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.

  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.

  The effects of nitriding treatment on rotating-bending fatigue properties were investigated on nine kinds of pearlitic ductile cast iron samples with Mo (0.1%), Cr (0.1%), V (0.1%), Al (0.1, 0.3, 0.5%), Al (0.1%) & Cr (0.1%), Al (0.1%) & V (0.1%) and without alloying element. The white layer of Fe4N nitride formed on the surfaces of all the samples was about 0.01mm in thickness. The practical nitrided depth and micro-Vickers hardness at 0.03mm below the surface in the nitride layer of the sample without alloying element were 0.148mm and 549HV, respectively. The addition of alloying elements to nitrided samples increased the practical nitrided depth and hardness in the vicinity of the surface. In the nitrided samples, fatigue existing in the higher stress range from 500 to 650MPa was found to be longer in the order of no addition, single addition, and double addition of alloying elements. 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 nitrided samples. The improvement of fatigue characteristic by the addition of the alloying element is considered to be efficient only in the higher stress range. The fatigue strength in the high stress range is considered to be related to the difference in the initiation time of the fatigue crack because the spacings of the striation formed on the fracture surfaces are more or less the same in all the samples. 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.