To examine the effects of nickel addition on not only static tensile properties and impact properties but also fracture toughness, castings of two sizes, namely heavy-section (600mm thickness) and Y-block (75mm thickness) spheroidal graphite cast irons, with the same chemical composition were prepared, and attempts were made to clarify and evaluate the fracture toughness. CT specimens of 25 mm in thickness were then obtained from the two materials by machining, according to ASTM regulations for heavy sectional castings, and fatigue pre-cracks were formed on the specimens by applying some 20 kilo-cycles of repeated axial loads using a hydraulic servo fatigue tester at room temperature. Next, 60 degree-V shaped side-grooves were formed on the CT specimens after inducing an approximately 2 mm large fatigue pre-crack at the machined crack tip, and CT testing was then performed on the specimens using an Instron type material tester at 296K (R. T.) and 233K. The applied load (L) and load-line-displacement (LLD) of the CT specimens were recorded on computer by the unloading elastic compliance method based on the JSME method. The results showed that two kinds of almost fully ferritic spheroidal graphite cast irons were obtained from heavy sectional sands molds. The microstructures and mechanical properties of these materials were that of typically normal spheroidal graphite cast irons ones. (ie : UTS > 300 MPa, Elongation > 20%, HV > 130)

  The obtained fracture toughness (J_Ic) of these specimens was 26-33 and 10-23 kN/m at R. T. and 223K, respectively. The converted fracture toughness (K_Ic) values from estimate equation showed > 45 MPa at 233K of heavy-section specimens added with 0.6% Ni. The calculated fracture toughness (converted K_Ic)) and Charpy impact characteristics of these materials showed good correlation with parameters when impact yield stress was applied in impact testing.

Arc welding is commonly applied to high-strength steel for buildings (H-SA700). However, the heat-affected zones of H-SA700 often become soft and the yield stress decreases compared to that of the base metal owing to the large heat input of arc welding. In this study, laser welding was applied to H-SA700. The residual stress, hardness, mechanical property and fracture toughness of H-SA700 after laser welding were examined and the results were compared with those of arc welding. As a result, the softened area of H-SA700 welded using laser welding was considerably smaller than that obtained using arc welding, and the yield stress was the same as that of the base metal. Fracture pass deviation (FPD) was observed in the Charpy impact test.

  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.

  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.

  The aim of the present work is to study the influence of copper content on impact characteristics and mechanical properties of ductile cast irons. Five melts with different chemical compositions from 0 (i.e. FCD) to 2mass% of copper content were produced using a high frequency induction furnace. The melt was poured into a Y-block CO₂ sand mould of 32mm in thickness.

  The chemical compositions of the samples are similar to each other in terms of Carbon (3.66mass%) and Silicon (2.52mass%), and the graphite nodularity is about 86% in all samples.

  In as-cast samples, the tensile strength (UTS), elongation, and Brinell hardness, of non-additive element samples (i.e. FCD) were 480MPa, 19.5%, and 157HBW, respectively. The Charpy impact value at room temperature (R.T.) of smooth samples was 95.2J/cm².

  The strength depends on the Cu contents, and showed the highest value of 958MPa for UTS, and the minimum value of 6.3% for elongation. However, the impact value of the sample containing 2mass% Cu was 25.8J/cm² at R. T. The impact transition temperature also tends to increase toward R. T. with increasing Copper contents. In order to improve toughness of cast irons for samples containing 1% Cu (named Cu1.0) and 2% Cu (named Cu2.0), normalization heat treatment was carried out in which the treatment temperature is decided based on the thermal expansion measurement results.

  As a result, the area fraction of pearlite in the Cu1.0 and Cu2.0 samples was found to decrease by heat treatment at 1073K. For Cu2.0 samples treated at 1173K, UTS was 951MPa and elongation was 9.5%. The fracture energy at R. T. of these samples was 70J/cm².