This study aims to research the development of high strengthened alloy tool steel (SKD11) castings with superior wear resistance. The mechanical properties of the steel castings are inferior to rolled steel with dispersed Cr carbides owing to Cr carbides with M₇C₃ type crystallized continuously in the matrix. Therefore, we examined the replacing of Cr carbides with MC carbide by means of adding carbide stabilizer elements such as V, Ti and Nb to standard composition of SKD11 castings. The steel casting, varying from 0.3mass% V to 5mass% V, 0.5mass% Ti to 4mass% Ti and 0.5mass% Nb to 5mass% Nb, were melted in a high frequency induction furnace.
  We systematically investigated the influence of amount, morphology, and distribution of M₇C₃ and/or MC type carbide on tensile and bending strength and wear resistance. With increasing amount of carbide stabilizer elements M, MC type carbides increased by the replacement from M₇C₃ carbides in the microstructure and continuity of Cr carbide tended to disappear. With the addition of the elements M, grained TiC crystallized randomly and dispersed finely, rod or petaline NbC crystallized randomly and aggregated locally, and rod VC crystallized around Cr carbides. This replacement effect is remarkable in the order to V ＜ Nb ＜ Ti. The tensile and bending strength of steel casting with the addition of V or Nb does not increase remarkably while the strength of the steel castings with about 2mass% Ti tends to improve remarkably. The abrasive wear properties of the steel castings with MC type carbides also improved significantly. From the viewpoint of improving wear resistance, fine TiC crystallized at random are more effective than NbC or VC. We found that the mechanical and wear properties of high strengthened alloy tool castings were closely related to the morphology and amount of crystallized carbides in the matrix.

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  The porosity of die cast aluminum alloy significantly affects its fatigue strength. However, the stress intensity around macroscopic pores is occasionally mitigated by its large curvature, so the maximum pore is not always guaranteed to be of a fatigue crack origin. Moreover, die cast materials include a large number of pores, therefore there is a need to consider the interaction of pores as well as the size of each pore. For macroscopic pores, we have proposed a methodology to evaluate the effect of pores on the fatigue strength by means of the stress concentration factor. In this study, as the first step to evaluate the stress enhancement around internal clustered pores, an empirical formulation of the elastic stress concentration factor around dual-pore was obtained from the numerical results of the three-dimensional finite element linear elastic analysis. Two spherical pores of the same radius in the middle of a sufficiently large region were considered to eliminate the effects of the outer surface. Uniform tensile stress was applied to the boundary of the analysis domain, and the stress concentration factor was evaluated as the ratio of the maximum principal stress to the applied nominal stress. The inter-pore distance and loading angle were systematically changed, and the variation of stress concentration factor was obtained numerically. After the selection of suitable function form for stress concentration factor prediction, the optimum coefficients were determined by least-square fitting to the numerical results, and its validity and availability were discussed from a mechanical standpoint.

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  Direct welding between spheroidal graphite cast iron (FCD700) and mild steel (SS400) was conducted using electron beam welding to study the microstructure and mechanical properties of the welds.
  Results showed that one-pass welding yielded an over-hardened fusion zone exhibiting acicular martensite structure (815HV) with cracks and porosities. Two-pass welding contained fewer porosities. Both one-pass and two-pass welding displayed acicular martensite and ledeburite within the microstructure of spheroidal graphite cast iron in the beat-affected zone. The tensile strength of both one-pass and two-pass welded joints was lower than that of mild steel base metals. These joints ruptured at the fusion zone or the mild steel bond. However, two-pass welded joints demonstrated tensile yield stress values greater than or equal to those of mild steel base metals. Hardening of the fusion zone and the heat-affected zone of spheroidal graphite cast iron made the impact strength values of two-pass welded joints conspicuously lower than those of the spheroidal graphite cast iron base metal. The fatigue strength of two-pass welded joints almost equaled that of mild steel base metal, with a fatigue limit of 209MPa.

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