The influence of V and Ta addition on solidification structure and hardness after quenching and tempering treatments was investigated for high speed steel type alloy (Fe- 1.9%C- 0.5%Mn- 4.9%Cr- 5.0%Mo- 5.0～7.2%V- 0.4～1.4%Ta). The compositions of V and Ta were systematically changed for the purpose of distributing hard MC carbides in the hypoeutectic range. EMPA and XRD analysis identified the lamellar structure as M₂C carbide containing mainly Fe and Mo, and the oval microstructure as MC carbide containing mainly V and Ta among austenite dendrite. The macrohardness of the quenched specimen gradually increased with solution treatment and increasing quenching temperature, and then decreased later. This indicates that the macrohardness of the quenched specimen depends on both the amount and hardness of the martensite matrix. All specimens which were tempered at 723K to 873K showed secondary hardening. Furthermore, hardening of the specimen was conspicuous when specimens containing large amounts of residual austenite were tempered at the optimum temperature, as seen with the increase in the hardening of Ta-contained specimen to around 900HV at the tempering temperature of 823K. These results suggest that the macrohardness of tempered specimens is governed by the amount of carbon in the austenite at quenching temperature, the degree of transformation from residual austenite to martensite, and the precipitation of secondary carbides.

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  The heat treatment characteristics of multi-component white cast irons containing cobalt content varying from 0mass% to 10mass% were investigated, and abrasive wear resistance was also studied by Suga-type wear test.
  The test specimens measuring 55^W × 55^H × 6^Tmm in size were cut from an ingot measuring 55^W × 55^H × 200^Lmm, after which annealing (1123K-18ks FC) was performed. Hardening (1323K-36ks FAC) and tempering (673K～873K-10.8ks AC) were then carried out on the annealed specimens. It was found that the amount of retained austenite (V_γ) in the specimens in the as-cast and as-annealed states decreases with increasing cobalt content, and no austenite could be observed in specimen with 10mass% Co. In the as-hardened state, the V_γ of specimens without cobalt was about 11%. The V_γ decreased to 6% with the addition of 2mass% Co. However, the V_γ value was more or less the same when the cobalt content was increased over 2mass%. The V_γ decreased with increasing tempering temperature regardless of the cobalt content and disappeared at tempering temperatures over 800K.
  The macro-hardness of specimens in the as-cast and as-annealed states decreased gradually with increasing cobalt content. In the hardened state however, the hardness kept to a slight decrease even when the cobalt content was changed, up to 10mass%.
  Secondary hardening occurred conspicuously in the tempered specimens regardless of the cobalt content, and the maximum tempered hardness increased with increasing cobalt content.
  Wear resistance was clearly related to the hardness and increased proportionally with hardness.

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  A ferritic spheroidal graphite cast iron (FCD450) is difficult to harden using a conventional surface hardening method, because the carbon content in the matrix is very low. In order to solve this problem, the friction stir processing (FSP) was used in this study as a new hardening method of cast irons. The authors have clarified in a previous study that the pearlitic cast iron, such as FC300 and FCD700, can be hardened using the friction stir processing and that there are several advantages, such as a higher hardness and no required post surface machining. In this study, it was clarified that a Vickers hardness of about 800HV is obtained due to the formation of fine martensite even for ferritic spheroidal graphite cast irons, although the optimal process range is much lower than that of the pearlitic cast iron due to the diffusion of carbon in the matrix which is required to obtain a hardness.

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