The machinability of gray cast iron is well-knwn to depend on its graphite morphology and matrix structure. However, machinablility occasionally changes even if graphite morphology or matrix structure has not varied. We have therefore focused attention on seasoning and stress relief annealing known to improve machinability without changing the graphite morphology or matrix structure, and carried out machining tests and a series of ultrasonic transmission speed measurements, tensile test, hardness test and structure evaluation while varying the conditions for seasoning and stress relief annealing. As a result, it was found that seasoning applied specimens subject to seasoning greatly vary in machinability despite small changes in the ultrasonic transmission speed, tensile strength, and hardness. Furthermore correlation between ultrasonic transmission speed and amount of machining tool wear was seen in some parts of the stress-relief-annealed specimens with little changes in the optical microstructure and mechanical properties.

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  Recently, Mn content in steel scrap is increasing due to the production of high strength steel for automobile body. The increase in Mn content particularly reduces the elongation of ductile cast iron.
  In the near future, this increase in Mn content is expected to make the production of high ductility cast iron in the cast condition extremely difficult. This paper aims at the removal of Mn from molten cast iron. First we confirmed that Mn in molten iron can be removed by adding sodium-sulfide in laboratory tests. The reproducibility was also checked in the manufacturing process. Furthermore, as a result of a study on the influence of each factor related to Mn removal, the optimal conditions for Mn removal processing were obtained.

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  It has been reported in present paper that 2mm thick section can be made without chill structures in vacuum depressurization processing. This paper describes a method to shorten the decompression processing time. Holding time in vacuum was decreased within five minutes with a addition of CaC₂ into molten iron in vacuum in this study. Total oxygen, dissolved oxygen and sulfur content in melt were decreased rapidly at a time. The greatest nodule count of 2mm thickness specimen was obtained with a addition of 1%CaC₂ and 2mm thickness specimen was produced without chill. When 2%CaC₂ was added into the melt, chill structure was observed in 2mm thickness specimen. The results demonstrated that there is an appropriate amount of CaC₂ addition.

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