Hitachi Metals, Ltd.

Kouhei Imanishi

Hideaki Nagayoshi

Fumio Obata

Hisashi Yasuda

Kiyoshi Suehara

Toshiki Yoshida

Seishin Ueda

A method of producing a spheroidal graphite cast iron article containing at least one pearlite stabilizing element selected from the group consisting of Mn, Cu, Sn, Sb and Pb and having a double layer structure which comprises a surface layer portion and an inner portion. The surface layer portion has a matrix 60% or more of which is ferrite phase and a thickness of at least 0.5 mm. The inner portion has a matrix substantially comprising pearlite phase. The method comprises: (1) heat-treating a starting spheroidal graphite cast iron at a temperature which transforms a whole part of a matrix of the starting spheroidal graphite cast iron substantially to austenite phase; (2) slowly cooling the austenitized spheroidal graphite cast iron at a cooling rate (5.degree. to 20.degree. C./min) which allows in a subsequent step the surface layer portion to be ferritized before the inner portion starts to be pearlitized; (3) holding the cooled spheroidal graphite cast iron at a temperature (710.degree. to 770.degree. C.) which allows the surface layer portion to be first ferritized and subsequently allows the inner portion to be pearlitized while preventing the ferrite phase of the surface layer portion from being transformed to another; and (4) cooling the spheroidal graphite cast iron thus transformed immediately after completion of the pearlitization of the inner portion.

Method of producing spheroidal graphite cast iron article

The spheroidal graphite cast iron member having a surface layer portion mostly composed of a ferrite phase and having a thickness of at least 1 mm, and an inner portion composed of a pearlite phase and a ferrite phase, the surface layer portion having a ferritization ratio of 70% or more which is larger than that of the inner portion by at least about 15% is produced by (a) pouring a spheroidal graphite cast melt into a casting mold; (b) removing the casting mold by shake-out after the completion of solidification of the melt, while substantially the entire portion of the resulting cast iron product is still at a temperature of its A.sub.1 transformation point or higher; (c) when the temperature difference between the surface layer portion and the inner portion has become 40.degree.-60.degree. C., introducing the cast iron product into a uniform-temperature furnace kept at 750.degree.-900.degree. C., where the cast iron product is held for such a time period as to produce the surface layer portion having a ferritization ratio of 70% or more which is larger than that of the inner portion by at least about 15%; and (d) transferring the cast iron product into a cooling furnace to cool the cast iron product at a cooling speed of 15.degree.-100.degree. C./min.

Spheroidal graphite cast iron member having improved mechanical strength

Growing community of inventors

Fukuoka, Japan

Kouhei Imanishi