Austempered steel for components requiring high strength and high ductility and/or fracture toughness, which has a silicon content of 3.1 weight-% to 4.4 weight-% and a carbon content of 0.4 weight-% to 0.6 weight-%. The microstructure of the austempered steel is ausferritic or superbainitic.

A ductile iron composition including, by weight: about 3.4% to about 4.0% Si; about 3.0% to about 3.5% C; about 0.5% to about 1.0% Cr; about 0.02% to about 0.05% Mo; up to about 0.01% S; up to about 0.5% Mn; and balance iron and incidental impurities. The composition has a a ferritic body center cubic microstructure and has a graphite nodule density of greater than 100 per mm.sup.2. A method for forming a ductile iron composition is also disclosed.

A ductile iron composition including, by weight: about 3.4% to about 4.0% Si; about 3.0% to about 3.5% C; about 0.5% to about 1.0% Cr; about 0.02% to about 0.05% Mo; up to about 0.01% S; up to about 0.5% Mn; and balance iron and incidental impurities. The composition has a a ferritic body center cubic microstructure and has a graphite nodule density of greater than 100 per mm.sup.2. A method for forming a ductile iron composition is also disclosed.

Provided is a method capable of producing a plated black heart malleable cast iron member having a plating layer with excellent adhesiveness to a black heart malleable cast iron member, while having less burden on the environment, compared to conventional methods.

Disclosed are a method for heat-treating a surface of a spheroidal graphite cast iron, particularly, heat-treating an uppermost surface of spheroidal graphite cast iron, and spheroidal graphite cast iron heat-treated by the same. The method may include first heat treating for forming ferrite and second heat treating for oxy-nitriding. The spheroidal graphite cast iron heat-treated includes an oxidation layer and a compound layer having a thickness of about 15 to 30 m, which may be uniform. The method of the heat-treating may decrease the pearlite fraction in the uppermost surface of spheroidal graphite cast iron and increase the ferrite fraction by forming ferrite, thereby forming a compound layer having a thickness of about 15 to 30 m during an oxy-nitriding heat treatment.

Disclosed are a method for heat-treating a surface of a spheroidal graphite cast iron, particularly, heat-treating an uppermost surface of spheroidal graphite cast iron, and spheroidal graphite cast iron heat-treated by the same. The method may include first heat treating for forming ferrite and second heat treating for oxy-nitriding. The spheroidal graphite cast iron heat-treated includes an oxidation layer and a compound layer having a thickness of about 15 to 30 m, which may be uniform. The method of the heat-treating may decrease the pearlite fraction in the uppermost surface of spheroidal graphite cast iron and increase the ferrite fraction by forming ferrite, thereby forming a compound layer having a thickness of about 15 to 30 m during an oxy-nitriding heat treatment.

A method for manufacturing an austempered ductile cast iron and a product made from the austempered ductile cast iron manufactured by the method are disclosed. In the method for manufacturing an austempered ductile cast iron, spheroidizing agent and primary inoculant are added to a raw molten metal to create homogeneous spheroidal graphite creation in a deep part of a matrix and the raw molten metal to which the spheroidizing agent and the primary inoculant are added is injected into a mold to which secondary inoculant is locally applied, to micronize spheroidal graphite of a local structure coated with the secondary inoculant into fine graphite that is easy to machine, thereby enhancing workability as compared with a conventional austempered ductile cast iron.

A method for manufacturing an austempered ductile cast iron and a product made from the austempered ductile cast iron manufactured by the method are disclosed. In the method for manufacturing an austempered ductile cast iron, spheroidizing agent and primary inoculant are added to a raw molten metal to create homogeneous spheroidal graphite creation in a deep part of a matrix and the raw molten metal to which the spheroidizing agent and the primary inoculant are added is injected into a mold to which secondary inoculant is locally applied, to micronize spheroidal graphite of a local structure coated with the secondary inoculant into fine graphite that is easy to machine, thereby enhancing workability as compared with a conventional austempered ductile cast iron.

A brake disk includes a basic material formed of gray cast iron, a decarburized layer formed on the basic material and formed via decarburizing, and a nitride compound layer formed on the decarburized layer and formed via nitriding of a nitride. A method of manufacturing a brake disk includes preparing a disk formed of gray cast iron, performing heat treatment of the disk to form a pre-decarburized layer and a base layer of gray cast iron over which the pre-decarburized layer is formed, and nitriding a portion of the pre-decarburized layer to form a nitride compound layer including a nitride and a decarburized layer over which the nitride compound layer is formed.

A brake disk includes a basic material formed of gray cast iron, a decarburized layer formed on the basic material and formed via decarburizing, and a nitride compound layer formed on the decarburized layer and formed via nitriding of a nitride. A method of manufacturing a brake disk includes preparing a disk formed of gray cast iron, performing heat treatment of the disk to form a pre-decarburized layer and a base layer of gray cast iron over which the pre-decarburized layer is formed, and nitriding a portion of the pre-decarburized layer to form a nitride compound layer including a nitride and a decarburized layer over which the nitride compound layer is formed.