Method for heat-treating uppermost surface of spheroidal graphite cast iron and spheroidal graphite cast iron heat-treated by the same

Abstract

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 μm, 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 μm during an oxy-nitriding heat treatment.

What is claimed is: 
     
         1 . A method for heat-treating a surface of spheroidal graphite cast iron, comprising:
 first heat treating the spheroidal graphite cast iron for forming ferrite; and   second heat treating for oxy-nitriding the spheroidal graphite cast iron.   
     
     
         2 . The method of  claim 1 , wherein the first heat treating comprises: heating the spheroidal graphite cast iron to a first heating temperature of about 940 to 1,060° C.; and maintaining the first heating temperature, wherein a total time for the first heat treating time is about 80 to 100 minutes. 
     
     
         3 . The method of  claim 2 , wherein the first heat treatment step further comprises: air-cooling the spheroidal graphite cast iron for about 110 to 130 minutes. 
     
     
         4 . The method of  claim 1 , wherein the second heat treating comprises:
 nitrocarburizing by introducing a gas;   oxidizing the spheroidal graphite cast iron; and   cooling;   wherein the nitrocarburizing is performed at a second heat treating temperature of about 550° C. to 570° C. for about 720 minutes to 1,200 minutes.   
     
     
         5 . The method of  claim 4 , wherein the cooling is furnace cooling. 
     
     
         6 . A spheroidal graphite cast iron comprising:
 an oxidation layer; and   a compound layer,   wherein the compound layer has a thickness of about 15 to 30 μm.   
     
     
         7 . The spheroidal graphite cast iron of  claim 6 , wherein the thickness of the compound layer is uniform. 
     
     
         8 . The spheroidal graphite cast iron of  claim 6 , wherein an uppermost surface of the spheroidal graphite cast iron comprises ferrite fraction in about 60 to 80%. 
     
     
         9 . The spheroidal graphite cast iron of  claim 8 , wherein the compound layer comprises:
 a pore layer; and   a nitride compound layer comprising a gamma prime phase and an epsilon phase.   
     
     
         10 . The spheroidal graphite cast iron of  claim 9 , wherein the nitride compound layer consists of the gamma prime phase and the epsilon phase. 
     
     
         11 . The spheroidal graphite cast iron of  claim 9 , wherein the nitride compound layer comprises the epsilon phase in about 80% or greater. 
     
     
         12 . The spheroidal graphite cast iron of  claim 9 , wherein the compound layer has the hardness of about HV 600 to 1000. 
     
     
         13 . The spheroidal graphite cast iron of  claim 9 , wherein the compound layer comprises the pore layer in a fraction of about 25 to 35%. 
     
     
         14 . The spheroidal graphite cast iron of  claim 10 , wherein the compound layer comprises the pore layer in a fraction of about 25 to 35%. 
     
     
         15 . The spheroidal graphite cast iron of  claim 9 , wherein the compound layer comprises the oxidation layer, the pore layer, the gamma prime phase and the epsilon phase from the uppermost surface of the spheroidal graphite cast iron. 
     
     
         16 . The spheroidal graphite cast iron of  claim 6 , the cast iron comprises the oxidation layer and the compound layer from an uppermost surface of the cast iron. 
     
     
         17 . A vehicle part comprising a spheroidal graphite case iron of  claim 6 . 
     
     
         18 . A vehicle comprising a vehicle part of  claim 17 .