Partial heat-treatment method using double metal layer

Abstract

Disclosed herein is a partial heat treatment method using a double metal layer, in which a surface of a workpiece is hardened by heat treatment, including the steps of: primarily plating the surface of the workpiece with a first metal layer; secondarily plating the surface of the first metal layer with a second metal layer; partially stripping the first metal layer and the second metal layer to expose a part of the surface of the workpiece; heat-treating the workpiece to harden the exposed surface of the workpiece; and removing the first metal layer and the second metal layer remaining on the surface of the workpiece.

What is claimed is: 
     
         1 . A partial heat treatment method using a double metal layer, in which a surface of a workpiece is hardened by heat treatment, comprising the steps of:
 primarily plating the surface of the workpiece with a first metal layer;   secondarily plating the surface of the first metal layer with a second metal layer;   partially stripping the first metal layer and the second metal layer to expose a part of the surface of the workpiece;   heat-treating the workpiece to harden the exposed surface of the workpiece; and   removing the first metal layer and the second metal layer remaining on the surface of the workpiece.   
     
     
         2 . The partial heat treatment method of  claim 1 , wherein the first metal layer has a different metal structure from that of the workpiece to be stripped without leaving remnants. 
     
     
         3 . The partial heat treatment method of  claim 2 , wherein the first metal layer includes at least one metal selected from among copper, chromium, nickel and tin. 
     
     
         4 . The partial heat treatment method of  claim 1 , wherein the second metal layer has a relatively compact metal structure compared to that of the first metal layer. 
     
     
         5 . The partial heat treatment method of  claim 4 , wherein the second metal layer includes at least one metal selected from among copper, chromium, nickel and tin, and the metal constituting the second metal layer is different from the metal constituting the first metal layer. 
     
     
         6 . The partial heat treatment method of  claim 1 , wherein, in the step of partial stripping, the workpiece is sequentially immersed in a solution to strip the first metal layer and the second metal layer. 
     
     
         7 . The partial heat treatment method of  claim 1 , wherein the step of heat-treating the workpiece is performed by salt bath heat treatment. 
     
     
         8 . The partial heat treatment method of  claim 7 , wherein, in the salt bath heat treatment, the workpiece is heat-treated by immersing the workpiece in an alkali salt-molten solution containing nitrogen and carbon compounds. 
     
     
         9 . The partial heat treatment method of  claim 1 , wherein the step of heat-treating the workpiece is performed by nitriding treatment. 
     
     
         10 . The partial heat treatment method of  claim 1 , wherein the first metal layer has a thickness of 15 to 50 μm. 
     
     
         11 . The partial heat treatment method of  claim 1 , wherein the second metal layer has a thickness of 15 to 50 μm. 
     
     
         12 . The partial heat treatment method of  claim 1 , wherein the workpiece is a ferrule for plugging a gap between pipes at the time of connecting the pipes. 
     
     
         13 . The partial heat treatment method of  claim 12 , wherein the ferrule includes a nose tightly sealing the pipes by irreversible deformation, and the surface of the nose is exposed in the step of partial stripping. 
     
     
         14 . A partial heat treatment method using a double metal layer, in which a surface of a ferrule is hardened by heat treatment, comprising the steps of:
 primarily plating the surface of the ferrule with a first metal layer;   secondarily plating the surface of the first metal layer with a second metal layer;   partially stripping the first metal layer and the second metal layer to expose a part of the surface of the ferrule;   heat-treating the ferrule to harden the exposed surface of the ferrule; and   removing the first metal layer and the second metal layer remaining on the surface of the ferrule.   
     
     
         15 . The partial heat treatment method of  claim 14 , wherein the first metal layer includes at least one metal selected from among copper, chromium, nickel and tin. 
     
     
         16 . The partial heat treatment method of  claim 14 , wherein the second metal layer includes at least one metal selected from among copper, chromium, nickel and tin, and the metal constituting the second metal layer is different from the metal constituting the first metal layer. 
     
     
         17 . The partial heat treatment method of  claim 14 , wherein, in the salt bath heat treatment, the ferrule is heat-treated by immersing the ferrule in an alkali salt-molten solution containing nitrogen and carbon compounds.