Nitrided valve lifter and producing method therefor

Abstract

A sliding surface ( 2 ) of a valve lifter ( 1 ) or a shim ( 3 ) is subjected to a gaseous nitriding to have a surface hardness of Hv 660 or higher and a thickness of a compound layer ( 6 ) of 1 to 5 μm. The nitrided layer on the surface is a dense compound layer ( 6 ) with a porosity of 5% or less.

1 . A valve lifter for an internal combustion engine which is subjected to gaseous nitriding or gaseous soft nitriding on at least an upper surface thereof, characterized in that a compound layer formed on an outermost surface of the upper surface by the nitriding process has a thickness of 1 to 5 μm and a surface roughness Ra thereof is 0.05 or less.  
   
   
       2 . A valve lifter according to  claim 1 , wherein the compound layer is composed of a γ′-phase and a mixture of a γ′-phase and ε-phase.  
   
   
       3 . A valve lifter according to  claim 1  or  2 , wherein the compound layer includes an equiaxed crystal.  
   
   
       4 . A valve lifter according to  claim 1 , wherein the compound layer has a porosity of 5% or less.  
   
   
       5 . A valve lifter according to  claim 1 , wherein a surface of the compound layer has a lot of protrusions of 0.5 μm or less in average diameter.  
   
   
       6 . A valve lifter according to  claim 1 , wherein the protrusions on the surface of the compound layer are formed by a nitride, or a nitride including at least one of carbon, oxygen, sulfur and compounds thereof.  
   
   
       7 . A valve lifter according to  claim 1 , wherein the upper surface thereof has a surface hardness equal to or higher than Hv 660.  
   
   
       8 . A combination of a valve lifter produced according to  claim 1  and a cam in slidable contact therewith, characterized in that a surface roughness of the cam is improved by a slidable contact of the upper surface of the valve lifter and the outer surface of the cam and the surface roughness of the valve lifter itself remains as it is.  
   
   
       9 . A method of producing a valve lifter for an internal combustion engine which has gaseous nitriding layer or gaseous soft nitriding layer on an upper surface of the valve lifter, characterized in that a surface roughness Ra of the upper surface before nitriding is finished in a range of 0.01 to 0.03 by a grinding process.  
   
   
       10 . A method according to  claim 9 , wherein a temperature of nitriding is 500-560° C.