Exhaust valve for an internal combustion engine, and a method of strengthening an annular valve seat area in an exhaust valve

Abstract

An exhaust valve for an internal combustion engine comprises a valve disc having a first side ( 14 ) and a second side ( 16 ), a valve stem extending from the first side of the valve disc, and an annular valve seat area ( 11 ) located at the first side of the valve disc. The annular valve seat area ( 11 ) is of a nickel-base alloy or a chromium-base alloy and has a valve seat surface. The annular valve seat area has at least a first annular portion ( 43 ) located in between two second annular portions ( 42 ). The first annular portion has lower hardness than the two second annular portions.

1 . An exhaust valve for an internal combustion engine, the exhaust valve comprising a valve disc having a first side and a second side, a valve stem extending from the first side of the valve disc, and an annular valve seat area located at the first side of the valve disc, wherein the annular valve seat area is of a nickel-base alloy or a chromium-base alloy and has a valve seat surface, wherein the annular valve seat area has at least a first annular portion located in between two second annular portions, and wherein the first annular portion has lower hardness than the two second annular portions. 
     
     
         2 . An exhaust valve as claimed in  claim 1 , wherein the average hardness at the valve seat surface at the middle of the first annular portion is at least 80 HV lower than the average hardness at the middle of the surface of at least one of the second annular portions. 
     
     
         3 . An exhaust valve as claimed in  claim 2 , wherein the material of the two second annular portions has a crystallographic grain structure of cold deformation and age hardening below recrystallization temperature. 
     
     
         4 . An exhaust valve as claimed in  claim 3 , wherein the material of the first annular portion has a crystallographic grain structure without cold deformation. 
     
     
         5 . An exhaust valve as claimed in  claim 4 , wherein the valve disc at its second side comprises a layer of a hot corrosion resistant material. 
     
     
         6 . An exhaust valve as claimed in  claim 5 , wherein the valve disc carrying the annular valve seat area comprises a base body of stainless steel. 
     
     
         7 . An exhaust valve as claimed in  claim 1 , wherein the material of the two second annular portions has a crystallographic grain structure of cold deformation and age hardening below recrystallization temperature. 
     
     
         8 . An exhaust valve as claimed in  claim 7 , wherein the material of the first annular portion has a crystallographic grain structure without cold deformation. 
     
     
         9 . An exhaust valve as claimed in  claim 8 , wherein the valve disc at its second side comprises a layer of a hot corrosion resistant material. 
     
     
         10 . An exhaust valve as claimed in  claim 9 , wherein the valve disc carrying the annular valve seat area comprises a base body of stainless steel. 
     
     
         11 . An exhaust valve as claimed in  claim 1 , wherein the material of the first annular portion has a crystallographic grain structure without cold deformation. 
     
     
         12 . An exhaust valve as claimed in  claim 11 , wherein the material of the two second annular portions has a crystallographic grain structure of cold deformation and age hardening below recrystallization temperature. 
     
     
         13 . An exhaust valve as claimed in  claim 12 , wherein the valve disc at its second side comprises a layer of a hot corrosion resistant material. 
     
     
         14 . An exhaust valve as claimed in  claim 13 , wherein the valve disc carrying the annular valve seat area comprises a base body of stainless steel. 
     
     
         15 . An exhaust valve as claimed in  claim 1 , wherein the valve disc at its second side comprises a layer of a hot corrosion resistant material. 
     
     
         16 . An exhaust valve as claimed in  claim 15 , wherein the valve disc carrying the annular valve seat area comprises a base body of stainless steel. 
     
     
         17 . An exhaust valve as claimed in  claim 15 , wherein the material of the first annular portion has a crystallographic grain structure without cold deformation. 
     
     
         18 . A method of strengthening an annular valve seat area in an exhaust valve for an internal combustion engine, said exhaust valve comprising a valve disc having a first side and a second side, a valve stem extending from the first side of the valve disc, and an annular valve seat area located at the first side of the valve disc, wherein at least the following steps are performed:
 a) a valve disc with a raw seat area is provided, said raw seat area having at least two annular protrusions separated by an intermediate annular area,   b) the two annular protrusions are plastically deformed,   c) the raw seat area is machined to the dimensions of the annular valve seat area,   d) the valve disc with the annular valve seat area is subjected to heat treatment involving at least an aging heat treatment.   
     
     
         19 . A method of strengthening an annular valve seat area in an exhaust valve as claimed in  claim 18 , wherein the plastic deformation in step b) involves rolling with a cylindrically shaped roll. 
     
     
         20 . A method of strengthening an annular valve seat area in an exhaust valve as claimed in  claim 19 , wherein a separate cylindrically shaped roll is provided for each annular protrusion. 
     
     
         21 . A method of strengthening an annular valve seat area in an exhaust valve as claimed in  claim 18 , wherein the separate cylindrically shaped rolls are carried by a common rolling tool, but are allowed to rotate during rolling with mutually different rotational speeds. 
     
     
         22 . A method of strengthening an annular valve seat area in an exhaust valve as claimed in  claim 18 , wherein the individual annular protrusion provided in step a) has a curved central portion.