Method of manufacturing nitrided valve seat insert with supersaturated nitrogen surface layer and product thereof

Abstract

A pre-surface hardened valve seat insert and a process for forming a pre-surface hardened valve seat insert useful for insertion into a cylinder head or an engine block of an internal combustion engine. The process comprises the steps of (a) forming the valve seat insert by casting an iron-chromium alloy having no more than 75 weight percent iron and at least 25 wt. % alloying elements, the alloying elements including at least 9 wt. % chromium; (b) precision machining the valve seat insert to precise tolerances so that the valve seat insert can be installed in the cylinder head or the engine block with or without the need for additional seat machining; and (c) treating the valve seat insert to form an exposed wear resistant surface layer of supersaturated nitrogen free of nitride compounds.

What is claimed is: 
     
         1 . A process for forming a surface hardened valve seat insert useful for insertion into a cylinder head or an engine block of an internal combustion engine, said process comprising the steps of:
 casting the valve seat insert from an iron-chromium alloy having no more than 75 weight percent (wt. %) iron and at least 25 wt. % alloying elements, the alloying elements including chromium in an amount of at least 9 wt. %;   precision machining the valve seat insert to precise tolerances so that the valve seat insert can be installed in the cylinder head or the engine block with or without the need for additional seat machining; and   nitriding the valve seat insert to produce an exposed supersaturated nitrogen region free of a white layer of nitride compounds.   
     
     
         2 . The process of  claim 1 , wherein the nitriding step includes ferritic nitrocarburizing the valve seat insert. 
     
     
         3 . The process of  claim 1 , wherein the nitriding step includes carbonitriding the valve seat insert. 
     
     
         4 . The process of  claim 1 , wherein the nitriding step includes ion nitriding the valve seat. 
     
     
         5 . The process of  claim 1 , wherein the alloy is melted and formed into the shape of a valve seat insert during the casting step. 
     
     
         6 . The process of  claim 5 , wherein the iron-chromium alloy includes 0.15-2.00 wt. % C, 0.3-1.0 wt. % Mn, 0.10-2.15 wt. % Si, 0.35-2.25 wt. % Ni, 9.8-33.5 wt. % Cr, up to 15.3 wt. % Mo, up to 2.6 wt. % V, up to 2.3 wt. % Nb, up to 4 wt. % W, up to 19 wt. % Co, up to 0.23 wt. % B, balance incidental impurities and 50-75 wt. % Fe. 
     
     
         7 . The process of  claim 5 , wherein the iron-chromium alloy includes 0.1-2.2 wt. % C, 0.1-1.5 wt. % Mn, 0.1-2.5 wt. % Si, 0.15-3.00 wt. % Ni, 9-35 wt. % Cr, up to 17 wt. % Mo, up to 3 wt. % V, up to 2.5 wt. % Nb, up to 5 wt. % W, up to 21 wt. % Co, up to 0.3 wt. % B, balance incidental impurities and 48-75 wt. % Fe. 
     
     
         8 . The process of  claim 5 , wherein the iron-chromium alloy includes 1.45-1.60 wt. % C, 0.35-0.45 wt. % Mn, 0.1-0.6 wt. % Si, 0.35-1.00 wt. % Ni, 9.8-13.0 wt. % Cr, 5.4-9.15 wt. % Mo, 1.25-1.30 wt. % V, 1.9-2.0 wt. % Nb, up to 4 wt. % W, up to 3.7 wt. % Co, up to 0.23 wt. % B, balance incidental impurities and 67.5-71.0 wt. % Fe. 
     
     
         9 . The process of  claim 5 , wherein the iron-chromium alloy includes 1.3-1.7 wt. % C, 0.3-0.5 wt. % Mn, 0.1-0.8 wt. % Si, 0.3-2.0 wt. % Ni, 9-15 wt. % Cr, 5-10 wt. % Mo, 1.0-1.5 wt. % V, 1.8-2.2 wt. % Nb, up to 5 wt. % W, up to 4 wt. % Co, up to 0.3 wt. % B, balance incidental impurities and 67-71 wt. % Fe. 
     
     
         10 . The process of  claim 5 , wherein the iron-chromium alloy includes 0.1-0.2 wt. % C, 0.3-0.5 wt. % Mn, up to 0.2 wt. % Si, up to 2 wt. % Ni, 14-16 wt. % Cr, 14-16 wt. % Mo, 2-3 wt. % V, 1-3 wt. % Nb, up to 0.1 wt. % W, up to 1 wt. % Co, up to 0.2 wt. % B, balance incidental impurities and 60-64 wt. % Fe. 
     
     
         11 . The process of  claim 5 , wherein the iron-chromium alloy includes 1.8-2.1 wt. % C, 0.1-0.5 wt. % Mn, 0.3-0.8 wt. % Si, 2-3 wt. % Ni, 14-18 wt. % Cr, 10-14 wt. % Mo, up to 0.2 wt. % V, up to 0.05 wt. % Nb, up to 0.3 wt. % W, 17-21 wt. % Co, up to 0.1 wt. % B, balance incidental impurities and 48-52 wt. % Fe. 
     
     
         12 . The process of  claim 5 , wherein the iron-chromium alloy includes 1.55-2.00 wt. % C, 0.35-0.40 wt. % Mn, 0.1-2.0 wt. % Si, 1.1-2.0 wt. % Ni, 20.0-33.5 wt. % Cr, up to 2 wt. % Mo, up to 0.05 wt. % V, up to 2.3 wt. % Nb, up to 1.5 wt. % W, up to 0.05 wt. % Co, up to 0.2 wt. % B, balance incidental impurities and 54-75 wt. % Fe. 
     
     
         13 . The process of  claim 5 , wherein the iron-chromium alloy includes 1.5-2.1 wt. % C, 0.3-0.5 wt. % Mn, 0.1-2.1 wt. % Si, 1.0-2.5 wt. % Ni, 20-35 wt. % Cr, up to 3 wt. % Mo, up to 0.1 wt. % V, up to 2.5 wt. % Nb, up to 2 wt. % W, up to 0.1 wt. % Co, up to 0.3 wt. % B, balance incidental impurities and 50-75 wt. % Fe. 
     
     
         14 . A valve seat insert made by the process of  claim 1 . 
     
     
         15 . The valve seat insert of  claim 14 , wherein the supersaturated nitrogen region has a nitrogen content of 1 to 5 weight percent. 
     
     
         16 . The valve seat insert of  claim 14 , wherein the valve seat insert is cast from a melt of an iron-chromium alloy including 1.45-1.60 wt. % C, 0.35-0.45 wt. % Mn, 0.1-0.6 wt. % Si, 0.35-1.00 wt. % Ni, 9.8-13.0 wt. % Cr, 5.40-9.15 wt. % Mo, 1.25-1.30 wt. % V, 1.9-2.0 wt. % Nb, up to 4 wt. % W, up to 3.7 wt. % Co, up to 0.23 wt. % B, balance incidental impurities and 67.5-71.0 wt. % Fe. 
     
     
         17 . The valve seat insert of  claim 14 , wherein the valve seat insert is cast from a melt of an iron-chromium alloy including 1.3-1.7 wt. % C, 0.3-0.5 wt. % Mn, 0.1-0.8 wt. % Si, 0.3-2.0 wt. % Ni, 9-15 wt. % Cr, 5-10 wt. % Mo, 1.0-1.5 wt. % V, 1.8-2.2 wt. % Nb, up to 5 wt. % W, up to 4 wt. % Co, up to 0.3 wt. % B, balance incidental impurities and 67-71 wt. % Fe. 
     
     
         18 . The valve seat insert of  claim 14 , wherein the valve seat insert is cast from a melt of an iron-chromium alloy including 0.1-0.2 wt. % C, 0.3-0.5 wt. % Mn, up to 0.2 wt. % Si, up to 2 wt. % Ni, 14-16 wt. % Cr, 14-16 wt. % Mo, 2-3 wt. % V, 1-3 wt. % Nb, up to 0.1 wt. % W, up to 1 wt. % Co, up to 0.2 wt. % B, balance incidental impurities and 60-64 wt. % Fe. 
     
     
         19 . The valve seat insert of  claim 14 , wherein the valve seat insert is cast from a melt of an iron-chromium alloy including 1.8-2.1 wt. % C, 0.1-0.5 wt. % Mn, 0.3-0.8 wt. % Si, 2-3 wt. % Ni, 14-18 wt. % Cr, 10-14 wt. % Mo, up to 0.2 wt. % V, up to 0.05 wt. % Nb, up to 0.3 wt. % W, 17-21 wt. % Co, up to 0.1 wt. % B, balance incidental impurities and 48-52 wt. % Fe. 
     
     
         20 . The valve seat insert of  claim 14 , wherein the valve seat insert is cast from a melt of an iron-chromium alloy including 1.5-2.1 wt. % C, 0.3-0.5 wt. % Mn, 0.1-2.1 wt. % Si, 1.0-2.5 wt. % Ni, 20-35 wt. % Cr, up to 3 wt. % Mo, up to 0.1 wt. % V, up to 2.5 wt. % Nb, up to 2 wt. % W, up to 0.1 wt. % Co, up to 0.3 wt. % B, balance incidental impurities and 50-75 wt. % Fe.