Corrosion and Wear Resistant Overlay, Method for Forming Corrosion and Wear Resistant Overlay, and Corrosion and Wear Resistant Valve

Abstract

The present invention is intended is to improve the corrosion resistance of an overlay used in a nuclear power plant, and to reduce dissolution of cobalt from an overlay. The corrosion and wear resistant overlay is formed along a surface of a base by laser lamination modeling, and is configured from a plurality of metal layers of a Co-base alloy. The thickness of carbide eutectics that precipitate in the metal layers is the largest in the metal layer closest to the base, and is gradually smaller in the metal layers farther away from the base. The intensity of the laser beam applied to form layers by laser lamination modeling is adjusted so that the carbide eutectics that precipitate in at least the outermost metal layer have a controlled size of 10 μm or less.

1 . A corrosion and wear resistant overlay, comprising:
 a plurality of Co-base alloy layers formed along a surface of a base;   wherein a carbide eutectic that precipitates at a boundary between crystals of a dendrite structure of the Co-base alloy in the plurality of layers has an average thickness that is thicker in layers closer to the base, or thinner in layers farther away from the base.   
     
     
         2 . The corrosion and wear resistant overlay according to  claim 1 , wherein the carbide eutectic that precipitates in at least the outermost layer of the plurality of layers has a maximum thickness of about 10 μm or less. 
     
     
         3 . The corrosion and wear resistant overlay according to  claim 2 , wherein the crystals of the dendrite structure, and the carbide eutectic in the outermost layer have a Cr concentration of about 17% or more. 
     
     
         4 . The corrosion and wear resistant overlay according to  claim 1 , wherein the plurality of layers is each formed by heating, melting, and depositing a Co-base alloy powder discharged through a nozzle. 
     
     
         5 . The corrosion and wear resistant overlay according to  claim 1 , wherein the plurality of layers is each formed by heating and melting a Co-base alloy powder deposited beforehand. 
     
     
         6 . The corrosion and wear resistant overlay according to  claim 4 , wherein the Co-base alloy powder heated and melted in forming the plurality of layers has a preset amount of heat input that is largest for the layer closest to the base, and that is gradually smaller for layers farther away from the base. 
     
     
         7 . The corrosion and wear resistant overlay according to  claim 1 , wherein the base is a carbon steel, or a stainless steel. 
     
     
         8 . A method for forming a corrosion and wear resistant overlay,
 the method comprising:   a first step of heating and melting a Co-base alloy powder to form a Co-base alloy layer; and   a second step of forming another Co-base alloy layer on a surface of the Co-base alloy layer by repeating the first step,   wherein an amount of heat input to heat and melt the Co-base alloy powder is reduced in repeatedly forming the Co-base alloy layer in the first step and the second step.   
     
     
         9 . The method according to  claim 8 , wherein the Co-base alloy layer is formed by heating, melting, and depositing the Co-base alloy powder discharged through a nozzle that moves along a surface of a base. 
     
     
         10 . The method according to  claim 8 , wherein the Co-base alloy layer is formed by heating and melting the Co-base alloy powder deposited beforehand. 
     
     
         11 . A corrosion and wear resistant valve, comprising:
 a pipe section as a conduit for a fluid;   a valve body that opens and closes the pipe section; and   valve seats provided for the valve body and the pipe section in portions that come into contact with each other when the pipe section is closed,   wherein the valve seats are configured from the corrosion and wear resistant overlay of  claim 1 .