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In a corrosion-resistant, double-wall pipe structure formed by tightly fitting a stainless-steel inner pipe in an outer pipe, the inner pipe at any portion thereof which is liable to be adversely affected by heat due to heat processing is replaced by an inner tube of a special metal having corrosion

In a corrosion-resistant, double-wall pipe structure formed by tightly fitting a stainless-steel inner pipe in an outer pipe, the inner pipe at any portion thereof which is liable to be adversely affected by heat due to heat processing is replaced by an inner tube of a special metal having corrosion resistance which is not easily impaired by the heat, the inner tube being joined integrally to the remainder portion of the inner pipe of the stainless steel. Examples of the special metal are Inconel 625, Incoloy 825, and Hastelloy.

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1. A corrosion-resistant double-wall pipe structure formed by interference-fitting a corrosion-resistant inner pipe in an outer pipe, said inner pipe, at a portion thereof which is liable to be adversely affected by heat due to heat processing to the pipe structure, being constituted by an inner tub

1. A corrosion-resistant double-wall pipe structure formed by interference-fitting a corrosion-resistant inner pipe in an outer pipe, said inner pipe, at a portion thereof which is liable to be adversely affected by heat due to heat processing to the pipe structure, being constituted by an inner tube of a nickel-base superalloy having corrosion resistance which is not easily imparied by said heat, said inner tube being joined integrally to the remaining, general portion of said inner pipe which is made of stainless steel, said inner tube being provided at an end of the pipe structure, the extreme end part of said outer pipe, at the end of said pipe structure provided with said inner tube, being replaced by an annular filling formed by overlay welding a corrosion-resistant metal around the end part of said inner tube and to the end face of said outer pipe, and tapered screw threads for screwing into a pipe coupling formed on the end portion of the pipe structure. 2. A corrosion-resistant double-wall pipe structure formed by interference-fitting a corrosion-resistant inner pipe in an outer pipe, said inner pipe, at a portion thereof which is liable to be adversely affected by heat due to heat processing to the pipe structure, being constituted by an inner tube of a nickel-base superalloy having corrosion resistance which is not easily impaired by said heat, said inner tube being joined integrally to the remaining, general portion of said inner pipe which is made of stainless steel, said inner tube being provided at an end of the pipe structure, and, at the end face of the end of said pipe structure provided with said inner tube, a sealing weld provided in the interface between said inner tube and said outer tube thereby to prevent corrosion at said interface. 3. A corrosion-resistant double-wall pipe structure formed by interference-fitting a corrosion-resistant inner pipe in an outer pipe, said inner pipe, at a portion thereof which is liable to be adversely affected by heat due to heat processing to the pipe structure, being constituted by an inner tube of a nickel-base superalloy having corrosion resistance which is not easily impaired by said heat, said inner tube being joined integrally to the remaining, general portion of said inner pipe which is made of stainless steel, said inner tube being provided at a position intermediate between the ends of said pipe structure, and a branch pipe to be connected at one end thereof by welding to the pipe structure at said position thereof, said branch pipe comprising a double-wall pipe structure having an end provided with an inner tube connected by welding to the first named double-wall pipe structure. 4. A corrosion-resistant double-wall pipe structure formed by interference-fitting a corrosion-resistant inner pipe in an outer pipe, said inner pipe, at a portion thereof which is liable to be adversely affected by heat due to heat processing to the pipe structure, being constituted by an inner tube of an iron-base superallow having corrosion resistance which is not easily impaired by said heat, said inner tube being joined integrally to the remaining, general portion of said inner pipe which is made of stainless steel, said inner tube being provided at an end of the pipe structure, the extreme end part of said outer pipe, at the end of said pipe structure provided with said inner tube, being replaced by an annular filling formed by overlay welding a corrosion-resistant metal around the end part of said inner tube and to the end face of said outer pipe, and tapered screw threads for screwing into a pipe coupling formed on the end portion of the pipe structure. 5. A corrosion-resistant double-wall pipe structure formed by interference-fitting a corrosion-resistant inner pipe in an outer pipe, said inner pipe, at a portion thereof which is liable to be adversely affected by heat due to heat processing to the pipe structure, being constituted by an inner tube of an iron-base superalloy having corrosion resistance which is not easily impaired by said heat, said inner tube being joined integrally to the remaining, general portion of said inner pipe which is made of stainless steel, said inner tube being provided at an end of the pipe structure, and at the end face of the end of said pipe structure provided with said inner tube, a sealing weld provided in the interface between said inner tube and said outer pipe thereby to prevent corrosion at said interface. 6. A corrosion-resistant double-wall pipe structure formed by interference-fitting a corrosion-resistant inner pipe in an outer pipe, said inner pipe, at a portion thereof which is liable to be adversely affected by heat due to heat processing to the pipe structure, being constituted by an inner tube of an iron-base superalloy having corrosion resistance which is not easily impaired by said heat, said inner tube being joined integrally to the remaining, general portion of said inner pipe which is made of stainless steel, said inner tube being provided at a position intermediate between the ends of said pipe structure, and a branch pipe to be connected at one end thereof by welding to the pipe structure at said position thereof, said branch pipe comprising a double-wall pipe structure having an end provided with an inner tube connected by welding to the first named double-wall pipe structure.