Nitridation and brazing of assemblies with titanium-containing iron based alloys

Claim 1

A method for producing a brazed composite of a thin gauge nitride-strengthened titanium-containing iron-base alloy member with a second iron-base alloy member brazed thereto, comprising the sequential steps: (a) applying to a pre-formed thin gauge titanium containing iron-base alloy member which is pre-assembled to a second iron-based alloy member, a powdered brazing alloy melting in the temperature range of 1700° to 2200° F. and wettable on the surface of said members following nitridation. (b) heating said assemblage to a temperature in the range of about 1100° to 1400° F. in the presence of an atomic nitrogen-yielding atmosphere in a retort, for a time sufficient to combine substantially all of the free titanium in said titanium-containing iron base alloy member, with nitrogen, (c) further heating said assemblage in said retort to a temperature in the range of about 1700° to 2200° F. in a non-oxidizing atmosphere which is substatially free of atomic nitrogen, at a heating rate which will denitride metallic nitrides other than titanium nitride before the melting temperature of said brazing alloy is reached, without substantially softening said titanium-nitride strengthened base material, and (d) holding said assemblage in said 1700° to 2200° F. range until brazing of said titanium strengthened iron-based alloy member to said second member is accomplished.

Claim 2

A method for producing a brazed composite of a thin gauge nitride-strengthened titanium-containing iron-base alloy member with a second iron-base alloy member brazed thereto, comprising the sequential steps: (a) applying to a pre-formed thin gauge titanium containing iron-base alloy member which is pre-assembled to a second iron-based alloy member, a powdered brazing alloy melting in the temperature range of 1700° to 2200° F. and wettable on the surface of said members following nitridation. (b) heating said assemblage to a temperature in the range of about 1100° to 1400° F. in the presence of an atomic nitrogen-yielding atmosphere in a retort, for a time sufficient to combine substantially all of the free titanium in said titanium-containing iron base alloy member, with nitrogen, (c) further heating said assemblage in said retort to a temperature in the range of about 1700° to 2200° F. in a non-oxidizing atmosphere which is substatially free of atomic nitrogen, at a heating rate which will denitride metallic nitrides other than titanium nitride before the melting temperature of said brazing alloy is reached, without substantially softening said titanium-nitride strengthened base material, and (d) holding said assemblage in said 1700° to 2200° F. range until brazing of said titanium strengthened iron-based alloy member to said second member is accomplished.

Claim 18

A method for producing a brazed heat exchanger structure comprised of a thin gauge nitride-strengthened titanium-containing iron-based alloy plate with a stainless steel fin brazed thereto, comprising the sequential steps: (a) applying a substantially boron-free brazing alloy powder melting in the temperature range of 1700° to 2200° F., to a pre-formed titanium containing iron-base alloy plate which is preassembled to a stainless steel fin, (b) heating said assemblage to a temperature in the range of about 1100° to 1400° F. in the presence of an atomic nitrogen-yielding atmosphere in a retort, for a time sufficient to combine with nitrogen, substantially all of the free titanium in said titanium-containing iron-base metal plate, (c) further heating said assemblage to a temperature in the range of about 1700° to 2200° F. in a non-oxidizing atmosphere which is substantially free of atomic nitrogen, at a heating rate which will denitride metallic nitrides other than titanium nitrides, before the melting temperature of said brazing alloy is reached, without substantially softening said titanium nitride-strengthened iron-base alloy plate, and (d) holding said assemblage in said 1700° to 2200° F. temperature range until brazing of said stainless steel fin to said pre-formed nitride-strengthened plate member is accomplished.

Claim 19

A method for producing a brazed heat exchanger structure comprised of a thin gauge nitride-strengthened titanium-containing iron-based alloy plate with a stainless steel fin brazed thereto, comprising the sequential steps: (a) applying a substantially boron-free brazing alloy powder melting in the temperature range of 1700° to 2200° F., to a pre-formed titanium containing iron-base alloy plate which is preassembled to a stainless steel fin, (b) heating said assemblage to a temperature in the range of about 1100° to 1400° F. in the presence of an atomic nitrogen-yielding atmosphere in a retort, for a time sufficient to combine with nitrogen, substantially all of the free titanium in said titanium-containing iron-base metal plate, (c) further heating said assemblage to a temperature in the range of about 1700° to 2200° F. in a non-oxidizing atmosphere which is substantially free of atomic nitrogen, at a heating rate which will denitride metallic nitrides other than titanium nitrides, before the melting temperature of said brazing alloy is reached, without substantially softening said titanium nitride-strengthened iron-base alloy plate, and (d) holding said assemblage in said 1700° to 2200° F. temperature range until brazing of said stainless steel fin to said pre-formed nitride-strengthened plate member is accomplished.

Claim 20

A method for producing a brazed heat exchanger structure comprised of thin gauge nitride-strengthened titanium containing stainless steel plate modified with about 0.9% free titanium, with a stainless steel fin brazed thereto, comprising the steps: (a) applying a nickel base brazing alloy powder consisting essentially of about 19-30% chromium, 9-10% silicon, 0-10% manganese, balance nickel, to a pre-formed titanium-containing stainless steel plate which is preassembled to a stainless steel fin member, (b) heating said assemblage to the temperature range of 1100° to 1400° F. in the presence of a mixture of ammonia and hydrogen in a retort, for time sufficient to combine with nitrogen, substantially all of the free titanium in said titanium-modified stainless steel plate, (c) further heating said assemblage in an atmosphere to which ammonia has not been added, to about 2000° F. at a heating rate which will denitride metallic nitrides other than titanium nitride before the melting temperature of said brazing alloy is reached, without substantially softening said titanium nitride strengthened plate member, and (d) holding said assemblage at a temperature of about 2050° F. to effect brazing of said fin member to said pre-formed nitride-strengthened plate member.

Claim 21

A method for producing a brazed heat exchanger structure comprised of thin gauge nitride-strengthened titanium containing stainless steel plate modified with about 0.9% free titanium, with a stainless steel fin brazed thereto, comprising the steps: (a) applying a nickel base brazing alloy powder consisting essentially of about 19-30% chromium, 9-10% silicon, 0-10% manganese, balance nickel, to a pre-formed titanium-containing stainless steel plate which is preassembled to a stainless steel fin member, (b) heating said assemblage to the temperature range of 1100° to 1400° F. in the presence of a mixture of ammonia and hydrogen in a retort, for time sufficient to combine with nitrogen, substantially all of the free titanium in said titanium-modified stainless steel plate, (c) further heating said assemblage in an atmosphere to which ammonia has not been added, to about 2000° F. at a heating rate which will denitride metallic nitrides other than titanium nitride before the melting temperature of said brazing alloy is reached, without substantially softening said titanium nitride strengthened plate member, and (d) holding said assemblage at a temperature of about 2050° F. to effect brazing of said fin member to said pre-formed nitride-strengthened plate member.