Polymer-based material for carbon deposition during brazing operations

A process for brazing a workpiece composed of a metallic substrate material having at least one exposed surface region having a metallic brazing material having a liquidus temperature, the metallic brazing material positioned in overlying relationship to the metallic substrate material, wherein the metallic brazing material overlies the exposed surface region defined in the substrate material, the process comprising the steps of:

• applying a thin film of a liquid, molten polymeric material into adhering contact with the exposed surface region of the metallic substrate material such that the metallic brazing material is interposed between the metallic substrate material and the applied polymeric coating;
• allowing the liquid polymeric material to cool and solidify on the exposed surface region;
• raising the temperature of the metallic substrate material to a first elevated temperature sufficient to initiate pyrolysis of the polymeric material and generation of volatilized pyrolysis products and a pyrolytic by-product consisting of a non-reactive particulate residue overlying the metallic brazing material, in contact with an anhydrous exothermic gaseous atmosphere;
• further raising the temperature of the metallic substrate material to a second elevated temperature, the second elevated temperature sufficient to attain the liquidus temperature of the brazing material, the temperature raising steps occurring in a controlled gaseous atmosphere capable of achieving and maintaining fluxing;
• and
• maintaining the metallic substrate material in contact with the anhydrous exothermic gaseous atmosphere at the second elevated temperature for an interval sufficient to permit fusion between the metallic brazing material and the metallic substrate material and to consume at least a portion of the volatilized pyrolysis products of the polymeric material.

A process of producing a brazed double-wall tube comprising the steps of:

• sequentially forming an unbrazed unsealed tube of an exterior circumference from an elongated sheet composed of a metallic substrate material, the sheet having at least one surface with a brazing metal in overlying relationship thereto;
• applying a liquid film of polymeric material to the exterior circumference of the unbrazed, unsealed tube, the polymeric material being a melt-processible thermoplastic material selected from the group consisting of acetal resins, nylon, polyesters, ethylene vinyl alcohol, and mixture thereof;
• allowing the liquid extruded polymeric material to cool and solidify on the exterior circumference;
• raising the surface temperature of the unbrazed, unsealed tube to a first elevated temperature sufficient to trigger pyrolysis of the applied polymeric material, and production of volatilized pyrolysis products and a non-reactive particulate film overlaying the exterior circumference;
• further raising the temperature of the unbrazed, unsealed tube to a second elevated temperature sufficient to trigger fusion between the brazing metal and the metallic substrate material and consumption of at least a portion of the volatilized pyrolysis products, the temperature elevation steps occurring in a controlled gaseous atmosphere.