Bright Hydrogen Annealing vs. Vacuum Annealing

Annealing is a heat treatment process which softens stainless steel and other tubing alloys, rendering them more ductile and workable. The metal’s microstructure changes at the atomic level when the tube reaches the prescribed annealing temperature (for 300 series stainless steel, this is 1950ºF). Once 300 series stainless steel (austenitic) tube is annealed, it can only be hardened to a high tensile strength by cold work processes like tube drawing, sinking and swaging for example.

Often, customers ask how the surfaces of tubes are kept looking bright when exposed to such high heat. After all, expose a piece of stainless steel to a clean open flame and the surface will become black, gray and blue---all signs of oxygen bonding with the metal at the surface of the tube. That oxide renders the resistive properties of stainless steel far less effective, nevermind the ugly appearance.

The alternatives are controlled processes in either a hydrogen atmosphere (what is referred to as bright annealed tube) or a vacuum atmosphere where all air (and in turn oxygen) is removed from the chamber before the tubes are heated. Bright annealing is most common because it is faster and more cost effective. Hydrogen is an excellent reducing agent---this means in simple terms that it is highly effective at not only displacing air (oxygen) in the chamber of the heat treating furnace, but it also scrubs any oxides off the surface of the tube, often leaving it looking brighter than when it went into the furnace. Improperly controlled flow of hydrogen or a poor purging process can result in discoloration that is objectionable. Alternative inert gases can be used in place of hydrogen, but the result will be a less shiny tube because these gases will not readily remove oxide. Further, many of those alternative gases cost considerably more than hydrogen, negating them from legitimate consideration.

Vacuum heat treating is a more limited process because the furnace can only accommodate stainless and nickel tubing lengths up to 72”, and the chamber is smaller due to the significant amount of energy required to create and maintain an environment free of air. To meet certain specifications, vacuum annealing may be warranted and can be accommodated. The surface of the tube post heat treatment in a vacuum is comparable to bright annealing, but slightly less shiny.

Both hydrogen and vacuum annealing will yield equivalent tempers/hardness values, and if you require a specific method for your application, please let us know.