Q: What is hydrogen furnace brazing?
A: Hydrogen furnace brazing is a technique where brazing is accomplished in a pure hydrogen atmosphere without flux. This process is used for many alloys, including carbon steel, copper and stainless steel. Hydrogen reduces the metal oxide on the surface and allows the brazing filler metal to diffuse or wet in to the base metal. For example, the reduction reaction for copper oxide is represented by CuO + H2 –> Cu + H2O. Since hydrogen is used as a reducing agent to remove the metal oxides, the parts are bright and clean after processing. Hydrogen brazing creates durable, flux-free joints of high structural integrity and allows for hardening or heat treating after brazing.
The ability of hydrogen to reduce the metal oxides on the surface is determined by temperature, hydrogen gas purity and pressure. Hydrogen furnace brazing is done in a pure dry hydrogen gas atmosphere at a slightly positive pressure. The brazing temperature is dependent on the base metal being joined and the filler metal used in the brazing process. The uniform heating in the furnace allows even complex shapes to be brazed with little distortion. Rigorous temperature uniformity is maintained on the part while being heated, drastically reducing the residual stresses by carefully controlling the heating and cooling cycle.
Safety is a concern when using hydrogen, since it is a highly explosive and flammable gas. Oxygen levels are constantly monitored throughout the brazing process to confirm the atmosphere is below the lower explosive limit (LEL). A constant flow of hydrogen is used during the entire brazing process to maintain the reduction reaction throughout the brazing process. The hydrogen gas exiting the furnace is simply burned off in a controlled manner.