What is Brazing?

Brazing is a process where two or more metal items are joined together by melting and flowing a filler metal into the joint.  During the brazing process, a brazed joint is formed by the filler metal melting and flowing via a capillary effect into the pores of the closely fitted surfaces of the joint to form an alloy of the metals upon solidification.

Preparation for Brazing

The key to successfully achieving a good brazed connection is surface preparation. The presence of contaminants or oxides prevents the filler metal from coming into contact with one of the surfaces to be brazed. In the case of minor oxidation, the pores of the surfaces to be brazed will be sealed by the oxide. This prevents the capillary action and, ultimately, the brazing from occurring. Hence, the initial cleanliness of the surfaces to be brazed is extremely important, but it is equally important that the cleanliness of these surfaces be maintained during the process.

Achieving and maintaining the necessary level of cleanliness is much more difficult for brazing stainless steel components than brazing steel components. The Chromium in the stainless steel forms a much more stable oxide at a much lower Oxygen level than iron.

The oxides present on the surface must be reduced prior to the part reaching the melting temperature of the filler metal. The reduction is typically achieved through a reaction of Hydrogen with the Oxygen present in the oxide to form water vapor.

The presence of too much water vapor or Oxygen in the system will prevent the reaction from proceeding. The dew point is used to determine the amount of water vapor in the system at given conditions. The dew point is the temperature at which an amount of water vapor in the system will saturate the atmosphere. The typical dew point required for brazing stainless steel joints in 100% Hydrogen is –50º F.

BRAZING ARTICLES

A REVIEW OF THE FUNDAMENTALS OF STAINLESS STEEL BRAZING IN CONTINUOUS STYLE, CONTROLLED ATMOSPHERE BRAZING FURNACES

Stephen L. Feldbauer, Ph.D.
Abbott Furnace Company

The use of continuous style, controlled atmosphere brazing furnaces to braze stainless steels is becoming more common as the aerospace and automotive producers increase the use of stainless steel components on existing designs and explore new designs. Although much of the metallurgy and thermodynamics of Iron, Chrome, Nickel and their compounds are well understood, a review of the application of the fundamentals to the current practices and equipment technology can help with improved product quality, expanded applications, and the optimization of costs for a net improvement in competitiveness.

STAINLESS STEEL BRAZING

Stephen L. Feldbauer, Ph.D.
Abbott Furnace Company

Atmosphere integrity and control are key to the successful brazing of stainless steel components. Due to the very high affinity that stainless has for Oxygen at high temperatures, the presence of Oxygen or moisture in the furnace will oxidize the surfaces to be brazed and result in a defective braze. Vacuum furnaces and Humpback furnaces have been the traditional systems used to braze stainless steel because of their ability to ensure an Oxygen-free atmosphere with a very low dew point. However, both furnace technologies bring with them issues that result in higher operational costs, increased maintenance costs, and other process related costs that can be avoided by using straight through continuous belt furnace technology.

Recent advances in furnace design and atmosphere control have made it possible for stainless steel to be brazed in straight through continuous belt furnaces. This step forward in technology now permits continuous processing at lower operational costs, less maintenance and higher yields than realized with traditional brazing systems. Oxygen levels of less than 10 ppm and dew points as low as –85ºF are common in a state-of-the-art straight through continuous belt brazing furnace. The adoption of this technology by some of the leading producers of brazed components is allowing the industry to become more competitive.

Hump Back Brazing Furnace by Abbott Furnace Company

ABBOTT BRAZING FURNACES

Abbott Furnace Company custom designs and manufactures continuous belt brazing furnaces in both straight or humpback designs.  Brazing furnaces are used in manufacturing process where there is a need to join simple or complex designs of one joint or multi-joint assemblies.