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Introduction to Furnace Brazing © Air Products and Chemicals, Inc Introduction to Furnace Brazing © Air Products and Chemicals, Inc. 2001 All rights reserved—no part of this document may be reproduced in any form without permission in writing from Air Products and Chemicals, Inc. While every effort was made to ensure accuracy in the preparation of this publication, the material used to compile the information presented was derived from many different sources and intended to serve only as a general guide, and not as a handbook for specific applications. As such, Air Products and the members of its Editorial Review Board assume no responsibility for technical accuracy or omissions due to editing or printing errors. Air Products’ Editorial Review Board Dr. Diran Apelian Executive Director, CHTE MPI Bruce Boardman, FASM Founding Member, CHTE Deere & Co. Roger Fabian, FASM President, ASM Heat Treating Society Bodycote-Lindberg Dan Herring Director of Technology Ipsen International, Inc. Harb Nayar, FASM President TAT Technologies Robert Peaslee Vice President Wall Colmonoy Corp. 3 4 Table of Contents Introduction to Furnace Brazing - 7 Definition of brazing - 7 History of brazing - 7 Differences between soldering, welding, and brazing - 7 Flame brazing vs. furnace brazing - 8 Furnace configurations - 9 A Look at Common Furnace Brazements - 11 Base metals - 11 Typical brazement parts/assemblies -11 Joint design and preparation - 13 Types of joints - 13 Selecting a base metal - 14 Selecting a filler metal - 17 Pre-assembly and fixturing - 17 Wettability -18 Considerations for Furnace Brazing -19 Physical -19 Metallurgical - 19 Furnace equipment - 20 Safety/environmental - 21 Furnace Brazing Technologies - 22 Controlled-atmosphere processing - 22 Vacuum furnace brazing - 29 Other brazing technologies - 30 Troubleshooting - 31 Potential metallurgical problems - 31 Atmosphere-related problems - 33 Summary - 35 Glossary - 36 Bibliography - 39 5 6 Introduction to Furnace Brazing What is brazing? The history of brazing As with brazing, soldering does not involve the The term “brazing” can be applied to any process Brazing is the oldest method for joining metals, melting of the base metals. However, the filler which joins metals (of the same or dissimilar com- other than by mechanical means. Initially, the metal used has a lower melting point (often position) through the use of heat and a filler metal process was most popular for joining gold and sil- referred to as “liquidus”) than that of brazing filler with a melting temperature above 840° F (450° C), ver base metals. Lead and tin, as well as alloys of metals (below approximately 840° F, or 450° C) but below the melting point of the metals being gold-copper and silver-copper, were used as filler and chemical fluxes must be used to facilitate joined. In furnace brazing, temperatures of 2050° metals because of their low melting points. Copper joining. to 2100° F (1120° to 1150° C) and above are not hydrates and organic gums were added later uncommon, especially when brazing stainless because of their reducing action, which helped to In soldering operations, heat may be applied in a steels with nickel-based filler metals or carbon minimize oxidation and improve the cosmetic number of ways, including the use of soldering steel with copper filler metal. Other very high appearance of the joint. Metallic salts were also irons, torches, ultrasonic welding equipment, temperature brazing applications include molybde- used. resistance welding apparatus, infrared heaters, or num with pure nickel as the filler metal and cobalt specialized ovens. A major advantage of soldering with a cobalt alloy filler metal. is its low-temperature characteristic which mini- mizes distortion of the base metals, and makes it A successfully brazed joint often results in a metal- the preferred joining method for materials that lurgical bond that is generally as strong or stronger cannot tolerate brazing or welding temperatures. than the base metals being joined. Modern brazing However, soldered joints must not be subjected to technology has extended the definition to include high stresses, as soldering results in a relatively the bonding of metal to non-metallic substrates, weak joint. including glass and refractory materials. However, this publication is limited to brazing of metals only, Welding, on the other hand, forms a metallurgical and, specifically, furnace brazing of metals. joint in much the same way as brazing. Welding filler metals flow at generally higher temperatures How does brazing join materials? than brazing filler metals, but at or just below the In furnace brazing, the parts or assemblies being melting point of the base metals being joined. joined are heated to the melting point of the filler metal being used. This allows the molten filler Fluxes are often employed to protect and assist in metal to flow via capillary action into the close- Figure 1. Eyeglass frames showing sequence of brazing wetting of the base-metal surfaces. Heating fitting surfaces of the joint and to form an alloy of operations. (Photo courtesy of Handy & Harman) sources include plasma, electron beam, tungsten the materials at the transition point upon solidifica- and submerged arc methods, as well as resistance tion. The base metals do not melt, but they can Later, alloys of brass and copper were introduced welding and, more recently, laser-based equipment alloy with the molten filler metal by diffusion to as filler metals because of their ability to produce and even explosive welding. form a metallurgical bond. higher-strength joints in copper and steel struc- tures, which were also able to withstand high tem- A disadvantage of welding is its requirement for Because the metallurgical properties at the brazed peratures. As brazing technology advanced, many higher temperatures, which melts the base metal joint may differ from those of the base metals, the other filler metals have evolved. at the joint area and can result in distortion and selection of the appropriate filler metal is critical. warpage of temperature-sensitive base metals and Depending on the desired properties of the appli- Differences between soldering, stress-induced weakness around the weldment cation, the brazing operation can be used to impart welding, and brazing area. It is generally used for joining thick sections where high strength is required and small areas of a leaktight seal and/or structural strength, with The joining techniques of soldering, welding, and large assemblies (spot welding) where a degree of excellent appearance characteristics, in addition to brazing have many similarities; however, each base-metal distortion is acceptable. Welding can joining for the purpose of extending section length, process has its own characteristics and specific also cause adverse changes in the mechanical and e.g., in piping or tubing materials. indications for use. Generally, the criteria for metallurgical properties in the base metals’ selecting one process over the other depend on the Heat Affected Zone (HAZ), requiring further physical and economic requirements of the base corrective heat treatments. metals and/or end-use of the assembly being joined. 7 In brazing operations, heat is generally supplied by Brazing as a joining technique has only a few dis- that allows the filler metal to liquify and fill the joint an oxyfuel-type torch (manual or automated), a advantages. As mentioned previously, it may not be gap via capillary action. Heat is removed and the controlled-atmosphere or vacuum furnace, a suitable for extremely large assemblies. Also, met- assembly is then cooled or allowed to cool to chemical dip (salt bath), or specialized equipment allurgical concerns may dictate using an alternate ambient temperature before further processing. using resistance, induction, or even infrared tech- joining method. It must be remembered that the nologies. Brazing is especially well suited to high- physical and chemical properties of a brazed joint Furnace brazing, however, offers distinct advan- volume production (automation) and for joining thin can differ from that of the base and filler metals at tages over flame brazing, especially in the areas of sections and parts with complex geometries. the joint transition, which is heterogeneous as a control, automation, repeatability, and flexibility. result of the molecular nature of the bond. Also, First commercialized in the early 1920’s, furnace Furnace brazing, as opposed to flame brazing in stresses caused by external loads are nonuniformly brazing usually takes place in a controlled gaseous air, does not generally require a chemical flux, distributed. These concerns are especially important atmosphere, in an evacuated chamber (vacuum which gives it a distinct advantage over welding when brazing cold-worked or hardenable steels. furnace), or in a specified low partial pressure and soldering by reducing or eliminating the need atmosphere (partial vacuum). As with flame braz- for cleaning the parts of flux residue. Table I compares the properties of soldered, weld- ing, furnace-brazed parts are heated to a specific ed, and brazed joints. brazing temperature until the filler metal flows. The Brazing filler metals flow at relatively low tempera- brazements are then cooled or “quenched,” usually tures and, thus, may be used with many popular Flame brazing vs. furnace brazing in a different zone of the furnace, or in a separate metals with minimal thermally-induced distortion Flame brazing is a process wherein the heat chamber, to produce the required material proper- of the
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