I am often asked about the differences between brazing and soldering. Perhaps this is a good time to describe the two processes in more detail so that readers can understand the significant differences between them.
There are some similarities between soldering and brazing, but many significant metallurgical differences. They are both used to join metals together to form a bond between the metals being joined, but the bonding mechanisms are very different. Let’s take a look at these two processes, and see how they compare.
Most soldering takes place at much lower temperatures than brazing. Typical soldering temperatures may be in the 300-500°F (150-260°C) range, although some types of soldering may be a bit higher, whereas brazing is typically done at 1200°F (650°C) to 2300°F (1260°C). Soldering is a joining process in which the filler metal melts completely below 450C (840F), whereas brazing is a joining process in which the filler metal melts completely at temperatures above 450°C (840°F). Both soldering and brazing use capillary action to distribute the molten filler metal between the un-melted base metals being joined, and those base metals must be very clean (free of oils, lubricants, dirt, etc.) prior to being joined.
Metallurgically, the higher temperatures required for brazing result in very different joint properties in service as compared to soldering. Whereas the lower temperatures of soldering usually allow only minimal inter-alloying of the filler metal with the base metals being joined, brazing often results in extensive interaction between the filler metal and base metal. Thus, the way they behave in service will usually be very different, in at least two ways:
1. Joint strength. In solder-joints, the base metals will typically be much stronger than the solder following the soldering process, and thus if the solder joint is highly stressed in service, failure will typically occur through the solder itself. A properly made braze-joint, however, should never fail through the BFM in the joint, but instead, because of the metallurgical alloying that has occurred at the much higher temperatures involved in brazing, any failure of a brazed assembly in service should always occur in the base metal outside of the brazed joint. At the higher temperatures involved in brazing, the metallurgical reactions and alloying are much more intense than those that occur at the much lower temperatures of soldering, thus well made brazed joints usually approach or exceed the strengths of the base metals being joined.
2. Fatigue resistance. A properly brazed joint (assuming the joint has been properly designed) should always be able to handle the stresses and fatigue placed on the joint by thermal cycling or mechanical shock, whereas solder joints under similar conditions would normally be expected to fail through the solder joint, since the degree of alloying with the base metal is usually so much less with soldering than with brazing.
Thus, care must be used when trying to compare the two processes of soldering and brazing. Although there are a number of similarities in the way parts are processed by these two joining methods, the very large temperature differences between these two joining processes result in very different behaviors when they are subjected to extreme service conditions.
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at firstname.lastname@example.org, and his website can be visited at: http://www.kaybrazing.com/
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