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 there are 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.
Soldering is a joining process in which the filler metal melts completely below 450°C (840°F), 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 unmelted base metals being joined, and those base metals must be very clean (free of oils, lubricants, dirt, etc.) prior to being joined.
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) or higher.
Metallurgically, the higher temperatures required for brazing result in very different joint properties in service as compared to soldering. The lower temperatures of soldering usually allow only minimal inter-alloying of the filler metal with the base metals being joined, but 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. If the solder joint is highly stressed in service, failure will typically occur through the solder. In contrast, a properly made braze joint should never fail through the BFM in the joint. Instead, if the brazed assembly is stressed to the point of failure, such failure should always occur in the base metal outside of the brazed joint.
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 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 the processes results in very different behaviors when they are subjected to extreme service conditions.
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