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Stainless steels are essentially iron-based alloys with at least 10.5% (or more) chromium added to it. There are many different types of stainless steels available to designers to consider, and austenitic stainless steels, which contain nickel as well as chromium, have been quite popular over the years for use in a wide range of brazement-designs due to their inherent corrosion resistance, brazeability, as well as the fact that they are often non-magnetic and do not need subsequent heat-treatment. These Fe/Ni/Cr alloys, designated as the 300-series of stainless steels, can be hardened by cold-working, but due to the temperatures involved in most brazing processes, are primarily used in the “annealed” (soft) condition in end-use service.

As has been discussed in previous blog-articles, stainless steels used in brazing (or welding) must be able to handle the high temperatures involved in those joining processes without losing any of their corrosion-protection properties. This corrosion protection depends on the presence of a strong, continuous layer of chromium-oxide on the surface of the stainless. But, it is widely known that the chemical bonding of chromium to oxygen is not as strong as the bond between carbon and chromium. Thus, at the elevated temperatures of brazing, any carbon present in the stainless steel will attempt to break up the chrome-oxide bond, steal the chromium, and form a chrome-carbon bond instead. Yes, carbon is a very active ingredient in steels, and at the temperatures involved in brazing (especially the longer cycles involved in furnace brazing), the carbon will readily react with chromium in the temperature range from 800-1500°F (425-815°C) to form chromium-carbides, which quickly tend to migrate into the grain-boundaries of the stainless, thereby greatly altering (depleting) the chromium-oxide layer on the surface. This can quickly lead to surface corrosion (rusting) on the surface of the stainless steel. by Dan Kay 

All brazed joints should be inspected after brazing to verify that the parts being brazed will be acceptable to the end-use customer. If the brazed-assemblies are complex, then 100% of the assemblies should be inspected to verify that the parts have been brazed properly and will meet end-use requirements.

This may include visual inspection of the exterior of all surfaces that have been brazed, and it may also involve one or more non-destructive testing (NDT) procedures to verify that each assembly will properly handle the end-use service conditions to which it will be subjected, such as fluid pressure, thermal cycling, or mechanical shock. Sometimes some destructive testing (DT) procedures are mandated, via an appropriate sampling plan, in order to physically examine the internal structure of some of the brazed joints. Is there any danger in using the examination of the cross-sectional microstructure of a brazed joint as an accept/reject criterion for the brazed tubular assembly? Yes, there is! Let’s see why…...by Dan Kay

But carbon-steels present a unique situation to designers and brazing companies, because when being heated all the way up to copper-brazing temperature, the metals will actually go through a temperature-range where the steel will actually be contracting (shrinking) while being heated, and then do just the opposite when cooling, thus potentially causing distortion and/or fracturing of brazements during a high-temp brazing cycle. Such a scenario is illustrated in Fig. 1 where an automotive fuel rail brazement failed to braze properly, because some unique CTE problems associated with carbon-steels was not properly taken into account during the furnace brazing cycle. by Dan Kay 

As mentioned in a previous blog-article, magnesium (Mg), often referred to simply as “mag”, is a highly effective “getter” that is used when vacuum-brazing aluminum. Because Mg is very effective at gettering (reacting with and removing) both oxygen and moisture that may be present in a vacuum-furnace atmosphere during aluminum-brazing operations, it can effectively prevent (or minimize) the reaction of these elements with aluminum, thus allowing aluminum-brazing to occur.

However, magnesium is a highly combustible metal, and when it condenses on the walls of a vacuum-furnace during aluminum brazing operations, extreme caution must be exercised in removing the condensed mag from the furnace walls during subsequent furnace clean-up, so that no sparks are generated which could cause rapid ignition of the condensed magnesium, resulting in explosive combustion, and even death. To prevent this, coating the walls of the vacuum furnace with a “non-stick” surface, may be highly effective. by Dan Kay 

Please note that a braze fillet is actually a casting along the outside of a braze joint that simply shows that the brazing filler metal (BFM) has melted and flowed along the edge of a braze joint. However, it doesn’t tell you if the BFM has adequately penetrated the joint. Caution is therefore strongly advised to anyone attempting to merely use the size of a braze-fillet as an inspection criteria for judging the overall quality of a braze joint. by Dan Kay 

A number of brazing shops today combine brazing and heat-treatment in their vacuum furnaces to join components together and then obtain certain desired base-metal properties in those brazed components via rapid cooling (quenching) immediately after brazing is done, and before the components are removed from the furnace. Thus, the same vacuum-furnace brazing cycle combines brazing and heat-treatment, yielding clean brazed components with special base-metal properties to meet unique end-use service conditions.

Vacuum furnaces today offer a number of options regarding the introduction and use of circulating gases in the furnace hot-zone during brazing processes. Gases typically introduced into the vacuum furnace are either argon or nitrogen, but a number of shops have found success with hydrogen and helium gases as well. Often gases are combined rather than just using one gas. Hydrogen/nitrogen and helium/nitrogen are typical combinations used in some brazing shops. by Dan Kay

Highly complex assemblies, for a variety of end-use applications in such diverse fields as automotive, aerospace, medical, electronics, and tooling (just to name a few), can be effectively made via brazing.

For this to happen, however, each of the individual components in these complex assemblies must be able to be held together in proper alignment, have the appropriate brazing filler metal (BFM) applied to it, and this assembly then moved into a brazing-furnace, where it can be heated until the BFM melts and flows into the joints by capillary action, thereby permanently joining the components together to make a complex assembly. by Dan Kay