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303 stainless steel is a machinable grade of 304-stainless steel. As mentioned in my earlier article (about 321-stainless), austenitic stainless steels are essentially iron-based alloys with at least 10.5% (or more) chromium added to it, as well as from 8-12% nickel, have inherent corrosion resistance, are usually very brazeable, are generally non-magnetic, and do not require (or effectively respond to) subsequent heat-treatment after brazing. They are primarily used in the “annealed” (soft) condition in end-use service.

303-Stainless is generally available in either of two chemistries, standard 303, or 303Se. The use of 303Se has apparently decreased significantly over the years, but it is still available. The standard grade of 303 contains a minimum of 0.15-percent sulfur added to its chemistry, the sulfur being added for machinability purposes. Notice in Table 1 that the other grades of austenitic stainless steels all are limited to no more than 0.030 sulfur maximum, which means that regular 303 stainless contains a minimum of six (6) times the usual amount of sulfur that is contained in all the other types of austenitic stainless steel. Remember, that’s a minimum amount; it can be higher! by Dan Kay

A number of companies who are currently using vacuum-furnaces for many of their brazing processes are also using induction-brazing equipment to join some of their other production parts. Let’s take a brief look at the induction-brazing process to see what it is, and how it can be effectively used by brazing shops today to meet some of their production needs.

This article is written without a lot of complex language in order to make this process as simple and easy to understand as possible, and to therefore encourage people to use it more. For a deeper, more thorough engineering-study of the principles and theory of induction heating, the reader is referred to other technical books and articles on the subject. This current article will give you a good, basic understanding of induction brazing, and how to apply it in your brazing shops. by Dan Kay

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