Question: We are brazing a couple of 316L sheet-metal parts together with a braze overlap equivalent to about six times the base-metal thickness. Our customer wants us to qualify to the AWS B2.2 spec, which requires (in their Table B.1) that it must have 95% of the tensile strength of the base metal when the joint is tested in tension. We have brazed samples with braze gaps from 0-0.002 inch at temperatures up to about 1975°F, holding it there for 90-105 minutes and then further diffusing the joint at 1600°F for two hours! Tensile samples still failed at strength levels much lower than the 95% requirement, and they never will pass. Can we ever get 95% of original base metal strength after brazing?
Answer: People occasionally ask me for information about how to determine the strength of their brazed assemblies, which involve different types of base metals and different kinds of brazing filler metals (BFMs). The answer that I always give to their question is: “What is the yield strength of that particular base metal in its annealed condition?” A properly brazed joint should always fail in the base metal and not in the brazed joint.
Why do I ask people this question? Because high-temperature brazing processes will always soften (anneal) the base metals being joined, and unless the base metal is heat treatable (austenitic stainless steels are not heat treatable), the softened base metal will yield first and break before the actual brazed joint does. At what strength level? At the yield-strength level of the base metal in its annealed condition.
“Yielding” of metals gets into complex discussions of preferred slip planes, dislocation movement, etc. Simply put, however, the very thin brazed joint just does not physically have enough BFM in it for slip planes to operate effectively. Instead, the BFM material must be pulled apart (i.e., ripped) by breaking metallurgical bonding rather than merely having atoms/molecules slide over other (elongate) like the more massive base-metal components can.
The base metals involved in the brazement can much more easily allow slip planes to operate with their much-more-massive structures (compared to the very thin brazed joint). Normal yielding (stretching) of the base metals can occur at strength levels clearly associated with the annealed condition of those base metals. Such information is readily found in most metallurgical handbooks for a wide variety of base metals.
Following brazing, the yield strength of the base metals in their annealed condition is a much more realistic strength level to specify for brazements than the tensile strength of the base metals in their as-rolled/drawn condition prior to brazing.
As a long-time member of AWS (almost 50 years) and an advisor to the AWS brazing-specification committee, it is my firm opinion that the requirement of Table B.1 in the AWS B2.2 spec should read “yield strength” rather than tensile strength. The tensile numbers shown in Table B.1 will NOT be reached when dealing with BFMs containing phosphorus (such as BNi-7, BNi-12, etc.) if such BFMs are used to braze ANY kind of ferrous metals. If you wish to braze a stainless using a BFM that contains NO phosphorus in it, then the numbers called for in the B2.2 spec are still a real stretch and, in my opinion, unrealistic. It certainly will NOT be achievable with phosphorous-containing BFMs on ferrous metals, as has been demonstrated over the years, both here in the U.S. as well as in European labs.
316L stainless steel has a yield strength of about 40,000 psi and a tensile strength of about 90,000 psi in its annealed state. Thus, if 316 stainless has been brazed, the assembly, under stress, may begin to yield (stretch) if external forces exceed about 40,000 psi on the assembly. I always suggest to people that this should be considered its “fail point” since the assembly is indeed failing if it is yielding (i.e., stretching) . I don't want it to completely break (tensile failure) before I call it “failing.” But tensile strengths of 316L stainless brazement joints using a phosphorus-containing BFM will never (in my opinion and in my experience) be able to reach or exceed the 95% of tensile-strength requirement shown in AWS B2.2, Table B.1.