Our customer used to send us assemblies made from 304 stainless steel for us to furnace braze, using pure copper as the brazing filler metal. The customer now wants us to use a 400 series stainless, specifically 409, due to its alleged better corrosion resistance at elevated temperatures and also because it can save money due to its much lower chromium and nickel content. Can this change from 304 to 409 affect our furnace brazing?
Be very careful when switching from a 304 stainless steel (austenitic) to a 409 stainless steel (ferritic) if you are brazing!
Some titanium (Ti) is often added to 409 stainless as a minor alloying addition, at a level of about six to eight times (6x to 8x) the expected carbon content. So, if the carbon content in the 409 is controlled to around 0.08% max, then there could be at least 0.50% or more of titanium in the 409 as well. Such a high level of titanium can indeed have a negative effect on the brazeability of the 409.
This situation is very similar to that often encountered with 321 stainless in that the titanium content added to each of these two base metals is primarily designed to tie up any extra carbon in the metals so as to prevent the formation of any chromium-carbide precipitates in the grain boundaries.
However, this excess titanium can hurt brazing because any excess titanium that is not tied up with carbon or nitrogen will then tie itself up to any available oxygen, thereby forming titanium oxides on the surface of the stainless. Since titanium oxides are very stable at brazing temps, they can prevent brazing filler metal (BFM) flow and are evidenced on the surfaces as a straw-color or a light gray color rather than a nice shiny look when coming out of the furnace after brazing.
So, if your customer wants to switch away from the 300 series and go to the 400 series, you should be sure that they use an alternate grade of 409 stainless, namely 409Nb, a grade that uses niobium (Nb) rather than Ti to tie up the carbon. The Niobium (Columbium) content in the 409Nb is very brazeable. Some suppliers have indicated that it's also cheaper than the standard grades of 409 with titanium.
Another factor to consider with 409 is that upon heating to high temperatures, such ferritic steels tend to exhibit grain growth and subsequent loss of ductility. So, if you were to try copper brazing of this alloy, you'd be high enough in temp (around 2050F/1100C or so) that this grain growth issue could become a real problem.
Do some test brazing with samples of 304 and 409 run in the same furnace load. Then, carefully analyze cross sections of these brazed samples to determine BFM wettability and flow and if grain growth will be an issue.