Now, were these 321-stainless steels strictly to be used only at ambient or room temperatures for making architectural trim, or automotive formed-products, etc., that would be fine. But many designers have opted for the use of 321-stainless steel in high-temp brazing applications, which means the 321-stainless will see temperatures as high as 2000°F (1100°C) or higher. At such temperatures, yes, there is enough titanium in the steel to tie up the carbon. But, since the amount of titanium added is at least five times the amount of carbon, what happens to all the rest of the titanium added to the steel? Some of that excess titanium may react with the oxygen tied to the chromium, or merely react with any free oxygen in the furnace atmosphere, or with the oxygen portion of any water molecules in the moisture in the furnace, to readily form titanium-oxides, which are much more robust than chromium-oxides.
Titanium-oxides, once formed, cannot be removed from the metal surfaces during brazing, and may strongly interfere with brazing, since there may now be a thin layer of tenacious titanium-oxide on the surface of the stainless that you want to braze, thereby preventing wetting of the metal surfaces by the brazing filler metal (BFM).
Yes, I know of brazing shops that have had lots of difficulty brazing 321-stainless because of this titanium content. Remember, the amount of difficulty found in brazing of this base metal will be directly dependent on the amount of titanium added by the manufacturer, as well as by the degree of cleanliness and leak-tightness of your vacuum furnace.
Thus, if you have a very clean vacuum furnace with a very low leak-rate (5-microns per hour or less), you may find that you can braze 321-stainless parts with no real problems (assuming the amount of titanium added to the stainless was not that great). But, if your vacuum furnace is marginal (cleanliness or leak-rate) you may find that you always seem to have problems trying to braze 321-stainless.
Use 304L, 316L, or 347 stainless steels for your brazing applications rather than 321. There have been too many problems over the years with 321-stainless in too many shops when it comes to brazing. Designers really must get to understand this, and stop using 321-stainless in their brazing designs, using one of the other very stable austenitic stainless steels instead.
Next month: We’ll look at the use of 303-stainless, and ask the same questions about why it is used, and why it’s often a problem in brazing!