Now, were these 303-stainless steels strictly to be machined and used at ambient or room temperatures, with no high-temp processing involved, then its use might be considered to be okay. However, many designers have opted to specify 303-stainless steel for machined components that will be used in high-temp brazing applications, which means the 303-stainless may see temperatures as high as 2100°F (1150°C) or higher during brazing. At such temperatures much of the sulfur may completely volatilize, forming bubbles (voids) in the joint, or perhaps this outgassing might combine with any hydrogen present in the brazing-furnace atmosphere to form hydrogen-sulfides (which can smell like rotten eggs). Additionally, if any sulfur combines with nickel at high temp, it can significantly weaken the grain boundaries in any of those nickel-containing base metals (hot short condition).
A number of brazing shops have had difficulty brazing 303-stainless components because of this sulfur content. Schrader-valves in automotive fuel-rail applications are a prime example. Remember, as with the titanium added to 321-stainless, the amount of difficulty found in brazing 303-stainless may be directly dependent on the amount of sulfur added by the manufacturer, and by the amount of oxygen and/or hydrogen present in any atmosphere used for hi-temp brazing.
Note: one option to consider might be to “bake out” the 303 valves when you receive them, and before you assemble them onto a component that will be brazed. By sending each valve through the furnace one or two times along with regular production parts, you may be able to “condition” them sufficiently so that when they are assembled onto the parts for brazing, they will not cause any outgassing or wettability problems. It is important that the valves be “bright” when they come out of the furnace. If they are dull, or grayish in color, they should be run through a furnace cycle again. If this works out okay to “clean them up” this way, you might then be able to stay with the 303 valves, and condition the surfaces adequately to allow for brazing. But this does not always work. You have to run a few to find out if it will work in your particular furnace.
It should cost you nothing to send the valves through the furnace along with regular production (only put a small number of valves through the furnace at a time, and keep them well spaced along with regular production loads in the furnace). By doing this, the tiny amount of sulfur that will outgas should have no effect on the brazing atmosphere in the furnace.
Therefore, I suggest that you try this bake-out procedure on a few valves to begin with, and then try them in your brazing process (IF the valves are able to come out bright from these clean-up runs). If you are happy with the brazing results, then you can consider sending a few valves at a time through the furnace with your regular production, as part of your regular production procedures, until you have satisfactorily cleaned up all the 303-fittings.
303Se – Stainless
The addition of selenium (Se) to 303 may be done instead of using sulfur, in order to eliminate any of the sulfur issues just described, since 303Se limits its sulfur content to 0.15 percent maximum, just like all the other austenitic stainless steels. Type 303Se has machinability rates that are approximately 25-30% faster than 304-stainless, and it is more corrosion resistant than regular 303.
However, at high temps, selenium has its own problems, since selenium is an oxygen-getter, and can form oxides, which in turn can causes de-wetting of any brazed surface, causing the brazing filler metal (BFM) to ball-up, or form ugly little lumps on the surface The higher the selenium content, the more de-wetting (un-brazing) can occur.
Some aerospace jet-engine manufacturing operations have discovered this over the years, and now place strict limits on the amount of Se allowed in any of the alloys they use in their production, as well as in the BFMs they allow to be used to joint those production components.
Recommendation: The use of a machinable grade of stainless steel may be great for producing parts that require a lot of machining, but such grades of machinable stainless may prove difficult to braze. Therefore, designers need to be made aware of this.
If a design calls for use of an austenitic grade of stainless, such as 304-stainless, then instead of using 303 or 303Se for any machined stainless component that will be brazed to the 304-stainless part, consider the total manufacturability of the entire part (including its need to be brazed), and perhaps stay with 304-stainless for the machined part as well. But to prevent work hardening and galling of the 304-stainless in machining, it is advisable to
use a slower machining speed, and a deeper cut.
Unfortunately, the additives that are put into stainless steel to make them more machinable apparently all have negative potential consequences for brazing. Those additives typically are sulfur, selenium, or phosphorus (the use of lead as a machinability additive has pretty much stopped). Always consider the effects of using machinable grades of stainless in high-temp brazing processes, and because those additives have a history of hurting brazing, give strong consideration to taking a bit more time to machine standard grades of stainless (so that the parts can braze well) rather than running the risk of hurting the brazing process with all its subsequent scrap-generation, lost time, re-braze requirements, etc.