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303 stainless steel -- Is it a good choice for brazing?

By Dan Kay

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.

Table-1 once again shows the chemistries of some of the more popular 300-series stainless steels used for brazing applications.

Table-1 sm
Table 1. Chemistries of some of the austenitic stainless steels. Values shown are maximums, unless otherwise noted in the table. Taken “Design Guidelines for the Selection and Use of Stainless Steel” (Handbook# 9014), courtesy of the Nickel Development Institute, and the American Iron and Steel Institute.

303-Stainless

This machinable version of 304-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!

In Fig. 1 we see the relative machinability of a number of stainless steels.. Notice the comparison of the 303 free-machining steels with the machinability of the standard 304/316 austenitic stainless steels. Increases of 25% or more
in machinability can be achieved by using those free-machining grades.

Also, sulfur forms inclusions that reduce the friction forces and transverse ductility of the chips, causing them to break off more readily.

Table-2 sm
Figure 1. Comparison of machinability of various stainless steels. Taken from “Design Guidelines for the Selection and Use of Stainless Steel” (Handbook# 9014), courtesy of the Nickel Development Institute, and the American Iron and Steel Institute.

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.

CONCLUSION: 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.


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Dan Kay - Tel: 860-651-5595: - Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/

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