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By Dan Kay
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| Table 1. Chemistries of some of the austenitic stainless steels. Values shown are maximums, unless otherwise noted in the table. |
Stainless steels are essentially iron-based alloys with at least 10.5% (or more) chromium added to it. There are many different types of stainless steels available to designers to consider, and austenitic stainless steels, which contain nickel as well as chromium, have been quite popular over the years for use in a wide range of brazement-designs due to their inherent corrosion resistance, brazeability, as well as the fact that they are often non-magnetic and do not need subsequent heat-treatment. These Fe/Ni/Cr alloys, designated as the 300-series of stainless steels, can be hardened by cold-working, but due to the temperatures involved in most brazing processes, are primarily used in the “annealed” (soft) condition in end-use service.
Table-1 shows the chemistries of some of the more popular 300-series stainless steels used for brazing applications.
“Sensitization” of stainless steels
As has been discussed in previous blog-articles, stainless steels used in brazing (or welding) must be able to handle the high temperatures involved in those joining processes without losing any of their corrosion-protection properties. This corrosion protection depends on the presence of a strong, continuous layer of chromium-oxide on the surface of the stainless. But, it is widely known that the chemical bonding of chromium to oxygen is not as strong as the bond between carbon and chromium. Thus, at the elevated temperatures of brazing, any carbon present in the stainless steel will attempt to break up the chrome-oxide bond, steal the chromium, and form a chrome-carbon bond instead. Yes, carbon is a very active ingredient in steels, and at the temperatures involved in brazing (especially the longer cycles involved in furnace brazing), the carbon will readily react with chromium in the temperature range from 800-1500°F (425-815°C) to form chromium-carbides, which quickly tend to migrate into the grain-boundaries of the stainless, thereby greatly altering (depleting) the chromium-oxide layer on the surface. This can quickly lead to surface corrosion (rusting) on the surface of the stainless steel.
Figure 1 shows the relative corrosion resistance of stainless steel based on its chromium content.
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| Fig. 1 is taken from p. 35 of Appendix B in “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. |
When stainless “sensitization” occurs and the chromium content of the steel is tied up by carbon, the effective amount of chromium left in the steel for oxidation protection may decrease dramatically, and therefore the corrosion rate of the stainless might increase. Please understand that I am NOT saying that Figure-1 was specifically developed based on “sensitization” studies, but instead, I’m suggesting that the effect of sensitization (chrome depletion) might lead to a similar trend in corrosion of stainless steels, namely, that it may corrode more rapidly under conditions that it might have nicely survived had there been no chromium depletion due to carbide-precipitation caused by the high percentage of carbon in the stainless steel.
To prevent this loss of corrosion protection, the carbon in the stainless must be strictly controlled, either by limiting the amount of carbon put into the metal, or by adding ingredients into the stainless that are more reactive with the carbon atoms than is chromium. Let’s see how this can be accomplished.
“L” version of stainless steels
Whenever brazing or welding is involved, the typical stainless steels used are the “L” version, which means they are the Low-carbon version of that alloy. As seen in Table 1, instead of allowing up to about 0.08% carbon in a typical 304 or 316 alloy, the carbon in a 304L or 316L alloy is limited to 0.03% maximum (about a third of the standard amount). In my experience, I’ve often seen such “L”-grade stainless steels have only about 0.008% carbon.
The effect of carbon content on carbide-precipitation is further illustrated in Fig. 2. When furnace brazing, a typical furnace cycle may run from 1-hr to 8-hrs long or more. In order to avoid carbide-precipitation, it can be seen from the chart that the carbon-content during such cycles needs to be below 0.03% for safe operation, i.e., operate within the circle whose curve is to the right of the total time you expect to use in your furnace cycle. Thus, if I were going to vacuum-furnace braze stainless above 1900°F (1040°C) for 8-hrs, I would want the carbon-content of the stainless to be 0.030 max.
321-Stainless
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| Fig. 2 shows the time/temperature relationship for carbide-precipitation to occur for various carbon contents in the stainless. Carbide-precipitation takes place in the region to the right of each circle. This chart is taken from p. 37 of Appendix B in “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. |
An interesting alloy was created when W. H. Hatfield of Thomas Firth & Sons (Sheffield, England) invented 321-stainless steel by adding titanium to stainless (in the late 1920’s) to act as a special “getter” to tie-up the excess carbon in the steel. In theory this is fine, since titanium is a very strong carbide former (stronger than chromium in that respect), and will rapidly pull any carbon in the steel to itself and thus prevent the carbon from going after the chromium. As can be seen in Table 1, titanium is added in an amount equal to at least 5-times (or more) than the amount of carbon in the metal. This will indeed be effective in tying up the carbon sufficiently so that no carbide-precipitation will occur.
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.
RECOMMENDATION: 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!
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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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