Over the years the brazing and welding industries have noticed something strange that sometimes happens when joining 304-stainless steel assemblies for a wide variety of applications exposed to outdoor weather. They noticed that sometime after the weldment or brazement was placed in service in situations where the stainless-assembly was exposed to moisture (such as in outdoor applications for automotive, aerospace, and tooling applications, etc.), the stainless steel started rusting, as if it were made from a regular carbon-steel rather than stainless-steel.
In the weldments, as shown in Fig. 1, the rust was limited to a rust-band up to about a half inch wide (a centimeter or more), located about that same distance away from, and parallel to, each side of the weld (i.e., along both sides of the weld). In the brazements, which has been furnace-brazed, the rusting was more general, generally spread over the entire exposed surface of the furnace brazed component.
In our discussion here, I’ll be using 304-stainless as my example. In reality, it can occur with other high-chrome containing base metals that also contain significant amounts of carbon (about 0.006% or more).
Stainless steels stain less than regular carbon steels because the chromium content in the 304-stainless (about 18% of the stainless’ total chemistry) reacts very readily with oxygen to form a tenacious and an even covering of corrosion-resistant chromium-oxides on its surface.
Therefore, care must be taken when joining 304-stainless components that are to used for applications requiring good corrosion-resistance in service. Austenitic stainless steels, such as 304, which contains both chromium and carbon, must be properly heated during welding or brazing to be sure that the heating cycles used do not cause carbide-precipitation (CP) to occur, since that can then bring about loss of corrosion resistance of the component.
How does CP occur within the 304-stainless?
CP may occur when the heating and cooling cycles used during the welding/brazing cycle allow the carbon and chromium in the base metal to chemically and metallurgically react with each other to form chromium-carbides. The thermodynamics of this process are such that during high-temp thermal processing of the stainless, such as during a high-temp furnace brazing cycle, the following things happen:
1. Carbon becomes a more desirable “partner” for chromium than does oxygen. Thus, the chromium-oxides can break apart, and the freed-up chromium can link up with carbon to form a chromium-carbide. Picture it in your mind as a “dance-team”, in which chromium and oxygen are dancing together on the dance floor. The carbon then comes over and taps oxygen on the shoulder and says “Let me take over this dance”, then steps in, replacing oxygen in the dance-team, and now chromium and carbon are dancing together.
2. These chromium-carbide teams, however, don’t stay on the dance floor, and instead, make their way off the floor, and out the doors, so to speak. Going out the door is actually their going out into the grain-boundaries of the 304-stainless structure. Thus, with less and less chromium-oxide dance-teams on the dance floor, so to speak, this process can result in loss of corrosion-resistance of the stainless component in service through intergranular-corrosion in particular, since the formation of the chromium-carbides tends to occur most rapidly in the grain-structure of the stainless that is nearest to, and along, the grain-boundaries, as shown in Fig. 2.
3. This reaction occurs between 800-1500°F (425-815°C)
4. Dwelling within this temperature range results in “sensitizing” the stainless, i.e., causing loss of chromium-oxides and formation of chromium-carbides instead, and is known as “sensitization of the stainless steel”, an undesirable process.
Thus, the cause of all this trouble is the fact that you are brazing a standard grade of 304-stainless that has a high carbon content in it, and you are bringing the stainless up to a temperature where all that carbon can chemically disturb the “delicate-balance” in the stainless and cause loss of corrosion-resistance.
So, how can you prevent this corrosion-loss when brazing 304-stainless steel? It’s not difficult. The problem can be solved by:
1. Heating/cooling very rapidly through the sensitization range, so that there is not enough time for the carbides to preferentially form.
2. Use a very low carbon grade of stainless (such as 304L, or 316L), in which the “L” means “low carbon” version of that steel. Whereas standard 304 has approximately 0.08% carbon in its chemistry, the 304L has typically no more than a quarter of that (often 0.01% or so, but no more than 0.03% max).
3. Use a “stabilized” grade of stainless, such as 347 (but not 321). 347-stainless uses Niobium to “tie-up” the carbon preferentially, so that the chromium is not depleted. 321-stainless uses titanium to do the same thing, but titanium has it’s own issues with brazing that cause it to be a non-preferred type of stainless for use in brazing! See. Fig. 3.
4. It is also possible to restore corrosion-resistance to brazed 304-stainless steel assemblies that have become sensitized by heating the parts up to 1850-2050°F (1010-1120°C), and then rapidly cooling to temps well below those of the “sensitization” range.
RULE OF THUMB: You should ALWAYS specify the low-carbon version of 304 or 316 when designing/building stainless assemblies that are to be brazed, especially if furnace brazing is to be employed. NEVER specify standard grades of 304 or 316 when welding or brazing. Yes, it may cost a bit more to get the correct grade of stainless for brazing (i.e., the “L” version), but the improved corrosion-resistance of the parts in service is worth it.
CAUTION: I’ve seen situations where purchasing-personnel have NOT been properly trained in the difference between metals such as 304 and 304L, and when asked by the engineers to buy 304L, they bought the wrong stainless, resulting in parts that performed poorly in service from a corrosion point of view. When asked later why they did not buy 304L, the buyers indicated they did indeed get what the engineers wanted. When challenged on that point by the engineers, the buyers told them: “You asked us to get 304 and that’s what we got. “ When the engineer replied that they did not buy 304L, the buyer responded with “304, 304L, they’re both 304. And the 304 I bought was less expensive”.
Like anyone else, buyers must understand what you need. It is YOUR job to be clear to the buyer what the difference is between 304 and 304L so that they understand what it is you’re trying to get!
Otherwise, stainless components may literally start to rust out in the field within a short period of time after being placed in service, and who do you think will “look bad” when that happens? Think again……
Footnote: Ed Craig is an expert in MIG/TIG welding, and lives in Asheville, North Carolina. He can be reached at his company, Welding Reality via e-mail, as firstname.lastname@example.org, and by phone at 828-658-3574.
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 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. Dan can be reached via e-mail at email@example.com, and his website can be visited at: http://www.kaybrazing.com/
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