A typical outdoor liquid nitrogen tank.
In AMS 2675G (“Brazing, using Nickel-Alloy Filler Metal”), paragraph 3.3 states that acceptable atmospheres for nickel-brazing are hydrogen, argon, or vacuum. No mention of nitrogen. People have asked me why nitrogen is apparently not allowed for furnace brazing with nickel-based brazing filler metals (BFMs), or for building up partial-pressures in a vacuum furnace for subsequent brazing.
Answer: Nitrogen, like hydrogen, can be reactive towards some of the metallic components in the base-metals and in the liquid BFM during brazing processes. As a safe-guard against any such problems, AMS 2675 excludes nitrogen from its list of acceptable furnace atmospheres for nickel-brazing.
Will nitrogen ALWAYS be a problem in nickel-brazing in a furnace atmosphere? NO! Please be aware that the exclusion of nitrogen is a “general safety” recommendation (suggestion), and is not to be taken as a prohibition against nitrogen for any and all nickel-brazing. Let’s take a closer look……
Here are a few of the issues that some people have expressed concern about when talking about the use of nitrogen atmosphere when brazing with a nickel-based BFM, and my response to those concerns:
I’ve heard a number of people express the concern that brazing in a nitrogen atmosphere will always cause nitriding of the parts being brazed. No, that is NOT correct! The mere use of nitrogen in a brazing furnace will not automatically result in “nitriding” of the metals being brazed! That’s a misconception that many people have. Nitriding is a process that uses ionic-nitrogen (N1), whereas the nitrogen you feed into a vacuum brazing furnace for quenching, rapid cooling, or for partial-pressure buildup, is molecular-nitrogen (N2). The molecule of molecular nitrogen must be broken up into ionic-nitrogen for effective nitriding to occur. To accomplish this, a combination of high-temperature and a special catalyst (something that will break the molecule into 2 separate atoms of nitrogen) is actually needed. Most brazing operations that I’ve been part of don’t have both of those necessary requirements present. Over the years I’ve furnace brazed many different components in nitrogen atmospheres with no problems whatsoever. So people should not, in my opinion, be overly concerned about nitriding the parts they are brazing just because there is a nitrogen atmosphere present.
Someone might then ask: “Okay, but isn’t nitriding and so-called nitrogen-embrittlement the same thing?” No, they are not. First of all, as just discussed, nitriding involves ionic-nitrogen (N1), whereas “nitrogen-embrittlement” involves molecular-nitrogen, and can occur during the manufacture or processing of certain base-metals that are process-annealed in a dissociated-ammonia atmosphere (a blend of hydrogen and nitrogen) or in a high-temp pure nitrogen atmosphere.
At high temp, when the atomic lattice-structure of the base-metals have opened up, the tiny atoms of ionic-nitrogen can easily move into the open spaces (interstices) between the base-metal atoms, and penetrate into the metal for the purpose of nitriding. It is much more difficult, however, for molecular-nitrogen (N2) to do so, since the molecule is twice as large as the single atom, and thus is pretty much limited to only infiltrating the top reaction layer on the surface of the base metals during brazing. Heating these metals to an intermediate temperature (above 1000°F) prior to brazing, should effectively remove the nitrogen from the surface of the metal, and allow excellent brazing thereafter.
Please note that such penetration by nitrogen (ionic or molecular) is NOT going to happen every time you try to nickel-braze in a nitrogen atmosphere. I merely suggest it MIGHT happen occasionally, and is something to be aware of. I’ve nickel-brazed many, many 304L stainless components in nitrogen atmospheres over the years, and have NOT had problems with such brazing.
So, if the metals you wish to braze have been previously processed in a nitrogen atmosphere prior to your nickel-brazing of those same parts, you may want to heat those components to an intermediate temp to remove any surface nitrogen prior to your actual brazing run. But, if the parts are free of nitrogen prior to your brazing run, then I don’t personally think you need to be overly concerned about nickel-brazing them in a nitrogen containing atmosphere. But of course, there are always “exceptions to the rule”, so to speak, and the following paragraph describes one of those exceptions.
Having described a little bit about the nitriding process, and how it is rare in most brazing processes when using a nitrogen atmosphere, let’s look at a specific situation were the combination of high-temperature and a nitrogen furnace-atmosphere and a specific base-metal/BFM element (“catalyst”) might cause some concern when nickel-brazing. It involves the use of nickel-based BFMs that contain the element “boron”.
AMS 2675 allows the use of a number of different nickel-based BFMs for nickel-brazing, including those BFMs that contain boron as a temperature-depressant. Boron is a highly effective temperature-lowering addition to a number of nickel-based BFMs, and because of its tiny size (its an interstitial atom, not a substitutional atom), boron can readily react with any compatible element it encounters, such as nickel, chrome, or nitrogen. No problem with nickel or chromium.
But there can be a potential problem when boron encounters nitrogen. Boron has the potential to disscociate molecular-nitrogen (N2) at the high temp of nickel-brazing, forming molecular nitrogen (N1) with which it can react to form boron-nitrides (a brazing stop-off). Thus, if you were brazing with a BFM such as AMS 4777 (which contains about 3.5% boron in its chemistry), and were heating and brazing the part in a nitrogen atmosphere, the nitrogen might react with the boron in the BFM to form a boron-nitride residue (black in color) on the surface of the metal, thus depleting some of the boron needed in the BFM to lower the melting point of the BFM. When this happens, the brazed-component, when removed from the furnace, may show some dark residue around the joint area, a lack of full melting of the BFM, and thus an incomplete BFM-flow in and around the joint.
3. Titanium-Nitride (et al)
There are a number of base-metal elements that can act as a catalyst to form nitrides, but not many have any real significance for brazing. The formation of titanium-nitrides and aluminum-nitrides may be of concern, but silicon-nitride formation (silicon is one of the ingredients in nickel-based BFMs) has apparently not been an issue.
Therefore, if you are brazing certain base-metals that contain small amounts of titanium and/or aluminum (such as Inconel 738 for example), and your furnace atmosphere during brazing contains nitrogen, then there might be problems with brazing, and such metals should be brazed only in vacuum (not in nitrogen, nor in pure dry hydrogen, or argon)! That’s because, as many of you are already aware, titanium is highly sensitive to the presence of any oxygen in any furnace atmosphere. Therefore, because all gaseous furnace atmospheres contain some moisture (measured as the dewpoint of that gas), and moisture represents oxygen in the atmosphere, it is never recommended that titanium-containing base metals ever be brazed in a gaseous furnace atmosphere, but should be strictly limited to vacuum furnaces that have very clean hot zones with very low leak-up rates.
4. Nitrogen Quench in Vacuum Furnaces?
Some people have asked if there is a potential problem with nickel-brazing in a vacuum furnace that uses nitrogen gas to rapid-cool or quench parts after brazing? No, there should be no problem whatsoever!
Note that when rapid-cooling or quenching, the nitrogen gas is not introduced into the furnace until after the brazing has already been completed. Thus the nitrogen atmosphere cannot have any effect on parts that have already been brazed prior to the introduction of the nitrogen gas to the system!
AMS 2675 is probably wise to refrain from placing nitrogen in the list of allowable atmospheres for high-temp nickel-brazing in a furnace, but only because it’s a safe precaution to prevent accidental use of nitrogen in a marginal
situation where boron in the BFM, or certain reactive components of the base-metal, might cause a problem.
Nitrogen should NOT be looked on as having been eliminated from the list of atmospheres because “nitrogen will always nitride the base metals and prevent nickel-brazing”. Such a statement would be highly misleading and inaccurate.
Remember — there is nothing inherent in nitrogen that makes it “completely unacceptable” for nickel-brazing in a furnace. It is only because of the potential for certain problems that might arise in a few types of scenarios that the
committee responsible for AMS 2675 felt it wise to NOT list it the spec as a recommended atmosphere.
Important: Please notice and understand that AMS 2675 does NOT “forbid” the use of nitrogen for any nickel-brazing situations. That’s because there are many situations where nickel-brazing in a furnace atmosphere of nitrogen will NOT cause any problems, and can be used.