trebor wrote:Regardless if it is the best choice, do you agree that 416 and 17-4 have no lower corrosion resistance after being properly nitrocarburized as they did before the process?
Good question. In fact, the links were to a relatively recent patent for salt bath nitriding of stainless through the use of a molten cyanate
salt, which is different from the classical molten cyanide
salt treatment. Essentially, if you put a part in this new bath at the right temperature you should only get Nitrogen diffusing inward (the balance of the cyanate
gassing off as Carbon Monoxide). Thus, we're talking about nitriding here, not nitrocarburizing. Note the title on the patent:
Low temperature nitriding salt and method of use
So, the more interesting question is why does one get better and the other worse? Well, let's review:
The 304 actually is unaffected at 750 F and only minimally degraded at 850 F. All treatments higher than that absolutely ravaged it.
The 416 actually is unaffected by the shorter time treatments but is improved with longer time treatments. Note that no corrosion data is given for the 750 F and 850 F treatments at all. Interestingly, the longer time at 950 F seems to give the best result.
So, what is happening here? In both cases temperatures below 1000 F seem to be the best improving or the least damaging. Higher temperatures seem to help less or actually make the problem worse. What is it about roughly 1000 F that changes the game? The answer lies in the cyanate
anion's decomposition products.
breaks down it forms Nitrogen and Carbon Monoxide. The nitrogen rapidly diffuses into the steel but the Carbon Monoxide does not. However, the Carbon Monoxide molecule can itself be decomposed. In fact, its decomposition occurs at roughly 550 C (1022 F) when CATALYZED by the presenc of iron (the number one ingredient in all steel, stainless included). It decomposes to form Carbon Dioxide (which gasses off) and solid Carbon, which promptly diffuses into the part.
So, this method at low temperatures is JUST a nitriding treatment. At higher temperatues it also does a bit of unintentional CARBURIZING. It is still a far cry in terms of carbon input when compared to carburizing (or carbonitriding), but it does occur.
But why should two different grades of stainless react to small amounts of carburizing so differently? The key lies in the crystal structure. The 304 is an austenitic (face-centered cubic crystalline structure) steel and as such the carbon is very soluble in it. As the carbon comes in it nicely dissolves. Meanwhile, the carbon already present gets all mobile due to the thermal energy being poured in. That carbon starts tying up the chromium alloying content as chromium carbides and voila, no more stainless steel (or at least greatly reduced corrosion resistance). This is EXACTLY why if you intend to weld 304 it is SO IMPORTANT to instead use 304L (the "L" indicates very low carbon content) so as to prevent weld area sensitization. Basically, heating up an austenitic stainless steel that has carbon in it ruins it. In the case of welding it only happens in the heat affected zone. In the case of a part submerged in a molten salt bath it happens to the whole bloody part.
On the other hand, 416 is a martensitic stainless steel. In point of fact, it likely never changes crystal structure at any of the reported temperatures and thus remains as martensite (body-centered tetragonal crystalline structure) throughout the processing. The key here is that Carbon is very much NOT soluble in martensite (martensite itself being a form of highly distorted/stressed ferrite due to the trapped carbon trying to precipitate out). Thus, any Carbon that might try to diffuse in has to go interstitial and form iron carbides with the small amount of free ferrite running around. This strengthens the surface (which mechanically aids corrosion resistance by keeping down the "rust bubbling" effect) without allowing a substantial amount of the dissolved chromium to get poisoned too much. Thus, 416 pretty much remains corrosion resistant chemically and picks up some mechanical help, too. Since the mechanical help is directly proportional to case depth, longer times help more. Longer times at temperatures just below where Carbon Monoxide can decompose help most of all.
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