Excess High Temperature Alloy Carburization

www.en.wikipedia.org/wiki/Carburization

I know a little about this, as I often work with nickel alloys, but it's late and I must go to bed now. I'll put my thoughts together and post sometime tomorrow. Help me though, what is a carbon/graphite particulate containing environment? What are you trying to make?

In this case small amounts of graphite particulate, in mostly helium as the fluid, with some other trace contaminants such as CO, CH4, H2O.

But in general, any industry's info on real experience with long term carburizing environments with negative consequences, such the Wikipedia article cited by bwire saying "known as precipitation strengthening — greatly improves the hardness but normally to the detriment of the host metals corrosion resistance".

Essentially you creating a case hardening situation, wouldn't be it fruitful to enquire as to the effects of over carburization in the case hardening production?

Interesting take on this, and perhaps a more controlled situation than in-place general industrial experience. I'll do some research from this perspective.

I've seen carburization resistance comparison data, but haven't known what to do with the mgm/sq.cm. and test duration time absolute value data. Those who are carburizing on purpose ought to know.

Thanks

You betcha!

this is a complex argument. every materials have different resistance to oxydation and carburization. You 've a particualr alloy in mind?

S.

Corrosion Control & Corrosion Protection

Under consideration are high temperature alloys such as 617, 230, 800H and Hastelloy X. Since these alloys are used in gas turbine combustors is there any history of adverse in-service carburization, I wonder?

The dynamics here are huge, but in a nut shell, any high temp environment that contains an active carbon species will violate an alloy. Remember that the NiCrMo alloy systems or a nickel based alloy will protect itself by forming a passive oxide film on its surface (chroma). A carburizing environment is a reducing environment where buy the lack of oxygen will prevent the alloy from reforming that passive surface oxide film. Once the film becomes compromised then the carbon will begin to activate the alloys surface. What will happen here, is the chrome in the alloy will combine with the carbon forming chrome carbide. The molly will form carbide as well. The remaining nickel will go to graphite. This process is referred to as metal dusting. One of the main reasons that coal gasification hasn't become more prevalent is because of this corrosion mechanism. The trend is to design alloy systems that contain elements that will not form carbides (at least at these pressures). The problem is that these alloys need to be strong and with the additions of copper and aluminum, well you get the drift. Remember that these materials will need to be fabricated as well. There is a ton of research being conducted by the DOE in relation to solving this problem. If you want a hydrogen economy, solve the dusting problem.

I'd seen your Avatar on materials elsewhere in CR4 and was hoping you'd lend an ear.

With respect to alloys like 617, in your opinion, would the contained cobalt be subject to "metal dusting" as described for Ni? And can the Ni & Co "dust" make it through the carbide layer and be exposed at the surface?

If you consider an "alloy" structure first, then the cobalt will support the nickel. Notice the aluminum content, it is there mainly to support the chroma scale. The chrome will form an oxide first and as the temperature rises and the aluminum will form its oxide below the chroma.

This is an old turbine coating bit, mostly used as a base coat under the thermal barrier ceramic coatings used in combustion liners. Thermal dynamics dictates that nickel first then cobalt (NiCrAlY's) just add cobalt when going reducing and your close. All this is very dependant upon the temperature and the fluctuations in the environment.

617 is one of the alloys being tested for the use in gasifiers. The problem would be solved if you only brought the equipment to its operating parameters and left it alone. Then you could design the one alloy that would work for that given environment. The reality is the constant changes that happen in that environment, mainly the cooling cycle and this is where the trouble begins. Add thermal expansion, creep, ageing, which disrupts that protective oxide scale causing the alloy to waste.

Now imagine you had a coating who's structure was porous and contained lets say tungsten, nickel, chrome,cobalt and understanding that the carbon will go after the tungsten to form carbides. These carbides would be precipitated within a NiCrCo matrix which would be unaffected by the carbon. The tungsten would basically use up the available carbon. At this point as long as the coating remains intact, then the base material would be protected. As the system swings from reducing to oxidizing the carbides will not oxidize and any internal metal within the coating would be passive.

This is something I have been working on. It is nice to know that this interest other people. I kind of feel like the omega man. Check out Air-Products, I believe there working on similar problems.

I think this article can give you some good advise on the issue of high temp corrosion:

hoep this help u

S.

Corrosion Protection & Corrosion Control

Thanks,

Nice article on the principles and effects of carburization including related failure mechanisms.

Looking through the other link as well.