Well, you're talking of a petrochemical furnace, where the tubes are in the vertical position. If I understood well, on top of two adjacent tubes there's a 180 degrees bend (that you call "fitting" and also "coupling") to interconnect them. The same is true for two adjacent tubes bottom. The bends are welded to the tubes.
You also say that when one 180° bend has failed, you must cut it out from the tubes and install a new one. You say that here comes the problem, because the tube ends, to which the 180° bend will be welded, are carburized and suspect to be cracked.
What do you mean by "carburized"? Is it a more or less thin (or more or less thick) layer of carbon deposited on the tube inside surface? Or is it carbon that has diffused into the tube material?
In the first case, the carbon layer is easy to see and not so easy, but perfectly possible, to be removed mechanically by means of a special tool whose name in English I don't know (in Portuguese is "broca").
In the second case, only a chemical test will show whether or not there's carbon diffused into the tube material. If there is, my advice is to replace the tube and sell the discarded one as scrap. 25% Cr and 35% nickel are always valuable, even if scrapped. Welding will be nearly impossible with that carbon content.
Question: why do you "suspect" that the tubes are cracked? Have you carried out an inspection on the metallurgical mycroscope?
If the answer to the question is "no", why don't you take a failed 180° bend, one of those you have replaced, and observe it on the metallurgical mycroscope? If it's cracked, the tube end wil also be, for the simple reason they were welded together. In this case, my advice is also to scrap the tube and sell it for the material value. Repairing the cracks one by one by welding will also be nearly impossible.
There's another possibility. You say that the furnace was designed by Linde, a very well known engineering company. Why don't you ask for advice to them? If they were the designers, they should be in position to give a qualified consultancy.
Giovanni S. Crisi
PS. Henry, do you think the information contained in this posting is enough for you to give a recommendation to hobart?
thank you Giovanni
your guides is very effective. first, I should say you have good cognition about these furnaces. second,I think carburization was in both forms. before welding, internal and external surface of tube was grinded to remove carburized layer but in many cases cracking was occured. deep diffusion of carbon into tubes at approximately 900 celsius, likely the reason of that. but you are right when you said "you should carry out an inspection on the metallurgical mycroscope".
tubes was manufactured by Manoir and suggestion of company in such cases is annealing the tube, expensive and time consuming process. but I have observed SMAW was reduced cracking than GTAW. I like understand the reason.
answer: I see those crack, very clear. let me say one important thing, I'm from Iran, you know my country is under International sanctions, because of inimical actions of Iran's goverment. then contact with european companies and buying from them is very very very difficult.
I should spend more time on metallurgical assessment.
thanks alot
First. It's not a good practice to remove tube scaling by grinding with a grinding wheel. By "scaling", I understand any layer of any material deposited on a tube inside surface. Examples: calcium carbonate deposited on the inside surface of a heat exchager tubes (water hardness is the reason); carbon deposited on your petrochemical furnace tubes. It's not a good practice because it's a job that must be carried out with extreme care, otherwise you'll grind also the tube material and its thickness will be reduced. As I told you on my former posting, there are special tools to do that, whose name in English I don't know, that will remove the scale but leave untouched the tube material.
Second. Yes, it's quite possible that carbon diffusion has also occurred, taking into account the high temperature the tube is subject to (900 Celsius). However, only a chemical test will confirm that suspicion. If it's true, then I believe it will be very difficult, if not impossible, to weld the tubes.
Third. I don't understand how annealing will reduce the carbon diffused into the tube material. Don't take Manoir word for granted; in my personal experience tube manufacturers know very well how to make a tube (or pipe) but know very little on how they perform in service.
Fourth. If the cracks are visible to the naked eye, then they are big enough to scrap the tube, unless you are patient enough to repair them one by one by welding.
Fifth. I confess my ignorance. I don't know the reason why SMAW is better than GTAW in your case.
Sixth. I insist. Ask Linde, the furnace designer, for recommendations. Politics has nothing to do in this case. They're the furnace designer, they've guaranteed their design and they have the obbligation to assist you in solving your problem, no matter if Iranian government is good or bad.
Giovanni S. Crisi
thank you very much giovanni
I'll try cotact with LINDE.
If possible, you're better off building a new one from scratch! Hey! You asked for an opinion!
