Abnormal grain growth during annealing (decarburization layer) 4

[AndreLuizPaiva]

Hello everybody,

I've been experiencing a problem with continuous furnace for annealing. The working cycle takes about 1,5h to reach its peak (750ºC (1-2 hours)), then decreases to 720ºC (5 hours).

In a particular sample, a SAE 1010 welded tube (6,3 mm thick, outer diameter 63,5 mm) , there is a large decarburized layer after the process, with a abnormal grain size (URL below).

http://postimg.org/image/871bzl409/

http://postimg.org/image/ygmeidpxl/

(This one is a SAE 1026 welded tube where i also find the decarburization, though is not that deep).

Can anybody guide me on this subject? I'm going to review some furnace parameters (dew point is not monitored yet) in order to avoid decarburization, but I would also like to know the reason for this huge grain growth.

I apologize for the bad english and thank your reply,

André Paiva

 

[weldstan]

You need to change the oxidizing atmosphere an exogas atmosphere (approximately 5% to 11% CO/5% to 10% CO2 + CH4 + H2 +N2). If you already are using a Lean gas switch to a rich one.

 

[EdStainless]

You need to be monitoring both dew point and carbon potential.
Obviously wither or both are too low in your furnace.
And why on earth do you have a 6.5 hr heat treat cycle? are these parts really 6" thick?

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Plymouth Tube

 

[metengr]

Looking at your information above you are performing an initial process anneal along with a sub-critical anneal if holding over 5 hours. This is why you have the issue of grain growth.

For process annealing the hold time should be 30 minutes at 750 deg C, followed by cooling in air. If you are performing a sub-critical anneal in lieu of the process anneal, this a different heat treatment and is performed below the austenitizing range to spheroidize the carbides. This requires extended time at temperature below 720 deg C.

 

[metengr]

Just a clarification and to provide consistent terminology, the process anneal I mentioned above is really an intercritical anneal. You have to decide on which anneal treatment will provide the desired results and avoid grain growth.

 

[tbuelna]

What process is used to weld the 1010 carbon steel tube? Is this 6.3mm thick wall, 63.5mm OD tube roll formed and resistance welded? And from what section of the welded tube were the metallurgical samples shown taken from?

 

[TVP]

Decarb on the inner diameter of tubes is a common problem. This occurs when the tubes are heated above the transformation temperature before the protective furnace atmosphere has completely filled the tubes. The rate of temperature increase along the tube needs to be reduced so that there is time for the gas to fill the tubes before the ID reaches the transformation temperature.

 

[AndreLuizPaiva]

weldstan, EdStainless, metengr and tbuelna,

- We are only using Nitrogen as the atmosfere

- Just measure the dew point on the line, -10ºC. Is that reasonable?

- We use this to process OAD Pulleys (example:

http://www.schaeffler.com/remotemedien/media/_shared_media/05_products_services/0001770B.jpg).

It iniciates with the annealing process, then it receives a phosphate coating in order to be cold formed; then the final machining of the part.
We need the long annealing cycle in order to spheroidize the carbides and homogenize the weld, to favor the cold forming process.

- The parts are actually 6 millimeters thick, but since this is a continuos process, they need to udergo the same furnace cycle as others items, which didn't showed decarburization yet (they are mostly SAE 10B22 though).

- Finally, EdStainless mentioned that the dew point is too low on the furnace. I assumed that lower dew points diminish the probability of decarburization. Am I wrong?

Thanks for the replys

 

[AndreLuizPaiva]

Tbuelna,

They use roll forming with high frequency induction welding on the tubes.

Since the tubes have small diameters, we analyze the whole transversal section (the laboratory operators cut the tube in half and preparate both samples for analysis). The images are from the opposite side of the weld.

 

[TVP]

You cannot process steel parts in only nitrogen, there needs to be some other gas present such as endogas or methanol. I suggest reading the following documents from Linde, as they comprehensively cover this subject:

http://heattreatment.linde.com/inte...sf/repositorybyalias/wp_annlg_13/$file/13.pdf

http://heattreatment.linde.com/inte...repositorybyalias/wp_anlghdng_11/$file/11.pdf

 

[AndreLuizPaiva]

TVP (Materials)

That's something i will try out. Thanks.

This explain why we got like 1,5mm decarburized layer in the ID and only 0,5mm on the OD. Luckily, we machine a lot from the ID, enough to avoid the decarburized layer, but not so much from the OD.

I runned some tests with oil and abrasive blasting on the samples. Trying to be clearer: The tubes are cutted here, and readily enter the furnace (with "cutting oil"). I didn't get to much of a difference between dry and wet samples, but do you guys think this could influence? Also, the abrasive blasting to remove any impurities on the surface of the tube didn't showed great results either. Then again, do you guys think i'm attacking the problem from the wrong angle?

 

[AndreLuizPaiva]

TVP (Materials)

- We use only Nitrogen because it's a protective atmosfere only. Also, we can't use neither endogas nor methanol because at temperatures below 800ºC there is an explosion risk.

-Also, I think is relevant the mention that we cut the tube 7mm length, so they are pilled up in a basket before entering the furnace (

http://postimg.org/image/rureb8dbf/).

 

[weldstan]

TVP,
Thanks for the Linde papers.

AndreLuizPaiva,
You need add an exogas mix to the Nitrogen. You will not have an explosion without adequate oxygen.

 

[AndreLuizPaiva]

TVP (Materials)

Thank you again for the papers, i'll check them right away.

weldstan (Materials)

Right now, the furnace have only Nitrogen and Natural gas (below 800ºC (furnace manual)) as options. I apreciate your answers, but i was willing to go for atmosfere review as a last resort. As I said before, almost every item in the company needs to be annealed, and changing the atmosfere due a few items would be risky, and would also take a lot of reasoning with superiors. You see, the puzzling thing about this is that only tubular items showed the descarburization until now (there weren't any prior to the annealing process).

 

[CoryPad]

Residual surface contamination like cutting fluids, etc. may prevent some decarburization, although I do not think the effect would be large at the time and temperature involved with your application.

Even though the parts are cut to a short length, your stacking operation results in the parts behaving like a long tube. There will not be uniform gas flow through the parts, which means the inner diameter surfaces are more likely to have decarburization.

If you cannot improve the atmosphere, then you will need to have larger blanks and machine the decarburized layer from the parts after heat treatment.

 

[EdStainless]

methanol/nitrogen is a common steel treating furnace atmosphere. It is easy to blend and control, and not very expensive.
There are specific balances of DP and carbon potential that you need to maintain.
You will need to continuously measure both.
If part of your problem is the tubes getting hot before the ID purges well then you may need higher furnace pressure/more gas flow.


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Plymouth Tube