welding the cracks in a stainless steel furnace jacket 1

[albertpaca]

I am about to help weld a furnace which has developed cracks in the stainless steel jacket, where it was formed to alleviate stresses. The cracks have already been welded once, and are now re-appearing, either on the weld or beside it. The owner said that the company who supplied it here did not tell them about the initial tempering procedure until after it had been used, hence it has not been properly tempered. Not sure if this is the cause of the cracking. I am not experienced in furnace work, but am good with basic metallurgy and procedures.
I would like to know if it is worth re-welding, or if there may be another solution (i was thinking of wrapping around the outside of the furnace in a heat-resisitant packing where the cracks are? unsure of the heat problems though....) the other option is of course a new jacket, but it is quite an expense....

the furnace is a Naberthem furnace, used to only 600 degrees where it is now, capable of 900, nitrogen atmosphere for annealing silver. the jacket is made from DIN1.4841 (X15CrNi2520) 310-314 equivalent stainless. It was previously welded with ER-310 rods (27Cr, 22Ni, 2Mn....) The muffle is approximately 50 x 70cms in throat, maybe 140cm deep.
Here are some pictures of the cracks - these first two of the cracks as they are now, the second two of as they first appeared (sorry - this last set was taken with a phone). The cracks are all adjacent to the original weld joining the top half of the furnace to the bottom, radiating out following the forming of the expansion ridges.....

any help or suggestions would be greatly appreciated, as this is stretching the realm of my experience!
in thanks,
sean.

http://files.engineering.com/getfil...9-9153-51fe4844751d&file=previous_general.jpg

http://files.engineering.com/getfil...c0-b191-980f0b615267&file=previous_detail.jpg

http://files.engineering.com/getfil...1993-4a38-b7dd-c07e7be54d4e&file=IMG_0058.jpg

http://files.engineering.com/getfil...bc8b-4f70-8f76-1d0b88887e18&file=IMG_0059.jpg

 

[metengr]

Based on the photographs you provided, there are two types of crack locations - within the center-line of the weld region and along the base metal heat affected zones of the jacket. The second photograph shows a close-up of two cracks and based on their appearance show brittle fracture features.

To me this looks like thermally embrittled material (from sigma-phase) and cracking induced by thermal fatigue.

Since this has been weld repaired before and has cracked again unfortunately, you will be chasing your tail on previously weld repaired regions.

Can you selectively replace the cracked portions of this jacket shown in the photographs versus the entire jacket? This approach would be similar to a flush patch, where new material is manufactured and spliced into the existing jacket using full penetration welds.

 

[EdStainless]

The ridges tend to stiffen the structure, so much that it loos like they lead to high local stresses.
Some of the re-cracking may because the repair welds are too strong compared to the old material. Not much that you can do about it.

I would favor cutting out sections with cracks and welding in larger patch pieces.

You need to look at your heating and cooling rates.
That is where the stress comes from.

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Plymouth Tube

 

[albertpaca]

Thankyou for your consideration metengr and EdStainless. So you both suggest cutting and patching, which makes sense. Probably to match the curved form also, which looks like it is formed to alleviate any stresses from expansion and contraction.... Again this would form strong areas and local stresses next to the welds - any suggestions as to some ways to minimise this?
Sorry for all the questions - you have helped alot....
In thanks,
Sean.

 

[racookpe1978]

What is the metal thickness?

1. I don't use DIN1.4841 every day, but I am going to assume it will be similar to many of the stainless steel varieties used in CT exhausts - which cycle through equal temperature extremes as they are started up and shutdown each day.

Those stainless steels MUST be back-purged with the same Ar or He purge gas - that is, the back of the weld must be covered continuously with the same level of purge gas that the front side of weld is receiving to prevent contamination and oxidation as the weld metal solidifies back from its molten-plasma state. You can replace the back-purge with a continuous backing strip that physically touches the back surface of the base metal the whole way.

2. Your repair should start with by grinding a full-penetration groove to good metal down the whole length of the crack - then past it a little longer than 1/2 inch (10-15 mm.) Then NDE (dye [penetrant testing) the ground out area to verify so hairline crack continue past the end of your grinding area.

 

[albertpaca]

racookpe1978 - thankyou - completely understood and completely obvious now you have said it. we will look into it. this makes much sense. greater than operating problems i think that the new welds are contributing greatly to the cracking. thankyou.

 

[EdStainless]

You might consider making the patches out of thinner material.
In thermal cycling you would rather have distortion. If the structure is so stiff that it doesn't distort then the stress will me very high.

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Plymouth Tube

 

[albertpaca]

edstainless, thankyou. you are right about the distortion. we need to have a look at the furnace and see whether patches or grinding out the old weld is best. obviously the existing form was stressed or it would not have cracked in the first place. i worry that patches and more welds (joining horizontally as well), may lead to us constantly chasing our tail....
i will talk to the owners about our options.
in thanks to all for your replies,
sean.

 

[unclesyd]

Excuse the late reply as I was checking on how our muffles were designed.

If this muffle is designed like some of ours it had to be essentially free floating. If it is in any way restrained it could could lead to problems.

I concur will all of the above and second the recommendation for the full penetration weld both in any repair and in an addition of a new section. The point is well taken concerning the heating and cooling especially if the temperature change is fairly rapid.
The cracking in the center of the weld is generally from insufficient weld metal.

From the second picture in your post is there anything welded on the outside at this point.

I'm not sure about the tempering or than a stress relief cycle at the operating temperature. We had many components made from 310 SS and would only thermal cycle the very intricate components to prevent distortion in operation. All this involved was get to operating temperature and immediately air cool.

 

[albertpaca]

unclesyd,
thanks for the reply. i will check whether there is anything welded behind, but I am pretty sure not.
interesting to know about the minimal thermal cycling. i think that the original cracks are from stresses in the original welding of top and bottom case of the muffle. would this be right, with the original cracks at 90 degrees to the original case weld? if this is an original problem, with embrittlement in the weld, we may be chasing our tails covering it up with more weld repairs....
i will get back to you soon,
sean.

 

[unclesyd]

This may seem strange but one thing you could do to check for brittleness of the metal is to make make a 1" inside diameter circular weld bead near the big cracks. If there is cracking around or inside the circle a repair will be very problematic. Welding this circle creates extremely high stresses in this area and thing that have a tendency to crack will crack.