Resistance Seam welding causing leaks

By Raviatluri Date 02-28-2017 18:18

We manufacture Solar wate heaters. We use SS 304 2B grade sheet metal (0.5mm thickness) to make inner tanks. Welding of this tank is the most crucial step in our manufacturing process. We are using Resistance Seam welding to make the tanks. Material thickness will be 0.5mm. We do both logitudanal seam and circumfrential seam welding on our machine. We weld the joints by overlapping the sheet ends for logitudal welding. We do leak test for every tank and no leaks were identified so far. Problem starts after the finished product is delivered to the customer and installed.

Major issue is that thease tanks start leaking after installing for hot water purpose. Water will be heated inside these tanks by capturing solar energy. When we open the problamatic tanks, we see that there are small pin holes just beside the welding joint where the sheet overlapping ends. some times on the welding joint aswell. There are absolutely no hole anywhere elese in the tank except beside the welded joints.

We have tried various options such as changing welding parameters (Current, speed, weld and cool time etc) , increasing the sheet overallping area etc. But the problem continues. We are looking for experts to help us as we are clue less.

We then tried manual TIG welding of the tanks and installed at the same place and there are no problems as part of our research. But manual welding is not scalable. What i fee is that it got to do something with the welding process. Our welding machine supplier is also clue less as he says the welding joint is excellent and strenght of the welding also is good. Point to note is that these tanks will have hot water inside 24hrs with temperate anywhere from 60degree Centigrade to 90Degree centigrade

Hope I get some help/guidance here from this forum. Please let me know incase you need any more data or information from me.

Regards

Ravi

By jwright650 (*****)

Date 02-28-2017 22:26

Ravi, are you sure that the leaks are due to pin holes(porosity) and not small cracks? I feel like you would have found the pin holes during your leak testing, cracking could have occurred after, possibly way after, the welding and leak testing.

By Raviatluri Date 03-01-2017 01:58

Yes John, as you rightly said, the leaks are happening after using the tanks for 20days to a month. for some customers, it happens after 2 to 3 months. I would have used wrong word Pin hole. But our observation is that there are a small holes (It might be small cracks and not holes) beside the welding joint. We test the tanks for leaks through pressure test. Any manufacturing defects would be captured there. But the product seems to be good even during our tests and turns bad after using it for 20 days or more at the cusotmer end. our other observation is that there is a corrossion around these small holes/crakcs and the rest of the tank looks perferct. We have to give 5 years warranty for these products. We stopped the production presenty and looking for the root cause to address. Its a major issue that we are trying to address.

Regards

Ravi

By mcostello (**)

Date 03-01-2017 03:28

Do all fail or just some? Maybe cycle testing of all product might be needed until the cause is found. Got to keep selling to stay in business. Just don't ship bad product.

By jwright650 (*****)

Date 03-01-2017 11:57

Hi Ravi,
Do you find the leaks only in the HAZ (heat affected zone)? This is the material beside the welded joint that is not actually melted during welding but is affected by the welding heat. I'm wondering if you could use a shielding gas on the backside of the material, if that would help until the weld has cooled down some to help prevent the rainbow coloring, it would be good if you could keep it from going beyond the straw color into those darker blues and purples if possible.
I'm not very familiar with resistance welding and welding a 304 material that is that thin.

By Tyrone

Date 03-01-2017 12:11

Hi Ravi,
Welcome to the AWS Forum.

The only way to find the root cause is to do a metallurgical examination of the defect. I start by LPI, then radiography of the pinhole/crack area to see if there's any other indications. Then perform fractography to determine the exact nature of the indications. This will tell you what the root cause is and what the fix should be.

Tyrone

By jwright650 (*****)

Date 03-01-2017 12:13 Edited 03-01-2017 12:17

Ravi, read through this old posting and see if any of this is similar to what you are experiencing: ie. heating tap water and 304 material

"Point to note is that these tanks will have hot water inside 24hrs with temperate anywhere from 60degree Centigrade to 90Degree centigrade"- quote by Ravi

https://app.aws.org/forum/topic_show.pl?pid=246022;hl=304%20corrosion

Also search this forum for "304" or "corrosion cracking"

EDIT: post a picture of one of your welded joints, if possible

By Raviatluri Date 03-01-2017 18:05 Edited 03-01-2017 18:07

Thanks John and Tyrone for your replys. Today I spent the whole day on analyzing the defects and our machine welding parameters. I requested support from a defence material research lab and we went though the whole product manufacturing process from the beginning till the end. I would like to share the critical discussion points here:

1. We weld the tanks using resistance seam welding and while welding it, we use coolant water on the copper wheels to reduce the heat. But the welding engineer from the research lab said that we should never use water while welding and preferably should use inert /argan gas to cool that area. We did that and observed no oxidation on the top and bottom of the welding is little goldern color. Not sure if that is going to address any issue.

I am surprised to know that we should use only inert gas and not water to cool the welding surface....But I see many videos where people use water to cool the welding wheels and the base material. Any thoughts on this?

2. Welding strength seems to be good. We are going to perfom lab testing of the cross sectional cut of the welded joint and see the percentage of material melted at the welding joint. other couple of test are also planned that I will definitely share here once I have the reports. We did the pull test of the welding joint and it looks good as the sheet is tearing off when pulling from bothe ends.

We see corrosion around the pin holes/cracks ( I am still not sure if I should call them pin hole or crack). I am attaching image of the defect I am talking about.

Thank you all for taking time and extending your support.

Regards

Ravi

By jwright650 (*****)

Date 03-01-2017 20:46

Hi Ravi,
I think the issue is the water or atmosphere in contact with the stainless steel while the material is still at an elevated temperature. Corrosion with stainless steel is a whole different problem when chlorides/chlorine is present.

Were you able to read any of the info presented in that older posting that I linked earlier? I think you may be experiencing some of the same problems the original poster did in that thread.

I've seen water used in cooling copper shoes while welding mild steel, but stainless steel is a whole different situation.

Edit: Thanks for posting a picture :cool:

By Tyrone

Date 03-02-2017 13:38

Ravi,
Looking at the picture, I don't think it's pinholes. I would call them indications.

I can't comment on the use of water vs inert gas on stainless, but if if it removed the problem, you just found the root cause.

Tyrone

By jwright650 (*****)

Date 03-03-2017 17:33

Hi Ravi...anything new going on with your testing?
Just curious what you may be finding out about this.

By Raviatluri Date 03-05-2017 07:18

Hi John,

I am waiting for the reports from the testing lab. In another two days I should get some insight. In the manwhile we are continuing with the manual TIG welding to continnue in business as that never gave us any issues since 5 years. But the production capacity will come down by 95% (high production impact). I will definitely share the outcome of our efforts to address this issue.

Regards

Ravi

By jwright650 (*****)

Date 03-05-2017 12:49

Hi Ravi,
Thanks for the update.