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Up Topic Welding Industry / Technical Discussions / cracking in rolling dss welding joint
- - By jiweiming (**) Date 08-20-2004 01:05
we use TIG welding in manufacturing of dulex stainless steel pressure vessel.the wall thickness is 6mm,and the material is SA240 S31803.the rod wire is ER2209 from Avesta,Sweden.after welding of the longitudinal seam of the cylinder,we use rolling machine to revise the roundness of the cylinder because of welding deformation. however, while rolling, we found some cracking in the welding seam.some appeared in HAZ,and some in the seam,etc.they are parallel to the welding line.it seems it is really difficult to weld dss compared to general austenitic stainless steel.
would someone kindly enough help me solve this problem?many thanks.

Parent - - By ssbn727 (*****) Date 08-20-2004 07:41
Hi Jiweiming!

Many questions need to be answered by you in order to get a better understanding regarding your problem...

The first one that I'd like to ask you is: What purity grade of argon are you using for welding and backing (purge) gas?
The reason for this is because unlike Austenitic stainless, Duplex is very sensitive to certain amounts of O2 in the argon content. In other words, not as pure as necessary for the type of stainless you're working with,
and must be monitored for proper O2 purity prior to welding in order to insure the absence of these types of discontinuites that, depending on the code or standard you're working from, can be determined as being defects which would require repair which is something that you definitely want to avoid in the first place.

If you do a search in the forum on Duplex, you'll find some helpful info and links to articles regarding proper welding procedures for DSS.
I'm sure that others will also chime in to offer more help.

Respectfully,
SSBN727 Run Silent... Run Deep!!!
Parent - By jiweiming (**) Date 08-21-2004 02:17
the gas purity of argon is 99.6%,while the amount of O2 is unknown.
it is said mis gas of Ar+2%N2 will be better(from Swden Avesta).is it reasonable?
would u kindly enough to tell me some forums on DSS? I guess it is not easy to weld DSS.
BEST REGARDS
Parent - - By GRoberts (***) Date 08-20-2004 20:40
The first question that comes to my mind when I hear about dupex weld brittleness (evidenced by cracking) is what sort of heat input was used. Both too high and too low can cause problems. Do you know what amps, volts, and travel speed you are using? Another question is how big are your cyclinders? Was the material severly deformed during rolling (i.e. a small diameter) or is it a large diameter?
Parent - - By jiweiming (**) Date 08-21-2004 02:12
yes,the welding current is120A,voltage is 12V,and the travel speed is about 23cm/min.
the diameter of the cylinder is 1000mm. with vee-type groove,first we weld from inside,then after carbon arc gouging(because there is no gas protection at back),continue to weld from outside.the total welding passes inside is 2,while the outside 2-3.after finishing outside,there is deformation towards inside.
to make it round,we use rolling machine.then we find cracking in the deposited metal area and HAZ.
in addition,what welding parameter is suitable for welding dss?
thanks for ur response.
Parent - By ssbn727 (*****) Date 08-21-2004 05:41
Hello again!

The fact that there's no gas protection when welding from the inside is the first part of your problem. the second part of your problem is the fact that you're backgouging is the second part of your problem in two ways.

One is excess heat input from the carbon arc or the weld passes themselves as GRoberts mentioned and Two is the addition of carbon from the gouging into the base metal and the previously welded side.
Upon further metallurgical examination this will bear fruit.

Now I'm not going to tell you what to do but, I can only recommend some changes that need to be made in order to minimize or even eliminate your problems.
First is to totally change your welding procedure whereby you do indeed protect the root side of the initially welded side of the joint with a high (less that 10ppm of O2) purity argon gas shielding (purging).
The same goes for the shielding gas for welding.
Get rid of the Carbon arc gouging on DSS PERIOD!!!
DSS= Duplex Stainless Steel

Get yourself an O2 monitor or gas analyzer so that you can constantly monitor the purity level (amount of O2 present in both the purging and shielding gas in parts per million) Do Not Weld unless the O2 content is at less than 10ppm.
On a side note: I do'nt trust blended gases for welding and shielding of DSS because of the fact that the purity levels cannot be maintained unless you blend them in real time and constantly monitor for O2 amounts in ppm.

Get yourself a DSS Weld/HAZ color comparison chart in order to "fine tune" your welding parameters (heat input) so that the amount of discoloration is minimized as much as possible. Avesta should be able to supply you with one.

Cleanliness is another TOP priority and should not be taken lightly especially with DSS, Ti and it's alloys or any other metal composition that would be extremely sensitive to any dirt, chemical residues, oils etc.

Passivating the welds/HAZ also needs to be performed after the welding is completed.

I wish I could give you some more recommendations but, due to the fact that a whole lot more information is needed from you regarding your situation, I'll stop here so, please let us know how you make out.
e-mail me at : hanklive39@hotmail.com so I can send you some .pdf's I have available on welding Duplex Stainless steels.

Respectfully,
SSBN727 Run Silent... Run Deep!!!


Parent - - By GRoberts (***) Date 08-25-2004 15:18
The first thing that pops out at me from your welding parameters is that your heat input is less than 10 kJ/in. (actual heat input you described is 9.5 kJ/in or .38 kJ/mm) This is very low. A typical minimum heat input in DSS welding specification is 15kJ/mm. You will have a high ferrite content in the HAZ with this heat input. Have you been able to do any ferrite testing? Ferrite should be below 60% in the weld and HAZ. A weld with high ferrite will be less ductile and have less toughness, which means it has less resistance to cracking. It will also have less corrosion resistance, which is why you are probably welding duplex in the first place.

The other issue is deformation. Duplex stainless steel does not like to be deformed over approx 5-10%. Deformation reduces corrosion resistance (especially to SCC), and can lead to cracking if too severe. However, with a 1m diameter, and a 5 pass weld with low heat input GTAW, (which leads me to believe you have a thin plate), I don't think that is the main issue, but check it out anyway.

I also don't completely agree with SSBNs comments. Since you back gouge the 2nd side, purging the root and 2nd pass is not essential. Purging is used to eliminate oxides on the base metal and weld, but you are removing those when you backgouge from the outside. Also, carbon arc gouging can be used sucessfully on duplex stainless steel. Use the same maximum interpass temperature for gouging as you do for welding. (should be 350F or less) Make sure you grind to bright metal afterwards though. And as SSBN mentioned, passivate afterwards and keep a clean (iron free) shop. What is the use of going to the expense of duplex sst if it is put in service with poor surface conditions.
Parent - By ssbn727 (*****) Date 08-29-2004 00:58
Hi GRoberts!

All I was trying to say is that this person does'nt need to back gouge the joint on the outside if there is a different setup involved, and if the proper changes were to be made by let's say - surrounding the outside diameter with copper tubing with small holes drilled into the tubing so that gas could flow through it to shield the root of the weld from the outside and shrouding the tubing so the gas does'nt escape, then there would be no need for back gouging, and very little prep work would be required for the outside weld.

Hence, a much more efficient procedure.

I do agree with you about the low amount of heat input and the concern about deformation which if excessive, can reduce corrosion resistance (Stress Corrosion Cracking=SCC)
Heck, I was just expressing my own personal opinion regarding the use of carbon arc gouging on DSS... Sometimes you cannot avoid using it but, in this case, it can be avoided.

I wonder how much heat input is being introduced to the joint from the backgouging? Since we do'nt know what this person is using as far as parameters for gouging are concerned, then it cannot be determined precisely what is the overall heat input after gouging...

If the gas had less O2 in ppm's then the concern for oxides forming in the root would be eliminated or least lessened to the point whereby grinding from the outside would take care of that provided that the correct type of grinding wheel is used for DSS...

Anywho, It's always a pleasure to read your opinion G!

Respectfully,
SSBN727 Run Silent... Run Deep!!!
Up Topic Welding Industry / Technical Discussions / cracking in rolling dss welding joint

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