Submerged Arc Weld Porosity in Duplex SS

Date 06-28-2004 23:54

In order to get porosity, the weld has to have too much of some kind of gas in it as it solidifies. The gas can come from contaminates, as you mention, from the base metal, wire, air, or flux. (I am assuming you are doing SAW since you mention flux in bags). You should consider baking the flux before you use it, even out of an unopened bag. It is easy to get small holes in the bag that let in air/moisture.

The ways you can eliminate porosity are to eliminate the contaminate, or allow more time for the gas to escape. If all else fails, slowing the welding travel speed down will slow the solidification, and could help reduce/eliminate the porosity, but if it is too severe, it would not eliminate it.

Also, too much or too little flux burden can contribute to porosity. Too much flux doesn't allow the gas to escape fast enough. Too little flux allows the air to be absorbed by the weld pool. Too fine of flux granules can also keep gas from escaping and cause porosity. Is your flux like a coarse sand or more like a powder? Parameter wise, make sure you are operating within the manufacturers guidelines. Excessive voltage can cause the arc to suck in air, but if you have too little flux, or too high voltage, you would usually notice because of the arc flashing through the flux instead of staying under it.

Another potential problem is the base metal. If it is rimmed steel, it could contain a lot of oxygen. This isn't that common anymore though, but worth checking if the others are dead ends.

By rodofgod (**)

Date 06-29-2004 04:18

Hi All!

First of all, You need to make sure your 'flux' is completely dry!!!
Next, make sure your parent material is dry and free from any oil/grease surface film.
All basic stuff, I know!

Have you taken into account the 'Nitrogen' content of Duplex?
This can affect SAW welding if the speed and heat input is too low/high! Cooling rate should be optimised between 1200-850 degrees!

Regards

By pipewelder_1999 (****)

Date 06-29-2004 10:27

Based on what I have read I am leaning towards a decrease in travel speed and and maybe a decrease in voltage. I have noticed that the smoke doesnt escape from the flux like I remember from using a semi auto gun years ago.

I'll let you know what happens.

Thanks for the help both you and Mr. Roberts

GA

By GRoberts (***)

Date 06-29-2004 04:34

Oops, I guess I forgot to read the title along with the body of the question. Most of my first reply can apply to duplex also, but I guess you can at least forget about the rimmed steel part. I did have a porosity problem with a SAW nickel overlay once, and cleaning and drying never helped. The manufacturer said that the flux should be good right out of the sealed bucket, and we tried baking it too, but could never get rid of the porosity at the travel speed we wanted to use, and had to end up slowing it down to get rid of the gas.

By CHGuilford (****)

Date 06-29-2004 13:11

Everyone has given some good advice to consider. I just want to add that if your parts were cut with plasma, you need to thoroughly remove the dross down to sound clean metal. If you don't, then that will be a likely source of porosity.

Other than that, I would try reducing the voltage. That can have a dramatic effect on porosity

Chet Guilford

By swnorris (****)

Date 06-29-2004 17:44

Hi All,

I haven't been around sub arc welding since the late 70's. I do have an old SAW troubleshooting chart for stainless weldments which indicates that possible causes for porosity and gas pockets are the following:
Shallow composition burden, improper cleaning, contaminated manual weld backing, insufficient penetration in double welds, improper fitup in welds with manual backing, and moisture on the plate or in the flux. It also indicates that once the arc is started, the welding current is the most influential variable, followed by welding voltage, and then travel speed. Also, I don't know if this is in any way related to your problems or not, but it goes on to state that the sub arc process is not recommended where a weld deposit is needed that is fully austenitic or is controlled to a low ferrite content below 4%.
The pock marking problem is addressed in the troubleshooting guide for carbon and low alloy steels, but not stainless. I don't know if the same corrective action can be used on stainless, but for what it's worth, it lists in order of importance to increase voltage, decrease current, decrease speed. It also goes on to say that heavier plate than normal will cause pocking.
Also, for what it's worth, here are a few causes of porosity in the sub arc welding of carbon and low alloy steel. Again, I don't know whether or not some of this could cause the same problems with stainless:
Contaminants in the joint: remove oil, dirt, moisture, etc., from the exterior surfaces and abutting edges, as Chet mentioned. Insufficient flux: if an inadequate amount of flux is used, the arc flashes through the flux and causes scattered surface porosity. The weld may also contain porosity if the slag spills off the weld before it has solidified. Contaminants in the flux: may be picked up by the flux recovery system and deposited in subsequent welds. Entrapped flux: the possibility of porosity exists wherever there is an opportunity for flux to become trapped between the bottom of the bead and the opposite side of the joint. Porosity from entrapped flux is most commonly encountered in butt welds. If the gap between plate edges is 1/32" or more, flux may spill into the gap ahead of the arc. The remedy is to back the joint with a manual or semiautomatic weld or a backing strip. The bead must penetrate the backing, or clear the backing by at least 5/32". Segregation: The composition of the base metal has a bearing on the porosity occurring in the weld. Even when composition is within acceptable limits, possible local segregation of constituents may promote porosity. Sulfur in particular tends to segregate within steel alloys. When it is necessary to weld steels with sulfur segregation, use procedures that give minimum admixture (low current, negative polarity, and large electrode diameters). Inappropriate polarity: the effect is slight and not normally an important factor, but positive polarity generally produces less porosity than negative polarity. However, negative polarity results in less porosity with sulfur bearing steels or with other alloys where penetration and admixture are to be minimized. Excessive travel speed: as GRoberts mentioned, reducing travel speed reduces porosity, because slow speeds allow gaseous materials to boil out of the molten weld metal. Slag residue: from some types of electrodes may cause porosity where tack welds are covered with sub arc welds. Lastly, the depth of the flux layer must also be considered. If it is too thin, there will be too much arcing through the flux or arc flash. This also may cause porosity.

By RonG

Date 06-29-2004 19:30

Gerald, We do a lot of SAW 410, 422, 309 and INCO not with Duplex SS.

I read thru the other post but can't recall all that was said, please forgive if I am redundant.

I would try increasing the voltage to get a wider puddle thus less dilution. Lower voltage tends be less forgiving in grooves.