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Hi 2008642!
First off, "Weldcome" to the AWS forum!!! :) :) :)
Now as far as your query is concerned, even though you provided some information with regards to your set up , there are still some unanswered questions that come to mind...
1.) Why is your CFH rate @ 45 to 50? Also, are you using a dual stage regulator/flowmeter as opposed to a single stage configuration?
2.) You mentioned that you are welding a 400 series stainless but you didn't mention specifically which grade and this is important because, as you already might know that "400 series" stainless can include both ferritic as well as martensitic stainless steel and unless you list a specific grade, we cannot just assume what you're welding together based on the filler metal grade you did specifically list so, please include this information. Also what is the thickness of the base members? I'm thinking not so thick because you're using a 98-2 mix instead of a 99-1 mix which is preferable to use on thicker base metals due to less possibility of undercut as well as other factors, and that's another reason why it's important to know the thickness of the base/parent metal. Also, if your heat input is too high and your travel speed insufficient then there is a possibility of thermal arc blow occurring in your set up when welding provided that everything else is correct.
3.) What is your CTWD (Contact Tip to Work Distance) and, contact tip set back, electrode extension and nozzle standoff distance? What size gas cup/nozzle are you using in your setup?
4.) Where is, and what type of work attachment do have on your set up?
5.) What are your cleaning/prep procedures for the base metal? What is the Joint configuration? What step(s) do you take to insure that the filler metal is protected from contamination?
6.) If you're using pulse spray @ 24 V and 300 A, what is your peak and background settings as well as you dwell timing, pulse frequency and while we're asking, what type of power source are you using in your configuration and what are the polarity settings for both torches and finally, are you using only one power source or two separate power sources?
7.) Do you use any other spool of a different type of filler wire such as another grade of metal other than the 439Ti MCAW grade you are using in the setup you're using?
8.) What type of liner/conduit are you using for your MIG gun/torch as well as the rest of the components along the wire path from the spool to the contact tip?
9.) "Several people assumed it was the gas, but both robots are split off the gas line." Now, when you mentioned the fact that both robots are split off the gas line, what exactly do you mean when you say this? Are you saying that you're only using one regulator/flowmeter for both GMAW torch/guns and split via a "Y" valve connection?
10.) What is your travel speed? When you mention your parameter settings @ 24V and 300A, are these your closed circuit output readings or your power source settings with an open circuit prior to welding? If you have this info, do you happen to know what are your Kilojoules per minute, hour or whatever you used to determine your heat input?
11.) Are the MIG gun/torches always six inches apart throughout the welding sequence? And what are the metal cored Filler metal Mfgr's recommendations for your application even if it's in the semi automatic mode only? And what is the total length of the joint?
I ask these questions to narrow down the possibilities of is causing this condition of porosity to occur on an intermittent basis throughout the length of the joint
My final question which could lead you towards narrowing down the cause and effect of you problem is to point you towards Lincoln Electric's " The Procedure Handbook of Arc Welding." I have the 14th edition but, I believe the 12th and 13th editions have this information available also... If you look at page 3.2-1 through 3.2-5 you'll be able to read the descriptions for Magnetic arc blow, thermal arc blow, and Arc blow with multiple arcs.
Arc blow with multiple arcs could be, or not be your problem but, since I do not know all of the specifics involving your set up, I can only describe what this means...
When two arcs are close to each other, their magnetic fields react to cause arc blow on both arcs. Multiple arcs are often used to increase the welding speed of the submerged arc welding process, and usually the arcs are less than one inch apart. When two arcs are close and are of opposite polarities the magnetic fields between the arcs add to each other. The strong field between the arcs causes the arcs to blow away from each other...
If the arcs are of the same polarity the magnetic fields between the arcs oppose each other. This results in a weaker field between the arcs, causing the arcs to blow towards each other. Now this may or may not be what is going on with your configuration but, I thought it was relevant enough to point this out to you also even though you're not using SAW and instead are using MCAW instead with a CV set up.
Thermal Arc Blow:
The physics of the electric arc reqires a hot spot on both the electrode and plate (base/parent metal) to maintain a continuous flow of current in the arc stream. As the electrode is advanced along the work, the arc will tend to lag behind. This natural lag of the arc is caused by the reluctance of the arc to move to the colder plate. The space between the end of the electrode and the hot surface of the molten crater is ionized and therefore, is a more conductive path than from the electrode to the colder plate.
When the welding is done manually, the small amount of "thermal back blow" due to the arc lag is not detrimental, but it may become a problem with the higher speeds of automatic welding, or when the thermal back blow is added to magnetic back blow. Both of these descriptions can be found on page 3.2-3 in the Procedure Handbook of Arc Welding published by the James F. Lincoln Foundation.
You also mentioned this: "(we use a house gas mix of 98%argon 2% Oxygen)" What steps do you take to insure the quality of gas you're using with respect to contamination and purity levels as well as safeguards to prevent moisture from encroaching the stored gases or during the mixing of the two gases, and are they mixed just prior to entering the pathway towards the regulator/flowmeter then towards the gun/torch nozzle or are the gases mixed and stored for some period of time prior to the start of welding?
I ask this because, I was reading my AWS Welding Handbook ninth edition vol. 2 Welding processes part one and I went to page 194, table 4.21 and it mentions this:
Possible Causes and Remedies for Porosity
Possible cause .1: Inadequate shielding gas coverage Corrective Actions: Optimize the gas flow, increase gas flow to displace all air from the weld zone. Decrease excessive gas flow to avoid turbulence and the entrapment of air in the weld zone. Eliminate any leaks in the gas line. (you did that already) Eliminate drafts ( from fans, open drafts etc.) blowing into the welding arc. Eliminate frozen or clogged regulators when using CO2 which doe not pertain to you. Reduce travel speed. Reduce nozzle to workpiece distance. Hold gun at end of the weld until molten metal solidifies.
Possible cause .2: Gas contamination. Corrective action: Use welding grade shielding gas.
Possible cause .3: Electrode Contamination. Corrective action: Use only clean and dry electrode (filler wire spool)
Possible cause .4: Workpiece Contamination. Corrective Action: Remove all grease, oil, moisture, rust, paint and dirt from workpiece surface beofre welding. use a more highly deoxidizing elecrode.
Possible cause .5: Arc voltage too high. Corrective action: Reduce voltage.
Possible cause .6: Excessive contact tip to work distance. Coirrective action: Reduce electrode extension.
Then I went to the troubleshooting section for process related problems encountered in GMAW (even though the process you're using is MCAW) found on page 197, Table 4.26.
As I was reading, I noticed that porosity in the weld could also be found in Tables 4.24 and 4.25 as well so, I decided to list all of them here...
We'll start with Table 4.24, Troubleshooting Electrical problems encountered in GMAW: Problem: "Porosity in weld" Possible cause: "Loose or broken wires to gas solenoid valve" Remedy: "Repair or Replace."
Now I'll go to Table 4.25, Troubleshooting Mechanical Problems encountered in GMAW: Problem: "Porosity in the weld bead" Possible cause: Failed gas valve solenoid, Gas cylinder valve closed (possible that one of the "Y" connection valves is closed or not fully open?), Insufficient shielding gas flow (I personally do not think it is applicable with 45-50 cfh), Leaks in gas supply lines (including the gun).
Finally, I look at Table 4.26, Troubleshooting Process Related Problems encountered in GMAW: Problem: "Heavily oxidized weld deposit" Possible cause" Weld joint area dirty, Improper gun/torch angle, Excessive nozzle to workpiece distance, Air drafts, Contact tube not centered in the gas nozzle distance." then I look down to the next problem and I find this: Problem: "Porosity in the weld bead" Possible cause: Dirty base material, Excessive wire feed speed, Moisture in shielding gas, Contaminated electrode (Filler wire spool).
In conclusion, I would have to say that you need to go over each and every one of the possible causes to either rule them out or single one of them out to find the root cause of this problem you're having and if non of these turn out to be the problem then, I would look towards the possibility of adjusting your electrical parameters and requalification of the WPS to meet the tolerances in the essential variables found in the code you're working t which you didn't mention either.
If you find that none of this is helpful, then the possible cause can be Magnetic arc blow and you can find that on page 3.2-1 thru 3.2-3 in "The Procedure Handbook of Arc Welding." You can also find Magnetic arc Blow in "The AWS Welding Handbook, Vol. 1, Welding Technology" on pages 48 and 49.
Hopefully this will help you narrow down the possible cause of your problem. ;)
Respectfully,
Henry