CO2 MIG WELDING 2

[pdprao]

WE PROPOSE TO USE CO2 MIG WELDING FOR WELDING CRANE GIRDERSO
OF 20 MM THICK FLANGS , 16MM WEB THINESS FULL PENETRATION GROVE WELDS OF RADOGRAPHIC QUALITY. WE WRE TO USE SUBMERGED ARC WELDING BUT AS AN ECONAMIC ALTERNATIVE 100% CO2 , MIG WELDING IS PROPOSED.
ANY EXPERIENCE AND OBSERVATIONS MAY BE INDICATED FOR SUCH APPLICAIONSFOR USE MIG WELDING WITH 100% CO2 SHIELDING GA
HERE THE PLATE MATERIALS ARE I.S 2062 I.E. PLAIN CARBON STEEL.
FOR APPLICATIONS LIKE CRANE GIRDERS WHICH ARE SYBJECTED TO DYNAMIC LOADING, IS THERE ANY RISTRICTION IN USE OF CO2 MIG FOR SHOP WELDS.
ALL QUALIFICATION PROCEDURES AS PER ASME WILL BE FOLLOWED

 

[metengr]

There have been posts from time to time on this forum regarding the pros and cons of MIG welding, especially in heavier wall components. The only advice I could give is that MIG welding requires significant welder experience to avoid lack of fusion (LOF) defects in weldments. Welders can sometimes push for maximum deposition rates with the process resulting in LOF defects.

Have you considered flux core arc welding (FCAW)? I have seen deposition rates with manual FCAW that are similar to MIG welding. FCAW is commonly used in heavy structural welding applications, and is cost effective.

 

[BillPSU]

My experience with full pentration radiographic weld is the problem of porosity in the root of the weld from no cover gas on the back side. Any process can produce a radiographic quality weld if performed properly. Your submerged arc process probably has a backup plate covering the back of the weld. The submerged arc process is more repeatable from operator to operator while CO2 Mig has more variables and thus more variation operator to operator. The CO2 process will probably require multiple passes to fill the groove

metengr suggestion of using a flux core process is excellent however this part will probably need two passes to fill the groove and good cleaning of the flux between passes will be imperative.

I assume someone wants to avoid using submerged arc welding for some financial or space conservation reason. Proceed carefully do testing of the process and verify it passes x-ray. You will learn a great deal about your current and proposed process by doing the testing.

I worked in the aerial device and digger derrick industry and know the risk of weld failures would caution you to do the testing. One of our customers added an x-ray requirement to all their machines and we discovered our welds were not as perfect as they looked on the outside. These welds were not full penetration welds but had to meet D1.1. We did extensive testing on several problem welds and on one we changed to a pulsed welding process which actually produced a fillet on the back side of the weld to stop bas infiltration.

 

[pdprao]

Mr BillPSU,
Thanks for your feedback. You are exactly right in your comments. The contractor wants to go for CO2 MIG for economic reason. I fully agree with u co2 mig has many variables which can affect the quality. The joints are be subjected to U.T/ radiography. As such the quality gets monitored. Ultimately it is doubtful wheter it will be economical

 

[MURZ]

You have received some good advice. I am confused about your economic reason for not using SAW. If you look at this from a cost standpoint, in welding cost means deposition rates. The higher the deposition rate the low your cost. SAW will give you the best deposition rates and therefore it is the cheapest process.

I would not use GMAW CO2 welding for these joints. This set-up will allow you to weld in the short-arc mode only. For what you are welding, this is not a very economical choice nor is it a good choice for a dynamically loaded structure for reasons that have been pointed out already (LOF concerns etc.). FCAW is a better choice as well as SAW. Lets look at some welding scenarios.

1) No matter what process you use you will need to put in a root pass. For this I would use the GMAW short arc process using 90%Ar -10% Co2 or 95%Ar-5% CO2. Use a 3/32-root opening with a 3/32 land. Lack of fusion in the root is less likely than for the fill and cap passes. Now I am not saying it will not happen just that is less likely. Finish the joint using spray mode with the same gas. Deposition rates for the short arc mode using 160 amps, 19 volts 180 ipm wire feed will yield about 4.6 lb/hr (low) Spray mode with 300 amps 27 volts 325 ipm wire feed will yields about 8.60 lb/hr not bad but you could do better. Total cost for 300 ft using $50/hr labor rate would be $3861.33. (I assumed 300ft and this cost is for welding ONLY. It is not for set-up, machining, handling, base material cost and so forth. It covers welding consumables and welding time.
2) GMAW short arc process using 90%Ar -10% Co2 or 95%Ar-5%co2. Use a 3/32-root opening with a 3/32 land for the root. FCAW for fill and cap passes using 0.052 wire and 75-25 gas. Deposition rates for the short arc mode using 160 amps, 19 volts 180 ipm wire feed will yield about 4.6 lb/hr. FCAW with 310 amps 30 volts 350 ipm wire feed will yield about 10.85 lb/hr. Total cost for 300 ft of weld using $50/hr labor rate would be $4241.77. A savings from number 1 above of $380.55. Not significant but it is a savings.
3) GMAW short arc process using 90%Ar -10% Co2 or 95%Ar-5%co2. Use a 3/32-root opening with a 3/32 land for the root. SAW for fill and cap passes using 0.062 (L-61) wire and Lincoln 860 Flux. Deposition rates for the short arc mode using 160 amps, 19 volts 180 ipm wire feed will yield about 4.6 lb/hr. SAW with 500 amps 33 volts 350 ipm wire feed will yield about 18.05 lb/hr. (now were talking!) Total cost for 300 ft of weld using $50/hr labor rate would be $2322.22. A savings from number 1 above of $1539.11.

As you can see SAW is a better choice from a cost standpoint. Keep in mind this is only for 300ft of weld. If you look at ft per year this would add up in a hurry. Also, this process can be used for more than just this project, so if capital cost is an issue the ROI time frame would be short.

Hope this helps

 

[unclesyd]

Here is a good source of information on GMAW and the gases>

http://www.weldreality.com/default2.htm

 

[stanweld]

pdprao,
As MURZ has so well stated, there is no way that CO2 MIG welding will be more economical than SAW; unless your Contractor does not possess SAW and the required ancillary equipment and the quantities involved are insufficient to warrant the investment. It is assumed that your Contractor had the low bid.

CO2 MIG should still be avoided per the comments of BillPSU.
FCAW with E70T-1 electrode and CO2 shielding or GMAW with E70S-3 wire and 90% Argon - 10% CO2 will prove economically attractive and will meet your quality requirements as verified by UT & RT.

 

[hacksaw]

so many excellent comments, why no stars?

 

[grigorius]

pdprao,

We were in a similar situation to you and chose GMAW. Initially we were disappointed with consumable costs but have managed to overcome it and are saving money dramatically with less down time etc.

Cheers,

Grigorius.

 

[optimum]

Spray arc with a tri-gas Argon-Co2-O2 will provide very good fusion and penetration and pass UT. Very fast Process