SAW flux size

Date 03-04-2008 18:54

Considerig that this looks like a homework question I'll still bite in this manner and briefly. I would suggest you invest in WRC Bulletin 190 Fluxes and Slags in Welding by Clarence Jackson. Also ASM Handbook Volume 6, AWS Welding Handbook, and the Lincoln Welding Handbook as well as info from TWI.
In general, higher current requires finer mesh. The result of too course a mesh for high (and I mean very high) currents is arc instability and deterioriation of surface quality.
Most SAW fluxes will come with current ratings (most up to 1000 to 1500 amps) that are (among other things) related to their mesh size. So unless you are operating at extreme high currents just go by manufacturers recommendations. If you want to learn more see para 1.

By lant Date 03-04-2008 19:30

Thank You.

However I would know if porosity can be entrepped with finer mesh?

Luis

By RBeldyk (**)

Date 03-04-2008 20:31

To say that a finer flux can directly cause porosity - No. Casues of SAW prosity are discussed here http://www.aws.org/cgi-bin/mwf/topic_show.pl?tid=14976

Finer fluxes do have a habit of being more easily trapped along the side of a gooved weld with the arc voltage is incorrect. This entrappment may result in porosity if welded over, without proper cleaning.
Fine Flux + improper parameters + improper cleaning + being welded over = porosity

By Fredspoppy (**)

Date 03-05-2008 13:32

Not that I am a name dropper, BUT, when Clarence Jackson was my advisor at OSU (many years ago) the most common causes of porosity (other than the obvious - moisture) in SAW was flux burden (excessive depth) and particle distribution (too many fines). JS55 is correct, the WRC Bulletin was good info then and still is.

By js55

Date 03-05-2008 15:24

Clarence belongs to that generation of gentlemen with tremendous knowledge wherein most of everything we have come to rely upon as standard in fusion welding originated. Their work runs so deep in the industry that little has come along since to change it.

By RBeldyk (**)

Date 03-05-2008 19:15

I do agree that excessive flux depth or too shallow of depth could cause porosity, but these conditions are quickly adjusted when setting up the equipment.

Excessive depth is usually very easily controlled so I don't think of it as a problem; one can quickly see the roughness and unevenness of the weld and adjust the flux depth. The same for too shallow of a flux burden one see the arcing and adjusts the flux depth.

The problems I have seen have been related to a poor weld profile trapping the fines along the toes and then adding a pass on top.

Definition - Recycled flux is the slag that is collected from the weld send out to a recycle, ground up, adding silicates and mixing with a percentage of virgin flux. also known as crushed slag.

Using recycled flux that has not be properly screened (eliminating the large and small particles) is a problem, many recyclers can screen out the large particles but the system they use for removing the fines (generally an electrostatic system) only removes about 1/3 of the fines, either the fines are have a reverse charge than the system or they have no charge at all (Gaussian distribution). I must admit some ground slags are worst then others in generating fines. The size of the silicates added to the ground slag needs to be monitored.

If the filtration section of a flux recovery system is not working correctly, you might obtain excessive fines, however, this should not be a problem if you are doing PM on your equipment, and know how to operate your machine.

By js55

Date 03-05-2008 19:40

Richard,
You're correct. Sorry for the sloppy language. Recycled as you have stated. Recovered as you have stated as well. We should keep these terms seperate. I'm as guilty as anybody. Good post. Good info.

By Joseph P. Kane (****)

Date 03-04-2008 20:50

Is it possible you have recycled flux? There, the "Fines" are often contaminents or broken up agglomerates. When these fines settle to one spot in theflux coverage trail, you can get porosity.

By js55

Date 03-04-2008 20:59

A finer mesh or original flux, or 'fines' as Joe mentioned can increase the possibility of porosity since the more compacted flux burden makes it more difficult for gases to escape. In fact, the appearance of small angled isolate porosity is a good visual indication that fines in recycled flux are too high of a volume percent. This is also common for people who have used agglomerated fluxes with concentric feeds and make a switch to fused fluxes whose mesh is generally smaller than agglomerated.
You will find also that fines and flux burden are not as common of issues with leading flux feeds as opposed to concentric flux feeds.

By lant Date 03-04-2008 21:18

Many thanks to all.

Regards

Luis