Porosity

Date 07-14-2000 18:52

Piping porosity (also referred to as wormhole or elongated porosity) is a term for elongated gas discontinuities.
This is AWS's definition from the Guide for the Visual Inpsection of Welds B1.11-88.

By - Date 07-16-2000 17:51

Most of us with much welding under our belts have see the worming(pipeing)..However i have never been able to get a difinate cause of it..Just theories of it..Im dealing with some right now on a job Im doing..Ive ran the Volts up and down..IPM up and down..Gas pressure..Now the latest theory I've heard is that the drive rolls may be the cause of it..Was told to go to knurled to reduce the wire tension that could cause the wire to deform..BTW..Wire is Lin 71m...035..with an80/20 mix..At first blamed the wire..After a new roll it was the same thing ..Very sporadice though..Wish someone could give me a solid answer..Thanks..Phil

By - Date 07-17-2000 02:21

Phil
A big problem with using gas shielded flux core wires is if the electrode stickout is not in the optimum range ( around 3/4"), wormholing will happen. I have experimented with playing around with this wire, which is an excellent product. (have you tried the new Lincoln 71M ELITE? fairly new on the market and I was involved in an open house/demo day last week and we tried this wire and it runs like butter. Excellent penetration and excellent workability in out of position welding) Anyway, the wormholes can be eliminated if you ,maintain the proper ESO. I haven't heard of wormholes being caused by drive roll problems before, but you should be using a knurled drive roll with flux cored wire anyway.
Hope this helped.

By ADK

Date 07-17-2000 15:16

Traditional folded flux cored wires often have excessive lubricant embedded in the folded seam. This lubricant is usually a hydrocarbon. In the weld pool it can cause porosity "worm tracking". If possible you might bake the spool, burning off the lubricant, to see if this eliminates your porosity. However; this may cause wire feeding problems. A better solution is the use of seamless flux cored wire such as MEGAFIL from Stein. There is no seam, the wire is copper coated for better current carrying capability, the wire is not sensitive to distortion caused by the wire feed rolls. See page 38 in the July 2000 Welding Journal.

By - Date 07-17-2000 22:35

I agree inpart that "worm-hole" porosity is used as a definition for "piping porosity" but usually the initial start of worm-hole porosity, as is being discussed preceedingly, begins as an elongated discontiuity from the root or depth of a weld. This will be eviedent while excavating the surface indication at the point where the worm-hole began.

From my experiences and as described by AWS B1.10-86R it resembles an elongated gas pocket extending from the root of fillet welds and groove welds (my observations) from the root upward toward the weld's surface.
I've encountered this type of porosity with SMAW while examining structural fillet welds and GMAW aluminum groove welds and GTAW inconel. When the welds were sectioned transverse through the weld, the elongated were very visable with the discontinuies resembling cone shaped cavities frozen in weld metal. That is, the very bottom of the discontinuity is much smaller in diameter the the top and is coned shaped. In structural fillet welds the discontinuity usually extends completey, or near completely throught the weld pass. In the aluminum plate groove welds the discontinuitities would never break the surface of the weld and when radiographed, the indications were veiwed as just being rounded indications in the image but upon sectioning the welds, the cone configurations were very evident.
Also I've encountered "worm-hole" porosity that was completely linear while GTAW welding sch 160 Inconel 690. It was a gas bubble trapped subsurface in a pass. Excavation proved that the indication was completely horizonal with no tail going verticle.