We are welding 6061-T6 to 6061-T6 pipe with an extended land-joint geometry, using ER5356 filler, GTAW process, and AC current. The operator is finding the welding excellent but a line circles the root of the weld around the circumference. It does not seem to be a crack, but it is definitely visible. What is this line?
The line is caused by the oxide on the inside corner of the weld preparation. Because the welding arc doesn't reach this area, it can't produce the cleaning necessary to remove oxide. Typically, this line shows up as an indication when the pipe is X-rayed. Using a deburring tool, put a slight chamfer on the pipe's internal diameter and the line should go away.

Can you summarize what happens when stainless steel welds produce "sugar" because of a lack of shielding when using the GTAW process?
"Sugaring" is used to describe the appearance of the backside of a stainless steel weld surface that has been exposed to oxygen. In this case, chromium at or near the surface combines with the oxygen to form a thick, porous oxide layer, depleting the chromium content.

If this depletion of chromium is great enough the corrosion protection is compromised because the resultant oxide layer is porous exposing the chromium-depleted areas to corrosive materials, leading to premature failure. At the same time, oxide debris provides crevices that also act as corrosion sites.

Prevent sugaring by properly using a backpurging gas. This purging gas eliminates the problematic oxygen by supplying an inert atmosphere during welding. The most frequently used purging gas in the U. S. is 100% Ar, but N has been successfully employed with austenitic stainless steels.

To properly purge the backside of the weld, determine the volume to be purged. This volume is defined by the system used to contain the purged gas. (Keep in mind that this system should have a vent to allow displaced air to escape.) Next, determine the purge-flow rate. Then, calculate the time required to purge the volume by dividing the volume to be purged (ft3) by the gas-flow rate (ft3/hr). Allow enough time for five to ten complete volume changes before welding. To avoid turbulence, purging flow should not exceed 80 ft3/hr.

Before welding, reduce the purging flow rate until a slight positive flow exists. This will prevent weld defects, such as concave root, due to excessive pressure. Typically, purging is only maintained during the first and second pass in pipe welding.

I have an old Lincoln SP-200 MIG welder with a Tweco No. 4 gun. I used it for welding aluminum several years ago and it seemed to work fine. Now, after reinstalling the liner and connecting to Ar shielding gas, I have a large amount of soot extending about an inch from either side of the weld. I have changed the Ar bottle and hooked it to the gun but the same soot pattern continues. Why does this occur? (I want to use the welder for constructing a welded aluminum boat.)
You are already on the correct track in addressing the shielding-gas issue. Contamination or disruption of the shielding-gas blanket causes sooting. Other variables causing sooting are: insufficient gas flow (set the flowmeter to 35 to 50 SCFH), breezes blowing away the Ar shielding gas, or operator mistakes including:

• The gas nozzle to work distance is too long. Keep this distance between 1/2 in. to 5/8 in.
• The gun angle is incorrect. Use about a 10 degree "push" angle. If a drag angle is used, the weld turns black immediately.

I need to weld a part made of 445 bronze light. What filler should be used?
The correct filler wire for welding your bronze part is ERCuSn-A or ERCuSn-C.

This column is sponsored by Penton and the Lincoln Electric Co., Cleveland. Dave Barton is a senior welding engineer in the Application Engineering Group of The Lincoln Electric Co. He oversees welding procedure development for both new technology and existing products, performs failure analyses for customers, and serves as a consultant on welding application problems. Barton has been with Lincoln Electric for 21 years. Send your questions for Mr. Barton in care of WDF by e-mail to: askdav@penton.com