? Tack welds for butt welded pipe:

Date 03-02-2005 19:46

Hi guys, great forum! I've been a lurker for awhile and this has been a great resource! Now I've got a couple of questions...

I'm trying to develop skill welding pipe and I have a question concerning the tack welds. Should the tack welds penetrate fully to the inside of the pipe or are they just supposed to go far enough in the groove to hold the work together and get burned through with the root pass?

I have been trying to make my tack welds penetrate fully. Then I grind them down even with the bottom of the groove with an 1/8" wheel. The reason I ask, is sometimes when I run the root pass over the tack weld it seems to cool the arc enough that sometimes it leaves a little void right after the end of the tack. Then it heats up and starts penetrating again. If the tack weld didn't penetrate and I don't burn through it, I still end up with a void.

Also when it all comes together and I hear that nice sizzle inside the pipe, I don't always seem to be getting the *keyhole*. Except if I slow down too much or get the rod at too much of an angle. Then it opens up and usually turns into a gopher hole! I'm using light pressure on the electrode inside the groove. Too much and I get stuck.

Do I need more heat, or is it something in my technique or the way I've prepaired the materials?

Here's some data:

4" sched. 40 pipe
30* bevel with 1/16" face and root
1/8" 5P electrode
90 amps, DC reverse
direction of weld: down
position: 5G, 6G

Scott

By SA-200

Date 03-03-2005 01:41

And what kind of welder are you using?

By stillman (*)

Date 03-03-2005 02:18

Miller Shopmaster 300. It's a 300 amp AC/DC power source run on 480V.
It has 0-100 'arc control' which I keep set at about 50.

By 357max

Date 03-03-2005 03:59

One trick to keep in mind when tack welding. Especially if you rotate the pipe to position for tack welds. Always tack weld full penetration in the direction of the final weld. If you are welding up from six o clock. Tack weld both sides in the uphill. Or, if you are going downhill downhill your tack welds. If you don't you will have voids and lack of fusion at the starts of the tack welds when you weld from finish to starts of those tack welds.

By stillman (*)

Date 03-03-2005 05:26

357max, thanks. I've been making my tack welds both pieces stacked so the joint is aligned horizontaly with a spacer in the middle to maintain my root opening. So far I'm just working on small practice pieces. I hadn't even considered the direction of the tacks and welds.

Scott

By SA-200

Date 03-03-2005 15:01

Your tack welds should be an inch long and fully penetrated. At the bottom tack across by starting on 1 side and ending on the other side to avoid ending a tack in the center on the bottom. Now grind your starts and stops so you can weld between the tacks and dont try to weld through the tacks as they are already penetrated. Just be sure you welded good into the ground start and stops.

By stillman (*)

Date 03-03-2005 17:10

Thanks SA-200. It sounds like the tack weld for the bottom may sometimes get longer than an inch. Would the same apply for the tack weld on the top too? I'm assuming I want to avoid stop/starts at the very bottom or the very top because that's the most likely place to burn through.

So when it's all tacked, ground and ready for the root pass: where's the best spot to start the tip of the electrode? In the center of the tack weld where it's the thickest, or on the area near the trailing edge where you've ground it to clean white metal?

Scott

By SA-200

Date 03-03-2005 18:31

The top only needs to be an inch. You shouldnt be blowing through if your gap, land, and current are all correct. Just start your weld in the groove ahead of where your starting and you will be going through your start as you progress down the pipe. Different arc welders have different arc characteristics. Some are not suitable for downhill pipe welding.