> In any case, a couple of classes ago I had a great chance to
>> practice welding thick beveled plate with 7018, and I think that I
>> have a grip now as far as how to avoid entrapping slag.
>
> I would suspect that if you didn't get to do a bend test on it, it's
> probably not as good as you think it is - at least if it was thick enough
> to require multiple passes. I know it sure fooled me. The welds looked
> very strong and clean to me, but when you bend them, even a small amount of
> trapped slag can cause it to break at the bend.
>
> Maybe with your experience you have mastered it, but like I said, is sure > fooled me.
>
Here are some pictures of beveled 3/4" plates welded with a multipass weld, that I did at school with 7018. I took the weldment home.
For the heck of it, I took it to a mill 5 minutes ago and milled a slot so that I could see how much slag I trapped. The result makes it a little hard to tell, but I do not see much trapped slag -- but maybe I was not looking hard enough.
Igor, I don't think that looks bad. I did the same with one of my welds a few years back and it looked pretty much the same.
You know that you can polish and etch a sectioned weld, which makes the weld stand out from the parent metal? I think you normally mill or file the cut, then smooth the surface on an oilstone, then etch for a few minutes with a mixture of nitric acid in ethanol.
The test is to cut a strip out of the material across the weld, polish it, and then BEND it at the weld.
If you want to do some for your own education, start with pieces 1" thick, bevel one at 45 deg., and weld multiple passes. Cut a strip 1/4" thick, clean and polish it, bend SIDEWAYS to the weld. Cut two more, about 1" wide. (Bending these will require some oomph.) ROOT-bend" one, FACE-bend" the other. Look closely and see what slag-inclusions really look like.
It's very hard for me to tell what I'm looking at in the picture, but it looks like you might have two defects in the cut (those small black spots). But like I said, I can't really tell what I'm looking at.
On a flat weld with stringer beads the trapped slag normally happens along the edge of the welds so that two spots could be tunnels of slag.
Any defect - even small spots the size of those spots - creates a weakness in the weld which is where the crack will start when the material is stressed in a bend test. What starts as a pin-sized spot, opens up to a
1/8" hole when bent or just causes the the piece to crack into two.
The correct procedure for a bend test is to cut two test strips, each 1
1/2" wide across the weld and then grind the weld down on both sides of the strip to turn it into a flat bar. It should be 8" or so long if possible. Round the edges of the strip as well because they tend to crack when you bend it. The final grind marks should run lengthwise along the test strip or else they could act as starting points for cracks.
Bend each strip in opposite directions around a circular mandrel about 1
1/2" in diameter to form a U shaped bend. The weld should be located in the center of the bend. One strip will be the face bend and one will be the root bend. The one with the face of the weld on the outside diameter of the bend is the face bend. The part on the outside diameter is what gets the most stress.
I've never bent anything over 3/8" so I don't know if you could bend that
3/4" material, but it might be fun to try. If you have a shop press you could probably rig up something to do the bend for you. (you seem to have just about everything at you house :)). Otherwise, a long pipe for leverage and strong vise might work for you.
For 1" material, the standard is to cut a thin slice out of the material and then do a side bends instead. I don't remember what the correct thickness is. Maybe 1/4"? So the side of the weld is being stressed on the outside of the bend.
If you do a bend test on it, you will get a very good feel of the quality of the weld because any small defects from trapped slag, or poor fusion, will cause it to open up big holes in the material when you bend it - or just break completely if there are too many defects.
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