If you have access to the back side of the flat stock, I'd drill a hole the size of your round tube, stick the tube most of the way through the flat stock, weld the outside, then weld the back side too. Or only weld the back side if you don't want to see the welds.
Full Answer
How to weld flat stock to round stock?
When finished, if your weld is sound, the flat stock will no longer be flat, the tube stock will no longer be round. I'd use DOM tubing thick enough to be adequate after it is deformed, then after welding, drill it and tap it cold. I would lay a couple pieces of round stock between, and weld them into the joint.
Should I bevel the top or bottom of a round stock?
If the round stock if fixed in a vertical position, only beveling the upper piece gives you a nice solid bottom to the joint to support the puddle. Otherwise it would be a little more common to bevel both.
Does the angle of the round stock matter for a puddle?
It won't much matter as long as the angle allows you enough access to get in and obtain good side wall fusion. If the round stock if fixed in a vertical position, only beveling the upper piece gives you a nice solid bottom to the joint to support the puddle.
How to draw steel?
Make the blows at 90° to the bar, with the swing toward the direction you want to draw the steel. Turn from side to side as you flatten and draw the rod. Third, once it is flattened out into something roughly bar shaped, work back from the edge as if drawing down flat stock into a bevel. Make a series of blows directly along the edge, ...
How to control a hammer?
One thing to mention on hammer control is to think of your arm working like a power hammer, the hammer comes down in the same spot every time and the piece you are hammering is the one that moves. Trying to aim or direct the hammer at the piece drastically reduces control. Also trying to over power the hammer reduces control, just hit moderately and consistently, while keeping the work hot and forging becomes so much easier and more effective.
Does Stacy use the horn of the anvil?
Stacy of course nailed it, but using the horn of the anvil can help too. It provides a rounded surface that isn't quite as extreme as the peen of the hammer.
