Warped

Making myself a rig for a table top drill I ran into a problem with warping of a rather thick mild steel plate welded to heavy peace of tube. I wasn't easy to find a solution (but many with a similar problem) and I got many a weird advice on the way. Eventually I got it straightened, and I thought I might shared my findings.
Thanks to Hotrodder's tip in a UK forum I found the knowledge and the courage to do what I intended to in the first place.

I used O/A to heat the plate on the opposite side of the welding, using a very narrow flame and trying not to melt the iron (for appearances sake). Discolouration is easier to grind away than melted scars.
I then left it to cool and were pleased that it had helped. I did several turns, and eventually the plate was almost flat again.
Here are some pictures to illustrate the task.

Attachment 5700Attachment 5701Attachment 5702

What you are doing is heat shrinking the material.
When you heat the material up to a red color in a local area, the thermal expansion in the local area actually yields the material in compression.
The material surrounding the hot area is much cooler, doesn't expand, and therefore imparts compression onto the hot area.
the hot area has a much lower yield strength and plastically deforms in compression.
When the part cools, and the thermal expansion equalizes, the area that was shrunk remains shrunk.

the technique is used a great deal in making large fixtures or other large structures.

Welcome to the forum WW, the stand looks sturdy.