hey- just wanted to share my findings i had when i was handed the scraps of some long steel beams where my boss decided to not order enough steel beams for a project
welding somewhat small steel beams has always been a struggle for me as they warp and bend when you weld but this time i got it right- or, at least very close to perfectly right

the procedure was to not clamp it and to lock the damn thing after its welded by cooling it!

first i placed the 2 segments on the table, chamfer maybe half to the center both sides. then get a long straight piece of aluminium flat"steel" to line it up
then 2 tacks on ONE SIDE of each the beams, the side facing me - off the edge of the table. the angle irons point is facing upwards
5mm gap- its roughly 10mm thick steel
then i bridge at the highest point, i believe i ran it at about 190 amps. tap welding it keeping it just on the verge of falling through the gap
no sidemovements. the angle of the pistol is pointing slightly upwards. moving it down just a bit for every blast but not so far down that the wire is underneath the weld
i make one pass and theres about 2mm of material lacking in the weld
at this point with a water spray i cool it down carefully, not too strongly as the steel might not like that- water MIST not water jet - until the water sticks to the metal, below 100*C
then i move onto the other side and do the same, starting at highest point and giving it long bursts of tack welds- cool it down
go back to the other side again, fill it up just a bit excess this time, keeping it nice and centered. cool it down. grind it down so its just a tiny bit excess still but nice and flat. finish the other side. cool down
then with a wrench i flip the beam at the very end, the tack welds simply break off. for a more smooth break you can weaken the tacks with cutting disc first as the breakage might bite into the material

flip the beam and tack it up like on the other side, you have to do long tacks so it really penetrates.
these tack pulses ive tested with bend test and its very doable to get perfect penetration, the 5mm gap and chamfer is what makes it easy to perfectly fill it up with material

now- this isnt PERFECT, the times ive done this the material mysteriously.. warps a bit in the opposite direction it would normally pull when having the first welds. maybe clamping it to table with some steel plates under center and letting it go while still hot or maybe welding / clamping it and then locking it by cooling it down before releasing the clamps/welds?

ive cooled down metal that i was correcting with water before and had great success with it as you dont have to worry what the last bit of heat warping will then leave you with
the first set of these i did was so messed up that i had to heat it up and put metal plates underneath the ends, pushing the center down by clamp after heating it redhot and then locking it by cooling it down, worked well!