RBrown
Hydraulic cylinder shaft is 1045/1050 that has been chromed. I use to work at Gannon MFG, they make tractor equipment we made our cylinders in house.
Sam
Get out of the box look around there is something out there I assure you. It will take you a long time to get that 4" square to weld temp then you will have to contend with the weight of that bar. Maybe easier to have the bar mounted solid and your plate on top and rig the forge with some handles and slip it over the bar. This way all you need to do is strike it when the forge has been removed. This will take some help, me I would have 1 guy for turning to propane on and off 2 guys to handle the forge and 2 guys to strike the plate. Have fun and most of all be safe.