Welding with the 309 or any other of this type of rod and weld a good deep weld helps , but it is not enough.
If the upper part of the die is flat and parralel to the bottom part when one put the hot steel in the dies open and they are not parraler any more.
Then when u operate the air or machanical hammer the hammer upper die will hit the high edge of the die tool and create a momentum on the weld and cause its tear appart.
I grind the upper part of the tool in a small radius and by then the hammer will hit the center of the tool and allways only one point of the tool is being forged and no breaking momentum.
If u do the welding right and the right radius grinding the tool will never fail
hofi