Attempting to weld is probably a waste of your time, Trying to weld heat-treated high-carbon steel items like shovels qualifies as abuse of welding...unless you're considering old-fashioned "forge welding" of the blacksmith trade.
High-carbon tools steels respond very poorly to fusion welding as a rule. they tend to develop lots of tiny cracks in the heat-affected zone around the weld. In any case the mechanical properties around the weld zone will be drastically altered leading to a bad combination of low strength and high brittleness in addition to the cracking.
A 6' spade used by hand?
If so, wire brush thoroughly, and lightly grind a 1" margin
around the crack. Cut a suitable strip of .060" galvanized steel
and fasten generously with parker screws.
Braze the patch.
While still cherry red, dip the blade quickly in cold water.
Wait 5 seconds and repeat.
You will not be able to arc weld. Gas welding would require skill,
and restoring original temper to something so thin is unlikely.
No you do not need to anneal it and if you do you will have to heat treat the whole thing. Just weld it and it will be okay
Put it in the oven at 450 long enough to get it to that temp then weld it up. If you have access to a torch you can heat it locally. To set the weld and make sure it doesn't crack use your torch heating the weld zone to a dull red and use a ball peen hammer to "forge" the weld.
