G'day mate, I am a cutler, (bladesmith) by trade and I get this sort of thing all the time.
https://www.buildsbybaz.com/blades-by-baz/
Not knowing EXACTLY what grade of steel it is, you could start here:
Your welding and grinding may cause different parts of the steel to become soft or brittle.
Leave your tool edge thick and sharpen it after heat treating.
Firstly, after you've done all your grinding/shaping./welding, you should normalise the steel so it is the same all the way through. To do this, use a forge or a torch with a wide, even flame and heat it up to a dull red, then let it air cool until it is cool enough to touch, or you can drip water on it and it doesn't sizzle. Do this three times to normalise the steel.
To harden, heat the whole thing up until it is a uniform bright red to dull orange, (815 Degrees C), then quench in vegetable oil for 5 to 9 seconds. If you have a large welding magnet nearby, it needs to be just hot enough to be non-magnetic. Clean the black oil off and make sure you can see some bare metal. Turn your kitchen oven on to 250 degrees C and leave it in for about an hour and a half to two hours to temper it. You should find the bare metal is a straw-yellow if you got the temper right.
Now you can sharpen your edge, but keep a bucket of water nearby and keep the edge cool as you grind or you will destroy your heat treatment.
If you get it too hot or not hot enough and you find it is not hard at the end, you can simply repeat the process to get it right. A good test is to run a rough metal file over a corner. It should skate off and not bite in. The sound should ring slightly and not be dull.
Baz.