Any welding on the hardened steel will cause problems, the welding process has to take the material to the melting stage of the material, say approximately 2600, 2700 degrees, this makes for a big change in the molecular structure of the steel, in affect you are wiping out a small portion of the heat treat, while the structure of the steel next to the weld area is still hard.
So we have hardened, and untempered steel and hardened, and tempered steel, plus large grain growth in the welded area, all this is taking place in the .00's of an inch.
When I used to do fit up work on tanks and boilers we would make fit up bars by sharpening an old piece of heavy leaf spring.
Some guys would weld handles of pipe on the end of theirs, it would always break, maybe not right away but it would break.
If the spring had been annealed, welded, annealed and then hardened..........
Remember, to leave the weld a bit proud, or full so you have enough metal to make a nice radius also.
Welding the guard and then grinding/filing a square corner in it would put a lot of stress on the weld.
Welds like to be smooth and radiused.
For example, turbine blades, aircraft and pressure vessell welds, must have no sharp corners, this according to Code.
Stress on a hard use knife should come under the same guidlines, no square corners on blade/tang junctions also.
my 2
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Sola Fide
