Why would heat treatment be necessary? If the rear bridge was tig welded then the receiver likely would not have been heated to a point to damage it's original heat treatment except at the weld lines in the rear bridge.

I'm guessing that Mannlicher actions where case hardened like Mauser receivers. This case hardening was done for the sole purpose of wear reduction of the sliding/rotating action of the bolt. The elasticity of the selected steel for the manufacturing of the receivers is what maintains the receiver shape/dimensions against the pressures of firing cartridges.

I'm not an engineer or machinist I just dabble so what the heck do I know.

Vlad