Hello all,
Andreas,
Welding does not cause the base metal to get harder unless one uses the incorrect type of welding rod/alloy for the material being welded i.e 7018 on cast iron... bad combination..hard and untouchable with HSS end mills, drills etc. but very weldable none the less. The coreect rod TO USE In this case would be NIROD (55% Nickle for a mild ,machinable weld on cast iron.) 7018 will make 4140 tougher as well so one must use a rod suitable for gun work and prefferably Tig weld for a more precise job.
The high heat generated by welding followed by slow cooling actually anneals the base metal.
So heat stop paste on the surroundind areas is required to preserve heat treatment and minimise distortions in the finished parts.

one can inadvertantly harden an area if it's not free of oil and or grease before welding.
Remember you have to be able to mill , drill or otherwise finish the parts you are welding?

Tom