Molybdenum solid solution strengthening

Molybdenum makes steel stronger and harder by embedding itself in the structure and straining the lattice. That stuff is good for strength, but when welding you have to be careful: if you have too much molybdenum in the material or you don't have the heat under control, the material becomes prone to hot cracking. Keep the welding parameters clean and stable, and if the part is supposed to be stress-relieved after welding, then actually do it. That prevents the structure from cracking or the weld from becoming brittle.