Work hardening

Cold-worked material has gained higher strength and hardness through forming. But the welding heat destroys this hardening in the heat-affected zone: the structure recovers and becomes softer there than the rest. This creates a weak zone next to the weld where the component yields first under load. So you need to know if your base material has been cold-worked. Keep the heat input as low as possible, then the soft zone stays narrow. If strength is critical there, the only help is to enlarge the cross-section or to re-harden the component specifically after welding.