By what means can a small area or stripe be selectively hardened on a large steel part?

A small area or stripe on a large steel part can be selectively hardened using techniques such as induction hardening, where an electromagnetic coil heats the specific area, followed by rapid cooling. Another method is laser hardening, which uses a focused laser beam to heat the targeted region, also followed by quenching. Both methods enable precise control over the hardened zone without affecting the surrounding material.