4140 steel has moderate weldability and is not classified as a free-welding steel.
4140 steel contains approximately 0.4% carbon and alloying elements such as chromium and molybdenum, resulting in strong hardenability. During the welding cooling process, the heat-affected zone is prone to forming a hard and brittle martensitic structure, which easily leads to welding cracks.
Therefore, it is necessary to reduce welding risks through preheating (200~300℃), using low-hydrogen welding electrodes, and timely tempering after welding.