AISI 4140 has poor to fair weldability due to its medium carbon content. It is prone to forming hard, crack-sensitive martensite in the Heat-Affected Zone (HAZ). Welding is generally not recommended for already heat-treated components unless followed by full re-heat treatment.
If welding is unavoidable:
Preheat: Essential. Typically 250-350°C for medium sections.
Low-Hydrogen Process: Use low-hydrogen electrodes (E10018-D2 type), and keep them perfectly dry.
Post-Weld Heat Treatment (PWHT): Mandatory. Immediately after welding, perform a stress relief anneal at ~600-650°C, or a full re-austenitize, quench, and temper cycle to restore properties and soften the HAZ.
Filler Metal: Often a lower-strength, more crack-resistant filler like AWS ER80S-D2 is used to under-match strength but ensure joint integrity.