42CrMo is considered difficult to weld due to its high hardenability. Procedures are strict:
Preheat: Mandatory. Typically 250-350°C, to slow the cooling rate in the HAZ and prevent formation of hard, brittle martensite.
Filler Metal: Use a low-hydrogen electrode or wire with matching or higher strength (e.g., EN ISO 16834-A G 69 6 M21 Mn2Ni1CrMo). Often an overalloyed filler (like a 2% Cr-Mo type) is used for better toughness.
Interpass Temperature: Maintained similar to preheat.
Post-Weld Heat Treatment (PWHT): Almost always required. A tempering treatment (e.g., at 600-650°C) is performed immediately after welding to temper the HAZ martensite, relieve stresses, and diffuse out hydrogen.
Major Risks: Without these steps: Hydrogen-Induced Cracking (HIC) in the HAZ due to hard martensite and hydrogen from moisture; loss of strength in the HAZ if overheated.