Cr-Mo steels are susceptible to hydrogen cracking. Remove all hydrogen sources such as moisture, oil, and grease from welding area.
Preheat minimum 180C for base metal up to 13mm thickness; and 210C for 13 to 25mm base metal thickness. Planed interruptions of the welding procedure in which the weld is allowed to cool to room temperature should be avoided.
Use E8018-B6 or ER80S-B6 for filler metal.
Heat the welded joint 710C to 760C immediately after welding and held it one hour for each 25mm thickness at this temperature.
Best Regards