We have been welding on a coupler ladder assembly for an ATB. (Articulated tug barge) It veries in thickness from 3"-1/2" up to 9" thick. The coupler ladder is structural casting ASTM A148-90-60 (Ce= 0.69 Pcm= 0.38) and is being welded to ABS Grade A 1"-1/2" plate and also ABS AH36 3/4" plate. The current WPS is for FCAW it states to use filler metal AWS Specification: A5.20 and AWS Classification: E71T-1CDH8 and to preheat to 300* F. We are having some cracking problems on the weld toe to the cast parent metal. On some of the research that I did it states to use an E81T1-Ni1C wire and to preheat to 400*F. In clause 3 of AWS D1.1 2010 on 3.5.1 base metal / thickness combination. (base on the category and thickness) shall be the highest of these minimum preheats. Would you suggest to increase the preheat to 400* F and use the E81T1 electrode?
The 70ksi electrode should work and the 300 degree preheat should be sufficient. You should have a PQR that was used to qualify these materials. If you do not then you should start there.
My experience with preheat...
When a 300 degree preheat is required keep in mind that the measurement to deturmine if the preheat's been met would be a minimum of 3 inches from the weld. That's 3 inches or the thickness of the material, which ever is more. You mentioned 9 inch material. That preheat would need to be measured 9 inches from the weld.
With toe cracks in high strength material (that A148 is a Class 3) I would suspect preheat and more then that, I would suspect a preheat that is not being applied the full material thickness distance.