I would not use 70 tensile strength filler metal as X70 has a minimum tensile strenth of 82 ksi. You might achieve that based on what brand/type of consumable you use, but not probable.
I assume you are not going to PWHT. In that case, I would use either E81T1-K2 (more than 1% Ni, so make sure NACE requriements for sour service are not imposed), or 8018-C3.
fakhmed,
As in most situations, there are numerous choices that will work. I've done as GRoberts has suggested and qualified a procedure using a low Ni electrode, but, Lincoln recommends among other things, 8018G. As to PWHT, you'll have to evaluate whether or not this is necessary based on other criteria.
8018-G really means nothing by itself. It just means it ia 80 minimum tensile strength, and of any chemistry. It could be a 12 Cr creep resistant electrode for all the 8018-G classification tells us. Each -G electrode has to be evaluated seperately with input from the manufacturer. There are literally tens if not hundereds of different kinds of -G electrodes out there, and they are all for different applications.
I've ordered already E81T1-K2M wire 1.2 and 1.6mm and we will proceed the welding with 80°C preheat and without PWHT for the pipe 1200mm diameter and 18mm thickness.