Nalla, answers to your questions based on my experience:
1. Use ER80S-D2 or ER100S-G for GTAW and E10018-D2 for SMAW. All welding cons must contain less than 1% nickel in order to comply with nace.
2. Use 1 - 1.5 kj/mm HI. Use 200 deg C min preheat with 350 deg C max interpass temp.
3. PWHT at 640 - 650 deg C for 2 to 4 hours soak time to achieve nace hardness requirements in the haz and weld metal.
Rgds