WELDING WPHY 70 WITH WPHY 52

All you need to do is develop a weld procedure to ASME Section IX using a weld filler metal of the appropriate tensile strength, such as any 80 ksi filler. It should be noted that ASTM A860 is not listed in ASME Section IX, but you should still use the methodology and requirements when developing a weld procedure. You can use any welding process. ASTM A860 states that the carbon equivalent of the base metal should not be more than 0.42, per the equation in ASTM A860. This means you will likely need to preheat and per ASTM A860 you should PWHT (there are several options in ASTM A860). Make all your choices wisely to meet your final strength and toughness requirements.