It strongly depends on the design code, and the minimum design temperature. According ASME B31.3 the common rule is PWHT above 19mm for carbon steels.
For X70 there may be other requirements, since it is a special high strength grade. I don't know if this material is listed in the ASME B31.3 in the first place.
You know API5L has not the same approach as ASME codes
The API5L require PWHT for high frequency electric welding process. But for the others refer to the manufacturer decision. Now it is very important to get other reference or sientific book as a guide for PWHT identification
Is this referring to the seam weld of SAW pipe? If yes, the need for PWHT will be driven by the specific application of the pipe (end user specified)and the need to meet any supplementary requirements that are imposed (hardness for example).