ASME IX does now cover B31 materials, but they are listed under S Numbers instead of P Numbers. Per ASME Table QW/QB-422, API 5L Grade X52 is an S No. 1, group 1 material. Since your WPS was qualified on P No. 1 material, it would be suitable for welding non-impact X52 material (it would not work the other way around though- qualifying on S-1 material, then welding P-1 material). Since you have impact requriements, your original qualification material would also have to be group 1, but you do not state whether it is or not.