What is the difference between J2 and JR? Can they be used as substitutes for each other?

J2 and JR refer to the material’s sub-grade – its fracture toughness – which is measured in
joules of a material’s toughness at a particular temperature.

J2 is a material with a minimum of 27 joules at -20C while JR is a material with a minimum of 27 Joules at +20C (room temperature).

A JR material is normally used in an internal environment whereas a J2 is for external use.