Let us start out by briefly explaining of these terms:
Radiography test is a nondestructive testing method of inspecting some hidden cracks in the material by using radio magnetic radiation to penetrate through the materials. The purpose is to ensure the structural integrity of the weld joint. As per ASME, there are four types of Radiography test (RT), i.e. RT1, RT2, RT3 and RT4.
Joint efficiency is the number used to define welded joint strength which basically depends on the RT type. This Joint efficiency plays a vital role in determining the thickness of the pressure vessel components in Mechanical calculation. For RT1 and RT2 joint efficiency is 1.0, for RT3 efficiency is 0.85 and for RT4 efficiency is 0.70.
The Weld Joint Category is how each weld on a vessel is classified to a Joint category, based on the criticality. As per ASME there are four types of weld joint categories, Category A, Category B, Category C, and Category D.
Now we can see how these three are interlinked as per ASME Sec VIII Div.1
As per ASME Sec VIII Div.1 the RT type 1 or 2 or 3 or 4 will be selected based on the type of weld joint category A or B or C or D. In turn the type of RT 1 or 2 or 3 or 4 will decide the Joint efficiency value 1.0 or 0.85 or 0.7. So this is how Radiography test, Joint efficiency and Weld Joint category are interlinked as per ASME Sec VIII Div.1.
To give an example, we have detailed the RT type with Joint efficiency based on weld category.
Category A and D butt welds shall be fully radiography (RT1) hence the Joint efficiency is 1.0.
Category B and C butt welds shall be spot radiography (RT3) hence the joint efficiency is 0.85.
This is because longitudinal joints are more critical than circumferential joints.
In general, heads connecting with shell will fall on Category B (RT-3 Spot Radiography), however in case of hemispherical head, the head with shell joint will fall on Category A (RT-2 full radiography), because this joint is more critical or it will be under double stress.
To help you better understand the weld joint category, we have taken a snap from ASME sec VIII Div.1 which shows the type of Joint category as shown below,