Therefore, it is necessary to study effective non thermal aging methods and test methods and scientific basis for the possibility of post weld heat treatment free of welded joints.

7. Classification by connection method (1) Threaded connection valve: the valve body is equipped with internal thread or external thread and connected with pipe thread. (2) Flange connected valve: the valve body is equipped with a flange, which is connected with the pipe flange. (3) Welding connection valve: the valve body is provided with a welding groove, which is welded with the pipeline. (4) Clamp connection valve: the valve body is provided with a clamp, which is connected with the pipe clamp. (5) Ferrule connection valve: ferrule connection is used to connect with the pipeline. (6) Wafer connection valve: the connection form in which the valve and two end pipes are directly clamped together with bolts.