Choice of head material

　　1. Comparison of mechanical properties of materials

　　According to the basic principles of pressure vessels, the material of the head is generally the same as that of the cylinder. That is to say, the material of the pressure test head of the pipeline is preferably the same as that of the pipe, so that its strength and welding performance are exactly the same. However, due to the small amount of materials used for the pressure test head and the special nature of the pipe material, it is very difficult to obtain the same steel plate as the pipe material, and the pressure vessel steel plate is very common. From the actual situation, the pressure vessel steel plate is used instead of the pipe steel plate. The head cover is a relatively simple and applicable solution. According to GB713-2008, the most commonly used pressure vessel steel plates are Q245R (20R) and Q345R (16MnR).

　　2. Matching of pipe and pressure test head material

　　For steel pipes made of Q235-A, B, C, and D of GB700, although GB3274 has corresponding materials, GB150 has cancelled Q235-A and has strict restrictions on the use of Q235-B and Q235-C.

　　3. Determination of the thickness of the pressure test head

　　Determine the calculated thickness δ according to the calculation formula of spherical head, standard elliptical head, and tapered head (optional according to actual conditions). The negative deviation C1 of the steel plate is taken as 0.8mm, and the corrosion margin is 1mm. In general, the pipeline pressure test is equal It is carried out at room temperature, and the room temperature range can be determined to be higher than -20 ℃ and lower than 100 ℃. The allowable stress of Q245R (20R) [σ] t =133MPa; the allowable stress of Q345R (16MnR) [σ] t =170MPa.

　　4. Need to explain

　　The butt joint of the pressure test head, the tapered transition section and the pipeline shall be grooved in accordance with the standard. The weld (including the longitudinal weld of the tapered transition section) must be 100% radiographically inspected, and the head must be the entire plate stamping.

　　Q245R (20R), Q345R (16MnR) and pipeline materials are recommended for welding process qualification to ensure the mechanical properties after welding.