These two types are the flange types transplanted from the American ANSI B16.5 pipe flange standard to the German system pipe flange standard when the HGJ pipe flange standard is prepared. In ISO7005-1, some flanges of PN0.25~4.0MPa are also included in the type of flat welding flange with neck. Compared with butt welding flanges with necks, the neck height of flat welding flanges or socket welding flanges is lower, so the production of flanges adopts rolling or die forging process, which is simpler than high neck flanges. The addition of a short neck to the flange is of great benefit to improve the rigidity of the flange and improve the bearing capacity of the flange. These two types of structures are commonly used in petrochemical plants introduced in recent years. On the other hand, the neck flat welding flange or socket welding flange adopts the structure of fillet welding or partial penetration fillet weld, which is more convenient for on-site installation. For construction units, the process of film inspection of welds can be omitted, so it is more popular. This revision refers to the provisions of ISO7005-1, combined with the actual use experience in the past few years, and adjusted the nominal pressure range of the flat welding flange with neck, from PN≤10.0MPa to PN≤4.0MPa. The nominal pressure of socket welding flange is still PN≤10.0MPa. Although the socket welding flange is only used for fillet welding on one side, because it is only used for small-diameter pipes below DN50, the load-bearing capacity of the weld is still sufficient after calculation. Due to the structure of the necked flat welding flange or socket welding flange, especially the characteristics of the welded structure, there are also restrictions on its use abroad. a. Weld neck flanges (ANSI B31.3 308.2.4) shall be used for flanges operating under severe cycling conditions. b. Weld neck flanges or socket weld flanges shall not be used in piping systems subject to frequent and large temperature cycling (ANSI B31.3 308.2.1). c. Socket welding flanges should not be used in crevice corrosion or strong corrosive media (ANSI B31.3 308.2.1) The above "severe cycle" refers to the displacement allowable stress of the pipeline's displacement stress σE ≥ 0.8 times [ σ]A, or when the equivalent cycle number N ≥ 7000, the above definition can be found in the American ANSI B31.3 or HGJ8-87 "Chemical Pipeline Design Code". Because the compression stress of the metal ring gasket is high, and the bearing capacity of the socket welding flange is worse than that of the neck butt welding flange, there is no ring connection surface for the socket welding flange. There are four types of sealing surfaces for flat welding flanges with necks: full surface, convex surface, concave-convex surface, and tongue-and-groove surface. Due to the single-side fillet welding of socket welding flanges, according to the requirements of relevant American specifications and the actual use experience in imported devices, although the maximum diameter of American ANSI B61.5 is DN80, it is actually used below DN50. For this reason, the maximum diameter of the socket welding flange in this standard is consistent with the HGJ pipe flange standard, which is still DN50. Because socket welding flanges are widely used in small diameter flanges, the lower limit of the nominal diameter is expanded in this revision, and DN10 socket welding flanges are added.