Pipe fittings are the general names of parts and components that play the role of connection, control, direction change, diversion, sealing and support in the pipeline system. Steel fittings are pressure fittings. According to the different processing technology, it is divided into four categories, namely, butt welding pipe fittings (divided into weld and no weld), socket welding and threaded pipe fittings, flange pipe fittings. According to the application: 1. pipe fittings for connecting pipes are: flanges, live joints, pipe hoops, clamps, clamps, throat hoops, etc. 2. changing the direction of the pipe: bends, bends. 3. pipe fittings changing pipe diameter: variable diameter (different diameter pipe), different diameter elbow, branch pipe table, reinforcing pipe. 4. pipe fittings with additional piping branches: three links, four links. 5. pipe fittings for pipe sealing: gasket, raw material belt, thread hemp, flange blind plate, pipe plug, blind plate, head, welding plug. 6. pipe fittings for pipe fixing: clasp, hook, ring, bracket, bracket, pipe clamp, etc. According to the connection :1, welded pipe fittings 2, threaded pipe fittings 3, clamping pipe fittings 4, clamping pipe fittings 5, socket pipe fittings 6, rubber ring connection pipe fittings According to the material divided into 1, cast steel pipe fittings 2, forged steel pipe fittings 3, alloy pipe fittings 4, stainless steel pipe fittings. Pipe fittings are widely used in municipal, petrochemical, west-to-east gas transmission, ships and nuclear power.