Analysis of the main reasons for the decline in the quality of the elbow and the method of attention: 　　1. In pure bending, the outer arm wall of the neutral layer of the tube is thinned by tensile stress &1 under the action of the external force M, and the inner side is under compressive stress. The combined forces N1 and N2 make the cross section of the tube change. Based on this factor, RX and SX are the main reasons that cause the quality of elbows to decrease. Therefore, in GBJ235-82, the RX value under various pressure levels and the amount of thinning on the outside are clearly specified, and the purpose is to control RX. And the value of SX, thus ensuring quality. 　　2. As mentioned in the previous article, when the pipe is bent, the outside of the material is stretched, and the position of the neutral axis under pressure on the inside is different from the bending method. In the top bending (compression bending) work, the neutral axis is about 1/from the outer wall. At 3 locations, the neutral axis is located 2/3 away from the outer wall when working in a rotary bending (return bending type). Therefore, for thin-walled pipes to be bent, it is beneficial to use the spin-bending method. 　　3. The precision of the bent tire is also one of the factors that affect the quality of the bent pipe. When we manufacture bent tires, in addition to the specification and size requirements to be controlled within a certain tolerance range, we also require users to choose the corresponding bent tires according to the bent pipe diameter during use.　 　 　　4. The bendability and surface corrosion of the pipe itself may also affect the quality of the elbow. During on-site construction, the operator also needs to understand the material, processing performance and surface corrosion of the pipeline to be processed.