Analysis of the main reasons for the decline of elbow quality and the methods for attention:

　　1. When the pipe is bend, the outer arm wall of the inner layer is thinned under the action of the outer force distance m, and the inner pressure stress is reduced. The resultant forces N1 and N2 change the cross section of the pipe. Based on this factor, the main reason for the quality reduction of bend is RX and SX. Therefore, in gbj235-82, the RX value under various pressure levels and the thinning amount on the outside are specified clearly, so as to control the value of Rx and SX, so as to ensure the quality.

　　2. As mentioned in the previous article, when bending, the material is pulled outside, and the position of the inner pressure neutral axis is different from the bending method. When the top bending (compression bending) works, the neutral axis is about 1/3 away from the outer wall, and the neutral axis is 2/3 away from the outer wall when the bending (bending back) is working. Therefore, it is beneficial to use the method of rotary bending for thin-walled pipe bending.

　　3. The accuracy of bending tire is also one of the factors affecting the quality of bending pipe. In the process of bending manufacturing, we need to select the corresponding bending tire according to the bending diameter when the specification and dimension requirements are controlled within a certain tolerance range.

　　4. The bendability and surface corrosion of the pipe itself may also affect the quality of the bend. In the field construction, the operator also needs to know the material, processing performance and judge the surface corrosion of the processed pipeline.