Function 1. Reduce the hardness of steel pipes, improve the plasticity, to facilitate  cutting machining and cold deformation. 2. Fine grains and homogeneous steel organization and composition, to improve the performance of steel, also to make preparation for the heat treatment. 3. Remove the residual internal stress in steel, to prevent deformation and cracking. Feature 1. No need pre-heating. Comparing traditional black annealing furnace, it only takes 10-15 minutes to reach the exact temperature. 2. No need re-straightening. Using� rotary tube delivering structure, there is only tiny deformation after heat treatment.  3. Automatic uploading tubes, with strict docking of front and rear tube's end. 4. Fully automatic running, saving labor cost. Large diameter welded pipe black annealing machine is a special machine for heating the steel pipe to solution temperature and then cool it to room temperature. This machine adopts the advanced induction heating power supply, the new type DSP+IGBT power supply. It uses IGBT inverter, DSP precision control, air-cooling structure( water cooling for induction coil), reduce the loss of system. It has perfect self-protection and self diagnostic function, saves 15% energy comparing with the similar product. It is the first machine that could annealing such large stainless steel welding pipes.