Take the pipe, mark with chalk the place that you need to straighten. Prepare metal straightening plate, put on it straightens the pipe so that the bend was from the top. A heavy hammer or small sledge hammer to strike the bulging part of the tube until it aligns. Beat not the tube itself, and through the wooden prorate. This method is difficult to carry out alone, you will need assistants to hold the pipe from turning. It is not necessary to strike - this can lead to deformation of the pipe diameter.
Put on the stove press the pipe upward. Hold the pipe in this position, giving her a punch press. In this case, the pipe in the curve can be heated with a blowtorch, but you can use a gas burner. Push the pipe under the pressure of a few inches and repeat the previous operation. Repeat this process until you straighten the pipe. It is recommended that before to straighten the pipe in this way, pour into the tube the sand to complete its filling. It tightly plugs on both ends (first from one end, stop, pour sand, then stop second). This will help to avoid strong deformations straighten pipe diameter.
Take a section of pipe with an internal diameter a few millimeters larger than the outer diameter of the pipe straighten, i.e., such that the tube fairly freely entered into this segment. Preferably, a thick-walled forged pipe with smoothly trimmed edges. Clean burrs from the inner edge of this segment. Clamp it in a large bench vise. Prepare straighten the pipe, why fill it with sand, as described above. Insert the pipe you want to straighten that section of pipe, which is clamped in a vise. Move the bending point to the edge segment and fueling his gas burner or blowtorch, slowly press in the direction opposite the bend.
If you straighten the pipe to enclose wood or some soft material, it will be less bounce, and you quickly reach the desired result.
All the work on straightening and straightening the pipes, no matter how minor they may seem, do in mittens. This will help you to avoid injuries.