Formation mechanism:
The assembled inner and outer pipes are drawn along the axis of the inner liner pipe through a mold with a taper (usually the taper is 1:25, 1:50) and the maximum outline outer circle size is fixed. The inner liner tube is compounded on the inner surface of the outer base tube in the diameter direction by extrusion and expansion of the drawing die, and the outer base tube is also in the range of elastic deformation by continuing to expand. When the external force is removed, the inner lining pipe is plastically deformed and cannot shrink, and the outer base pipe is elastically deformed and shrinking, but limited by the inner lining pipe, the inner surface of the outer base pipe is strongly embedded on the outer surface of the inner lining pipe, and the composite take shape.
Features:
① The forming process is simple and effective.
② The inner surface of the composite pipe has good roundness.
Factors Affecting Quality:
① Due to the irregular shape of the inner surface of the outer base tube, the uneven height affects the passage of the full stroke of the maximum outer circle size of the mold profile.
The largest contour of the drawing die is a machined full circle, which must be parallel to the axis line. The material of the mold and the lining pipe is hard steel or mild steel. During the drawing and expansion process, the mold is embedded in the inner surface of the lining pipe. When the irregular inner surface of the outer base tube affects the passage of the drawing die, it will cause the axis of the drawing die to form an angle with the axis of the liner tube, which enlarges the maximum contour diameter of the die. When there is a symmetrical uneven height, due to the force acting on the drawing die, it is symmetrical in the 3600 direction, which will cause the drawing die to be avoided. Both phenomena will increase the bending deformation of the liner tube where it is squeezed, causing the surface of the liner tube to be cut at the maximum contour of the drawing die, causing damage to the surface finish, or even failing to pass. This situation is more pronounced when compounding larger diameter bimetallic clad pipes.
In order to ensure the surface quality and efficacy, the maximum contour size of the drawing die is usually reduced, so the combined bimetallic composite pipe has a small bonding force. Usually only between 0.20.3MPa.
② Due to the low bonding force, the gap between the annular joint surfaces of the inner and outer pipes is large, and the inner lining pipe will be repeatedly twisted and bent by the alternating pressure during the medium transportation process at the welded part of the pipe end, resulting in material fatigue and cracking at the joint. Lead to a decrease in corrosion resistance - (warping phenomenon).
③ Since the lining pipe does not achieve sufficient plastic deformation, the surface compressive stress generated by the action and reaction forces on the inner surface of the lining pipe cannot be fully reflected, and the surface compressive stress is hardly reflected. At this time, the straight seam weld in the liner pipe is still in a state of tensile stress, so the overall surface of the liner pipe, especially the straight seam weld, has a reduced thermal stress corrosion resistance. In addition, using the drawing process, the drawing die moves in a straight line, and the surface of the lining pipe is always in a state of tensile stress.
