What is the reason for the formation of holes in the production of spiral steel pipes?

The rolled parts of spiral steel pipe are repeatedly rolled in the deformation zone because of the spiral motion. Due to the influence of the stress-deformation state, the center of the rolled piece has a longitudinal internal tear, which is called the hole cavity. Under certain process deformation conditions (processing temperature, deformation speed, tool design, etc.), the diameter shrinkage of the tube blank reaches a certain critical value E, and then longitudinal micro-cracks appear along the axis, and then the cavity is formed. Premature formation of a cavity in front of the head will cause a large number of internal folding defects, deteriorate the surface quality of the spiral steel pipe, and even become waste products. Therefore, in the perforation process to avoid premature formation of cavity. The reason of cavity formation is ultimately determined by the stress state of the metal.

What is the reason for the formation of holes in the production of spiral steel pipes?

There are two reasons for the actual formation of the cavity:

(1) The influence of the ' outer end ' . A large number of experimental studies show that when the primary radial compression rate of round forging is below 6%, the maximum plastic deformation zone only occurs near the surface in contact with the tool, and the axial deformation is very small, which is similar to the characteristics of double drum deformation. The characteristics of single drum deformation only occur when the primary diameter shrinkage rate reaches more than 10%, but the double drum deformation here occurs inside a cross section. There are also ' outer ends ' with small deformation on both sides of the severe deformation zone I and III, so that the transverse flow of the metal in the zone I and II plays a ' wedge ' effect on the outer ends of the two sides, making the axis zone II bear a strong additional transverse tensile stress.

a- compression rate of less than 6%; b- compression rate of more than 10%

(2) Surface deformation. Under the condition of spiral steel pipe cross rolling, the plastic deformation of the surface metal is severe, and the metal flows continuously along the axial and tangential direction, which inevitably pulls the metal in the axial zone to the surface continuously as a whole, and forms a three-way additional tensile stress on the axis. In this way, the working stress state of the axial area of the solid body of the spiral steel pipe is the compressive stress in the direction of external force, and the other two directions are the tensile stress. Because the additional stress caused by the two factors is in the same direction, the value of the transverse tensile stress is the highest and the growth rate is the fastest. It is impossible to completely restore recrystallization after each roll processing of spiral steel pipe under the condition of diagonal rolling, so the additional stress will be partially retained in the form of residual stress, and will accumulate and increase with the increase of repeated processing times. No matter how the rolled piece rotates, the basic phase of this stress field in the axis region is constant. Therefore, when the working stress state develops to a certain limit value, the shear deformation begins to occur in the direction of the maximum shear stress relative to the principal stress of about 45°. After many repeats, due to the existence of work hardening and internal defects in the crystal, these parts will crack under the action of maximum transverse tensile stress, and gradually develop into an axial loose area to form a cavity.

There are many explanations about the formation mechanism of cavity in spiral steel pipe. It is generally believed that the formation of the cavity is caused by the alternating shear stress and transverse tensile stress of the rolled piece. When spiral steel pipe is being rolled, the rolled piece moves helically, and the central part of the rolled piece is subjected to alternating shear stress and transverse tensile stress. The shear stress causes the metal to slip and form micro-cracks, and the tensile stress causes micro-cracks to expand and form macroscopic cracks, which expand and connect to form cavity.