1. Roll changing

1) Base level distance: One end of the bearing bush has a fixed end, the fixed end has a base level, or on the stand frame, or on the bearing bush, in order to determine the vertical distance between the rolling center base level and the rolling center.

2) In order to ensure the correct installing position of the roll, it is necessary to ensure that the base level of each horizontal frame is in the same level and shall not be loosened.

3) The height of each lower horizontal shaft shall be exactly the same so as to ensure the correct horizontal position of rolls.

4) Matters need attention of stands installation:

  (1) Pay attention to the direction of the bearing bush under the open frame to avoid reverse loading. Do no install the upper and lower shafts in reverse.

  (2) Make sure each adjusting part can slide and can be adjusted conveniently.

  (3) Each fasten part shall not be loose.

  (4) Check the size and surface of rolls as well as those of each closed hole guiding ring.

  (5) Roll installation and fixation should be fasten. Axial tandem motion and radial run out is not allowed. Check whether bearings are damaged or loose.

2. Adjustment after roll changing

1) Rolling center line verify.

  (1) The principle of adjusting mill is considering the lower horizontal roll as base level to verify the rolling center line.

  (2) Pull one center line from the first forming stand to the last sizing stand, contain certain tension and the bottom of pass groove. Pay attention that the center line shall not contact any part exclude the lower horizontal roll.

  (3) Each pass center of lower horizontal roll shall be conform with the center line position.

  (4) Each groove bottom of lower horizontal roll shall be closed to the center line.

  (5) Adjust the horizontal position of rolls. Check transversely whether the upper and lower roll shaft center line of each horizontal roll is horizontal or not. Whether there is the tilting phenomenon that one end higher and the other lower, if so, adjust to horizontal by the pressing down device.

  (6) Adjust the roll gap between each stand correctly. Adjust the roll gap between each horizontal and vertical roll according to pass drawing and technological procedure, normally it is the thickness of the strip. If the gap is too large, forming will be not insufficient, and will cause sliding and twisting of strip in the pass; if it is too small, it will increase the forming load and damage the stand.

2) Vertical roll adjusting.

  (1) Be symmetric with the rolling center.

  (2) The end face shall be level.

  (3) The second, third and fourth vertical roll bottom shall be higher than the rolling center line.

  (4) Others shall be a little bit lower than the rolling center line from oval to round.

3) Adjusting principle

  (1) Vertical roll higher: cause the upwarp of strip head, and the running strip may be made an arch between vertical rolls, which increase the wearing of pass bottom, so the edge may damage the strip.

  (2) Vertical roll lower: it’s bad for the deformed strip to enter the pass, and may damage the strip edge, thus cause welding imperfection.

  (3) Guiding roll: adjust the lower roll pass groove bottom to slightly higher than the center line. Function: eliminate crease, increase current concentration.

  (4) Eight-roll adjusting: inset a finished pipe into the eight-roll and try to center the rolls. Adjust pass position, parallel the pipe and rolling center line, push the pipe and could insert the sizing part correctly.

  (5) Extruding roll adjusting

      A. The output pipe is oblate, that is, the elevation is smaller than the plane.

      B. The pipe seam is in the roll gap and shall not be buried in the pass.

      C. The edges of the pipe should be well connected and should not be misplaced.

      D. The head runs stably and is not allowed to deviate from the turn seam.

3. Adjusting during production

1) Dislocation and torsion

(1) Horizontal roll

      A. Fine centering performance, but slightly worse parallelism of the upper and lower rolls, which will cause higher pressure on one side of the strip with small shaft gap, and make it deflect to the reverse direction. Under this situation, we should increase the pressure on the deflection side of the strip. (Positive adjusting, small adjusting amount)

      B. Fine parallelism of the upper and lower rolls, but worse centering performance. The upper roll tilt inwards, which causes higher pressure of the strip inside. We should deflect to the outside and decrease the pressure of strip deflection side. (Negative adjusting, large adjusting amount)

          This fault often occurs in the fourth and fifth stands, check the parallelism the upper and lower rolls first!

       C. The two side rolls seriously damage or guiding ring damage on the upper roll guiding ring of each closed pass stand.

(2) Vertical roll

       A. Before returning to the circle, the vertical roller moves in the same direction as the seam turning.

       B. Closed vertical roll large dislocate, which may cause one side of the pipe seam running closed to the former stand guiding ring, and causes the pipe seam turn reversely.

       C. Vertical multi-direction dislocation will cause the irregular seam turning of the pipe, which cause unsteady running of the pipe.

       D. The wearing of horizontal rolls will decrease the rolling center line, which will cause the relative rise of vertical roll, head upwarp and push the pipe into the seam.

       E. The pressure is not enough before turning to circle, the pipe head enters into roll gap.

       F. Vertical roll of the two sides on each stand are not symmetric or high and low movement.

(3) Extrusion roll

       A. Slight dislocation, reversely rotate the guiding roll according to the pipe seam direction.

       B. Bigger dislocation, one side if pipe edge runs cling to the guiding plate because of extrusion roll center dislocation. Under this situation, the welded junction will turn the seam reversely, that is, dislocate direction will be the opposite to the seam turning direction.

       C. Adopts the flat pass design of extrusion roll.

       D. Keep level, no elevation angle.

       E. Keep appropriate extruding pressure.

(4) The strip has sickle bend.

(5) If the running of strip is not steady, eliminate by increase pressing amount.

2) Swell

(1) Check whether the rolling bottom line is reasonable or not, bottom diameter of each lower roll reasonable or not, roll pass design reasonable or not.

(2) When adjusting, increase the pressure of the vertical roller set before the closed pass, increase the pressure of the closed pass, and increase the pressure of the preformed opening pass.

(3) Swell is because of the edge stretch of plate in forming section, normally adjust by gradually increase the pipe bottom line of fin-pass section.

3) Indentation and scratches

(1) Roll defect, broken roll, drop clip or iron chips and other sundries stick on the roll.

(2) The moving speed of pipe are not equal to the roll speed, relative sliding occurs. We can disassemble the transmission shaft and let the roll rotate freely.

(3) Roll diameter is not reasonable, bearing defect, bad chamfering of the roll ring.

4. Effect of operation on welding quality

1) Inductor

(1) Distance between the inductor and steel pipe should be 3mm to 5mm

(2) Multi-turns inductor should have 2 to 4 turns, wrap into a round barrel by round or square copper wire and cool down by infill water.

(3) Single-turn inductor width:

        When weld pipe Dia. below 1.5 inches, it is 1.5 times of the pipe Dia.

         When weld pipe Dia. between 2-3 inches, it is 1.2 times of the pipe Dia.

         When weld pipe Dia. above 4 inches, it is the same as the pipe Dia.

         Multi-turn inductor width: select refer to single-turn, slightly narrower than single-turn.

(4) Setting position: inductor and pipe put concentrically and as close as possible to the extrusion roll. To increase efficiency, we can put the inductor at an angle to the welding seam.

2) Impeder (ferrite core)

(1) Gap: the gap between pipe shall be 6mm to 15mm

(2) Length:

          When weld pipe Dia. below 1.5 inches, the length shall be 150mm to 200mm;

           When weld pipe Dia. between 2 to 3 inches, the length shall be 250mm to 300mm;

           When weld pipe Dia. above 4 inches, the length shall be 350mm to 400mm.

(3) Setting position: the head shall be coincide with extrusion roll center line, the distance from the tail to the center of the inductor should be greater than the length from the head to the center of the inductor.

(4) To increase efficiency, we can add additional impedor to both sides of the inductor.

3) Welding system

(1) Solid phase plastic pressure welding

           In this way, the edge of the tube billet is heated to a high temperature of 1300~1350 degrees Celsius, but it has not yet reached the melting state. Due to the strong pressure of the extrusion roller, the oxide film at the edge is extruded from the weld, and the solid phase is crystallized at high temperature, so that the two edges are welded together. This method requires the extrusion pressure of the extrusion roller is large, which should not be less than 4~5 kg/mm & Sup2; . The burr height is small, but the surface is smooth and uniform. It is characterized by no spark splashing during welding.

(2) Semi-fusion welding

           In this way, the edges of the tube billet are heated to a semi-melting state at the intersection point. The welding temperature is higher than that of the first kind of solid phase plastic pressure welding, about 1350~1400 degrees Celsius. With the help of the pressure of the extrusion roll, the liquid metal with oxide can be easily extruded from the weld and welded in the semi-melting state. This method requires less, about 2~3 kg/mm & Sup2; . The burr height is small, but the surface is not too smooth and uniform. It is characterized by a slight spark splash at the intersection when welding.

(3) Fusion welding

           In this way, the edges of the billet are heated to a molten state on the way to the intersection, and the welding temperature is about 1400 degrees Celsius or more. Due to the early melting of the edge metal, the edge lintel occurs before the intersection point (the center of the extrusion roller). The current is highly concentrated here, so that local vaporization occurs at the lintel, and strong spark splashing occurs under the pressure of the extrusion roller. It is characterized by periodic intense spark splashing before the intersections during welding. The extrusion pressure in this way is similar to the second, but the inner burr and the outer burr are larger, and the periodic hills are strong.

(4) Opening angle

           A. Increase of opening angle: the current decrease is not obvious, and the welding quality is easy to be guaranteed.

           B. Small opening Angle: produce large particles flash spatter, heat loss increases, slag is not easy to remove, welding defects.

           C. Thin wall thickness, increase the opening Angle appropriately.

            D. Thick wall thickness, the welding speed is the main contradiction, and the opening angle should be appropriately reduced.

            E. When welding large pipe, the loop impedance is large, and the opening angle should be larger.

            F. When welding small pipe, the opening angle should be smaller.

            G. Normally the opening angle is 2 to 6 degree.

(5) Welding speed: It is more suitable to weld at the maximum allowable speed of the mechanical equipment and welding device of the pipe welding unit.

(6) Pipe billet shape: Edge shape affects the metal flow rising angle, affects the internal and external burr, and affects the welding strength of the entire welding section. Should be X row, followed by I row.