Defect in Rolling-Drawing-Extrusion

Rolling defects:

Mill spring is a defect in which the rolled sheet is thicker than the required thickness because,

the rolls get deflected by high rolling forces. Elastic deformation of the mill takes place. If we

use stiffer rolls, namely roll material of high stiffness or elastic constant, we could avoid mill

spring. Normally elastic constant for mills may range from 1 to 4 GNm

Wavy edge/ Zipper cracks  :Roll elastic deformation may result in uneven sheet thickness across. Roll material should have high elastic modulus for reducing the roll deformation. For producing very thin gage sheets like  foils, small diameter rolls are used. They are supported with larger rolls. We can say the minimum thickness of rolled sheets achieved is directly proportional to roll radius, friction, flow stress. Flatness of rolled sheets depends on the roll deflection. Sheets become wavy as roll deflection occurs.

If rolls are elastically deflected, the rolled sheets become thin along the edge, whereas at

centre, the thickness is higher. Similarly, deflected rolls result in longer edges than the centre.

Edges of the sheet elongate more than the centre. Due to continuity of the sheet, we could say

that the centre is subjected to tension, while edges are subjected to compression. This leads to

waviness along edges. Along the centre zipper cracks occur due to high tensile stress there.

Cambering of rolls can prevent such defects. However, one camber works out only for a

particular roll force.

           Zipper cracks                                       Wavy edge                  Edge defect due to heavy reduction

Centre crack:In order to correct roll deflection for a range of rolling conditions, hydraulic jacks are used, which control the elastic deformation of rolls according to requirement

                        

                Centre crack                                                                                          Edge cracks

If rolls have excess convexity then the center of the sheet metal will have more elongation than the edges. This leads to a defect called centre buckle.

  Edge cracks: Small thickness sheets are more sensitive to roll gap defects leading to greater defects. Thin strips are more likely to undergo waviness or buckling. These defects are corrected by doing roller leveling or stretch leveling under tension. Stretch leveling is carried out between roller leveler rolls.

During rolling the sheet will have a tendency to deform in lateral direction. Friction is high at the centre. Therefore, spread is the least at the centre. This leads to rounding of ends of the sheet. The edges of the sheet are subjected to tensile deformation . This leads to edge cracks. If the center of the sheet is severely restrained and subjected to excess tensile stress, center split may happen.

Residual stress in rolling: Compressive stress is induced on the surface of rolled product if small diameter rolls are used or if smaller reductions are affected during rolling. Stress in the bulk of the strip is tensile in the above case. Larger reductions or rolling using large diameter rolls leads to tensile stress on the skin and compressive stress in the bulk of the metal. Stress relieving operation can be used to relieve the residual stresses of rolled products.

Barrel: Due to friction at the edges of the product barrel action takes place. Surface in contact

experience severe friction as compared to center of the work. Hence, with heavy reduction in the work the center tends to expand laterally more than the outer surfaces in contact with the dies and produces barreled edges.

Alligator Cracks Non-homogeneous material deformation across the thickness leads to high secondary tensile stress along edge. This leads to edge cracks. Secondary tensile stresses is due to bulging of free surface. Edge cracks can be avoided by using edge rolls. Due to non homogeneous flow of material across the thickness of the sheet, another defect called allegatoring occurs. This is due to the fact that the surface is subjected to tensile deformation and centre to compressive deformation. This is because greater spread of material occurs at center.

       

Extrusion Defects

Surface cracking (also called hot shortness)

              •If temperature, friction or speed is too high, intergranular cracks occur

              •Common in aluminum, magnesium, and zinc alloys

  •Bamboo defects are periodic surface cracks that develop due to the extruded product                                                     sticking to the die land

Pipe defect (also called tailpipe or fishtailing)

•Metal flow pattern draws surface oxides and impurities toward the center of the billet,  like a funnel

•To prevent, modify flow pattern to be more uniform, control friction and minimize temperature gradients, remove scale and impurities by machining or chemical etching prior to extrusion

Internal cracking (also called center burst, chevron cracking)

           •Due to hydrostatic tensile stress at centerline of deformation zone

            (similar to necking in a tensile test specimen)

             •Increases with increased die angle, impurities

            •Decreases with increased extrusion ratio and friction

Defects in rod and wiredrawing

Defects in the starting rod (seams, slivers and pipe).

Defects from the deformation process, i.e., center burst or cracking (cupping).

             • This defect will occur for low die angles at low reductions.

           • For a given reduction and die angle, the critical reduction to prevent fracture                  increases  with the friction.