31 61 ME 4563 Fall ’05 Dr. S. Haran • • Use Lower Reductions Use Lower Reductions This may increase the number of required passes, thus This may increase the number of required passes, thus requiring: requiring: More time More time Possibly more equipment requirements Possibly more equipment requirements Possibly more energy Possibly more energy • • Increase the Work piece Temperature Increase the Work piece Temperature This lowers the yield point This lowers the yield point This allow This allow recrystallization recrystallization and reduces strain hardening and reduces strain hardening • • Apply Tension Apply Tension Either side will reduce the force Either side will reduce the force If applied at the entrance, termed back tension If applied at the entrance, termed back tension If applied at the exit, termed front tension If applied at the exit, termed front tension Reducing the Roll Force Reducing the Roll Force 62 ME 4563 Fall ’05 Dr. S. Haran Lubricants Lubricants … … Normally none used for rolling Ferrous alloys; Graphite Normally none used for rolling Ferrous alloys; Graphite may be used sometimes may be used sometimes Water Water - - based solutions are used to cool the rolls based solutions are used to cool the rolls Non Non - - ferrous alloys are hot ferrous alloys are hot - - rolled with variety of rolled with variety of lubricants such as oils and fatty acids lubricants such as oils and fatty acids Cold rolling is carried out with low Cold rolling is carried out with low - - viscosity lubricants viscosity lubricants such such as mineral oils, paraffin, etc. as mineral oils, paraffin, etc. Rolling Mills Rolling Mills - - Lubricants Lubricants 32 63 ME 4563 Fall ’05 Dr. S. Haran • • Significant effect on product Quality and Productivity Significant effect on product Quality and Productivity • • Chatter (self excited vibration) can occur in rolling as well Chatter (self excited vibration) can occur in rolling as well as as other processes (extrusion, drawing, machining, etc.) other processes (extrusion, drawing, machining, etc.) • • Related to the dynamics of the rolling action Related to the dynamics of the rolling action • • In Rolling In Rolling – – predominantly occurs in Tandem mills predominantly occurs in Tandem mills • • variation in thickness, poor surface finish, high levels of variation in thickness, poor surface finish, high levels of scrap scrap • • frequencies related to the resonance as well as other natural frequencies related to the resonance as well as other natural frequencies of the system frequencies of the system • • Controlled by process parameters: distance between roll sets, Controlled by process parameters: distance between roll sets, width of job, speed of rolling, increasing damping, etc. width of job, speed of rolling, increasing damping, etc. Rolling Rolling – – Vibration and Chatter Vibration and Chatter 64 ME 4563 Fall ’05 Dr. S. Haran • • The forces of rolling will flatten the rollers elastically The forces of rolling will flatten the rollers elastically – – similar to tires on the road surface similar to tires on the road surface (Roll radius, (Roll radius, Modulus of elasticity, roll forces) Modulus of elasticity, roll forces) • • This flattening causes an increase in the contact This flattening causes an increase in the contact area area • • The increased contact area causes the forces to The increased contact area causes the forces to increase increase • • Roll Flattening can be reduced by: Roll Flattening can be reduced by: • • Using a roller material with a higher modulus of Using a roller material with a higher modulus of elasticity elasticity • • Reducing the roll force Reducing the roll force Rolling Rolling – – Problems with Rolls Problems with Rolls Roll Flattening Roll Flattening 33 65 ME 4563 Fall ’05 Dr. S. Haran Roll Deflection Roll Deflection • Roll forces tend to bend the rolls: leading to a strip that is thicker at the center than at the edges •Solution: Grind the rolls such that the diameter at the center is slightly larger than at the edges • This is known as giving them CAMBER –In practice, this would generally be around 0.25 mm or less on the radius. Camber is only correct for a • given load on the roll • thickness of the strip Hence normally done for a large batch, due to economic reasons Rolling Rolling – – Problems with Rolls Problems with Rolls 66 ME 4563 Fall ’05 Dr. S. Haran (a) (a) Bending of straight Bending of straight cylindrical rolls, cylindrical rolls, caused by the roll caused by the roll force. force. (b) (b) Bending of rolls Bending of rolls ground with camber, ground with camber, producing a strip producing a strip with uniform with uniform thickness. thickness. Roll Roll Deflection Deflection Rolling Rolling – – Problems with Rolls Problems with Rolls 34 67 ME 4563 Fall ’05 Dr. S. Haran Spreading Spreading • • Rolling plates sometimes causes the Rolling plates sometimes causes the width to increase considerably width to increase considerably • • This increase is called This increase is called SPREADING SPREADING • • Controlled with: Controlled with: • • high width high width - - to to - - thickness ratio thickness ratio • • roll radius to strip thickness roll radius to strip thickness ratio ratio • • decreasing friction between rolls decreasing friction between rolls and material and material Spreading can be observed when Spreading can be observed when dough is rolled with a rolling pin. dough is rolled with a rolling pin. Rolling Rolling – – Problems with Rolls Problems with Rolls 68 ME 4563 Fall ’05 Dr. S. Haran Successful Rolling depends upon various factors Successful Rolling depends upon various factors including material properties, process variables, and including material properties, process variables, and lubrication lubrication • • Defects: Defects: • • on the on the surface surface of the rolled material of the rolled material • • structural defects structural defects within the rolled material within the rolled material • • Surface Defects: Surface Defects: • • inclusions and impurities in the material inclusions and impurities in the material • • scale, rust, dirt, roll marks, etc., caused due to scale, rust, dirt, roll marks, etc., caused due to prior treatment and working of the material prior treatment and working of the material • • Remedy: Remedy: precondition the material by cleaning precondition the material by cleaning it by it by various means various means Defects due to Rolling Defects due to Rolling 35 69 ME 4563 Fall ’05 Dr. S. Haran • • Structural Defects: Structural Defects: distort or affect the integrity of distort or affect the integrity of the rolled product the rolled product • • Wavy Edges Wavy Edges – – Edges are thinner than the center Edges are thinner than the center caused by bending of the rolls caused by bending of the rolls • • the edges elongate more then the center and the edges elongate more then the center and are restrained from expanding freely, they are restrained from expanding freely, they “ “ buckle buckle ” ” • • Cracks Cracks in the Rolled Product in the Rolled Product – – usually caused by usually caused by low ductility and barreling (in the edges & middle) low ductility and barreling (in the edges & middle) • • Alligatoring Alligatoring – – resulting from inhomogeneous resulting from inhomogeneous material deformation during rolling and defects in material deformation during rolling and defects in the original cast ingot itself the original cast ingot itself Defects due to Rolling Defects due to Rolling 70 ME 4563 Fall ’05 Dr. S. Haran Schematic Schematic illustration of illustration of typical defects in typical defects in flat rolling: flat rolling: (a) (a) wavy edges; wavy edges; (b) (b) zipper cracks in zipper cracks in the the center center of the of the strip; strip; (c) (c) edge cracks; edge cracks; and and (d) (d) alligatoring alligatoring . . Defects due to Rolling Defects due to Rolling 36 71 ME 4563 Fall ’05 Dr. S. Haran Defects due to Rolling Defects due to Rolling 72 ME 4563 Fall ’05 Dr. S. Haran Defects due to Rolling Defects due to Rolling 37 73 ME 4563 Fall ’05 Dr. S. Haran Taking care of wavy edges Taking care of wavy edges Defects due to Rolling Defects due to Rolling 74 ME 4563 Fall ’05 Dr. S. Haran Defects due to Rolling Defects due to Rolling Taking care of wavy Taking care of wavy edges edges 38 75 ME 4563 Fall ’05 Dr. S. Haran Hot Rolling Hot Rolling 76 ME 4563 Fall ’05 Dr. S. Haran • • Hot Rolling is done between 100 and 200 degrees F Hot Rolling is done between 100 and 200 degrees F above the re above the re - - crystallization temperature for crystallization temperature for most metals most metals • • Hot rolling gives a final product with a fine grain Hot rolling gives a final product with a fine grain size and little strain hardening size and little strain hardening • • This provides increased material yield strength This provides increased material yield strength • • This also provides increased ductility, by breaking This also provides increased ductility, by breaking grain grain boundaries, closing internal defects and boundaries, closing internal defects and breaking inclusions breaking inclusions Hot Rolling Hot Rolling 39 77 ME 4563 Fall ’05 Dr. S. Haran Stresses lower Stresses lower Forces smaller Forces smaller Power requirements less Power requirements less No work hardening No work hardening Large deformations possible Large deformations possible Breaks up the cast structure into preferable forms Breaks up the cast structure into preferable forms Closes porosity Closes porosity Sometimes the only way to create sheet Sometimes the only way to create sheet Hot Rolling Hot Rolling Advantages Advantages 78 ME 4563 Fall ’05 Dr. S. Haran Higher friction Higher friction Rolls need to be cooled Rolls need to be cooled Material handling difficult Material handling difficult Personnel must be protected from heat Personnel must be protected from heat Hot Rolling Hot Rolling Disadvantages Disadvantages 40 79 ME 4563 Fall ’05 Dr. S. Haran • • Causes changes in the grain structure of cast or of Causes changes in the grain structure of cast or of large large - - grain wrought metals grain wrought metals • • Hot rolling is an effective way to reduce grain size in Hot rolling is an effective way to reduce grain size in metals, for improved strength and ductility metals, for improved strength and ductility Hot Rolling Hot Rolling 80 ME 4563 Fall ’05 Dr. S. Haran Hot Rolling Hot Rolling – – some points some points • • The final product has little directionality, however: The final product has little directionality, however: • • If there are non If there are non - - metallic inclusions, they will not metallic inclusions, they will not recrystallize recrystallize and and may impart directionality may impart directionality • • Alloying elements that have a high Alloying elements that have a high recrystalliztion recrystalliztion temperature temperature may cause the same effect may cause the same effect • • Thinner sheets will have more directional characteristics Thinner sheets will have more directional characteristics • • The final product has few residual stresses, however: The final product has few residual stresses, however: • • Non Non - - uniform cooling can induce substantial residual stresses uniform cooling can induce substantial residual stresses • • The edges may warp due to rapid cooling, especially in more The edges may warp due to rapid cooling, especially in more complicated shapes (I complicated shapes (I - - beam, Flanges do this) beam, Flanges do this) • • The final surface is rougher, surface will contain difficult to The final surface is rougher, surface will contain difficult to remove remove high high - - temperature oxides (termed mill scale), tolerances can temperature oxides (termed mill scale), tolerances can usually be held to 2 usually be held to 2 - - 5% of the specified dimension 5% of the specified dimension . Rolling 36 71 ME 45 63 Fall ’05 Dr. S. Haran Defects due to Rolling Defects due to Rolling 72 ME 45 63 Fall ’05 Dr. S. Haran Defects due to Rolling Defects due to Rolling 37 73 ME 45 63 Fall ’05 Dr Rolling Defects due to Rolling 74 ME 45 63 Fall ’05 Dr. S. Haran Defects due to Rolling Defects due to Rolling Taking care of wavy Taking care of wavy edges edges 38 75 ME 45 63 Fall ’05 Dr. S. Haran Hot. increasing damping, etc. Rolling Rolling – – Vibration and Chatter Vibration and Chatter 64 ME 45 63 Fall ’05 Dr. S. Haran • • The forces of rolling will flatten the rollers elastically The