Chapter 18 Flashcards
Primary processes
Reduce a cast material into intermediate shapes, such as slabs, plates, or billets
Secondary processes
Converts these shapes into finished or semifinished products.
Bull deformation processes
Those in which the thicknesses or cross sections are reduced or shapes are significantly changed.
Common bulk forming operations
Rolling, forging, extrusion, cold forming, and wire, rod, and tube drawing.
Sheet forming operations
Involve the deformation of a material where the thickness and surface area remain relatively constant.
Common sheet forming processes
Shearing or blanking, bending, and deep drawing. Most sheet forming operations are performed cold.
Bull deformation processes
Rolling, forging, extrusion, wire, rode, and tube drawing, cold forming, cold forging, and impact extrusion, piercing, and other squeezing processes.
Rolling operations
Reduce the thickness or change the cross section of a material through compressive forces exerted by rolls.
Rolling starting stock
Can be rolled into blooms, billets, or slabs, or these shapes can be obtained directly from continuous casting.
Bloom
Has a square or rectangular cross section.
Billet
Usually smaller than a bloom and has a square or circular cross section.
Slab
Rectangular solid where the width is greater than twice the thickness.
Plate, sheet, strip
Slabs can be further rolled to produce these.
Sheet and strip can be fabricated into products or further cold roller into
Foil
Blooms and billets can be further rolled into finished products, such as
Structural shapes, rail road rail, or semifinished shapes such as bar, rod, tube, or pipe.
Rolling process
Metal is passed between two rolls that rotate in opposite directions. Friction along the contact interface propels the metal. The metal is then squeezed and elongates.
Hot rolling
Starting material should be heated to a uniform elevated temperature. The material must first be brought to the desired rolling temperature, usually in gas soaking pits or furnaces.
Hot rolling operations are terminated
When the temperature falls to about 50 to 100 degrees Celsius above the recristallisation temperature. Such a finishing temperature ensure the production of a uniform fine grain size.
Cold rolling
Can be used to produce sheet, strip, bar, and rod products with extremely smooth surfaces and accurate dimensions.
Cold rolled sheet and strip can be obtained in various conditions
Skin rolled, quarter hard, half hard, full hard
Two high non reversing mill
The material can only pass through the mill in one direction.
Two high reversing mill
Permits back and forth rolling, but the folks must be stopped, reversed, and brought back to rolling speed between each pass.
Three high mill
Eliminates the need for roll reversal, but requires some form of elevator on each side of the mill to raise or lower the material and possibly some form of mechanical manipulators.
Cluster mills
Multiple rollers. Foil is almost always rolled on these. These rollers have a small diameter
Pack rolling
Two or more layers of metal are rolled simultaneously as a means of providing a thicker input material.
Continuous or tandem rolling mill.
Used when the volume of a product justifies the investment. Billets, blooms, or slabs are heated and fed through an integrated series of no reversing rolling mill stands. As the material is reduced in size, the rolls must turn faster than the preceding one.
Ring rolling
One roll is placed through the hole of a thick walled ring, and a second roll presses in from the outside. The wall thickness is reduced and the diameter of the ring increases. Can be used in products such as rockets, turbines, airplanes, pipelines, and pressure vessels.
Thread rolling
Alternative to the cutting of threads.
Hot rolled products
Uniform and if dependable quality. The surfaces are usually a bit rough, however, and are originally covered with a tenacious high temperature oxide known as mill scale. This can removed be an acid pickling operation
Cold rolled products exhibit
Superior surface finish and dimensional precious, and can offer the enhanced strength obtained through strain hardening.
If a crowned or barrel shaped roll
Is subjected to the designed load, it will deflect into flatness.
Thermomechanical processing time
Controlled rolling is an example. This integrates deformation and thermal processing into a single process that produces the desired shape and desire properties.
Forging
Induces plastic deformation through localized compressive forces applied through dies. Equipment could be hammers, presses, or special forging machines, deformation can be performed in all temperature regimes.
The variety of forging processes offers a wide range of capabilities.
The metal may be drawn out to increase its length and decrease its cross section. Upset to decrease the length and increase the cross section, or squeezed in closed impression dies to produce multidirectional flow.
Common forging processes
1) open die drop gamer forging. 2) impression die drop hammer forging. 3) press forging. 4) orbital forging. 5) Upset forging. 6) automatic hot forging. 7) roll forging. 8) swaging. 9) Net shale and near net shape.
Open die hammer forging
The metal is first heated to the proper temperature, then an impact is delivered by some Thor of mechanical hammer. The simplest industrial hammer is a gravity drop hammer.
Computer controlled hammers
Can provide blows of differing impact speed for different products.
Open did hammer forging is a simple and flexible process but
It is not practical for large scale production. It is a slow operation and the shape and dimensional precision of the resulting workpiece are dependent on the skill of the operator.
Impression Die or closed die forging
Overcomes the difficulties of open die hammer forging, by using shapes dies to control the flow of metal.
Process of impression die hammer forging
Heated metal is positioned in the lower cavity and struck one or more blows by the upper die.
Flash
In impression die forging, excess metal is squeezed out along the parting line to form a flash around the periphery of the cavity. Metal fills all of the die cavity.
