Rolling Flashcards
Hot Rolling
- rolling at an elevated temperature
- Smaller forces and stresses than in cold rolling.
- Less power requirements.
- No strain hardening.
- Breaks up cast structure and closes internal defects.
Cold Rolling
- rolling at ambient temperature
- Improved mechanical properties.
- Excellent surface finish
- High dimensional precision.
- High power requirements.
- Sometimes have non-uniform stress distribution.
Draft
reduction of thickness in 1 roll pass
Maximum draft is dependent on
coefficient of friction and roll radius
Spreading
increase in width as strip is rolled
Forward Slip
Relative sliding between roll and strip along the roll gap
Contact length of Roll Strip
the length of the roll circumference that is in contact with the strip
Recrystallisation Temperature
Is a particular value below the melting point of a material where, with enough force, you can easily change the grain structure of the material. When metals reach such a temperature, the grains make them to behave in a plastic manner, allowing for deformation to occur with much less force required.
The velocity of the roll strip
increases from before entering the rolls, to maintain a constant volume flow rate. The final velocity is the initial velocity multiplied by the ratio of change in width and ratio of change in height.
friction
The friction must be controlled with lubricants. A coefficient of friction greater than 0.7 and the metal would stick to the rolls, and ‘alligatoring’ will occur. You can control friction by changing the temperature of the material.
2-high non-reversing
Simplest design with material passing through in one direction.
2-high reversing:
The mill is stopped, reversed and brought up to speed when all material has passed through. Allows for several passes on the same mill (back-and-forth rolling).
3-high
The material is passed through the bottom 2 rolls, then back the other way between the top 2. There is no need for reversal, but some form of elevator is required to lift the material up to the top mill. The centre roll has material either side of it at the same time so may wear down quicker.
4-high non-reversing
Uses small rolls with large backup rolls pressing against them. Allows for a small roll gap whilst still maintaining a large force.
Cluster mill
Same idea as 4-high, but uses several backup rolls surrounding more backup rolls. Often have 2 backup rolls per smaller roll. Small work rollers are easier to replace and maintain. Backup rolls don’t need maintaining as much. Rolls with a narrow diameter can flex easily due to their reduced stiffness compared to larger rolls. Backup rolls also support the work rolls, helping them to maintain their shape.
Tandem rolling
Has lots of rolling mills thinning the same stretch of material one after another in series. The same volume must pass through each stand at the same time so roll speed increases as you go down the line.
Wavy edges
Non-uniform width. This is caused by roll bending, if the rolls flex due to their material not being strong enough. Can also be down to roll bearings not being strong enough. The strip is thinner along its edges than at the centre, causing the edges to be more elongated.
Cracks
Zipper cracks form in the centre of the strip, looks a bit like a zip down the middle of the material. Edge cracks can form along the sides of the strip. Caused by; poor material ductility at the rolling temperature, incorrect preparation i.e. poor casting method, incorrect roller speed (too fast).
Alligatoring
Where the strip is pulled apart down the middle horizontally (opens like an alligators’ mouth). Caused by; non-uniform bulk deformation of the billet during rolling, defects in the original casting, friction between rolls and material being too high.
Shape rolling
Rolls vary in diameter along their length and material is at an elevated temperature. It is used to manufacture straight and long structural shapes such as I beams and solid bars. Also known as profile rolling.
Skew rolling
Rolls are not parallel to each other and have a screw like thread on them that increases in size toward the exit side of the mill. Round wire is fed into the roll gap at a non-perpendicular angle to either of the rolls. Spherical blanks (ball bearings) are made.
Ring rolling:
A hole is punched in a circular blank, then a ring roller can be used to get it to the correct diameter by reducing its thickness (achieved by moving rolls closer together). The volume of the ring stays constant, so this must be calculated to determine what size hole to punch in the blank. This method is used where you don’t want a weld or join (weak point) in the ring. Edging rolls are used to ensure barrelling doesn’t occur (ensures it still has a square cross section).
Rotary tube piercing
Uses tensile stress during rolling to purposely create a void in a tube. This void is then manipulated into a correct size using a mandrel. Can manufacture long thick-walled seamless pipe and tubing (good for high pressure). The axes of the rolls are orientated at an angle from the axis of the billet, to help pullet through the rolls and mandrel.
Thread rolling
Thread is formed with each stroke of a pair of reciprocating (back-and-forth) flat dies or rotary dies (rolls), usually done by cold working. It’s a way of making stronger screws than on a mill or lathe. This is because no material is removed, the grains aren’t cut they are just deformed. This is used for screws that must withstand high forces, such as wheel nuts or the threads on a towing eye.
