Question

1. What are the reasons why the bulk deformation processes are important commercially and technologically? 2. Name the four basic bulk deformation processes. 3. What is rolling in the context of the bulk deformation processes? 4. In rolling of steel, what are the differences between a bloom, a slab, and a billet? 5. List some of the products produced on a rolling mill. 6. What is draft in a rolling operation? 7. What is sticking in a hot rolling operation? 8. Identify some of the ways in which force in flat rolling can be reduced. 9. What is a two-high rolling mill? 10. What is a reversing mill in rolling? 11. Besides flat rolling and shape rolling, identify some additional bulk forming processes that use rolls to effect the deformation.

Answer 1

1. Bulk deformation denotes large movement of material by deformation. For example, in hot rolling. In hot working, metals are worked well above the recrystallization temperature which makes the working on metals easier. In commerical sense, this process consumes less energy while deforming process. And also handling hot metal is lot easier than cold-worked material handling. The term "Bulk" means final parts having relatively small surface area to voulme ratio.

2. Four basic bulk deformation processes are : a). Forging , b). Rolling , c). Extrusion and d). Drawing and e). Swaging

3. Rolling involves applying compressive forces on a part through rolls to change cross-section or thickness of the part.

Maximum bulk deformation processes comes under rolling The rolls apply compressive load onto the parts therby reducing the metal's shape by bulk deformation.

4. Bloom - It is a part with square cross-section of 150 mm by 150 mm or larger.

Slab - It is formed from ingot or bloom having rectangular cross-section. Minimum thickness is 40 mm and minimum width is 250 mm.

Billet - It is formed from a bloom. It has square cross-section. Minimum size is 40 mm.

5. Products produced on a rolling mill :

a. Railroad tracks

b. bars and rods

c. plates, sheets and strips

These products are further processed and moved for machining, and welding operations.

6. Draft :

Draft is the amount of reduction of thickness in rolling process.

draft , d = ( initial thickness ) - ( final thickness )

7.   Sticking in a hot rolling operation :

Hot surface of the workpiece gets sticks roller arcs. This is called sticking. When hot surfaces stick to the rollers, top layers move at the same speed as rollers. This causes severe deformation in the mid-portion of the workpiece.

8. Force in a flat rolling can be reduced in following ways.

F = (Avg. flow stres )*(width of the work)*(length of contact between rolls and work)

In the above equation, F is the rolling force

It can be seen from above equation that, if we decrease contact length and width we can reduce rolling force.

9. Two-high rolling mills:

In this type, there are two opposing rolls and have diameters of 0.6 to 1.4 mm. And also this can be revering or non-reversing.

10. Reversing mill :

This mill allows reversal of rotation of rollers. i.e. work can be moved in both horizontal direction. So, progressive reduction of cross-section is possible on same set of rolls.

11. Besides shape and flat rolling, we have

a). Thread rolling

b). Ring rolling

c). Gear rolling

d). Roll Piercing