Metalizado de superficies

Rolling is a process where the metal is compressed between two rotating rolls for reducing its cross sectional area. This is one of the most widely use of all the metal working processes. Because of its higher productivity and low cost. Rolling would be able to produce components having cross sectional through out its length. May shape such as itl and channel section are possible , but not very complex shapes.

Rolling is normally a hot working process unless specifically mensions as cold working. The metal is taken into rolls by friction and subsequently compressed to obtain the final shape.

The thickness of the metal that can down into rolls depends on the roughness of the roll surface. Rougher rolls would be able to achive greator reduction than smoother rolls. The reduction that would be achieve with a given set of roll is designed as the angle of the bite. cold rolling is done bellow the recrystallization temperature of the metal , hot rolling is done when the metal pieces at uniform working temperature.

Types of Rolling mills

Rolling mills may be classified according to the number and arrangement of the rolls.

(a): Two high rolling mills

(b): Three high rolling mills

(c): Four high rolling mills

(d): Tandem rolling mills

(e): Cluster rolling mills

1: Two high rolling mills

Two high rolling mills may further classified as

· Non reversing mill

A two high rolling mill has two rolls only.

Two high reversing mill:

In two high reversing rolling mills the rolls rotate ist in one direction and then in the other, so that rolled metal may pass back and forth through the rolls several times. This type is used in pluming and slabing mills and for roughing work in plate , rail , structural and other mills.

These are more expensive compared to the non reversing rolling mills. Because of the reversible drive needed.

Two high non reversing mill:

In two high non reversing mills as two rolls which revolve continuously in same direction therefore smaller and less costly motive power can be used. However every time material is to be carried back over the top of the mill for again passing in through the rolls. Such an arrangement is used in mills through which the bar passes once and in open train plate mill.

2: Three high rolling mill:

It consists of a roll stand with three parallel rolls one above the other. Adjacent rolls rotates in opposite direction. So that the material may be passed between the top and the middle roll in one direction and the bottom and middle rolls in opposite one.

In three high rolling mills the work piece is rolled on both the forward and return passes. First of all the work piece passes through the bottom and middle rolls and the returning between the middle and the top rolls.

So that thickness is reduced at each pass. Mechanically operated lifted tables are used which move vertically or either side of the stand. So that the work piece fed automatically into the roll gap.

Since the rolls run in one direction only a much less powerful motor and transmission system is required. The rolls of a three high rolling mills may be either plain or grooved to produce plate or sections respectively.

3: Four high rolling mill:

It has a roll stand with four parallel rolls one above the other. The top and the bottom rolls rotate in opposite direction as do the two middle rolls. The two middle are smaller in size than the top and bottom rolls which are called backup rolls for providing the necessary rigidity to the smaller rolls.

A four high rolling mill is used for the hot rolling of armor and other plates as well as cold rolling of plates, sheets and strips.

4: Tandem rolling mills:

It is a set of two or three stands of roll set in parallel alignment. So that a continuous pass may be made through each one successively with change the direction of material.

5: Cluster rolling mills:

It is a special type of four high rolling mill in which each of the two working rolls is backup by two or more of the larger backup rolls for rolling hard in materials. It may be necessary to employ work rolls of a very small diameter but of considerable length. In such cases adequate of the working rolls can be obtained by using a cluster mill.

Difference between Hot rolling and cold rolling processes

1: Metal is fed to the rolls after being heated above the recrytallization temperature.

1: Metal is fed to the rolls when it is below the recrystallization temperature.

2: In general rolled metal does not shows work hardening effect.

2: The metal shows the working hardening effect after being cold rolled.

3: Co-efficient of friction between two rolls and the stock is higher, it may even caused shearing of the metal in contact with rolls.

3: Co-efficient of friction between two rolls and the stock is comparatively lower.

4: Experiment measurement are difficult to make.

4: Experiment measurement can be carried out easily in cold rolling.

5: Heavy reduction in area of the work piece can be obtained.

5: Heavy reduction is not possible.

6: Mechanical properties are improved by breaking cast structure are refining grain sizes below holes and others, similar deformation in ingot (get welded) and or removed the strength and the toughness of the job should increases.

6: Hotness increased excessive cold working greatness crackers ductility of metal reduction. Cold rolling increased the tensile strength and yield strength of the steel.

7:Rolls radius is generally larger in siz.

7: Rolls radius is smaller.

8: Very thin sections are not obtained.

8:Thin sections are obtained.

9: Hot roll surface has(metal oxide) on it , this surface finish is not good.

9: The cold rolled surface is smooth and oxide free.

10: Hot rolling is used un ferrous as well as non ferrous metals such as industries for steel , aluminum, copper , brass, bronze , alloy to change ingot into slabs.

10: Cold rolling is equally applicable to both plain and alloys steels and non ferrous metals and their alloys.

11: Hot rolling is the father of the cold rolling.

11: Cold rolling follows the hot rolling.

Hot rolling	Cold rolling
1: Metal is fed to the rolls after being heated above the recrytallization temperature.	1: Metal is fed to the rolls when it is below the recrystallization temperature.
2: In general rolled metal does not shows work hardening effect.	2: The metal shows the working hardening effect after being cold rolled.
3: Co-efficient of friction between two rolls and the stock is higher, it may even caused shearing of the metal in contact with rolls.	3: Co-efficient of friction between two rolls and the stock is comparatively lower.
4: Experiment measurement are difficult to make.	4: Experiment measurement can be carried out easily in cold rolling.
5: Heavy reduction in area of the work piece can be obtained.	5: Heavy reduction is not possible.
6: Mechanical properties are improved by breaking cast structure are refining grain sizes below holes and others, similar deformation in ingot (get welded) and or removed the strength and the toughness of the job should increases.	6: Hotness increased excessive cold working greatness crackers ductility of metal reduction. Cold rolling increased the tensile strength and yield strength of the steel.
7:Rolls radius is generally larger in siz.	7: Rolls radius is smaller.
8: Very thin sections are not obtained.	8:Thin sections are obtained.
9: Hot roll surface has(metal oxide) on it , this surface finish is not good.	9: The cold rolled surface is smooth and oxide free.
10: Hot rolling is used un ferrous as well as non ferrous metals such as industries for steel , aluminum, copper , brass, bronze , alloy to change ingot into slabs.	10: Cold rolling is equally applicable to both plain and alloys steels and non ferrous metals and their alloys.
11: Hot rolling is the father of the cold rolling.	11: Cold rolling follows the hot rolling.