What is Hot Rolling – Advantages and Disadvantages
Hot Rolling Process
Hot rolling is a metalworking process in which metal is heated above the recrystallization temperature to plastically deform it in the working or rolling operation.
This process is used to create shapes with the desired geometrical dimensions and material properties while maintaining the same volume of metal. The hot metal is passed between two rolls to flatten it, lengthen it, reduce the cross-sectional area and obtain a uniform thickness. Hot-rolled steel is the most common product of the hot rolling process, and is widely used in the metal industry either as an end product or as raw material for subsequent operations.
Hot rolling improves:
- Toughness and strength
- Resistance to vibration and shock
Hot-rolled steel products are classified into four groups:
The rolling of metals above the recrystallisation temperature is called hot rolling. The temperature at which new grains are formed in the metal is known as recrystallisation temperature. This temperature should not be too high; otherwise the metal will burn and become unsuitable for use.
The process of hot rolling is shown in Fig. It consists of heating of ingots to about 1200°C in soaking pits. Hot ingots then passed through a gap between rollers in many stages to get blooms.
The gap between the rollers is adjusted to conform to the desired thickness of the rolled product. The blooms are rolled to billets and the billets to the desired section like flat, square, rectangular, hexagonal, angle rails, I, U etc.
The angle of bite is usually 40° at the area of contact of rollers and metal. The rollers impart compression stresses, while squeezing the metal. These results in changes of grains, grain fragmentation, and lattice distortion.
In hot rolling the coarse grains are converted into smaller grains. Care has been taken that the correct range of temperature is maintained during the process. In this process, the speed of emerging metal is higher due to elongation.
Rolling is normally a hot working process unless specifically mentioned as cold rolling. It is a most rapid method of converting large sections into smaller stocks. By hot rolling we can produce bars, plates, sheets, rails, angles, structural sections, I, L, T, C, shapes etc.
The materials commonly hot rolled are steel, copper, magnesium, aluminum and their alloys. All hot rolled steel has a reddish blue surface.
Advantages of Hot Rolling:
1. Mechanical Properties Improved:
The mechanical properties like toughness, resistance to shock, ductility, and percentage elongation are improved.
2. Grain Structure Refined:
The grain structure of the metal is refined i.e., the coarse grains are converted into fine grains which changes the properties of metal.
3. Porocity Eliminated:
The porocity of the metal is eliminated, thus producing strong and uniform structure.
4. Impurities Uniformly Distributed:
The impurities like slag are evenly distributed throughout the metal. Otherwise, they concentrated at one position inside the metal, shall make it weaker at that position.
5. Low Pressure Required:
The deformation of metal is easy due to higher temperature of metal being rolled. This reduces the pressure requirements.
Disadvantages of Hot Rolling:
1. Expensive Process:
Hot rolling is an expensive process because it requires expensive tools, great skill to handle and maintain it.
2. Correct Temperature Difficult to Maintain:
The correct temperature range for working is difficult to maintain.
3. Poor Surface Finish:
The higher temperature of working causes to rapid oxidation and scale formation on the metal surface. This gives a poor surface finish and loss of strength of the material.
4. Difficult to Maintain Close Tolerance:
Due to poor surface finish, close tolerances cannot be maintained.
Typical applications of hot-rolled steel are:
- Automotive structural parts such as frames
- Tabular products such as pipe and gas cylinders
- Machine structures such as saws and springs
- Agriculture equipment
- Metal buildings
- Guard rails
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