Hot working is the process of plastically deforming a metal above its recrystallization temperature or below its melting point to improve its mechanical properties and grain structure. The hot working process helps modify the existing grain structure of the metal, and plastic deformation helps give it the desired shape and improve or create new mechanical properties in the material.
Advantages of Hot Working
- Metal porosity is largely eliminated.
- The grain structure of the metal is refined.
- Impurities such as slag are compressed into fibers and dispersed in the metal.
- Grain refinement improves mechanical properties such as toughness, ductility, elongation, necking, impact resistance, and vibration resistance.
Disadvantages of Hot Working
- Requires expensive tools.
- This results in poor surface finish due to rapid oxidation and scale formation of the metal surface.
- Poor surface quality makes it impossible to meet tight tolerances
Hot Working Processes
Following are some of the Hot Working Processes used in the Industry:
The metal is heated to a plastic state and pressure is applied to form it into the desired shape and size. The pressure is applied intermittently instead of continuously as in hot rolling. Pressure can be applied by hand hammers, power hammers, or forging machines.
Hot forging is the most common forging process. The hot forging process involves forging above the metal’s recrystallization temperature (the temperature at which new grains form in the metal). This type of extreme heat is necessary to avoid the work hardening of the metal during deformation.
Hot forging is a forging process that plastically deforms metal above its recrystallization temperature and retains the deformed shape upon cooling.
As the name “hot forging” suggests, the hot forging process takes place at very high temperatures (up to 1150 °C for steel). This temperature is necessary to avoid strain hardening of the metal during deformation.
This process typically involves heating a billet of steel (above its recrystallization point) and then pressing it under a die. Depending on your requirements, this mold can be heat treated to make it hard enough to withstand severe impacts. Because the billet is hot, it “moves” and allows for more precise shapes than cold forging.
After the hot forging process is complete, the part must be cooled rapidly to “solidify” the metal into a new shape. Cooling must be done with care, but too rapid cooling of hot forged products can cause warping. Normally, hot forgings are simply left at room temperature. The hot forging process produces very little material waste, making it one of the most efficient metal-forming processes.
- Less energy is required.
- Ductility is increased during this process.
- Pores may shrink or close completely during hot forging and subsequent forming.
- Used in making Engine Parts such as Piston, Camshafts, Crankshafts, and Connecting Rods.
- Used in making Brake system.
- Used in making of Levers, Steering Levers, and Front Axle Beams.
The hot rolling process is the fastest way to convert large sections into the desired shape. It consists of passing a hot ingot through two rollers rotating at the same speed in opposite directions. The distance between rolls is adjusted to correspond to the desired thickness of the rolled part. Thus, the rollers crush the passing billet to reduce its cross-section and increase its length. Hot rolling is used to form bars, plates, sheets, rails, angles, I-beams, and other profiles.
Scale, rust, or contamination can cause surface defects. Scale occurs when hot surfaces are oxidized by air. This mill scale can reduce corrosion when the steel is exposed to air for a short time but increases corrosion when exposed for a long time. A large amount of mill scale also causes severe pitting when the surface comes in contact with water.
The surface scale is typically removed mechanically using high-pressure water or pickling, followed by lubrication to reduce rust.
It improves ductility, toughness, strength, Formability, Weldability, and resistance to vibration & shock.
Used in making of Hollow Seamless Tubes, Screw, Bolts, Railroads, Partition Beams.
It consists of forcing metal into a chamber and extruding it under high pressure through an orifice shaped to provide the desired shape of the finished product. Most commercial metals such as steel, copper, aluminum, nickel, and their alloys are directly extruded at high temperatures. Rods, tubes, structures, ground strips, and lead-clad cables are typical extrusion products.
Types of hot Extrusion
Direct / Forward Extrusion
The diagram consists of a pressure-actuated ram and a cylinder or container in which the workpiece is placed for containment. A blind block is placed between the ram and the hot metal. Under ram pressure, the metal is first plastically filled into a cylindrical mold and then forced out of the mold opening until a small amount remains in the container. Next, it is cut next to the matrix and the blunt ends are removed.
Indirect / Backward Extrusion
Indirect extrusion is like direct extrusion, except that the extruded part is pushed into a hollow punch, as shown in the figure. The billet does not move the container, so there is no friction between the metal billet and the walls of the container. Compared to direct extrusion, less overall force is required, but the equipment used is mechanically more complex to allow the extruded shape to pass through the center of the hollow punch.
This is a form of direct extrusion but uses a mandrel to shape the inside of the tube. After the heated billet is placed in a container, a die containing a mandrel is pushed into the billet. The punch is then advanced to extrude metal through a die and around a mandrel as shown in the figure.
Extrusions are also made by pounding chunks of metal into a shape with a strong impact.
- Complex shapes can easily be created.
- Tooling costs are comparatively low.
- Set-up times are minimum.
Used in making Rods, Tubes, Frames, Doors, and Windows.
It is mainly used for making thick-walled seamless tubes and cylinders. Usually, he is carried out in two stages. The first step is to use dies and punches to squeeze the cup out of the hot disk. In the second stage, the drawn cup is reheated and further drawn to the desired length with the required wall thickness. In the second drawing process, several dies with smaller diameters are arranged in order so that the wall thickness gradually decreases.
Hot Drawing: Types
A wire is a circular, flexible rod of a small diameter. Wire drawing is a cold-forming process. This is the process of producing wires of various sizes within certain tolerances. In this process, a thick wire is reduced in diameter by passing it through a series of wire drawing dies, with the next die having a smaller diameter than the previous one. Most dies are made from chilled iron, tungsten carbide, diamond, or other tool materials. Maximum wire area reduction is less than 45% in a single pass.
Rod drawing is similar to wire drawing, except that it is stiffer and has a larger diameter than wire. This process allows the wire to be coiled but requires heavier equipment compared to wire drawing as the bar must be kept straight. The workpiece is first inserted into the mold and pulled by the carriage to increase its length and reduce its cross-section.
Tube drawing is similar to the other two methods, except that a mandrel is used to reduce the wall thickness and cross-sectional diameter of the tube. This mandrel is placed in the mold and the workpiece is drawn by the carriage system as described in Drawing the Bar. Tubes are either round or rectangular. Also, multiple passes were required to complete the drawing process.
Components made from this process have better toughness, better ductility, and better resistance to vibration & shocks.
- Used in making Wire of Copper, Aluminium, GI.
- Used in making of Spring.
- Used in making Paper clips.
- Used to produce rods & tubes of small diameters.
This process is used to produce seamless tubes. In that operation, a heated cylindrical steel ingot is passed between two conical rollers that work in the same direction. A mandrel is provided between these rollers to aid drilling and control hole size as the billet is forced across the hole.
- The process is cost-effective.
- Efficient & Reliable.
Used in making of Seamless Tubes & Pipes.
The metal is heated to forging temperature and machined into the desired shape on a lathe. This process produces parts with circular cross-sections that are symmetrical about the axis of rotation.
- The spinning process uses a simple tool. Therefore, it is economical in small lots.
- Large parts are much easier to make by spinning than by drawing.
- Complicated shapes that cannot be achieved by drawing can be easily created by spinning.
- Pressed parts do not require post-processing such as bending.
Bowl, Cap, Cone, Hopper, Utensil Lid, Roof Drain, Metal Housing, etc.