Methods Adopted for Hot Working of Metals | Industries | Metallurgy

The following points highlight the top three methods adopted for the hot working process of metals. The methods are: 1. Piercing 2. Drawing or Cupping and Bending 3. Hot Spinning.

Method # 1. Piercing:

Piercing process is employed for the production of seamless tubes. It offers the most economical mechanical working process for the manufacture of seamless tubes. It consists of passing the hot rolled billet at 1100°C between two conical rollers and over a mandrel which helps in piercing and controlling the size of bore as the billet is forced over it.

The solid billet is first centre-punched and then heated to 1100°C in a furnace before being pierced. It is then pushed into two piercing rollers, which impart both axial and rotational motion. Due to the alternate squeezing and bulging operations, a seam is formed in the centre and the piercing mandrel controls the size and shape of the hole.

The first pass makes a rather thick-walled tube which is again passed between grooved rollers over a plug held by a mandrel and is converted into a longer tube with specified wall thickness. The tube is again passed through a reeling machine which helps in straightening and sizing to the final dimensions.

The tubes are the cooled and placed in the pickling bath of dilute sulphuric acid to remove the oxide and scale formation. The one pass-process is applicable to seamless tubes of 15 centimetres diameter. Large tubes upto 35 centimetres diameter are then given a second pass on piercing rollers.

Method # 2. Drawing or Cupping and Bending:

Bending and drawing deal with pressures applied on two sides of a sheet. The range of application is large and the thickness of materials which can be hot-shaped may vary widely, provided equipment is available for required pressures and size.

Maximum plate thickness that can be easily (economically) formed is about 200 mm. Details desired in the finished products can be faithfully reproduced if care is taken in the design of the tools and steps taken not to disregard certain conditions required by metals at elevated temperature.

Drawing is defined as a process of making cup-shaped parts from sheet metal blanks by pulling it into dies with the help of punch. Sheet-blanks are heated to working temperatures to provide sufficient plasticity. These are then placed on the die or cavity as shown in Fig. 5.53 and the punch is hammered over the heated blank, which pushes the metal through die to form a cup.

For long thinned cylinders or tubes, repeated heating and drawing operations are carried out. This drawing operation is carried out in a horizontal draw-bench, consisting of several dies, of successively decreasing diameter as shown in Fig. 5.53 (c).

In this way finished cylinders of required thickness can be manufactured. For production of tubes, the closed end is cut off and then sizing and finishing operations are carried on reeling machine as in the case of piercing operation.

(1) No chemical and mechanical properties are impaired.

(2) Reduction in area can be carried out upto 50% of original area.

Design Considerations of Drawing Operation:

It must be remembered that attendant heat in hot working brings some reduced tolerance considerations. Therefore, tolerances must be greater. Fitting operations may be necessary to bring the part to the proper size.

Minimum radii need to be considered carefully in hot forming so that excessive reduction in the thickness of the metal does not occur at these points.

Owing to the wide plastic range in hot materials, any locking or holding conditions in tools must be carefully avoided or controlled. Hot-shaped parts must be removed from dies quickly after they are formed so that contraction lock-up at any point, which, might cause fracture of the material when it cools, does not occur.

The speed of the tools should be regulated to avoid rupture due to shear during hot shaping; and similarly pressure of holding devices controlled so that the metal is not thinned too much in thickness.

The chemical composition of metal plays an important part in the requirements for shaping and in subsequent operations such as hardening, and must be taken into consideration. As a general rule if the carbon content is more than 0.25% and the total alloying elements are 1%, steels should best be hot-formed.

Because an oxide coating usually forms on a hot steel surface, careful consideration should be given to atmospheres or methods of heating. Extreme precaution in equipment, tools and processes must be taken in the handling of hot material because of it & low hardness at elevated temperatures.

Method # 3. Hot Spinning:

Hot spinning is carried out on thick sheet work to impart circular and desired contours. This process is used to form a dish or head from circular, heavy steel plates. It is also used to neck or close the ends of tubes. The operation is performed on a lathe which imparts rotational motion to job. The heated circular blank of sheet metal is lightly held against the chuck made of plaster or metal and is revolved on the spindle of the lathe.

Shaping is done with a blunt tool under pressure, which contacts the surface of the rotating part and causes the metal of flow in the desired direction. During the operation a considerable friction heat is generated, which keeps the already hot blank in the plastic state.

If large parts are made from this metal, spinning is performed without the aid of a form (pattern). A hard wooden bar is commonly used as a back-up support opposite the spinning tool to avoid wrinkling at the outer-edge.