TYPES OF FORGING
OPERATIONS
Forging was the first
of the indirect compression-type process and it is probably the oldest method
of metal forming.
It involves the
application of a compressive stress, which exceeds the flow stress of the
metal. The stress can either be applied quickly or slowly. The process can be
carried out hot or cold, choice of temperature being decided by such factors as
whether ease and cheapness of deformation, production of certain mechanical
properties or surface finish is the overriding factor.
There are two kinds of
forging process:
1.
impact forging and
2.
press
forging.
In the former, the
load is applied by impact, and deformation takes place over a very short time.
Press forging, on the
other hand, involves the gradual build up of pressure to cause the metal to
yield. The time of application is relatively long.
Over 90% of forging
processes are hot.
1. IMPACT FORGING
Impact forging can be
further subdivided into three types:
- Smith forging,
- Drop forging,
- Upset forging.
Smith Forging
This is undoubtedly
the oldest type of forging, but it is now relatively uncommon.
The impact force for
deformation is applied manually by the blacksmith by means of a hammer. The
piece of metal is heated in a forge and when at the proper temperature is
placed on an anvil. This is a heavy mass of steel with a flat top, a horn which
is curved for producing different curvatures, and a square hole in the top to
accommodate various anvil fittings. While being hammered the metal is held with
suitable tongs.
Formers are sometimes
used; these have handles and are held onto the work piece by the smith while
the other end is struck with a sledgehammer by a helper.
The surfaces of the
formers have different shapes and are used to impart these shapes to the
forgings.
One type of former,
called fuller, has a well-rounded chisel-shaped edge and is used to draw
out or extend the work piece. A fuller concentrates the blow and causes the
metal to lengthen much more rapidly than can be done by using a flat hammer
surface. Fullers are also made as anvil fittings so that the metal is drawn out
using both a top and bottom fuller. Fittings of various shapes can be placed in
the square hole in the anvil.
The working chisels
are used for cutting the metal, punches and a block having proper-sized holes
are used for punching out holes. Welding can be done by shaping the surfaces to
be joined, heating the two pieces then adding a flux to the surfaces to remove
scale and impurities. The two pieces are then hammered together producing
welding.
The easiest metals to
forge are the low and medium carbon steels and most smith forgings are made of
these metals. The high carbon and alloy steels are more difficult to forge and
require great care.
Drop Forging
This is the modern
equivalent of smith forging where the limited force of the blacksmith has been
replaced by the mechanical or steam hammer.
The process can be
carried out by open forging where the hammer is replaced by a tup and the metal
is manipulated manually on an anvil.
The quality of the products depends very much
on the skill of the forger.
Open forging
Open forging is used
extensively for the cogging process where the work piece is reduced in size by
repeated blows as the metal gradually passes under the forge.
The cogging of a
prismatic bar can be used to assess the parameters involved and how they are
controlled.
The objective is to
reduce the thickness of the work piece in a stepwise sequence from end to end.
Several passes may be required to complete the work and edging is usually
carried out to control the width. The reduction in thickness is accompanied by
elongation and spreading. The relative amounts of elongation and spread cannot
be calculated theoretically but they have been determined experimentally for
mild steel.
Die drop forging
Closed-die drop
forging is widely used and the tup and anvil are replaced by dies. Matching
dies fit into the anvil and the tup. The dies have a series of grooves and
depressions cut into them and the work piece is passed in sequence through a
shaping series.
These stations have
names such as fullering, blocking, edging, bending and cut off. Where several
stages are involved, care must be taken to ensure that the metal does not
become excessively chilled before the last station is reached. To ensure that
the die cavity is completely filled the volume of the starting billet is
greater than that of the final forging.
The excess metal
appears as a "flash" at each stage, this is a thin fin around the
perimeter of the forging at the parting line. This flash is cut away in a
further press operation generally at a high temperature. The weight of flash
may be a small percentage of the total weight for forgings of simple shapes but
may exceed the weight of the actual forging for those of complex shape.
Each size and shape of
forging will thus require a separate set of forging and trimming dies. The
production tolerance for the initial metal must involve excess, e.g. ~10 mm.
The over-tolerance metal is accommodated by a gutter around the die cavity
which allows the formation of the fin referred to earlier.
Upset Forging
This process was
developed originally to gather, or upset metal to form heads on bolts. Today
the purpose of this machine has been broadened to include a wide variety of
forgings.
It is essentially a
double-acting press with horizontal motions rather than vertical.
The forging machine
has two actions. In the first, a movable die travels horizontally towards a
similar stationary die. These two dies have semi-circular horizontal grooves,
which grip the bars. A bar heated at the end is inserted between the movable
and stationary die. While thus held, the end of the bar is upset or pressed
into the die cavity by a heading tool mounted on a ram, which moves towards the
front of the machine.
If hexagon heads are
desired, a heading tool will upset some of the metal into a hexagon-shaped die
cavity. For more complex forgings, as many as six different dies and heading
tools may be used in turn in a similar manner to the different stations in die
drop forging.
2. Press Forging
Whereas impact forging
usually involves a mechanical press, press forging, on the other hand, requires
hydraulic power.
The largest forgings
are invariably produced on large hydraulic presses. These have vertically
moving rams, which move down slowly under considerable pressure.
A typical press forge
would be capable of loads of the order of 6000 to 10000 tons.
Forgings up to 100
tons weight can be handled easily in this forge and the highest-quality
products are manufactured by this technique.
Structure and Properties of Forgings
Forgings are
invariably produced by the hot-working process and this controls the resultant
structure and properties. There are, however, important differences in forgings
produced by different techniques.
The fact that the impact forge applies a stress for a
very short period compared to the long period for the press forge results in
totally different structures in the product.
In the case of impact,
the mechanical working is concentrated in the surface layers, since rapid
removal of the stress after the blow results in metal relaxation before the
effect of the blow has penetrated into the center.
Impact forging of a
large "as cast" piece of metal at high temperature will result in a
very inhomogeneous structure, the outside layers showing a typical hot-worked
structure whilst the center is still as cast. Any attempt to achieve greater
penetration by increasing the impact load usually leads to internal cracking.
Impact forging is therefore limited to relatively small work pieces.
Press forging invariably results in total penetration of the effect of the applied
stress into the center of the work piece. The process is generally less severe
on the metal than impact. The end result is a more homogeneous product having
very high quality. Since the process is much slower and the equipment used is
much larger, press forged articles are more expensive than impact forged
components.
