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March, 1925 forging- Stamping - Heat Treating 79
reduction of area on the outer portion of the bend. To
avoid the extensive reduction of area on the outer side
of the specimen, which would eventually lead to the
rupture of the metal on this side of the article, the
metal on the concave surface which is already under
compression tends to flow across the bar towards the
convex face, thus providing metal to compensate for
the deficiencies of the material that are produced by
the plastic flow on the convex surface. The metal on
the inner surface is already under compression and is
predisposed to flow. It is also sufficiently plastic to
flow easily, and as a result it tries to travel in a general
way towards the convex surface. The probability
of this type of flow can readily be seen by bending the
bar in the cold. The metal must have some moderate
supply of ductility, as otherwise bending experiments
are rather absurd. If such a bar is bent it will be found
that the metal flows evenly on the convex face and for
a short time evenly on the concave face. Very soon
crinkles appear on the concave side and spread into
the bar more and more as the bending is increased.
Naturally some material is in this instance being
pushed outwards, i.e., towards the radius of the bend.
This means that towards the inwards, apices of the
crinkles there is a stress tending to drive the metal
inwards, i.e., towards the convex face. In the cold
bar the metal is too rigid to flow far, but in the hot,
easily deformed metal, this stress operates and does
actually drive the metal towards the convex face.
The nature of the action here described can be understood
easily by a reference to the drawings in Fig.
1. The original bar is shown in Fig. 1A. In Fig. IB
is shown the theoretical way in which the metal might
be expected to flow during the bending. In Fig. 1C
is shown what very often does occur and what always
occurs if the conditions are suitable. This figure indicates
very clearly that a kink forms on the concave
side of the bar by reason of the way in which the
metal flows from this position towards the convex
side in order to neutralize the deficiency of the metal
in the part which is being stretched.
Fig. 2 shows an actual section from a bend made in
this way and indicates the defect that has been produced
upon the inner surface of the bent article.
How can this kink be avoided? The best method
undoubtedly is to arrange not to bend a uniform section.
The kinking can be prevented fairly definitely
by increasing the diameter of the bar at the point
where it is to be bent. If a kneecap of metal is put
onto the bar on the inner side, more metal is available
to flow across the canvex side during bending. This
prevents the excessive flow of the metal at one place
that eventually leads to a kink. Similarly, if there is
an accumulation of metal on the outer surface there is
more material to extend plastically than in a plain bar.
The outer surface can then be deformed without creating
a marked deficiency at any one place and therefore
without calling for the flow of metal from the concave
to the convex surface. Both these actions are shown
in Fig. 3. The most reliable method is to combine the
two actions, thus giving an excess of metal on both
faces, but particularly on the convex side.
If a bar has kinked in the way already described,
the defect may or may not be subsequently incorporated
in the forging. There is as great a probability that
it will be, as that it will not be, for the eventual position
of the defect is very considerably governed by the
amount of metal that is extruded from the pattern during
forging in the dies. The location of the defect is
also affected by the quantity of metal on both sides
of the bend — convex and concave — as will be shown
later. It is perfectly possible, however, to produce
the same type of defect in the finished forging from a
bar which has not kinked during preliminary forging.
This is likely to arise in several ways, but two definite
examples can be examined. Both of them really arise
from the same cause, which is, put briefly, a shortage
of metal in the bar at the time that it is introduced
between the dies.
Imagine that an article containing a corner like that
shown in Fig. 4A is to be made. The dies for the corner
will be approximately as shown in Fig. 4A, and it
can be assumed that a bent bar has been prepared.
As soon as this bar is placed between the dies, work
will be done upon it, and the tendency of the metal
will be to flow towards the corner X. As the metal in
the bar will always attempt to flow in this direction,
the movement of this convex surface will tend to draw
the remainder of the metal after it away from the concave
corner, just as has already been described. At
this side X, the excess of metal is extruded from the
impression. The general action of the dies is therefore
to decrease the radius at the corner, which is
FiG.4 Fic.e
FIG. 4 (A)—The pattern in the dies showing form of corner.
(B) The defect in the resulting forging.
FIG. 5—Outline of typical crankshaft.
FIG. 6—Diagram of type of defect produced in the
crankshaft at X.
equivalent to making the bend more severe, or to
bending the bar through a larger angle. The effects
produced are therefore quite similar to those that have
already been described with a plain bar. If there is
sufficient metal in this portion of the bar to fill the
dies completely and to make each, portion of the die
do its share of work, it is probable that the dies will
be kept properly filled. Under these conditions there
will be an excess of metal to be extruded both at
the point X and at the point Y. If on the other hand
there is any deficiency of metal it will become most
evident at the point Y, which is the concave side of
the bend. From the position Y the metal will flow
towards the position X and will draw in and kink and
form a gall at Y, as shown in Fig. 4B, in just the same
way as occurs in the simple bending outside the dies.
This action may occur in a bar which is quite free
from any initial kink or shut, the actual operation of
forging being quite sufficient to cause it to take place.
The obvious remedy for the production of this type of
defect is to have sufficient metal on the bar, in the
part that is likely to form a gall, and consequently to
avoid the sucking action of the metal towards the convex
side of the pattern.
This explanation of the spontaneous production
of a gall when forging a bar that is initially free from