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.'I l( I Forging - S tamping - Heat Treating
length or m several places. The surfaces of these
pipe cavities are usually oxidized, which prevents any
possibility of welding in subsequent forging operations.
The two forgings shown in Fig. 1. which have been
cut in two, show examples of hidden pipe. In the
case of both forgings the defects shown are due to
secondary piping, for ample material had been cut
from the top of the ingots to remove all normal
shrinkage.
Gas pockets or blowholes are due-to the rejection
of gas by the molten steel at the time of solidification.
If the solidification is normal, that is, progressing from
FIG. 2—Ghost lines in the machined surface of a shaft.
the outside inward and from the bottom upward, the
gas is carried into the sink head. But if the freezing
takes place above molten metal and this gas collects,
there will be a gas pocket which will cause trouble.
It considerable gas is present, a number of small
bubbles will probable form just below the skin of
the ingot and escape outward, leaving tiny "worm
holes." The holes arc usually oxidized and cause
surface seams on forging or rolling. The extent of
these seams is dependent on the depth of the gas
"worm holes."
Cracks.
If the surface of the ingot is not properly designed
or the surface of the mold is rough or full of holes,
natural contraction cannot take place and the ingot
will crack.
Solidification begins about various centers which
are at the lowest temperature, which results in the
formation of crystal skeletons. These skeletons rapidly
develop branches which become more and more
coated with crystallizing matter until the whole resembles
a tree or is, as it is termed, "dendritic."
In the case of an ingot the formation of a number
of dendrites proceeds simultaneously from the chilled
surface of the iron mold until they interlock and the
whole material is solid. Impurities, having the lowest
freezing temperature, will be carried in the mother
liquid until final solidification, when they will be
trapper between the dendritic branches.
If the dendritic formation proceeds from two surfaces
it forces the non-metallics in the molten material
to th«ir extremities, until, when the dendrites
interlock, a plane of inclusions is formed.
June, 1925
This plane .of inclusions is a plane of weakness and
can be the cause of a serious crack in an ingot and
the forgings therefrom; and it is for this reason that
the shape of the ingot mold, as regards surface, is
designed so carefully, in order to obtain the maximum
cooling surface with the minimum of corner
segregation.
Slag Lines and Ghost Lines.
"Ghost line" is a rather indefinite term applied to
a frequently encountered defect. Ghost lines are
streaks varying in composition from the major portion
of the metal and usually containing slag impurities.
They usually show up in the machined surface
of the forging as light or dark marks, depending on
the angle at which the light strikes the machined surface.
As mentioned before, the material in these lines
differs in composition from the rest of the steel, being
usually verv low in carbon and high in phosphorus.
These lines differ in hardness from the surrounding
material, which causes the tool to jump in machining
and the lines to show. The depth of these marks is
usually verv small, so that with a very slight removal
of material they disappear. However, when present
they usually occur in many groups, and when one is
removed, another appears.
The danger from these lines is a much-debated
question. It can be safely said that in straight tension
the strength of the material containing the lines
differs little, if at all, from that of adjacent material
containing no lines. However, the action under reversed
stress is another matter, and that under temperature
changes appear to the author to be detrimental.
Being of radically different composition, the
expansion and contraction are different from what
they are in the surrounding material. It is because
of these two unknown variables, reversed stresses and
temperature variations, that ghost lines are prohibited
in high-class equipment.
Fig. 2 shows the machined surface of a shaft illustrating
a bad condition of ghost lines.
FIG. 3—Shafts showing the clink type of defect.
Defects due to fabrication may be classified under
the following heads: 1, Laps; 2, Star cracks; 3,
Clinks; 4, Improper reduction; 5, Heat treatment.
Laps.
Laps are the folding over of the surface of the
metal in the forging operation. This type of defect
is usually due to working the metal too much in one
direction before rotating it, or to the use of improper
forging dies. It can also occur with proper working