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396 Forging- Stamping - Heat Treating
Soft places produced on grinding hard surfaces are
easily disclosed by means of etching with an alcoholic
solution of nitric acid.
Distinct results can sometimes be produced by verydilute
solutions of nitric acid in water. A solution of
a few tenths per cent of nitric acid contained in a receptacle
in which the bright object can be suspended
for 10 minutes or more will often give clear evidence
when stronger solutions acting more rapidly will be
confusing-.
FIG. 13—Deep-seated cracks on tread of "shelled" tire.
The hardening of drop-forging die blocks offers
many problems and brief reference to this subject
might be of interest. The direction in which the
forging grain lies and the general soundness and
suitability of material for die blocks are now being
taken care of by a recently drafted specification. But
how hard should the hardened dies be made, and howdeep
will the hardening effect penetrate? On examining
a series of broken dies two kinds of defects
recurred with great frequency. The hardened faces
were too shallow and the dies had been insufficiently
tempered before using.
Fig. 16 is a contact print of a section through a
circular block which had cracked badly in both radial
and peripheral directions. The depth of hardness penetration
is indicated byr the dotted line, but is otherwise
made visibile by the diffused condition of the
surface material. The layer of hardened steel is most
shallow in the depression where the working stresses
FIG. 14—Transverse cracks on flange of tire due to
heavy braking.
are greatest. On Brinelling a case-hardened object
the hard surface is damaged because it cannot follow
the distortion of the softer material underneath it without
cracking. In the same way the hardened surface
of a die block if too shallow, is readily distorted into
a crack by temperature effects or it may be directly
stressed into a crack by the hammer blows. It is
obvious that the die block represented by Fig. 21
November, 1925
would be most likely to break at the bottom of the
semi-circular impression because in that position the
steel was only superficially hardened.
Of a score hardened die blocks examined eight of
them at least had probably gone into service either
untempered or tempered only at low temperatures.
This seemed apparent from the high Brinell hardness
numbered (500 to 600) of the material on the working
face but not in the actual impression. In the impres-
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FIG. IS—Cracks in layer of hardened steel caused by
rash grinding.
sion itself the material was much softer and it was
generally softest in the largest part of the impression.
These observations suggest that the hardened and insufficiently
tempered dies were further tempered in
and about the impression byr the hot steel brought into
contact with these parts. As no condition could be
more favorable to cracking, the fact that each block is
reported to have cracked after a very short life is not
surprising.
The impression in a hardened die must inevitably
be tempered during the actual forging operation and
the tempering effect will ultimately reach a maximum
and proceed no further. If such tempering is inevita-
FIG. 16—Macro print from cracked die block. Note shallow
hardness penetration on working face indicated by dotted
line.
ble it would be better to temper the whole block to
that extent under controlled conditions.
A final reference is made to the burning of steel
because makers of billets, bars, rods and tubes, as
well as forgers, burn material without knowing it, and
are, therefore, prone to believe that such a mishap
does not occur in their furnaces. It would help greatly
if every person responsible for the hot working of
(Concluded on page 411)