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It is quite evident that the proper time to reject
steel is before it is put into forgings — at a time before
the drop forger has put money into processing,
and at a time when the steel manufacturer may salvage.
Furthermore, before forging, the responsibility
for defects can be definitely established. After forging
there is often difficulty in definitely assigning correct
origin of a defect.
Defects in Rolled Steel.
A brief review of the most prevalent defects in
rolled steel, found by regular bar inspection, may be
of some interest. They are pipe, unsound center and
bursts or ruptures on the inside, and seams, laps, scabs
and livers on the outside, and occasionally evidence
of burning. Pipe is a central defect originating in the
shrinkage cavity in the top of the ingot and is due to
insufficient croppage. Unsound center or excessive
porosity may consist of the segregated and impure
metal immediately beneath this zone, but may also
be characteristic of the heat throughout and due to
improper refining of the metal, or over-oxidation. Pipe
may be usually distinguished by shear tests or fracture
tests, but in some cases may be so faint as to
necessitate a coarse or fine etch on a properly prepared
surface. Secondary pipe which may occur in
material rolled from small-end-up ingots is most difficult
to detect, as perfectly sound steel is found on
either side of it in the bar. For internal flaw in bars
such as unsound or segregated center, shearing tests
are usually inadequate and etch tests are necessary.
Internal ruptures or bursts are more prevalent in
steels of higher alloy content and may be detected by
fracture or etch. Most surface defects on bars are
classed as seams, or scabs and slivers, although the
former cover imperfections of various origins. Defects
known generally as seams may be due to over-fills,
under-fills, rolling laps, rolled in chip marks, guide
scratches, crossed rolls, poor roll surface, etc., or they
may originate in the ingot, elongate in rolling to billits
and persist through the next conversion into bars.
Transverse cracks in the skin of an ingot roll out into
seams in finished bars if not chipped out in the bloom
or billet form. Sub-cutaneous gas pockets form light
sub-surface seams. Scabs and slivers usually may be
attributed to poor ingot surface. The extent to which
these defects in bars give trouble in forging in most
cases is not difficult to forecast. Guide scratches
which are often mistaken for seams should cause absolutely
no trouble unless unusually deep. Slight
over-fills, if not lapped, do not open in forging, but
most other forms, unless so light as to scale off in the
heating furnace, are very likely to cause serious trouble
in the finished forging.
In man}- cases the above, which may be considered
primary inspection and consisting of check analysis,
dimensional check, examination for soundness and
surface condition, is supplemented by shear inspection
on such types where analysis is such as to make
cold shearing difficult if not hazardous. Recently, tvpes
of steel have been satisfactorily cold sheared where
formerly other than hot shearing was considered out
of the question. The steel plants, through closer temperature
control on the finishing mills, and more definite
regulation of cooling of their product have accom •
plished much in shearing quality.
Case Hardening Test.
As a rule after the steel has been subjected to these
tests it mav be stocked with assurance that "it will
forging- Si amping - He>af Treating
January, 1925
meet all requirements. However, for special purposes
there are other qualifications which must be
met. A number of specifications are now received
incorporating the McQuaid-Ehn test, of which all
should have a comprehensive idea of its intent and
general features. A thorough explanation of this test
would be quite long and highly technical, but a few
words should be able to convey its purpose together
with the general method of procedure and observations
in its execution- The McQuaid-Ehn test is to
pre-determine whether a particular heat of steel will
give satisfactory results in case hardening. Often, for
some unknown reason, soft spots are encountered in
the case on case hardened parts, characteristic of a
particular heat of steel. The analysis is correct, the
steel is sound, it is satisfactorily free from microscopic
defects. Likewise no fault can be attributed to case
hardening operations, which show satisfactory response
on other heats of steel. The McQuaid-Ehn
test will show conditions characteristic of heats which
will harden satisfactorily and of those in which trou-
14e is encountered. Furthermore, in gears made from
certain heats, distortion after hardening is much greater
than in others, and more difficult to control. This
test enables a forecast on this condition. The mechanical
procedure is as follows : Carburizing of suitable
samples at definitely prescribed temperatures for
a sufficient length of time to obtain a hyper-eutectoid
(approximately 1.00 to 1.10 carbon) case, cooling in
pots after carburizing, polishing and etching these
samples, and finally studying under the microscope.
Microscopic Study.
From a microscopic study the steel is classed
as "normal" steel, which should give satisfactory results
in case hardening, and "abnormal" steel which
may give unsatisfactory results in case hardening.
There are, of course, almost unlimited transitional
stages between "normal" and "abnormal" and their
classification must necessarily depend on experience
and personal judgment. Briefly, characteristics of
"normal" steel are, in the hyper-eutectoid case, large,
well defined, pearlite grains with excessive carbides
or cementite in the form of coarse network around
the pearlite grains and grading down into a core consisting
of large angular grains of pearlite and ferrite.
In "abnormal" steel the hyper-eutectoid case consists
of much smaller and less uniform grains of pearlite
with the excess cementite occurring in smaller envelopes
around the pearlite, and in some cases as
patches of massive cementite. In the most "abnormal"
instances the pearlite breaks down into patches of
cementite and ferrite. The core in such steel shows
much finer grained pearlite and ferrite than in "normal'^
steel, and is often quite banded. The cause of
the "abnormal" condition has been attributed to overoxidized
condition of the steel, but this has not bee i
universally accepted. In checking material for Mc­
Quaid-Ehn characteristics tests may be taken by the
consumer from finish rolled material. The time for
the making of tests by the steel manufacturer should
be when material is in the semi-finished form and before
approving a heat for application on a particular
order- A convenient method of such sampling is in
the form of chips from blooms. Sonv steel manufacturers
have based tests on ladle sample, which I consider
less reliable. The McQuaid-Ehn test, if properly
conducted and interpreted, is a valuable contribution
in quality control, but does not solve all case harden-