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354 Forging- Stamping - Heat Tieahng
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. Some steel manufacturers
have based tests on ladle sample, which is
considered less reliable. The McQuaid-Ehn test, if
properly conducted and interpreted, is a valuable contribution
in quality control,-but does not solve all case
hardening troubles, the fault may not and often does
not lie with the steel.
Soundness.
For soundness, in relatively few cases the porosity
or macro-etch test has been used, and this test at
times otters great advantages in predetermining the
quality of steel. The chief disadvantage in its application
are lack of standardization (acids, acid
strengths, time and temperature factors, etc.) and lack
of ability to properly interpret and put to practical
use the results. In many cases, such as investigations
for internal rupture, snow-flakes, bursts, etc., the test
is infallible. The practical execution of this test consists
in submitting to action of acid solution properly
prepared surfaces of longitudinal and transverse sections
cut from material submitted for examination.
< )l different acids and percentages of dilution, a 50
per cent hydrochloric solution is preferable for general
purposes, varying the time in accordance with the
nature of the material examined.
The microscope is at times used as a means of
establishing the suitability of heats for certain important
uses. It offers an excellent means for the
study of structure and freedom of steel from sonims.
but it may be abused, and in the hands of the inexperienced
cause more harm than good. It is a very difficult
matter to base the acceptance or rejection of a
heat of steel solely on microscopic examination for
impurities, when the field or fields examined represent
such an infinitesimal portion of the material involved.
Furthermore, no standards are available and
any- decisions must necessarily- be based on the personal
opinions of the investigators, and such opinions
are wide and varied.
Fibre tests, but rarely used, embrace the study of
fractures made after subjecting material to a sorbitizing
treatment.
X-Ray and Magnetic Testing.
There appears in the near future but slight probability
of the general adoption of magnetic or X-ray
testing for other than special work, where the cost of
the finished product is very great as compared with
steel costs or where a failure would result disastrously.
Magnetic testing has evolved through the defectoscope,
a means of determining defects or flaws;
through the magnetoscope, composition and mechanical
properties. A most valuable feature of such control
is that no material is destroyed or disfigured in
the execution of tests. This method has been put into
practical use on turbine bucket wheels, gears, saws,
elevator cable, rails, etc. While there is at present
neither prospects for the steel manufacturer being
able to run the product of his mills through a defectoscope,
nor a probability that the forging manufacturer
will so test his vast production, it does appear
that apparatus of this type may be advantageously
applied for special requirements.
October, 1925
X-ray examination has been put to practical usage
for the detection of internal defects, but the time and
expense involved render its scope too limited for
quantity production.
The foregoing has dwelt more particularly in consideration
of stock for forging purposes.
Shearing the Stock.
Before using under the hammer, stock must be
first sheared to suitable lengths or multiples. It is
impossible on steels of higher alloying content to
draw a definite line as to what analyses will and what
will not satisfactorily cold shear. Such variables as
size, condition of shears, high or low side of range in
hardening elements, and even weather conditions
have pronounced effect. Under most favorable conditions
it may be assumed that standard analyses of
chrome-vanadium, chrome, carbon, low-chromc-nickcl
and 3y per cent nickel steels will cold shear in sizes
up to 2y in. round or square with a carbon content
ti]) to .45 maximum. With carbon as high as .50 or
.55 these types can usually be sheared cold with
safety in sizes up to \y in. round or square. These
limits are given more in the nature of a warning than
as a recommendation. A little heating Before shearing
steel approximating these limits is by no means
an extravagance. In extremely cold weather much
improvement may even be effected on certain types
of steel by raising temperatures but slightly, from
yard to room temperature for instance, or just sufficient
to relieve the intense chill from the steel. Requiring
special attention in cold shearing are capacity
of shears, their alignment, condition of knives and
hold down.
Trouble in cold shearing may be evidenced in several
forms. The stock may break off sharply, or spall
on the corners; it may show a very fine crack across
the sheared surface or it may be strained to such an
extent upon shearing that while no crack is perceptible,
one opens from a few hours to a few days after.
The latter condition may prove the most serious, as
shear inspection will not reveal the trouble and such
stock may rupture further when brought to forging
heat.
Fins or ragged edges should be avoided in shearing
as they may "lap in" during forging. Often it is
necessary to grind badly finned edges. This difficulty
can usually be eliminated by closer alignment of shears
and maintenance of proper knife edges.
To Reduce Variety of Taper Roller Bearings*
As the result of a conference held on June 15 at
the suggestion of the division of simplified practice, it
was conceded that there are too many varieties of
taper roller bearings, and it was deemed feasible and
advisable to eliminate some of these existing varieties.
The best procedure to be followed was divided into
three steps: (1) Representatives of leading producers
of taper roller bearings to meet and formulate recommended
standard "current" and "service" lists; (2)
these lists to be sent to the Society of Automotive Engineers
for approval in accordance with the established
methods of the society: (3) the above society to request
the co-operation of the division of simplified
practice in securing nation-wide adoption of such
approved lists as are found to be acceptable to manufacturers,
distributors, and consumers.
•From Technical News Bulletin. Bureau of Standards.