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236 rorging- Stamping - Heat Treating
July, 1925
T e s t s o n S t e e l a t E l e v a t e d T e m p e r a t u r e s
Short-Time Static Experimental Results for High-Strength Steel
Are Compared with Long-Time Static Results on the
T H E use of metals at elevated temperatures has
become an important question to builders and
users of steam generating machinery and internal
combustion engines. Certain parts of such machinery
are subjected to elevated temperatures, and these temperatures
have steadily increased in recent years with
the improvement in size and efficiency of such equipment
until, at the present time, metal under considerable
stress is subjected to temperatures which keep
them constantly at from 600 to 900 deg. F Certain
metal parts used at elevated temperatures, such as
steam piping, valves, boilers, and turbine cases, are
subjected in general to static stresses only. Other
metal parts, such as turbine wheels, turbine blades,
piston rods, and cylinders of internal combustion engines,
are subjected to reversed stresses which may
be repeated many times.
The metals in general use at elevated temperatures
are almost all ferrous, and consist in the main
of wrought steel, cast steel and cast iron. It is gradually
becoming known that certain ferrous alloys are
capable of withstanding stress at high temperatures
better than others. Such metals are usually high in
tungsten, nickel, chromium, or some satisfactory combination
of these alloys. Fig. 1 gives a general idea
of the relative values of steels containing these alloys
when tensile strength at elevated temperatures is considered.
These data are from annealed steels and cast
iron, and are largely drawn from results by Harper
and MacPherrant. The results of tests on steels at
elevated temperatures show that in general the static
properties other than strength are affected by temperature
in the following manner: the higher the
strength for any particular steel, the lower the percentages
of reduction of area and elongation and the
higher the hardness factors.
In obtaining strength data at elevated temperatures
for use in actual design, care must be exercised
to approach as nearly as possible the conditions under
which the metal is to be used. The method usually
adopted in testing metals at ordinary temperatures
has been to make a test which lasts less than 10 minutes,
and expect this to represent the strength and
ductility factors to which the metal will conform
when subjected to stress for months, or even years.
For wrought and cast ferrous metals at ordinary
atmospheric temperatures, this assumption may be
made without serious error, but at elevated temperatures
an error of from 30 to 50 per cent depending on
the temperature used may result when this procedureis
followed. Static testing at elevated temperatures.
therefore, is not so simple, nor can it be as expedi­
*A paper presented at the twenty-eighth annual meeting
of the American Society for Testing Materials, held at Atlantic
City, N. J.. June. 1925.
tSpecial Research Assistant Professor of Engineering Materials.
University of Illinois.
{Bulletin Xo. 141. Allis-Chalmers Manufacturing Comnany,
1922.
Same Steel Under Similar Conditions
By T. McLEAN JASPERf
tiously carried out as at ordinary atmospheric temperatures.
At ordinary atmospheric temperatures steel is a
crystallin substance which, within its elastic range
under static load, acts as an isotropic, elastic substance.
Under elevated temperatures, however, this
general state of affairs no longer exists, and as the
temperature is increased the metal gradually loses certain
of its elastic properties and at the same time
assumes a state approaching that of a plastic amorphous
material. As this condition is approached, the
steel tends to continually increase its stretch or strain
without an accompanying increase of load, and the
result is that long-time tensile strength varies from
the short-time tensile strength in an increasing percenage
as the temperature is increased. The effect of
this is well illustrated in Fig. 2, which shows the
variation of the static properties of a quenched metal
and indicates the values of the tensile strength at"
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FIG. 1—Curves showing the effect of certain ingredients on
the tensile strength of various annealed steels and cast iron
at elevated temperatures.
elevated temperatures under ordinary test conditions
and under long-time test conditions. In the shorttime
test the material was continuously loaded to its
tensile strength within a period of about five minutes
after it was raised to the correct temperature. The
values of the tensile strengths in this case are shown
by the upper tensile strength curve. In the long-time
test, the specimen was tested to its proportional limit
fairly rapidly and then increments of load were added
only when strain or stretch had become zero, or
almost so. for each increment of load. In this manner,
the time necessary to break a long-time test specimen
varied from 12 to 72 hours, depending on the
material and on the temperature at which it was being
tested. The values of the long-time tensile strengths
are shown by the lower portion of the tensile strength
curve. It will be noticed that, as the values of the
temperature of specimens are increased, the ductilityvalues
are also increased and the strength values are