ASTM B78-90(1995)e1

NOTICE: This standard has either been superseded and replaced by a new version or
withdrawn. Contact ASTM International (www.astm.org) for the latest information.
e1
Designation: B 78 – 90 (Reapproved 1995) An American National Standard
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method of
Accelerated Life of Iron-Chromium-Aluminum Alloys for
Electrical Heating
This standard is issued under the fixed designation B 78; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript
epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Section 13 was added editorially in June 1995.
1. Scope junctions shall be 3 in. (76.2 mm) lower than the plane of the
top of the enclosure.
1.1 This test method covers the determination of the resis-
tance to oxidation of iron-chromium-aluminum electrical heat-
4. Apparatus
ing alloys at elevated temperatures under intermittent heating
4.1 The apparatus shall be similar to the requirements
using a constant-temperature cycle test. This test is used for
specified in 4.2 to 4.8, inclusive, and shall be connected as
internal comparative purposes only.
shown in Fig. 2.
1.2 The values stated in inch-pound units are to be regarded
4.2 Power Supply—The transformer or motor generator set
as the standard. The metric equivalents of inch-pound units
shall be capable of delivering a controlled voltage of from 10
may be approximate.
to 35 V to the circuit. It shall have a continuous current
1.3 This standard does not purport to address all of the
capacity of at least 20 A/specimen.
safety concerns, if any, associated with its use. It is the
4.3 Voltage Control—The automatic voltage control shall be
responsibility of the user of this standard to establish appro-
capable of maintaining across the bus bars a constant voltage
priate safety and health practices and determine the applica-
within6 0.5 %.
bility of regulatory limitations prior to use.
NOTE 1—It has been found impossible to make accurate tests without
2. Significance and Use
voltage control, as changes in line voltage are sufficient to cause
2.1 This test method is used by producers of electrical considerable variation in the results obtained (see Annex A1).
heating alloys to measure the cyclic oxidation resistance of
4.4 Variable Transformer—The variable transformer shall
these alloys.
be capable of adjusting the voltage across the specimen to
2.2 Because of the effect of environment, design and use,
within approximately 0.25 % of any desired value within the
the life values obtained from this test method may not correlate
working range and shall have a continuous current rating of
with that of an appliance or industrial heating unit.
approximately 25 A.
3. Test Panel
NOTE 2—A variable transformer having a working range of adjustabil-
ity from approximately 0 to 20 V, provides for testing wires within a
3.1 Size and Location—The dimensions of the test panel
considerable range of size and resistivity.
shall be similar to those shown in Fig. 1. The test panel shall be
located in a position free from drafts of air.
4.5 Ammeter and Voltmeter—The ammeter and voltmeter
3.1.1 The enclosure shall fit tightly on the panel and the shall have an accuracy of 1 % of normal test deflection
glass slide shall fit snugly to prevent leakage of air at this point
(approximately 15 A and 15 V respectively). For alternating
during the operation of the test, as even a slight draft of air in current the range shall be such as to give a reading above the
contact with the specimen will cause excessive variation in
lower fifth of the scale range. The ammeter has appreciable
length of life. A screen of 40 wire mesh, 0.010-in. (0.025 mm) resistance. A compensating resistance shall be cut into the
wire diameter, market grade, may be used as a cover over the
circuit to replace the resistance of the ammeter so that the
individual stations. over-all resistance of the circuit is not changed. This resistance
3.2 Terminals—The two terminals shall be spaced 2 in.
shall be inserted in series with the contact of the upper switch
(50.8 mm) apart, center to center and shall be so positioned that shown in Fig. 2.
the wire specimen when secured therein shall be in a U-shaped
4.6 Optical Pyrometer or Infrared Thermometer—The op-
pattern as described in Section 5. The specimen terminal tical system shall be such as to provide a magnification of at
least four diameters. This may be accomplished by means of a
special lens or combination of two standard lenses in the
This test method is under the jurisdiction of ASTM Committee B-4 on Metallic
objective to provide a short focal length and the desired
Materials for Electrical Heating, Electrical Resistance, and Electrical Contacts.
magnification (see Annex A1). These instruments must have an
Current edition approved June 29, 1990. Published August 1990. Originally
published as B 78 – 59 T. Last previous edition B 78 – 81. accuracy of 610°F (5.5°C) and UBS traceability.
B78
Metric Equivalents
in. mm
⁄2 12.7
5 ⁄32 131.0
7 ⁄4 184.2
21 ⁄2 546
22 559
52 1321
64 1626
FIG. 1 Test Panel
NOTE 3—It is highly important that the temperatures of the test
been selected for test. Particular care shall be taken to see that
specimen be adjusted as accurately as possible, as small variations in
the specimen selected is free from kinks. This precaution is
temperature result in considerable variation in length of life. Optical
necessary, since a kink, even though later removed may cause
pyrometer or infrared pycnometer makes it possible to determine the
burnout at that point.
temperature at any particular point on the wire, and with the arrangement
described, the temperature of a comparatively small wire may be taken
NOTE 4—It is also very desirable to select and keep as a reference
quite readily.
standard for comparison, a spool or coil of wire that is uniform in cross
section from one end to the other. Tests may then be made at any time on
4.7 Interrupter—Some form of apparatus shall be used as
the reference standard, and if conditions have changed they will be noted
an interrupter to open and close the circuit.
by the length of life of the standard. Comparisons between tests made at
4.8 Apparatus for Recording Time of Burnout—If no appa-
different times on the standards and other wires may be correlated in this
ratus is available for recording the time of burnout, arrange-
manner (see Annex).
ments shall be made for hourly observations for burnouts.
6. Mounting Specimen
5. Test Specimen
6.1 A straight length of the specimen shall be shaped into a
5.1 The test specimen shall be 0.0254 in. (0.645 mm) in “U” pattern, 10 in. (254 mm) of which shall be mounted
diameter. The length of the wire selected for test shall be such between the terminals described in 2.2. The “U” pattern shall
as to give a test length of approximately 10 in. (254 mm). consist of two parallel legs and a semicircle of 1-in. (25.4-mm)
5.2 The test specimen shall be representative with regard to radius, the plane of which is parallel to and equidistant from
the surface of the average of the coil or spool of wire which has the front and back panels, and the legs of which are equally
B78
9.9 Measure the voltage and the current and record the
values together with the starting temperature and time of
starting the test.
9.10 Start the interrupter, the timing device of which shall
have been previously regulated so that the “on” period and the
“off” period shall be equal and shall each have a duration of 2
min.
NOTE 7—Various cycles have been tried varying from 10 min on and 5
min off to 30 s on and 30 s off, whence it was found that the 2-min-on and
2-min-off c
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