ASTM B78-90(2019)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: B78 − 90 (Reapproved 2019)
Standard Test Method of
Accelerated Life of Iron-Chromium-Aluminum Alloys for
Electrical Heating
This standard is issued under the fixed designation B78; the number immediately following the designation indicates the year of original
adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.1 The enclosure shall fit tightly on the panel and the
glass slide shall fit snugly to prevent leakage of air at this point
1.1 This test method covers the determination of the resis-
during the operation of the test, as even a slight draft of air in
tance to oxidation of iron-chromium-aluminum electrical heat-
contact with the specimen will cause excessive variation in
ing alloys at elevated temperatures under intermittent heating
length of life.Ascreen of 40 wire mesh, 0.010-in. (0.025-mm)
using a constant-temperature cycle test. This test is used for
wire diameter, market grade, may be used as a cover over the
internal comparative purposes only.
individual stations.
1.2 The values stated in inch-pound units are to be regarded
3.2 Terminals—The two terminals shall be spaced 2 in.
as standard. The values given in parentheses are mathematical
(50.8 mm) apart, center to center, and shall be so positioned
conversions to SI units that are provided for information only
that the wire specimen when secured therein shall be in a
and are not considered standard.
U-shaped pattern as described in Section 5. The specimen
1.3 This standard does not purport to address all of the
terminaljunctionsshallbe3in.(76.2mm)lowerthantheplane
safety concerns, if any, associated with its use. It is the
of the top of the enclosure.
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
4. Apparatus
mine the applicability of regulatory limitations prior to use.
4.1 The apparatus shall be similar to the requirements
1.4 This international standard was developed in accor-
specified in 4.2 to 4.8, inclusive, and shall be connected as
dance with internationally recognized principles on standard-
shown in Fig. 2.
ization established in the Decision on Principles for the
4.2 Power Supply—The transformer or motor generator set
Development of International Standards, Guides and Recom-
shall be capable of delivering a controlled voltage of from 10
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee. to 35 V to the circuit. It shall have a continuous current
capacity of at least 20 A/specimen.
2. Significance and Use
4.3 VoltageControl—The automatic voltage control shall be
2.1 This test method is used by producers of electrical capable of maintaining across the bus bars a constant voltage
heating alloys to measure the cyclic oxidation resistance of within 60.5 %.
these alloys.
NOTE 1—It has been found impossible to make accurate tests without
voltage control, as changes in line voltage are sufficient to cause
2.2 Because of the effect of environment, design, and use,
considerable variation in the results obtained (see Annex A1).
the life values obtained from this test method may not correlate
4.4 Variable Transformer—The variable transformer shall
with that of an appliance or industrial heating unit.
be capable of adjusting the voltage across the specimen to
3. Test Panel within approximately 0.25 % of any desired value within the
working range and shall have a continuous current rating of
3.1 Size and Location—The dimensions of the test panel
approximately 25 A.
shall be similar to those shown in Fig. 1.The test panel shall be
located in a position free from drafts of air.
NOTE 2—Avariable transformer, having a working range of adjustabil-
ity from approximately 0 to 20 V, provides for testing wires within a
considerable range of size and resistivity.
4.5 Ammeter and Voltmeter—The ammeter and voltmeter
This test method is under the jurisdiction of ASTM Committee B02 on
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
shall have an accuracy of 1 % of normal test deflection
B02.10 on Thermostat Metals and Electrical Resistance Heating Materials.
(approximately 15 A and 15 V, respectively). For alternating
Current edition approved April 1, 2019. Published April 2019. Originally
current, the range shall be such as to give a reading above the
approved in 1959. Last previous edition approved in 2013 as B78 – 90 (2013). DOI:
10.1520/B0078-90R19. lower fifth of the scale range. The ammeter has appreciable
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B78 − 90 (2019)
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
described, the temperature of a comparatively small wire may be taken
resistance. A compensating resistance shall be cut into the
quite readily.
circuit to replace the resistance of the ammeter so that the
overall resistance of the circuit is not changed. This resistance
4.7 Interrupter—Someformofapparatusshallbeusedasan
shall be inserted in series with the contact of the upper switch
interrupter to open and close the circuit.
shown in Fig. 2.
4.8 Apparatus for Recording Time of Burnout—If no appa-
4.6 Optical Pyrometer or Infrared Thermometer—The opti-
ratus is available for recording the time of burnout, arrange-
cal system shall be such as to provide a magnification of at
ments shall be made for hourly observations for burnouts.
least four diameters. This may be accomplished by means of a
special lens or combination of two standard lenses in the
5. Test Specimen
objective to provide a short focal length and the desired
magnification(seeAnnexA1).Theseinstrumentsmusthavean
5.1 The test specimen shall be 0.0254 in. (0.645 mm) in
accuracy of 610 °F (5.5 °C) and UBS traceability.
diameter. The length of the wire selected for test shall be such
as to give a test length of approximately 10 in. (254 mm).
NOTE 3—It is highly important that the temperatures of the test
specimen be adjusted as accurately as possible, as small variations in
5.2 The test specimen shall be representative with regard to
temperature result in considerable variation in length of life. Optical
thesurfaceoftheaverageofthecoilorspoolofwirewhichhas
pyrometer or infrared pycnometer makes it possible to determine the
temperature at any particular point on the wire, and with the arrangement been selected for test. Particular care shall be taken to see that
B78 − 90 (2019)
NOTE 6—Do not operate the interrupter while the temperature is being
adjusted.
9.3 Close the switch in series with the specimen.
9.4 Adjust the variable transformer until the specimen is at
a low red heat.
9.5 Adjust the temperature of the specimen to 2000 °F
(1093 °C).
9.6 Allow the specimen to operate under this condition until
2 h have elapsed, in order to bring the emissivity nearer to a
black body condition.
9.7 Set the temperature of the specimen to 2200 °F
(1204 °C). Maintain this balance until 1 min has elapsed.
9.8 After another 13 min, readjust the temperature of the
specimen to 2250 °F (1232 °C). Final adjustment shall be
completed within the next minute or a total of 15 min. The end
of this 15-min period is the start of the test. It is important to
FIG. 2 Electrical Circuit Diagram for Accelerated Life Test
maintain this time schedule.
9.9 Measure the voltage and the current and record the
the specimen selected is free from kinks. This precaution is
values together with the starting temperature and time of
necessary since a kink, even though later removed, may cause
starting the test.
burnout at that point.
NOTE 4—It is also very desirable to select and keep as a reference 9.10 Start the interrupter, the timing device of which shall
standard for comparison, a spool or coil of wire that is uniform in cross
have been previously regulated so that the “on” period and the
section from one end to the other. Tests may then b
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