ASTM B76-90(2001)

Designation: B 76 – 90 (Reapproved 2001) An American National Standard
Standard Test Method for
Accelerated Life of Nickel-Chromium and Nickel-Chromium-
Iron Alloys for Electrical Heating
This standard is issued under the fixed designation B 76; 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.
1. Scope post or through the upper bus bar. This provides for adjustment
laterally and vertically, as shown in Fig. 1.
1.1 This test method covers the determination of the
3.3 Lower Terminal—A 10-g weight shall be attached to the
resistance to oxidation of nickel-chromium and nickel-
lower end of the specimen. A flexible silver foil (approximately
chromium-iron electrical heating alloys at elevated tempera-
0.375 in. (9.52 mm) wide and 0.0015 in. (0.038 mm) thick)
tures under intermittent heating. Procedures for a constant-
connected to the 10-g weight shall constitute the lower
temperature cycle are provided. This test method is used for
terminal.
internal comparative purposes only.
1.2 The values stated in inch-pound units are to be regarded
NOTE 2—Experiments have shown that with high temperatures alloys
as the standard. The metric equivalents of inch-pound units
of nickel-chromium and nickel-chromium-iron are subject to plastic flow
when under relatively light load. The weight specified in 3.3 does not
may be approximate.
cause appreciable increase in length during the test.
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4. Apparatus
responsibility of the user of this standard to establish appro-
4.1 The test apparatus shall be similar to the requirements
priate safety and health practices and determine the applica-
specified in 4.2 to 4.8, inclusive, and shall be connected as
bility of regulatory limitations prior to use.
shown in Fig. 2.
2. Significance and Use 4.2 Power Supply—The transformer or motor generator set
shall be capable of delivering a controlled voltage of from 10
2.1 This test method is used by producers of electrical
to 20 V to the circuit. It shall have a continuous current
heating alloys to measure the cyclic oxidation resistance of
capacity of at least 20 A/specimen.
these alloys.
4.3 Voltage Control—The automatic voltage control shall be
2.2 Because of the effect of the environment, design, and
capable of maintaining across the bus bars a constant voltage
use, the life values obtained from this test method may not
within6 0.5 %.
correlate with that of an appliance or industrial heating unit.
NOTE 3—It has been found impossible to make accurate tests without
3. Test Panel
voltage control, as changes in line voltage were sufficient to cause
considerable variation in the results obtained (see Annex A1).
3.1 Size and Location—The dimensions of the test panel
shall be similar to those shown in Fig. 1. The test panel shall be
4.4 Variable Transformer—The transformer shall be ca-
located in a position free from drafts of air.
pable of adjusting the voltage across the specimen so that
current is controlled to approximately 0.25 % of desired value,
NOTE 1—The enclosure shall fit tightly on the panel and the glass slide
and shall have a continuous current rating of approximately 25
shall fit snugly to prevent leakage of air at this point during the operation
of the test, as even a slight draft of air in contact with the specimen will
A.
cause excessive variation in length of life. A screen of 40 wire mesh,
4.5 Ammeter and Voltmeter—The ammeter and voltmeter
0.010-in. (0.025-mm) wire diameter, market grade, may be used as a cover
shall have an accuracy of 1 % of normal test deflection
over the individual stations.
(approximately 15 A and 15 V, respectively). For alternating
3.2 Upper Terminal—The upper terminal shall consist of a
current the range used shall be such as to give a reading above
binding post attached to a rod passing through another binding
the lower fifth of the scale range. The ammeter has appreciable
resistance. A compensating resistance shall be cut into the
This test method is under the jurisdiction of ASTM Committee B02 on
circuit to replace the resistance of the ammeter so that the
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
overall resistance of the circuit is not changed. This resistance
B02.10 on Thermostat Metals.
shall be inserted in series with the blade of the upper switch
Current edition approved May 25, 1990. Published July 1990. Originally
published as B 76 – 29 T. Last previous edition B 76 – 81. shown in Fig. 2.
Further information on this test method is given in a paper by F. E. Bash and
4.6 Optical Pyrometer or Infrared Thermometer—The op-
J. W. Harsch, “Life Tests on Metallic Resistor Materials for Electrical Heating,’’
tical system shall be such as to provide a magnification of at
Proceedings, ASTEA, American Society for Testing and Materials. Vol 29, Part II,
least four diameters. This may be accomplished by the use of
1929, p. 506.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
B76
Metric Equivalents
in. 1 2 55 32 71 4 21 1 2 52 64
/ / / /
mm 12.7 131.0 184.2 546.1 1321 1626
FIG. 1 Test Panel for Accelerated Life Test
a special lens or combination of two standard lenses in the
objective to provide a short focal length and the desired
magnification. (See Annex A1.) These instruments must have
an accuracy of 610°F and NIST traceability.
NOTE 4—It is highly important that the temperature of the test specimen
be adjusted as accurately as possible, as small variations in temperature
result in considerable variation in length of life. An optical pyrometer or
infrared thermometer makes it possible to determine the temperature at
any particular point on the wire and with the arrangement described the
temperature of a comparatively small wire may be taken quite readily.
4.7 Interrupter—Some form of apparatus shall be used as
an interrupter to open and close the circuit.
4.8 Apparatus for Recording Time of Burnout—If no appa-
ratus is available for recording the time of burnout, arrange-
ments shall be made for hourly observations for burnouts.
Some form of electric-clock mechanism which can be con-
nected into the circuit may be used.
FIG. 2 Electrical Circuit Diagram for Accelerated Life Test
4.9 Apparatus for Measuring Length Changes—Any form
of optical apparatus such as a traveling microscope, an optical
B76
by the length of life on the standard. Comparisons between tests made at
projection system, a projection microscope, or a contact
different times between the standards and other wires may be correlated in
microscope may be used for determining changes in length of
this manner (see Annex A1).
the test specimen. A type of apparatus that has been found
satisfactory consists of a telescope with a horizontal cross hair
6. Mounting of Specimens
and leveling bubble. The apparatus may be mounted on a frame
so that it can be readily moved from one position to another for 6.1 The test specimens shall be mounted on the test panel in
a vertical position, as shown in Fig. 1, and shall have the
examining specimens in the life test. The telescope mount
should be adjustable in the vertical plane on guides by means following typical spacing:
of a threaded member. A movement of 2.5 in. (64 mm) is Distance between test panel and 2 in. (50.8 mm)
specimens
desirable. A piece of cross-section paper, 4 in. (102 mm) in
Distance between specimens 6 in. (152.4 mm)
length by 3 in. (76 mm) in width, calibrated 20 lines to 1 in.
Distance between specimen and shield 2 in. (50.8 mm)
(25.4 mm) should be mounted on the test panel so that the Distance between upper bus bar and silver foil 20 in. (508 mm)
contact . approximately
lower edge is below a horizontal line drawn across the top of
the lowest possible position of the weight attached to the lower
NOTE 6—This recommendation is based on a series of tests run in four
end of the wire under test. A satisfactory arrangement is shown laboratories to determine the best position for the specimen in which
horizontal mounting, catenary mounting, and vertical mounting were
in Fig. 3. A steel scale 18 in. (457 mm) in length, calibrated to
compared. The results of the tests indicated that the vertical mounting
0.01 in. (0.25 mm), may be used for length measurements.
gave the best results and was most convenient. It might be expected that
the vertical wire would be a great deal hotter near the top than near the
5. Test Specimen
bottom. This does not appear to be the case due to the fact that convection
5.1 The test specimen shall be No. 22 Awg, 0.0253 in. (0.64
currents are greater near the top, and therefore largely compensate for
mm). The length of wire selected for test shall be such as to
variations that otherwise would occur.
permit the use of a 12-in. (305-mm) test length between the
6.2 In mounting a test specimen, one end of the specimen
two terminals.
shall be inserted in the upper terminal and the weight attached
5.2 The test specimen shall be representative, as regards
to the other end. The upper terminal shall then be adjusted to
surface, of the average of the coil or spool of wire which has
give a test length of the wire of approximately 12 in. (305 mm)
been selected for test. Particular care shall be taken to see that
between the two terminals. Care shall be taken to see that the
the specimen selected is free from kinks. This is necessary, as
weight will be able to move freely after the specimen has
a kink, even though later removed, may cause burnout at that
expanded upon heating.
point.
6.3 Number of Test Specimens:
NOTE 5—It is also very desirable to select and keep as a reference
6.3.1 The life value shall be the average of three simulta-
standard for comparison a spool or coil of wire which is uniform in cross
neous determinations on wire specimens of the material being
section from one end to the other. Tests may then be made at any time on
the reference standard, and if conditions have changed they will be noted tested.
FIG. 3 Apparatus for Measuring Length Changes During Life Test
B76
7. Ballast Resistance “off’’ period shall be equal and shall each have a duration of 2
min.
7.1 The voltage between the bus bars shall be adjusted so
that it will not be necessary to make the ballast resistance in
NOTE 8—Various cycles have been tried varying from 10 min on and 5
series with the specimen greater than 20 % of the resistance of
min off to 30 s on and 30 s off, when it was found that the 2 min on and
2 min off cycle gave the shortest life for a given temperature. It appears
the specimen.
that sufficient cooling time has to be allowed to permit the specimen to
reach a low enough temperature to cause any loosening or cracking of
8. Temperature of Test
scale which will occur due to variations in coefficient of expansion of the
8.1 In the test method all temperatures are true tempera-
scale and the metal. The heating and cooling operation is more injurious
tures.
to wire than maintaining it at a definite temperature.
8.2 For alloys of nominal composition 80 % nickel and
9.1.13 Adjust the temperature to the test temperature after 5
20 % chromium, the temperature of test shall be 2200°F
h and 24 h total elapsed time. Record the voltage and current
(1204°C); for alloys of nominal composition 60 % nickel,
after each resetting. Stop the interrupter before each resetting
15 % chromium and 25 % iron, the temperature of test shall be
and start it again after making the observation.
2200°F (1204°C); for alloys of nominal composition 35 %
9.1.14 After the first 24-h period, allow the test to run
nickel, 20 % chromium, and 45 % iron, the temperature of test
without readjustment for the next 24 h. At the end of this period
shall be 2050°F (1121°C).
and every 24 h thereafter until burnout, readjust the tempera-
ture so that it will be the same as the test temperature. After
9. Procedure
each adjustment of the temperature, observe and record the
9.1 Carry out the procedure as described in 9.1.1 to 9.1.13,
current and voltage measurements.
inclusive.
9.1.15 Length Changes of Specimen—Changes in length of
9.1.1 Support the temperature measuring instrument so that
the test specimen may be determined with an accuracy of 0.5 %
it can be quickly adjusted and read.
by the following procedure: First measure with a steel scale the
9.1.2 Set the series variable transformer at minimum volt-
length between the point at which the specimen wire leaves the
age resistance.
binding post and the top of the weight attached to the lower end
9.1.3 Close the switch in series with the specimen.
of the specimen wire. This measurement should be accurate to
9.1.4 Adust the variable transformer until the specimen is at
60.02 in. (0.51 mm) (see 9.1.6). If the apparatus described in
a low red heat.
4.9 is used, adjust the telescope in the vertical plane until the
9.1.5 Grasp the weight to apply a slight tension, sufficient to
cross hair is directly lined up with the top of the weight
straighten the wire.
attached to the wire (see 9.1.6). Estimate the readings on the
9.1.6 If change of length measurements are to be made as
cross section paper to the nearest 0.01 in. (0.25 mm). Other
specified in 9.1.15, open the switch in series with the specimen
readings through the telescope may be taken in the same
and make the initial length measurement of the unheated
manner and noted. Calculate the changes in length of the
specimen. Then again close the switch to reheat the specimen.
specimen as the difference between the first reading made on
9.1.7 Adjust the temperature of the specimen to 200°F
the cross-section paper and the subsequent readings.
lower than the test temperature.
10. Record
NOTE 7—The interrupter shall not be in operation while the temperature
10.1 Measurements and observations shall be recorded on a
is being adjusted.
data sheet similar to that shown in Fig. 4 (see Annex A1 for
9.1.8 Allow the specimen to operate under this condition
reference to useful life).
until 2 h have elapsed in order to bring the emissivity nearer to
a black body condition.
11. Report
9.1.9 Adjust the temperature of the specimen to the test
11.1 Report the following information:
temperature. Maintain this balance until 1 min has elapsed.
11.1.1 Nominal analysis,
9.1.10 After another 13 min readjust the temperature of the
11.1.2 Identification of specimen,
specimen to the test temperature. Final adjustment shall be
11.1.3 Cross-sectional dimensions of the specimen,
completed within the next minute or a total of 15 min. The end
11.1.4 Life of the s
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