ASTM B76-90(2007)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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
Designation: B76 − 90 (Reapproved2007)
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 B76; 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.
NOTE 1—The enclosure shall fit tightly on the panel and the glass slide
1. Scope
shall fit snugly to prevent leakage of air at this point during the operation
1.1 This test method covers the determination of the
of the test, as even a slight draft of air in contact with the specimen will
resistance to oxidation of nickel-chromium and nickel- cause excessive variation in length of life. A screen of 40 wire mesh,
0.010-in.(0.025-mm)wirediameter,marketgrade,maybeusedasacover
chromium-iron electrical heating alloys at elevated tempera-
over the individual stations.
tures under intermittent heating. Procedures for a constant-
temperature cycle are provided. This test method is used for 3.2 Upper Terminal—The upper terminal shall consist of a
binding post attached to a rod passing through another binding
internal comparative purposes only.
post or through the upper bus bar.This provides for adjustment
1.2 The values stated in inch-pound units are to be regarded
laterally and vertically, as shown in Fig. 1.
as the standard. The metric equivalents of inch-pound units
may be approximate. 3.3 Lower Terminal—A10-g weight shall be attached to the
lowerendofthespecimen.Aflexiblesilverfoil(approximately
1.3 This standard does not purport to address all of the
0.375 in. (9.52 mm) wide and 0.0015 in. (0.038 mm) thick)
safety concerns, if any, associated with its use. It is the
connected to the 10-g weight shall constitute the lower
responsibility of the user of this standard to become familiar
terminal.
with all hazards including those identified in the appropriate
Material Safety Data Sheet (MSDS) for this product/material
NOTE2—Experimentshaveshownthatwithhightemperaturesalloysof
as provided by the manufacturer, to establish appropriate nickel-chromium and nickel-chromium-iron are subject to plastic flow
when under relatively light load. The weight specified in 3.3 does not
safety and health practices, and determine the applicability of
cause appreciable increase in length during the test.
regulatory limitations prior to use.
4. Apparatus
2. Significance and Use
4.1 The test apparatus shall be similar to the requirements
2.1 This test method is used by producers of electrical
specified in 4.2 to 4.8, inclusive, and shall be connected as
heating alloys to measure the cyclic oxidation resistance of
shown in Fig. 2.
these alloys.
4.2 Power Supply—The transformer or motor generator set
2.2 Because of the effect of the environment, design, and
shall be capable of delivering a controlled voltage of from 10
use, the life values obtained from this test method may not
to 20 V to the circuit. It shall have a continuous current
correlate with that of an appliance or industrial heating unit.
capacity of at least 20 A/specimen.
4.3 Voltage Control—The automatic voltage control shall be
3. Test Panel
capable of maintaining across the bus bars a constant voltage
3.1 Size and Location—The dimensions of the test panel
within 6 0.5 %.
shall be similar to those shown in Fig. 1.The test panel shall be
NOTE 3—It has been found impossible to make accurate tests without
located in a position free from drafts of air.
voltage control, as changes in line voltage were sufficient to cause
considerable variation in the results obtained (see Annex A1).
4.4 Variable Transformer—Thetransformershallbecapable
This test method is under the jurisdiction of ASTM Committee B02 on
of adjusting the voltage across the specimen so that current is
Nonferrous Metals and Alloys and is the direct responsibility of Subcommittee
B02.10 on Thermostat Metals and Electrical Resistance Heating Materials.
controlled to approximately 0.25 % of desired value, and shall
Current edition approved May 1, 2007. Published May 2007. Originally
have a continuous current rating of approximately 25 A.
approved in 1929. Last previous edition approved in 2001 as B76 – 90 (2001). DOI:
10.1520/B0076-90R07.
4.5 Ammeter and Voltmeter—The ammeter and voltmeter
Further information on this test method is given in a paper by F. E. Bash and
shall have an accuracy of 1 % of normal test deflection
J. W. Harsch, “Life Tests on Metallic Resistor Materials for Electrical Heating,”
(approximately 15 A and 15 V, respectively). For alternating
Proceedings, ASTEA, American Society for Testing and Materials. Vol 29, Part II,
1929, p. 506. current the range used shall be such as to give a reading above
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B76 − 90 (2007)
Metric Equivalents
1 5 1 1
in. ⁄2 5 ⁄32 7 ⁄4 21 ⁄2 52 64
mm 12.7 131.0 184.2 546.1 1321 1626
FIG. 1 Test Panel for Accelerated Life Test
the lower fifth of the scale range. The ammeter has appreciable a special lens or combination of two standard lenses in the
resistance. A compensating resistance shall be cut into the objective to provide a short focal length and the desired
circuit to replace the resistance of the ammeter so that the
magnification. (See Annex A1.) These instruments must have
overall resistance of the circuit is not changed. This resistance
an accuracy of 610°F and NIST traceability.
shall be inserted in series with the blade of the upper switch
NOTE4—Itishighlyimportantthatthetemperatureofthetestspecimen
shown in Fig. 2.
be adjusted as accurately as possible, as small variations in temperature
4.6 Optical Pyrometer or Infrared Thermometer—The opti-
result in considerable variation in length of life. An optical pyrometer or
cal system shall be such as to provide a magnification of at
least four diameters. This may be accomplished by the use of
B76 − 90 (2007)
the specimen selected is free from kinks. This is necessary, as
a kink, even though later removed, may cause burnout at that
point.
NOTE 5—It is also very desirable to select and keep as a reference
standard for comparison a spool or coil of wire which is uniform in cross
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
by the length of life on the standard. Comparisons between tests made at
different times between the standards and other wires may be correlated in
this manner (see Annex A1).
6. Mounting of Specimens
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
following typical spacing:
Distance between test panel and 2 in. (50.8 mm)
specimens
Distance between specimens 6 in. (152.4 mm)
Distance between specimen and shield 2 in. (50.8 mm)
FIG. 2 Electrical Circuit Diagram for Accelerated Life Test
Distance between upper bus bar and silver foil 20 in. (508 mm)
contact . approximately
NOTE 6—This recommendation is based on a series of tests run in four
infrared thermometer makes it possible to determine the temperature at
laboratories to determine the best position for the specimen in which
any particular point on the wire and with the arrangement described the
horizontal mounting, catenary mounting, and vertical mounting were
temperature of a comparatively small wire may be taken quite readily.
compared. The results of the tests indicated that the vertical mounting
4.7 Interrupter—Someformofapparatusshallbeusedasan
gave the best results and was most convenient. It might be expected that
interrupter to open and close the circuit. the vertical wire would be a great deal hotter near the top than near the
bottom. This does not appear to be the case due to the fact that convection
4.8 Apparatus for Recording Time of Burnout—If no appa-
currents are greater near the top, and therefore largely compensate for
ratus is available for recording the time of burnout, arrange-
variations that otherwise would occur.
ments shall be made for hourly observations for burnouts.
6.2 In mounting a test specimen, one end of the specimen
Some form of electric-clock mechanism which can be con-
shall be inserted in the upper terminal and the weight attached
nected into the circuit may be used.
to the other end. The upper terminal shall then be adjusted to
4.9 Apparatus for Measuring Length Changes—Any form give a test length of the wire of approximately 12 in. (305 mm)
of optical apparatus such as a traveling microscope, an optical
between the two terminals. Care shall be taken to see that the
projection system, a projection microscope, or a contact weight will be able to move freely after the specimen has
microscope may be used for determining changes in length of
expanded upon heating.
the test specimen. A type of apparatus that has been found
6.3 Number of Test Specimens:
satisfactory consists of a telescope with a horizontal cross hair
6.3.1 The life value shall be the average of three simulta-
andlevelingbubble.Theapparatusmaybemountedonaframe
neous determinations on wire specimens of the material being
so that it can be readily moved from one position to another for
tested.
examining specimens in the life test. The telescope mount
should be adjustable in the vertical plane on guides by means
7. Ballast Resistance
of a threaded member. A movement of 2.5 in. (64 mm) is
7.1 The voltage between the bus bars shall be adjusted so
desirable. A piece of cross-section paper, 4 in. (102 mm) in
that it will not be necessary to make the ballast resistance in
length by 3 in. (76 mm) in width, calibrated 20 lines to 1 in.
series with the specimen greater than 20 % of the resistance of
(25.4 mm) should be mounted on the test panel so that the
the specimen.
lower edge is below a horizontal line drawn across the top of
the lowest possible position of the weight attached to the lower
8. Temperature of Test
end of the wire under test.Asatisfactory arrangement is shown
8.1 In the test method all temperatures are true tempera-
in Fig. 3.Asteel scale 18 in. (457 mm) in length, calibrated to
tures.
0.01 in. (0.25 mm), may be used for length measurements.
8.2 For alloys of nominal composition 80 % nickel and
5. Test Specimen
20 % chromium, the temperature of test shall be 2200°F
(1204°C); for alloys of nominal composition 60 % nickel,
5.1 The test specimen shall be No. 22Awg, 0.0253 in. (0.64
15 % chromium and 25 % iron, the temperature of test shall be
mm). The length of wire selected for test shall be such as to
permit the use of a 12-in. (305-mm) test length between the
two terminals.
The apparent temperature is lower than the true temperature by approximately
20°F (11.1°C) and 30°F (16.7°C) for the 80-20 type alloy and the 60-15-25 type
5.2 The test specimen shall be representative, as regards
alloy, respectively: reference, Roeser, W. F., “Spectral Emissivity (At 0.65 µ) of
surface, of the average of the coil or spool of wire which has
Some Alloys for Electrical Heating Elements,” Proceedings, ASTEA, American
been selected for test. Particular care shall be taken to see that Society for Testing and Materials, Vol 39, 1939, p. 780.
B76 − 90 (2007)
FIG. 3 Apparatus for Measuring Length Changes During Life Test
2200°F (1204°C); for alloys of nominal composition 35 % of this 15-min period is the start of the test. It is important to
nickel, 20 % chromium, and 45 % iron, the temperature of test maintain this time schedule.
shall be 2050°F (1121°C). 9.1.11 Measure the voltage and the current and record the
values together with the starting temperature and time of
9. Procedure
starting the test.
9.1.12 Start the interrupter, the timing device of which shall
9.1 Carry out the procedure as described in 9.1.1 to 9.1.13,
inclusive. have been previously regulated so that the “on” period and the
“off” period shall be equal and shall each have a duration of 2
9.1.1 Support the temperature measuring instrument so that
it can be quickly adjusted and read. min.
9.1.2 Set the series variable transformer at minimum volt-
NOTE 8—Various cycles have been tried varying from 10 min on and 5
age resistance.
min off to 30 s on and 30 s off, when it was found that the 2 min on and
9.1.3 Close the switch in series with the specimen. 2 min off cycle gave the shortest life for a given temperature. It appears
that sufficient cooling time has to be allowed to permit the specimen to
9.1.4 Adjustthevariabletransformeruntilthespecimenisat
reach a low enough temperature to cause any loosening or cracking of
a low red heat.
scale which will occur due to variations in coefficient of expansion of the
9.1.5 Grasp the weight to apply a slight tension, sufficient to
scale and the metal. The heating and cooling operation is more injurious
straighten the wire.
to wire than maintaining it at a definite temperature.
9.1.6 If change of length measurements are to be made as
9.1.13 Adjust the temperature to the test temperature after 5
specified in 9.1.15, open the switch in series with the specimen
h and 24 h total elapsed time. Record the voltage and current
and make the initial length measurement of the unheated
after each resetting. Stop the interrupter before each resetting
specimen. Then again close the switch to reheat the specimen.
and start it again after making the observation.
9.1.7 Adjust the temperature of the specimen to 200°F
9.1.14 After the first 24-h period, allow the test to run
lower than the test temperature.
withoutreadjustmentforthenext24h.Attheendofthisperiod
NOTE7—Theinterruptershallnotbeinoperationwhilethetemperature
and every 24 h thereafter until burnout, readjust the tempera-
is being adjusted.
ture so that it will be the same as the test temperature. After
each adjustment of the temperature, observe and record the
9.1.8 Allow the specimen to operate under this condition
until 2 h have elapsed in order to bring the emissivity nearer to current and voltage measurements.
a black body condition. 9.1.15 Length Changes of Specimen—Changes in length of
9.1.9 Adjust the temperature of the specimen to the test thetestspecimenmaybedeterminedwithanaccuracyof0.5 %
temperature. Maintain this balance until 1 min has elapsed. by the following procedure: First measure with a steel scale the
9.1.10 After another 13 min readjust the temperature of the length between the point at which the specimen wire leaves the
specimen to the test temperature. Final adjustment shall be bind
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