ASTM B76-90(2013)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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 (Reapproved 2013)
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 standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only 3.3 Lower Terminal—A10-g weight shall be attached to the
lowerendofthespecimen.Aflexiblesilverfoil(approximately
and are not considered standard.
0.375 in. (9.52 mm) wide and 0.0015 in. (0.038 mm) thick)
1.3 This standard does not purport to address all of the
connected to the 10-g weight shall constitute the lower
safety concerns, if any, associated with its use. It is the
terminal.
responsibility of the user of this standard to become familiar
with all hazards including those identified in the appropriate
NOTE2—Experimentshaveshownthatwithhightemperaturesalloysof
Material Safety Data Sheet (MSDS) for this product/material nickel-chromium and nickel-chromium-iron are subject to plastic flow
when under relatively light load. The weight specified in 3.3 does not
as provided by the manufacturer, to establish appropriate
cause appreciable increase in length during the test.
safety and health practices, and determine the applicability of
regulatory limitations prior to use.
4. Apparatus
4.1 The test apparatus shall be similar to the requirements
2. Significance and Use
specified in 4.2 to 4.8, inclusive, and shall be connected as
2.1 This test method is used by producers of electrical
shown in Fig. 2.
heating alloys to measure the cyclic oxidation resistance of
4.2 Power Supply—The transformer or motor generator set
these alloys.
shall be capable of delivering a controlled voltage of from 10
2.2 Because of the effect of the environment, design, and
to 20 V to the circuit. It shall have a continuous current
use, the life values obtained from this test method may not
capacity of at least 20 A/specimen.
correlate with that of an appliance or industrial heating unit.
4.3 Voltage Control—The automatic voltage control shall be
3. Test Panel
capable of maintaining across the bus bars a constant voltage
within 60.5 %.
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
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, 2013. Published May 2013. Originally
have a continuous current rating of approximately 25 A.
approved in 1929. Last previous edition approved in 2007 as B76 – 90 (2007). DOI:
10.1520/B0076-90R13.
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 (2013)
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
resistance. A compensating resistance shall be cut into the
circuit to replace the resistance of the ammeter so that the
overall resistance of the circuit is not changed. This resistance
shall be inserted in series with the blade of the upper switch
shown in Fig. 2.
4.6 Optical Pyrometer or Infrared Thermometer—The opti-
cal system shall be such as to provide a magnification of at
least four diameters. This may be accomplished by the use of
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.
NOTE4—Itishighlyimportantthatthetemperatureofthetestspecimen
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
FIG. 2 Electrical Circuit Diagram for Accelerated Life Test
any particular point on the wire and with the arrangement described the
temperature of a comparatively small wire may be taken quite readily.
B76 − 90 (2013)
FIG. 3 Apparatus for Measuring Length Changes During Life Test
4.7 Interrupter—Someformofapparatusshallbeusedasan 5.2 The test specimen shall be representative, as regards
interrupter to open and close the circuit. surface, of the average of the coil or spool of wire which has
been selected for test. Particular care shall be taken to see that
4.8 Apparatus for Recording Time of Burnout—If no appa-
the specimen selected is free from kinks. This is necessary, as
ratus is available for recording the time of burnout, arrange-
a kink, even though later removed, may cause burnout at that
ments shall be made for hourly observations for burnouts.
point.
Some form of electric-clock mechanism which can be con-
nected into the circuit may be used.
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
4.9 Apparatus for Measuring Length Changes—Any form
section from one end to the other. Tests may then be made at any time on
of optical apparatus such as a traveling microscope, an optical
the reference standard, and if conditions have changed they will be noted
projection system, a projection microscope, or a contact
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
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
andlevelingbubble.Theapparatusmaybemountedonaframe
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
examining specimens in the life test. The telescope mount a vertical position, as shown in Fig. 1, and shall have the
following typical spacing:
should be adjustable in the vertical plane on guides by means
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.Asatisfactory 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.Asteel 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
currents are greater near the top, and therefore largely compensate for
5.1 The test specimen shall be No. 22Awg, 0.0253 in. (0.64
variations that otherwise would occur.
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 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
B76 − 90 (2013)
to the other end. The upper terminal shall then be adjusted to 9.1.10 After another 13 min readjust the temperature of the
give a test length of the wire of approximately 12 in. (305 mm) specimen to the test temperature. Final adjustment shall be
between the two terminals. Care shall be taken to see that the completed within the next minute or a total of 15 min. The end
weight will be able to move freely after the specimen has of this 15-min period is the start of the test. It is important to
expanded upon heating. maintain this time schedule.
9.1.11 Measure the voltage and the current and record the
6.3 Number of Test Specimens:
values together with the starting temperature and time of
6.3.1 The life value shall be the average of three simulta-
starting the test.
neous determinations on wire specimens of the material being
9.1.12 Start the interrupter, the timing device of which shall
tested.
have been previously regulated so that the “on” period and the
“off” period shall be equal and shall each have a duration of 2
7. Ballast Resistance
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
min off to 30 s on and 30 s off, when it was found that the 2 min on and
series with the specimen greater than 20 % of the resistance of
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
to wire than maintaining it at a definite temperature.
tures.
9.1.13 Adjust the temperature to the test temperature after 5
8.2 For alloys of nominal composition 80 % nickel and
h and 24 h total elapsed time. Record the voltage and current
20 % chromium, the temperature of test shall be 2200°F
after each resetting. Stop the interrupter before each resetting
(1204°C); for alloys of nominal composition 60 % nickel,
and start it again after making the observation.
15 % chromium and 25 % iron, the temperature of test shall be
9.1.14 After the first 24-h period, allow the test to run
2200°F (1204°C); for alloys of nominal composition 35 %
withoutreadjustmentforthenext24h.Attheendofthisperiod
nickel, 20 % chromium, and 45 % iron, the temperature of test
and every 24 h thereafter until burnout, readjust the tempera-
shall be 2050°F (1121°C).
ture so that it will be the same as the test temperature. After
9. Procedure each adjustment of the temperature, observe and record the
current and voltage measurements.
9.1 Carry out the procedure as described in 9.1.1 to 9.1.13,
9.1.15 Length Changes of Specimen—Changes in length of
inclusive.
thetestspecimenmaybedeterminedwithanaccuracyof0.5 %
9.1.1 Support the temperature measuring instrument so that
by the following procedure: First measure with a steel scale the
it can be quickly adjusted and read.
length between the point at which the specimen wire leaves the
9.1.2 Set the series variable transformer at minimum volt-
bindingpostandthetopoftheweightattachedtothelowerend
age resistance.
of the specimen wire. This measurement should be accurate to
9.1.3 Close the switch in series with the specimen.
60.02 in. (0.51 mm) (see 9.1.6). If the apparatus described in
9.1.4 Adjustthevariabletransformeruntilthespecimenisat
4.9 is used, adjust the telescope in the vertical plane until the
a low red heat.
cross hair is directly lined up with the top of the weight
9.1.5 Grasp the weight to apply a slight tension, sufficient to
attached to the wire (see 9.1.6). Estimate the readings on the
straighten the wire.
cross section paper to the nearest 0.01 in. (0.25 mm). Other
9.1.6 If change of length measurements are to be made as
readings through the telescope may be taken in the same
specified in 9.1.15, open the switch in series with the specimen
manner and noted. Calculate the changes in length of the
and make the initial length measurement of the unheated
specimen as the difference between the first reading made on
specimen. Then again close the switch to reheat the specimen.
the cross-section paper and the subsequent readings.
9.1.7 Adjust th
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