ASTM B76-90(1995)e1

Standard Test Method for Accelerated Life of Nickel-Chromium and Nickel-Chromium-Iron Alloys for Electrical Heating

SCOPE
1.1 This test method covers the determination of the resistance to oxidation of nickel-chromium and nickel-chromium-iron electrical heating alloys at elevated temperatures under intermittent heating. Procedures for a constant-temperature cycle are provided. This test method is used for internal comparative purposes only.  
1.2 The values stated in inch-pound units are to be regarded as the standard. The metric equivalents of inch-pound units may be approximate.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Historical
• Publication Date: 31-Dec-2000
• Technical Committee: B02 - Nonferrous Metals and Alloys
• Drafting Committee: B02.10 - Thermostat Metals and Electrical Resistance Heating Materials

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or
withdrawn.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 76 – 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 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.
e NOTE—Section 14 was added editorially in June 1995.
over the individual stations.
1. Scope
1.1 This test method covers the determination of the 3.2 Upper Terminal—The upper terminal shall consist of a
binding post attached to a rod passing through another binding
resistance to oxidation of nickel-chromium and nickel-
chromium-iron electrical heating alloys at elevated tempera- post or through the upper bus bar. This provides for adjustment
laterally and vertically, as shown in Fig. 1.
tures under intermittent heating. Procedures for a constant-
temperature cycle are provided. This test method is used for 3.3 Lower Terminal—A 10-g weight shall be attached to the
internal comparative purposes only. lower end of the specimen. A flexible silver foil (approximately
1.2 The values stated in inch-pound units are to be regarded 0.375 in. (9.52 mm) wide and 0.0015 in. (0.038 mm) thick)
as the standard. The metric equivalents of inch-pound units connected to the 10-g weight shall constitute the lower
may be approximate. terminal.
1.3 This standard does not purport to address all of the
NOTE 2—Experiments have shown that with high temperatures alloys
safety concerns, if any, associated with its use. It is the
of nickel-chromium and nickel-chromium-iron are subject to plastic flow
responsibility of the user of this standard to establish appro-
when under relatively light load. The weight specified in 3.3 does not
priate safety and health practices and determine the applica- cause appreciable increase in length during the test.
bility of 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
within6 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).
NOTE 1—The enclosure shall fit tightly on the panel and the glass slide
shall fit snugly to prevent leakage of air at this point during the operation
4.4 Variable Transformer—The transformer shall be ca-
of the test, as even a slight draft of air in contact with the specimen will
pable of adjusting the voltage across the specimen so that
cause excessive variation in length of life. A screen of 40 wire mesh,
current is controlled to approximately 0.25 % of desired value,
0.010-in. (0.025-mm) wire diameter, market grade, may be used as a cover
and shall have a continuous current rating of approximately 25
A.
This test method is under the jurisdiction of ASTM Committee B-4 on Metallic 4.5 Ammeter and Voltmeter—The ammeter and voltmeter
Materials for Thermostats and for Electrical Resistance, Heating, and Contacts and
shall have an accuracy of 1 % of normal test deflection
is the direct responsibility of Subcommittee B04.01 on Electrical Heating and
(approximately 15 A and 15 V, respectively). For alternating
Resistance.
current the range used shall be such as to give a reading above
Current edition approved May 25, 1990. Published July 1990. Originally
published as B 76 – 29 T. Last previous edition B 76 – 81.
the lower fifth of the scale range. The ammeter has appreciable
Further information on this test method is given in a paper by F. E. Bash and
resistance. A compensating resistance shall be cut into the
J. W. Harsch, “Life Tests on Metallic Resistor Materials for Electrical Heating,’’
circuit to replace the resistance of the ammeter so that the
Proceedings, ASTEA, American Society for Testing and Materials. Vol 29, Part II,
1929, p. 506. overall resistance of the circuit is not changed. This resistance
B76
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
shall be inserted in series with the blade of the upper switch
shown in Fig. 2.
4.6 Optical Pyrometer or Infrared Thermometer—The op-
tical 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.
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.
FIG. 2 Electrical Circuit Diagram for Accelerated Life Test
4.8 Apparatus for Recording Time of Burnout—If no appa-
ratus is available for recording the time of burnout, arrange-
B76
NOTE 5—It is also very desirable to select and keep as a reference
ments shall be made for hourly observations for burnouts.
standard for comparison a spool or coil of wire which is uniform in cross
Some form of electric-clock mechanism which can be con-
section from one end to the other. Tests may then be made at any time on
nected into the circuit may be used.
the reference standard, and if conditions have changed they will be noted
4.9 Apparatus for Measuring Length Changes—Any form
by the length of life on the standard. Comparisons between tests made at
of optical apparatus such as a traveling microscope, an optical
different times between the standards and other wires may be correlated in
projection system, a projection microscope, or a contact
this manner (see Annex A1).
microscope may be used for determining changes in length of
the test specimen. A type of apparatus that has been found
6. Mounting of Specimens
satisfactory consists of a telescope with a horizontal cross hair
6.1 The test specimens shall be mounted on the test panel in
and leveling bubble. The apparatus may be mounted on a frame
a vertical position, as shown in Fig. 1, and shall have the
so that it can be readily moved from one position to another for
following typical spacing:
examining specimens in the life test. The telescope mount
Distance between test panel and 2 in. (50.8 mm)
should be adjustable in the vertical plane on guides by means
specimens
of a threaded member. A movement of 2.5 in. (64 mm) is
Distance between specimens 6 in. (152.4 mm)
Distance between specimen and shield 2 in. (50.8 mm)
desirable. A piece of cross-section paper, 4 in. (102 mm) in
Distance between upper bus bar and silver foil 20 in. (508 mm)
length by 3 in. (76 mm) in width, calibrated 20 lines to 1 in.
contact . approximately
(25.4 mm) should be mounted on the test panel so that the
NOTE 6—This recommendation is based on a series of tests run in four
lower edge is below a horizontal line drawn across the top of
laboratories to determine the best position for the specimen in which
the lowest possible position of the weight attached to the lower
horizontal mounting, catenary mounting, and vertical mounting were
end of the wire under test. A satisfactory arrangement is shown
compared. The results of the tests indicated that the vertical mounting
in Fig. 3. A steel scale 18 in. (457 mm) in length, calibrated to
gave the best results and was most convenient. It might be expected that
0.01 in. (0.25 mm), may be used for length measurements.
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
5. Test Specimen
currents are greater near the top, and therefore largely compensate for
5.1 The test specimen shall be No. 22 Awg, 0.0253 in. (0.64
variations that otherwise would occur.
mm). The length of wire selected for test shall be such as to
6.2 In mounting a test specimen, one end of the specimen
permit the use of a 12-in. (305-mm) test length between the
shall be inserted in the upper terminal and the weight attached
two terminals.
to the other end. The upper terminal shall then be adjusted to
5.2 The test specimen shall be representative, as regards
give a test length of the wire of approximately 12 in. (305 mm)
surface, of the average of the coil or spool of wire which has
between the two terminals. Care shall be taken to see that the
been selected for test. Particular care shall be taken to see that
weight will be able to move freely after the specimen has
the specimen selected is free from kinks. This is necessary, as
expanded upon heating.
a kink, even though later removed, may cause burnout at that
point. 6.3 Number of Test Specimens:
FIG. 3 Apparatus for Measuring Length Changes During Life Test
B76
6.3.1 The life value shall be the average of three simulta- values together with the starting temperature and time of
neous determinations on wire specimens of the material being starting the test.
tested. 9.1.12 Start the interrupter, the timing device of which shall
have been previously regulated so that the “on’’ period and the
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
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
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
is being adjusted.
10.1 Measurements and observations shall be recorded on a
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).
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