1、Designation: B76 90 (Reapproved 2013)Standard Test Method forAccelerated Life of Nickel-Chromium and Nickel-Chromium-Iron Alloys for Electrical Heating1This standard is issued under the fixed designation B76; the number immediately following the designation indicates the year of originaladoption or,
2、 in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method2covers the determination of theresistance to oxidation of nicke
3、l-chromium and nickel-chromium-iron electrical heating alloys at elevated tempera-tures under intermittent heating. Procedures for a constant-temperature cycle are provided. This test method is used forinternal comparative purposes only.1.2 The values stated in inch-pound units are to be regardedas
4、standard. The values given in parentheses are mathematicalconversions to SI units that are provided for information onlyand are not considered standard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of thi
5、s standard to become familiarwith all hazards including those identified in the appropriateMaterial Safety Data Sheet (MSDS) for this product/materialas provided by the manufacturer, to establish appropriatesafety and health practices, and determine the applicability ofregulatory limitations prior t
6、o use.2. Significance and Use2.1 This test method is used by producers of electricalheating alloys to measure the cyclic oxidation resistance ofthese alloys.2.2 Because of the effect of the environment, design, anduse, the life values obtained from this test method may notcorrelate with that of an a
7、ppliance or industrial heating unit.3. Test Panel3.1 Size and LocationThe dimensions of the test panelshall be similar to those shown in Fig. 1. The test panel shall belocated in a position free from drafts of air.NOTE 1The enclosure shall fit tightly on the panel and the glass slideshall fit snugly
8、 to prevent leakage of air at this point during the operationof the test, as even a slight draft of air in contact with the specimen willcause excessive variation in length of life. A screen of 40 wire mesh,0.010-in. (0.025-mm) wire diameter, market grade, may be used as a coverover the individual s
9、tations.3.2 Upper TerminalThe upper terminal shall consist of abinding post attached to a rod passing through another bindingpost or through the upper bus bar. This provides for adjustmentlaterally and vertically, as shown in Fig. 1.3.3 Lower TerminalA 10-g weight shall be attached to thelower end o
10、f the specimen.Aflexible silver foil (approximately0.375 in. (9.52 mm) wide and 0.0015 in. (0.038 mm) thick)connected to the 10-g weight shall constitute the lowerterminal.NOTE 2Experiments have shown that with high temperatures alloys ofnickel-chromium and nickel-chromium-iron are subject to plasti
11、c flowwhen under relatively light load. The weight specified in 3.3 does notcause appreciable increase in length during the test.4. Apparatus4.1 The test apparatus shall be similar to the requirementsspecified in 4.2 to 4.8, inclusive, and shall be connected asshown in Fig. 2.4.2 Power SupplyThe tra
12、nsformer or motor generator setshall be capable of delivering a controlled voltage of from 10to 20 V to the circuit. It shall have a continuous currentcapacity of at least 20 A/specimen.4.3 Voltage ControlThe automatic voltage control shall becapable of maintaining across the bus bars a constant vol
13、tagewithin 60.5 %.NOTE 3It has been found impossible to make accurate tests withoutvoltage control, as changes in line voltage were sufficient to causeconsiderable variation in the results obtained (see Annex A1).4.4 Variable TransformerThe transformer shall be capableof adjusting the voltage across
14、 the specimen so that current iscontrolled to approximately 0.25 % of desired value, and shallhave a continuous current rating of approximately 25 A.4.5 Ammeter and VoltmeterThe ammeter and voltmetershall have an accuracy of 1 % of normal test deflection(approximately 15 A and 15 V, respectively). F
15、or alternatingcurrent the range used shall be such as to give a reading above1This test method is under the jurisdiction of ASTM Committee B02 onNonferrous Metals and Alloys and is the direct responsibility of SubcommitteeB02.10 on Thermostat Metals and Electrical Resistance Heating Materials.Curren
16、t edition approved May 1, 2013. Published May 2013. Originallyapproved in 1929. Last previous edition approved in 2007 as B76 90 (2007). DOI:10.1520/B0076-90R13.2Further information on this test method is given in a paper by F. E. Bash andJ. W. Harsch, “Life Tests on Metallic Resistor Materials for
17、Electrical Heating,”Proceedings, ASTEA, American Society for Testing and Materials. Vol 29, Part II,1929, p. 506.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1the lower fifth of the scale range. The ammeter has appreciableresistance
18、. A compensating resistance shall be cut into thecircuit to replace the resistance of the ammeter so that theoverall resistance of the circuit is not changed. This resistanceshall be inserted in series with the blade of the upper switchshown in Fig. 2.4.6 Optical Pyrometer or Infrared ThermometerThe
19、 opti-cal system shall be such as to provide a magnification of atleast four diameters. This may be accomplished by the use ofa special lens or combination of two standard lenses in theobjective to provide a short focal length and the desiredmagnification. (See Annex A1.) These instruments must have
20、an accuracy of 610F and NIST traceability.NOTE 4It is highly important that the temperature of the test specimenbe adjusted as accurately as possible, as small variations in temperatureresult in considerable variation in length of life. An optical pyrometer orinfrared thermometer makes it possible t
21、o determine the temperature atany particular point on the wire and with the arrangement described thetemperature of a comparatively small wire may be taken quite readily.Metric Equivalentsin.mm1212.75532131.0714184.22112546.1521321641626FIG. 1 Test Panel for Accelerated Life TestB76 90 (2013)24.7 In
22、terrupterSome form of apparatus shall be used as aninterrupter to open and close the circuit.4.8 Apparatus for Recording Time of BurnoutIf 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 mechan
23、ism which can be con-nected into the circuit may be used.4.9 Apparatus for Measuring Length ChangesAny formof optical apparatus such as a traveling microscope, an opticalprojection system, a projection microscope, or a contactmicroscope may be used for determining changes in length ofthe test specim
24、en. A type of apparatus that has been foundsatisfactory consists of a telescope with a horizontal cross hairand leveling bubble. The apparatus may be mounted on a frameso that it can be readily moved from one position to another forexamining specimens in the life test. The telescope mountshould be a
25、djustable in the vertical plane on guides by meansof a threaded member. A movement of 2.5 in. (64 mm) isdesirable. A piece of cross-section paper, 4 in. (102 mm) inlength by 3 in. (76 mm) in width, calibrated 20 lines to 1 in.(25.4 mm) should be mounted on the test panel so that thelower edge is bel
26、ow a horizontal line drawn across the top ofthe lowest possible position of the weight attached to the lowerend of the wire under test. A satisfactory arrangement is shownin Fig. 3. A steel scale 18 in. (457 mm) in length, calibrated to0.01 in. (0.25 mm), may be used for length measurements.5. Test
27、Specimen5.1 The test specimen shall be No. 22 Awg, 0.0253 in. (0.64mm). The length of wire selected for test shall be such as topermit the use of a 12-in. (305-mm) test length between thetwo terminals.5.2 The test specimen shall be representative, as regardssurface, of the average of the coil or spo
28、ol of wire which hasbeen selected for test. Particular care shall be taken to see thatthe specimen selected is free from kinks. This is necessary, asa kink, even though later removed, may cause burnout at thatpoint.NOTE 5It is also very desirable to select and keep as a referencestandard for compari
29、son a spool or coil of wire which is uniform in crosssection from one end to the other. Tests may then be made at any time onthe reference standard, and if conditions have changed they will be notedby the length of life on the standard. Comparisons between tests made atdifferent times between the st
30、andards and other wires may be correlated inthis manner (see Annex A1).6. Mounting of Specimens6.1 The test specimens shall be mounted on the test panel ina vertical position, as shown in Fig. 1, and shall have thefollowing typical spacing:Distance between test panel andspecimens2 in. (50.8 mm)Dista
31、nce between specimens 6 in. (152.4 mm)Distance between specimen and shield 2 in. (50.8 mm)Distance between upper bus bar and silver foilcontact . approximately20 in. (508 mm)NOTE 6This recommendation is based on a series of tests run in fourlaboratories to determine the best position for the specime
32、n in whichhorizontal mounting, catenary mounting, and vertical mounting werecompared. The results of the tests indicated that the vertical mountinggave the best results and was most convenient. It might be expected thatthe vertical wire would be a great deal hotter near the top than near thebottom.
33、This does not appear to be the case due to the fact that convectioncurrents are greater near the top, and therefore largely compensate forvariations that otherwise would occur.6.2 In mounting a test specimen, one end of the specimenshall be inserted in the upper terminal and the weight attachedto th
34、e other end. The upper terminal shall then be adjusted togive a test length of the wire of approximately 12 in. (305 mm)between the two terminals. Care shall be taken to see that theweight will be able to move freely after the specimen hasexpanded upon heating.6.3 Number of Test Specimens:6.3.1 The
35、life value shall be the average of three simulta-neous determinations on wire specimens of the material beingtested.7. Ballast Resistance7.1 The voltage between the bus bars shall be adjusted sothat it will not be necessary to make the ballast resistance inseries with the specimen greater than 20 %
36、of the resistance ofthe specimen.8. Temperature of Test8.1 In the test method all temperatures are true tempera-tures.38.2 For alloys of nominal composition 80 % nickel and20 % chromium, the temperature of test shall be 2200F(1204C); for alloys of nominal composition 60 % nickel,15 % chromium and 25
37、 % iron, the temperature of test shall be2200F (1204C); for alloys of nominal composition 35 %nickel, 20 % chromium, and 45 % iron, the temperature of testshall be 2050F (1121C).3The apparent temperature is lower than the true temperature by approximately20F (11.1C) and 30F (16.7C) for the 80-20 typ
38、e alloy and the 60-15-25 typealloy, respectively: reference, Roeser, W. F., “Spectral Emissivity (At 0.65 ) ofSome Alloys for Electrical Heating Elements,” Proceedings, ASTEA, AmericanSociety for Testing and Materials, Vol 39, 1939, p. 780.FIG. 2 Electrical Circuit Diagram for Accelerated Life TestB
39、76 90 (2013)39. Procedure9.1 Carry out the procedure as described in 9.1.1 to 9.1.13,inclusive.9.1.1 Support the temperature measuring instrument so thatit can be quickly adjusted and read.9.1.2 Set the series variable transformer at minimum volt-age resistance.9.1.3 Close the switch in series with
40、the specimen.9.1.4 Adjust the variable transformer until the specimen is ata low red heat.9.1.5 Grasp the weight to apply a slight tension, sufficient tostraighten the wire.9.1.6 If change of length measurements are to be made asspecified in 9.1.15, open the switch in series with the specimenand mak
41、e the initial length measurement of the unheatedspecimen. Then again close the switch to reheat the specimen.9.1.7 Adjust the temperature of the specimen to 200Flower than the test temperature.NOTE 7The interrupter shall not be in operation while the temperatureis being adjusted.9.1.8 Allow the spec
42、imen to operate under this conditionuntil 2 h have elapsed in order to bring the emissivity nearer toa black body condition.9.1.9 Adjust the temperature of the specimen to the testtemperature. Maintain this balance until 1 min has elapsed.9.1.10 After another 13 min readjust the temperature of thesp
43、ecimen to the test temperature. Final adjustment shall becompleted within the next minute or a total of 15 min. The endof this 15-min period is the start of the test. It is important tomaintain this time schedule.9.1.11 Measure the voltage and the current and record thevalues together with the start
44、ing temperature and time ofstarting the test.9.1.12 Start the interrupter, the timing device of which shallhave been previously regulated so that the “on” period and the“off” period shall be equal and shall each have a duration of 2min.NOTE 8Various cycles have been tried varying from 10 min on and
45、5min off to 30 s on and 30 s off, when it was found that the 2 min on and2 min off cycle gave the shortest life for a given temperature. It appearsthat sufficient cooling time has to be allowed to permit the specimen toreach a low enough temperature to cause any loosening or cracking ofscale which w
46、ill occur due to variations in coefficient of expansion of thescale and the metal. The heating and cooling operation is more injuriousto wire than maintaining it at a definite temperature.9.1.13 Adjust the temperature to the test temperature after 5h and 24 h total elapsed time. Record the voltage a
47、nd currentafter each resetting. Stop the interrupter before each resettingand start it again after making the observation.9.1.14 After the first 24-h period, allow the test to runwithout readjustment for the next 24 h.At the end of this periodand every 24 h thereafter until burnout, readjust the tem
48、pera-ture so that it will be the same as the test temperature. Aftereach adjustment of the temperature, observe and record thecurrent and voltage measurements.9.1.15 Length Changes of SpecimenChanges in length ofthe test specimen may be determined with an accuracy of 0.5 %by the following procedure:
49、 First measure with a steel scale thelength between the point at which the specimen wire leaves thebinding post and the top of the weight attached to the lower endof the specimen wire. This measurement should be accurate toFIG. 3 Apparatus for Measuring Length Changes During Life TestB76 90 (2013)460.02 in. (0.51 mm) (see 9.1.6). If the apparatus described in4.9 is used, adjust the telescope in the vertical plane until thecross hair is directly lined up with the top of the weightattached to the wire (see 9.1.6). Estimate the readings on thecros
