1、Designation: B76 90 (Reapproved 2018)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 appropriateSafety Data Sheet (SDS) for this product/material as providedby the manufacturer, to establish appropriate safety, health,and environmental practices, and determine the applicabilityof regulatory limitations p
6、rior to use.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization Technica
7、lBarriers to Trade (TBT) Committee.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 n
8、otcorrelate with that of an appliance 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 th
9、e glass slideshall fit snugly 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
10、 a coverover the individual stations.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
11、be attached to thelower end of 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-chromi
12、um-iron are subject to plastic 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 F
13、ig. 2.4.2 Power SupplyThe transformer 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 acros
14、s the bus bars a constant voltagewithin 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
15、 adjusting the voltage across the specimen so that current is1This 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.Current edition approv
16、ed Nov. 1, 2018. Published November 2018. Originallyapproved in 1929. Last previous edition approved in 2013 as B76 90 (2013). DOI:10.1520/B0076-90R18.2Further information on this test method is given in a paper by F. E. Bash andJ. W. Harsch, “Life Tests on Metallic Resistor Materials for Electrical
17、 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 StatesThis international standard was developed in accordance with internationally recogni
18、zed principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1controlled to approximately 0.25 % of desired value, and shallhave
19、 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). For alternatingcurrent the range used shall be such as to give a reading abovethe lower fifth of th
20、e scale range. The ammeter has appreciableresistance. 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
21、. 2.4.6 Optical Pyrometer or Infrared ThermometerThe 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 desiredmagnif
22、ication. (See Annex A1.) These instruments must havean accuracy of 610F and NIST traceability.Metric Equivalentsin.mm1212.75532131.0714184.22112546.1521321641626FIG. 1 Test Panel for Accelerated Life TestFIG. 2 Electrical Circuit Diagram for Accelerated Life TestB76 90 (2018)2NOTE 4It is highly impo
23、rtant 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 to determine the temperature atany particular point on the wire an
24、d with the arrangement described thetemperature of a comparatively small wire may be taken quite readily.4.7 InterrupterSome 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
25、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.4.9 Apparatus for Measuring Length ChangesAny formof optical apparatus such as a traveling microscope, an opticalprojection system,
26、a projection microscope, or a contactmicroscope may be used for determining changes in length ofthe test specimen. 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 rea
27、dily moved from one position to another forexamining specimens in the life test. The telescope mountshould be adjustable 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) i
28、n width, calibrated 20 lines to 1 in.(25.4 mm) should be mounted on the test panel so that thelower edge is below 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
29、 scale 18 in. (457 mm) in length, calibrated to0.01 in. (0.25 mm), may be used for length measurements.5. Test 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 thetw
30、o terminals.5.2 The test specimen shall be representative, as regardssurface, of the average of the coil or spool 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 c
31、ause burnout at thatpoint.NOTE 5It is also very desirable to select and keep as a referencestandard for comparison 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
32、 be notedby the length of life on the standard. Comparisons between tests made atdifferent times between the standards 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 F
33、ig. 1, and shall have thefollowing typical spacing:Distance between test panel andspecimens2 in. (50.8 mm)Distance 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 reco
34、mmendation is based on a series of tests run in fourlaboratories to determine the best position for the specimen in whichhorizontal mounting, catenary mounting, and vertical mounting werecompared. The results of the tests indicated that the vertical mountingFIG. 3 Apparatus for Measuring Length Chan
35、ges During Life TestB76 90 (2018)3gave 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. This does not appear to be the case due to the fact that convectioncurrents are greater near the top, and therefo
36、re 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 the other end. The upper terminal shall then be adjusted togive a test length of the wire of approximately 12 in. (
37、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 life value shall be the average of three simulta-neous determinations on wire specimens of the material beingtest
38、ed.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 % of the resistance ofthe specimen.8. Temperature of Test8.1 In the test method all temperatures are true tempera-t
39、ures.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 % iron, the temperature of test shall be2200F (1204C); for alloys of nominal composition 35 %nickel, 20 % chromi
40、um, and 45 % iron, the temperature of testshall be 2050F (1121C).9. 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-a
41、ge resistance.9.1.3 Close the switch in series with 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
42、, open the switch in series with the specimenand make 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 t
43、he temperatureis being adjusted.9.1.8 Allow the specimen 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 Af
44、ter another 13 min readjust the temperature of thespecimen 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
45、current and record thevalues together with the starting 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 8Variou
46、s cycles have been tried varying from 10 min on and 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 temperatur
47、e to cause any loosening or cracking ofscale which will 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
48、5h and 24 h total elapsed time. Record the voltage and 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 e
49、very 24 h thereafter until burnout, readjust the tempera-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: 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 sho
