1、Designation: B 76 90 (Reapproved 2007)Standard Test Method forAccelerated Life of Nickel-Chromium and Nickel-Chromium-Iron Alloys for Electrical Heating1This standard is issued under the fixed designation B 76; the number immediately following the designation indicates the year of originaladoption o
2、r, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method2covers the determination of theresistance to oxidation of ni
3、ckel-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 regarded
4、as the standard. The metric equivalents of inch-pound unitsmay be approximate.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 this standard to become familiarwith all hazards including those identified i
5、n 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 to use.2. Significance and Use2.1 This test method is used by producers of
6、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 appliance or industrial heating unit.3. Test Panel3.1 Size and LocationThe
7、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 to prevent leakage of air at this point during the operationof the test,
8、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 stations.3.2 Upper TerminalThe upper terminal shall consist of abinding pos
9、t 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 of the specimen.Aflexible silver foil (approximately0.375 in. (9.52 mm) wid
10、e 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 alloysof nickel-chromium and nickel-chromium-iron are subject to plastic flowwhen under relatively light load. The weight specified in 3.3 does n
11、otcause 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 transformer or motor generator setshall be capable of delivering a controlled
12、 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 voltagewithin 6 0.5 %.NOTE 3It has been found impossible to make accurate tes
13、ts 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 ca-pable of adjusting the voltage across the specimen so thatcurrent is controlled to approximately 0.25 % of d
14、esired value,and shall have a continuous current rating of approximately 25A.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
15、abovethe lower fifth of the scale range. The ammeter has appreciable1This 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
16、 approved May 1, 2007. Published May 2007. Originallyapproved in 1929. Last previous edition approved in 2001 as B 76 90 (2001).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 Heating,”Proceedings,
17、ASTEA, American Society for Testing and Materials. Vol 29, Part II,1929, p. 506.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.resistance. A compensating resistance shall be cut into thecircuit to replace the resistance of the ammet
18、er 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 op-tical system shall be such as to provide a magnification of atleast four diameters. This ma
19、y 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 havean accuracy of 610F and NIST traceability.NOTE 4It is highly important that the temperature of
20、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 and with the arrangement describ
21、ed thetemperature of a comparatively small wire may be taken quite readily.Metric Equivalentsin.mm1212.75532131.0714184.22112546.1521321641626FIG. 1 Test Panel for Accelerated Life TestFIG. 2 Electrical Circuit Diagram for Accelerated Life TestB 76 90 (2007)24.7 InterrupterSome form of apparatus sha
22、ll be used asan interrupter 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 mechanism which can be con-nected into th
23、e 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 specimen. A type of apparatus that has be
24、en 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 adjustable in the vertical plane on
25、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 below a horizontal line drawn across t
26、he 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 Specimen5.1 The test specimen shall
27、 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 spool of wire which hasbeen selected f
28、or 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 comparison a spool or coil of wire which i
29、s 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 standards and other wires may be corr
30、elated 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)Distance between specimens 6 in. (152.4
31、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 specimen in whichhorizontal mounting, cate
32、nary 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. This does not appear to be the case
33、 due to the fact that convectioncurrents are greater near the top, and therefore largely compensate forvariations that otherwise would occur.FIG. 3 Apparatus for Measuring Length Changes During Life TestB 76 90 (2007)36.2 In mounting a test specimen, one end of the specimenshall be inserted in the u
34、pper 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. (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 heati
35、ng.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 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 ins
36、eries 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-tures.38.2 For alloys of nominal composition 80 % nickel and20 % chromium, the temperature of test shall be 2200F(1204C); for alloys of nominal co
37、mposition 60 % nickel,15 % chromium and 25 % 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).9. Procedure9.1 Carry out the procedure as described in 9.1.1 to 9.1.13,inclusiv
38、e.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 the specimen.9.1.4 Adjust the variable transformer until the specimen is ata low red heat.9.1
39、.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 make the initial length measurement of the unheatedspecimen. Then again close the switch to rehe
40、at 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 specimen to operate under this conditionuntil 2 h have elapsed in order to bring the emissivity n
41、earer 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 thespecimen to the test temperature. Final adjustment shall becompleted within the next minute or
42、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 starting temperature and time ofstarting the test.9.1.12 Start the interrupter, the timing device
43、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 5min off to 30 s on and 30 s off, when it was found that the 2 min on and2 min off cycle gave
44、 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 will occur due to variations in coefficient of expansion of thescale and the metal. The heatin
45、g 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 and currentafter each resetting. Stop the interrupter before each resettingand start it again
46、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 tempera-ture so that it will be the same as the test temperature. Aftereach adjustment of the te
47、mperature, 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 leav
48、es thebinding post and the top of the weight attached to the lower endof the specimen wire. This measurement should be accurate to60.02 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 to
49、p of the weightattached to the wire (see 9.1.6). Estimate the readings on thecross section paper to the nearest 0.01 in. (0.25 mm). Otherreadings through the telescope may be taken in the samemanner and noted. Calculate the changes in length of thespecimen as the difference between the first reading made onthe cross-section paper and the subsequent readings.10. Record10.1 Measurements and observations shall be recorded on adata sheet similar to that shown in Fig. 4 (see Annex A1 forreference to useful life).11. Report11.1 Report the following informati
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