1、Designation: B 78 90 (Reapproved 2007)Standard Test Method ofAccelerated Life of Iron-Chromium-Aluminum Alloys forElectrical Heating1This standard is issued under the fixed designation B 78; the number immediately following the designation indicates the year of originaladoption or, in the case of re
2、vision, 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 method covers the determination of the resis-tance to oxidation of iron-chromium-alumin
3、um electrical heat-ing alloys at elevated temperatures under intermittent heatingusing a constant-temperature cycle test. This test is used forinternal comparative purposes only.1.2 The values stated in inch-pound units are to be regardedas the standard. The metric equivalents of inch-pound unitsmay
4、 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 in the appropriateMaterial Safety Data Sheet (MSDS) for this pr
5、oduct/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 electricalheating alloys to measure the cyclic oxidation resis
6、tance ofthese alloys.2.2 Because of the effect of environment, design and use,the life values obtained from this test method may not correlatewith 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.
7、 1. The test panel shall belocated in a position free from drafts of air.3.1.1 The enclosure shall fit tightly on the panel and theglass slide shall fit snugly to prevent leakage of air at this pointduring the operation of the test, as even a slight draft of air incontact with the specimen will caus
8、e excessive variation inlength of life. A screen of 40 wire mesh, 0.010-in. (0.025 mm)wire diameter, market grade, may be used as a cover over theindividual stations.3.2 TerminalsThe two terminals shall be spaced 2 in.(50.8 mm) apart, center to center and shall be so positioned thatthe wire specimen
9、 when secured therein shall be in a U-shapedpattern as described in Section 5. The specimen terminaljunctions shall be 3 in. (76.2 mm) lower than the plane of thetop of the enclosure.4. Apparatus4.1 The apparatus shall be similar to the requirementsspecified in 4.2 to 4.8, inclusive, and shall be co
10、nnected asshown in Fig. 2.4.2 Power SupplyThe transformer or motor generator setshall be capable of delivering a controlled voltage of from 10to 35 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
11、of maintaining across the bus bars a constant voltagewithin 60.5 %.NOTE 1It has been found impossible to make accurate tests withoutvoltage control, as changes in line voltage are sufficient to causeconsiderable variation in the results obtained (see Annex A1).4.4 Variable TransformerThe variable tr
12、ansformer shallbe capable of adjusting the voltage across the specimen towithin approximately 0.25 % of any desired value within theworking range and shall have a continuous current rating ofapproximately 25 A.NOTE 2A variable transformer having a working range of adjustabil-ity from approximately 0
13、 to 20 V, provides for testing wires within aconsiderable range of size and resistivity.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 shall be such as to give a rea
14、ding above thelower fifth of the scale range. The ammeter has appreciableresistance. A compensating resistance shall be cut into thecircuit to replace the resistance of the ammeter so that theover-all resistance of the circuit is not changed. This resistanceshall be inserted in series with the conta
15、ct 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 may be accomplished by means of a1This test method is under the jurisdiction of ASTM Committee B02 onNonferrous Metals an
16、d Alloys and is the direct responsibility of SubcommitteeB02.10 on Thermostat Metals and Electrical Resistance Heating Materials.Current edition approved May 1, 2007. Published May 2007. Originallyapproved in 1959. Last previous edition approved in 2001 as B 78 90 (2001).1Copyright ASTM Internationa
17、l, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.special lens or combination of two standard lenses in theobjective to provide a short focal length and the desiredmagnification (seeAnnexA1). These instruments must have anaccuracy of 610F (5.5C) and UBS traceabil
18、ity.NOTE 3It is highly important that the temperatures of the testspecimen be adjusted as accurately as possible, as small variations intemperature result in considerable variation in length of life. Opticalpyrometer or infrared pycnometer makes it possible to determine thetemperature at any particu
19、lar point on the wire, and with the arrangementdescribed, the temperature of a comparatively small wire may be takenquite readily.4.7 InterrupterSome form of apparatus shall be used asan interrupter to open and close the circuit.4.8 Apparatus for Recording Time of BurnoutIf no appa-ratus is availabl
20、e for recording the time of burnout, arrange-ments shall be made for hourly observations for burnouts.5. Test Specimen5.1 The test specimen shall be 0.0254 in. (0.645 mm) indiameter. The length of the wire selected for test shall be suchas to give a test length of approximately 10 in. (254 mm).5.2 T
21、he test specimen shall be representative with regard tothe surface 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 precaution isnecessary, since a kink, even though later removed may
22、 causeburnout at that point.NOTE 4It is also very desirable to select and keep as a referencestandard for comparison, a spool or coil of wire that 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 wi
23、ll be notedMetric Equivalentsin. mm12 12.75532 131.0714 184.22112 54622 55952 132164 1626FIG. 1 Test PanelB 78 90 (2007)2by the length of life of the standard. Comparisons between tests made atdifferent times on the standards and other wires may be correlated in thismanner (see Annex).6. Mounting Sp
24、ecimen6.1 A straight length of the specimen shall be shaped into a“U” pattern, 10 in. (254 mm) of which shall be mountedbetween the terminals described in 2.2. The “U” pattern shallconsist of two parallel legs and a semicircle of 1-in. (25.4-mm)radius, the plane of which is parallel to and equidista
25、nt fromthe front and back panels, and the legs of which are equallyspaced from the side panels.7. Ballast Resistance7.1 The ballast resistance in series with the specimen shallbe at least 60 % of the specimen resistance at the beginning ofthe test, but shall not be greater than the specimen resistan
26、ce.8. Test Temperature8.1 The true temperature of the test shall be 2250F(1232C).NOTE 5The apparent temperature is approximately 50F (27.8C)lower than the true temperature for the type alloy composed of 23 %chromium, 5 % aluminum, and the remainder essentially iron.29. Procedure9.1 Support the tempe
27、rature measuring instrument so that itcan be quickly adjusted and read.9.2 Set the variable transformer at minimum voltage.NOTE 6Do not operate the interrupter while the temperature is beingadjusted.9.3 Close the switch in series with the specimen.9.4 Adjust the variable transformer until the specim
28、en is ata low red heat.9.5 Adjust the temperature of the specimen to 2000F(1093C).9.6 Allow the specimen to operate under this condition until2 h have elapsed, in order to bring the emissivity nearer to ablack body condition.9.7 Set the temperature of the specimen to 2200F(1204C). Maintain this bala
29、nce until 1 min has elapsed.9.8 After another 13 min, readjust the temperature of thespecimen to 2250F (1232C). 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.9.9 Mea
30、sure the voltage and the current and record thevalues together with the starting temperature and time ofstarting the test.9.10 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 durati
31、on of 2min.NOTE 7Various cycles have been tried varying from 10 min on and 5min off to 30 s on and 30 s off, whence 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 toreac
32、h 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 heating and cooling operation is more damagingto wire than maintaining it at a definite temperature.9.11 Adjust the temperature to the t
33、est temperature after 5 hand 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.9.12 After the first 24-h period and each 24 h thereafter untilburnout, adjust the temperature of the sp
34、ecimen to the testtemperature. After each temperature adjustment, observe andrecord the voltage and current.10. Record10.1 Measurements and observations shall be recorded on adata sheet similar to that shown in Fig. 3.11. Report11.1 Report the following information:11.1.1 Nominal analysis,11.1.2 Ide
35、ntification of specimen,11.1.3 Cross-sectional dimensions of the specimen,11.1.4 Life of the specimen in hours (total elapsed timefrom the end of the first 15-min aging period to burnout), and11.1.5 The increase in resistance as noted for the lastresistance reading before burnout.12. Precision and B
36、ias12.1 The life test is an individual internal comparative testonly. Too many variables exist to define a unit that wouldenable precision and bias to be defined. Insufficient need existsto alter the existing test method. Therefore, precision and biasare not defined.12.2 Reproducibility:In the cours
37、e of the development ofthis test method, it has been found that the major source of2For reference see Roeser, W. F., “Spectral Emissivity (at 0.65) of Some Alloysfor Electrical Heating Elements,” Proceedings, ASTM, Vol 39, 1939, p. 780.FIG. 2 Electrical Circuit Diagram for Accelerated Life TestB 78
38、90 (2007)3irregularity is in respect to temperature measurement. There-fore, no attempt shall be made to run standard accelerated lifetests until it has been definitely proved that the operator canobtain consistent results with specimens taken from the samesample coil or spool. Four or five specimen
39、s shall be tested atthe same time to make sure that no variables, such as errors intemperature measurement, would affect one test and notanother. Consecutive tests also shall be run. If the tests areproperly made and controlled, the life of a number of speci-mens cut from the same spool should not v
40、ary more than610 % from the average.13. Keywords13.1 accelerated life test; heating alloys; iron-chromium-aluminum; life test; oxidation resistanceFIG. 3 Data Sheet for Accelerated Life Test Record for Iron-Chromium-Aluminum AlloysB 78 90 (2007)4ANNEX(Mandatory Information)A1. PRECAUTIONSA1.1 Genera
41、lA1.1.1 Following are a few of the general considerationsand precautions that should be kept in mind in the setting upand operation of the life test equipment and in the making ofthe accelerated life test.A1.2 TemperatureA1.2.1 Temperature is one of the most important variablesin a life test. The pr
42、obable life of a wire varies inversely withan exponential function of the temperature. This is readilyunderstandable when it is considered that the life of the wirewhen operated at a temperature close to the melting point willbe but a few hours, while at lower temperatures, as forexample, about a re
43、d heat, it will endure for several thousandhours.A1.2.2 In general, it has been found desirable to use atemperature for any particular alloy which will result in a totallife of approximately 100 h. Experience has indicated that thisis a sufficient length of life to give a fair index of the quality o
44、fthe material. A measure of the necessary length of time to geta good life test is shown by the consistency of the results; forexample, if the test temperature is too high, the results arelikely to be inconsistent since the excessive temperature causesaggravated hot spot conditions.A1.2.3 To determi
45、ne the temperature accurately, if thedisappearing filament-type optical pyrometer is being used, thedesired magnification can be obtained by substituting for thestandard objective lens, another lens having approximately onehalf its focal length. The temperature of the wire under test isvery greatly
46、affected by drafts. The enclosure, therefore, shouldbe left in place at all times during the temperature observationand the pyrometer reading made directly through the glassfront of the enclosure. In taking this reading through the glassfront, it is necessary to allow for the reflection and absorpti
47、onof the glass. Reflection on the surface of the glass is theprincipal cause of error. This effect is nearly independent of thekind and thickness of the glass. Reflection from external lightsources shall be avoided. A correction of approximately 10F(5.5C) or a correction as determined by a specific
48、test for theconditions involved shall be added to the temperature asobserved. The glass slide shall be kept clean at all times toavoid increase in the absorption of light.A1.3 Voltage ControlA1.3.1 In a number of cases, it has been attempted to makelife tests, using the regular line voltage with no
49、regulation, butthis has never succeeded in producing uniform results. It isabsolutely essential that voltage control be used.ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and
