1、Designation: B78 90 (Reapproved 2013)Standard Test Method ofAccelerated Life of Iron-Chromium-Aluminum Alloys forElectrical Heating1This standard is issued under the fixed designation B78; the number immediately following the designation indicates the year of originaladoption or, in the case of revi
2、sion, 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 method covers the determination of the resis-tance to oxidation of iron-chromium-aluminum
3、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 standard. The values given in parentheses are mathematicalconv
4、ersions 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 this standard to become familiarwith all hazards including those
5、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 to use.2. Significance and Use2.1 This test method is used by p
6、roducers of electricalheating alloys to measure the cyclic oxidation resistance 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 Locat
7、ionThe 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.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
8、 test, as even a slight draft of air incontact with the specimen will cause 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
9、) apart, center to center and shall be so positioned thatthe wire specimen 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 simi
10、lar 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 voltage of from 10to 35 V to the circuit. It shall have a continuous currentcapacity of at least 20 A/
11、specimen.4.3 Voltage ControlThe automatic voltage control shall becapable 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 t
12、he results obtained (see Annex A1).4.4 Variable TransformerThe variable transformer 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
13、 transformer having a working range of adjustabil-ity from approximately 0 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 resp
14、ectively). For alternatingcurrent the range shall be such as to give a reading 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
15、 is not changed. This resistanceshall be inserted in series with the contact of the upper switchshown in Fig. 2.1This 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
16、Resistance Heating Materials.Current edition approved May 1, 2013. Published May 2013. Originallyapproved in 1959. Last previous edition approved in 2007 as B78 90 (2007). DOI:10.1520/B0078-90R13.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. Unit
17、ed States14.6 Optical Pyrometer or Infrared Thermometer The opticalsystem shall be such as to provide a magnification of at leastfour diameters. This may be accomplished by means of aspecial lens or combination of two standard lenses in theobjective to provide a short focal length and the desiredmag
18、nification (see AnnexA1). These instruments must have anaccuracy of 610F (5.5C) and UBS traceability.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. Opt
19、icalpyrometer or infrared pycnometer makes it possible to determine thetemperature at any particular 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 as aninterrupter to o
20、pen 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.5. Test Specimen5.1 The test specimen shall be 0.0254 in. (0.645 mm) indiameter. The length of the wi
21、re selected for test shall be suchas to give a test length of approximately 10 in. (254 mm).5.2 The 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 sele
22、cted is free from kinks. This precaution isnecessary, since a kink, even though later removed may causeburnout at that point.Metric Equivalentsin. mm12 12.75532 131.0714 184.22112 54622 55952 132164 1626FIG. 1 Test PanelB78 90 (2013)2NOTE 4It is also very desirable to select and keep as a references
23、tandard 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 will be notedby the length of life of the standard. Comparisons between tests made atdifferent t
24、imes on the standards and other wires may be correlated in thismanner (see Annex A1).6. Mounting Specimen6.1 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 paral
25、lel legs and a semicircle of 1-in. (25.4-mm)radius, the plane of which is parallel to and equidistant 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 s
26、pecimen resistance at the beginning ofthe test, but shall not be greater than the specimen resistance.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 o
27、f 23 %chromium, 5 % aluminum, and the remainder essentially iron.29. Procedure9.1 Support the temperature 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.
28、3 Close the switch in series with the specimen.9.4 Adjust the variable transformer until the specimen 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 t
29、o ablack body condition.9.7 Set the temperature of the specimen to 2200F(1204C). Maintain this balance 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
30、 this 15-min period is the start of the test. It is important tomaintain this time schedule.9.9 Measure 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 previousl
31、y regulated so that the “on” period and the“off” period shall be equal and shall each have a duration 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
32、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 heating and cooling operation is more
33、 damagingto wire than maintaining it at a definite temperature.9.11 Adjust the temperature to the test 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
34、After the first 24-h period and each 24 h thereafter untilburnout, adjust the temperature of the specimen 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
35、 shown in Fig. 3.11. Report11.1 Report the following information:11.1.1 Nominal analysis,11.1.2 Identification 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 Th
36、e increase in resistance as noted for the lastresistance reading before burnout.12. Precision and Bias12.1 The life test is an individual internal comparative testonly. Too many variables exist to define a unit that would2For reference see Roeser, W. F., “Spectral Emissivity (at 0.65) of SomeAlloysf
37、or Electrical Heating Elements,” Proceedings, ASTM, Vol 39, 1939, p. 780.FIG. 2 Electrical Circuit Diagram for Accelerated Life TestB78 90 (2013)3enable precision and bias to be defined. Insufficient need existsto alter the existing test method. Therefore, precision and biasare not defined.12.2 Repr
38、oducibilityIn the course of the development ofthis test method, it has been found that the major source ofirregularity is in respect to temperature measurement.Therefore, no attempt shall be made to run standard acceleratedlife tests until it has been definitely proved that the operator canobtain co
39、nsistent results with specimens taken from the samesample coil or spool. Four or five specimens 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 areproperl
40、y made and controlled, the life of a number of speci-mens cut from the same spool should not vary 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
41、-Chromium-Aluminum AlloysB78 90 (2013)4ANNEX(Mandatory Information)A1. PRECAUTIONSA1.1 GeneralA1.1.1 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
42、.2 TemperatureA1.2.1 Temperature is one of the most important variablesin a life test. The probable 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
43、 melting point willbe but a few hours, while at lower temperatures, as forexample, about a red 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. Experienc
44、e has indicated that thisis a sufficient length of life to give a fair index of the quality ofthe 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 incons
45、istent since the excessive temperature causesaggravated hot spot conditions.A1.2.3 To determine 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 a
46、pproximately onehalf its focal length. The temperature of the wire under test isvery greatly 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
47、this reading through the glassfront, it is necessary to allow for the reflection and absorptionof 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
48、avoided. A correction of approximately 10F(5.5C) or a correction as determined by a specific 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 numb
49、er of cases, it has been attempted to makelife tests, using the regular line voltage with no 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
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