ASTM_B_78_-_90_2019.pdf

Designation:B7890(Reapproved 2019)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 revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()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 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 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 this standard to establish appro-priate safety,health,and environmental practices and deter-mine the applicability of regulatory limitations prior 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 TechnicalBarriers 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 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.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 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)apart,center to center,and shall be so positionedthat the wire specimen when secured therein shall be in aU-shaped pattern as described in Section 5.The specimenterminal junctions shall be 3 in.(76.2 mm)lower than the planeof the top of the enclosure.4.Apparatus4.1 The 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 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 of maintaining across the bus bars a constant voltagewithin 60.5%.NOTE1It 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 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.NOTE2A variable 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,respectively).For alternatingcurrent,the range shall be such as to give a reading above thelower 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 approved April 1,2019.Published April 2019.Originallyapproved in 1959.Last previous edition approved in 2013 as B78 90(2013).DOI:10.1520/B0078-90R19.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 recognized 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.1 resistance.A compensating resista