ASTM_D_3026_-_94.pdf

Designation:D 3026 94Standard Test Method forSinter Index of Electrical Grade Magnesium Oxide for Use inSheathed Heating Elements1This standard is issued under the fixed designation D 3026;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon(e)indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers the determination of sinterindex which predicts the sintering characteristics of electricalgrade magnesium oxide for use in manufacturing sheathedheating elements.1.2 The values stated in SI units are to be regarded as thestandard.The values given in parentheses are for informationonly.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 and health practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:C 266 Test Method for Time of Setting of Hydraulic-Cement Paste by Gillmore Needles2D 2755 Test Method of Sampling and Reduction to TestWeight of Electrical Grade Magnesium Oxide3D 3347 Test Method for Flow Rate and Tap Density ofElectrical Grade Magnesium Oxide for Use in Sheathed-Type Electric Heating Elements3E 220 Method for Calibration of Thermocouples by Com-parison Techniques43.Terminology3.1 Descriptions of Terms Specific to This Standard:3.1.1 sinter index,na number that characterizes the de-gree of sintering and numerically equals the weight in gramsthat causes a specially shaped probe to penetrate the sinteredbody of a fired magnesium oxide specimen to a prescribeddepth.3.1.2 sintering,nthe welding together and growth ofcontact area between two or more initially distinct particles attemperatures below the melting point,but above one-half ofthe melting point.The rate of sintering is greater with smallerparticles than with larger particles.4.Summary of Test Method4.1 The specimen of magnesium oxide is placed in amagnesium oxide combustion boat and exposed to an elevatedtemperature in an oxidizing atmosphere for a specified time.Itis then cooled and subjected to a weighted,specially shaped,probe that penetrates the body of the specimen.5.Significance and Use5.1 The degree of sintering of the magnesium oxide atelevated temperatures experienced in manufacturing sheathedheating elements is important to the user.A high degree ofsintering increases the tendency of the insulation to fractureduring bending of the heating element but a low degree ofsintering reduces the thermal conductivity.6.Apparatus6.1 Electric Furnace,capable of heating the entire combus-tion boat(6.3),to 10936 5C(2000 6 9F).The furnace shallbe provided with a power supply and control to permitadjustment and maintenance of the desired test temperature.The use of a nonswitching-type power supply is optional.Thefurnace shall be of one of the following types:6.1.1 Tube Furnace,having a minimum 305 mm(12 in.)uniform heat zone,with a straight bore,open-ended,ceramiccombustion tube measuring approximately 32 mm(114in.)inoutside diameter,by 25.4 mm(1 in.)in inside diameter,by 610mm(24 in.)in length.6.1.2 Muffle Furnace,having a minimum internal dimensionof 133-mm(514-in.)width by 203-mm(8-in.)depth and95-mm(334-in.)height,and capable of heating twelve com-bustion boats simultaneously.The furnace cavity should bechecked to ensure a uniform heat pattern.6.2 Combustion Plate,102-mm(4-in.)by 203-mm(8-in.)by 13-mm(12-in.)thick to support boats in the muffle furnace.The combustion plate must be capable of withstanding thetemperature and thermal shock associated with the test.6.3 Combusion Boat,5refractory type,magnesium oxide,95mm(334in.)long,and having a flat bottom for at least half its1This test method is under the jurisdiction of ASTM Committee D-9 onElectrical and Electronic Insulating Materials and is the direct responsibility ofSubcommittee D09.14 on Electrical Heating Unit Insulation.Current edition approved June 15,1994.Published August 1994.Originallypublished as D 3026 72.Last previous edition D 3026 86.2Annual Book of ASTM Standards,Vol 04.01.3Annual Book of ASTM Standards,Vol 10.02.4Annual Book of ASTM Standards,Vol 14.03.5Magnesium oxide combustion boats are available from Saxonburg Ceramics,100 Isabella St.,Saxonburg,PA 16056.1AMERICAN SOCIETY FOR TESTING AND MATERIALS100 Barr Harbor Dr.,West Conshohocken,PA 19428Reprinted from the Annual Book of ASTM Standards.Copyright ASTMNOTICE:This standard has either been superceded and replaced by a new version or discontinued.Contact ASTM International(www.astm.org)for the latest information.width(Fig.1).Prior to use,the boat shall be fired at 109365C(2000 6 9F)for 1 h to remove trace material that mayaffect the testing to follow.Magnesium oxide boats arereusable until fractured.6.4 Sint