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| NATURE OF ISOTHERMAL TRANSFORMATION BETWEEN ROOM TEMPERATURE AND MARTENSITE POINT IN A BALL-BEARING STEEL |
| CHANG PEI-LIN AND HSU TSUI-CHANG Institute of Metal Research;Academia Sinica |
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Cite this article:
CHANG PEI-LIN AND HSU TSUI-CHANG Institute of Metal Research;Academia Sinica. NATURE OF ISOTHERMAL TRANSFORMATION BETWEEN ROOM TEMPERATURE AND MARTENSITE POINT IN A BALL-BEARING STEEL. Acta Metall Sin, 1956, 1(4): 347-366.
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Abstract The isothermal transformation of austenite in a ball-bearing steel within thetemperature range between martensite point M and room temperature has been investigated by electrical resistance measurements.The steel employed was ofcommercial production containing 1.43% Cr and 1.02% C and its martensite pointis 159℃ when quenched from 1100℃.The specimen was first quenched toroom temperature(in which state it contained 17% retained austenite),and thenupquenched to isothermal bath in which its transformation was followed.Theeffect of tempering of martensite was eliminated by employing two specimens ofdifferent martensite contents.1)By comparing the kinetics of isothermal transformation below and abovemartensite point,it was concluded that isothermal transformation below M pointtakes place by the martensite process and not by intermediate transformation(bainitic),in.spite of the presence of large amount of martensite and relativelyhigh temperature at which the transformation takes place.2)From kinetics considerations it appears that isothermal transformationof retained austenite to martensite takes place not by the formation of new nuc-lei but mainly by the growth of already existing martensite.This would alsoinfer that coherency between martensite and austenite is not destroyed(or notcompletely destroyed)during quenching transformation.3)The tempering of martensite appears to be the controlling process forthe isothermal transformation of retained austenite to martensite,such that theelastic strain energy relaxed by tempering of martensite exceeds the difference offree energies between the two phases.The activation energy of tempering ofmartensite within the temperature range considered(below 150℃)was found tobe 13,300 cal/mol,which compares favourably well with the value of 16,000 cal/molas given by Averbach and Cohen in 1953.4)The results show that isothermal keeping for prolonged duration at theneighbourhood of 100℃ brings about maximum amount of transformation of re-tained austenite.That is,at either higher or lower temperatures and for a speci-fied time,the amount of austenite-martensite transformation is less.This is signifi-cant for practical heat-treating of precision gauges and tools for which dimen-sional stability is important and high temperature tempering may not be de-sirable for hardness consideration.
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Received: 18 April 1956
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