The effect of quench rate on the properties and morphology of ferrous martensite
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AND
G. S. ANSELL
The effect of high quench rate on the M s temperature, percent transformed, martensite morphology and austenite hardness has been studied for several F e - N i - C steels. For these steels the quench rate was varied only in the austenite region. The M s t e m p e r a ture was found to increase with increased quench rate for both high and low carbon steels while the percent transformation increased or decreased depending upon the morphology of the steel. No variations in martensite hardness were found in the as-quenched condition, but a difference in tempering rate was found between fast and slow quenched specimens. Austenite hardness decreased slightly with increasing quench rate while the m a r tensite morphology changed from lath to plate. Parallel aligned plate s t r u c t u r e s were observed which r e s e m b l e a twinned lath morphology. It was demonstrated that the actual difference between this morphology and a true lath morphology is the self-accommodating nature of the lath structure. The morphology changes were compared to the measured changes in martensite properties in o r d e r to identify the mechanism of the morphology shift. It was concluded that for these alloys the morphology was controlled by the austenite shear mode. R . E C E N T investigations have reported a number of effects of ultrahigh quench rate on the martensite transformation in F e - C and other atloy steels. These effects occur as changes in three martensite p r o p e r ties: a) M s temperature and the extent of t r a n s f o r m a tion, l'e b) martensite hardness, s and c) martensite morphology. ~'4 QUENCH RATE EFFECTS ON M TEMPERATURE
s
Changes in M s temperature with quench rate have been reported as " s - s h a p e d " for simple alloy s y s tems I and as linear for more complex systems. 1'2 In all systems in which changes have been observed, the M s temperature and the extent of transformation has increased with increasing quench r a t e . For s i m ple alloy systems, there are two a r e a s of constant Ms. At low quench rates, no change in M s is observed with quench rate, producing a lower plateau which corresponds to the nominal M s t e m p e r a t u r e of the steel. At v e r y high quench rates another plateau of constant M s temperature is observed, approximately 150 to 250~ higher than the nominal M s t e m perature. At intermediate quench rates, the M s t e m perature increases with increasing quench rate to produce the characteristic " s - s h a p e d " curve. The theory of Ansell and Arrott ~ has been used as a basis for an explanation of these phenomena. This theory assumes that a nonuniform distribution of carbon exists in austenite prior to the martensitic transformation because of segregation occurring at defects (predominantly dislocations) in the lattice. The size of the atmospheres surrounding such a defect is temperature dependent, being small at high S. J. DONACHIEis with International NickelCo., Suffern, N. Y. G. S. ANSELLis Dean of Engineeringand R. W. Hunt Professorat Rensselaer Poly
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