APPLICATION OF QUANTITATIVE MICROSCOPY TO THE STUDY OF TRANSFORMATION KINETICS AND MICROGEOMETRY OF MARTENSITE IN STEELS

PENG-HENG CHANG, Purdue University

Abstract

Quantitative stereology was applied to study the transformation kinetics and the microgeometry of athermally transformed martensite in Fe-1.4 w/o C-0.02 w/o P alloy and commercial 01 tool steel. The effects of prior austenite grain boundary and the existing martensite plates on the nucleation of martensite were found to be different. The grain boundary dominates the nucleation events primarily in the early stages of the transformation and the effect is the same for both steels studied. The existing martensite plates were found to have the same autocatalytic effect throughout the entire course of the transformation. The autocatalytic nucleation of martensite transformation was characterized by the Gurland's contiguity ratio defined as the fraction of total martensite surface in contact with other martensite plates. This ratio of 15% is insensitive to grain size for the two steels studied and is much lower than the 41% obtained from the data of Guimaraes in Fe-31 w/o Ni-0.02 w/o alloy which transformed to martensite by burst.^ The average martensite plate thickness and radius were defined by means of the total volume, total midplane area and total midplane periphery length of the martensite plates. The average plate thickness was found to be constant during the transformation except for coarse-grained 01 tool steel after (TURN) 60% transformation. However, the plate radius for all cases decreased slowly as the transformation progressed. The plate radius is independent of the mean free path in austenite initially, but decreased as the mean free path in austenite decreases during the latter stages of the transformation. ^ The distributions of martensite plate thickness and plate diameter were determined by kernel method combined with the appropriate stereological analysis. The average plate thickness and the average plate diameter obtained from the distribution curves are in good agreement with those obtained by using the global quantities described previously. The distribution curves revealed that, although most of the plates have thickness of less than 1 (mu)m and diameter of less than 4 (mu)m, a few plates may be as large as 4 (mu)m in thickness and 30 (mu)m in diameter.^ The experimental standard deviation of various stereological quantities were compared with the theoretical standard deviation. The agreement between the two is generally good provided that the microstructural features are randomly distributed in the specimen. ^

Degree

Ph.D.

Subject Area

Engineering, Metallurgy

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