New crystallographic and morphological observations on the martensite transformation in AISI 4340 type steel

H. K. D. H. Bhadeshia and D. V. Edmonds
University of Cambridge


AISI 4340 is a high-strength, medium-carbon low-alloy steel used extensively in the quenched and tempered condition. Its chemical composition places it in a domain where both plate and lath martensites may occur. Our observations show that the lath packet is the fundamental transformation unit rather than individual laths. Thus, the packet can be considered as a low aspect ratio plate formed at the highest of martensite transformation temperatures. An observed degeneracy in the plate tip is thought to be attributable to the need to minimise the high interface curvature accociated with a singularly tipped plate.

Phase Transformations, York Conference, April 1979 The Institution of Metallurgists, Series 3, No. 11, Vol. 2, pp. IV-9 to IV-13

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