The Variations of Martensitic Morphology and Habit Plane in High Carbon Steel

Abstract

The martensites in steel T8A containing 0.8 pct C were examined by light microscope and TEM after martensitic austemper. There were boundary plate martensite, 12251 , butterfly martensite, { 2251, plate martensite, { 1 1 1 ], plate martensite, { 111 ], butterfly martensite and ( Ill], lath martensite observed in the steel. With the decrease of transformation temperature, the morphologies of the martensites formed changed from butterfly or plate to lath, the substructures from partial twin to dislocation and the habit planes from 12251, to ill1 1 ,. All these changes were controlled by the relief of elastic strain energy in the parent phase austenite. In a microregion of lath martensite, the martensites nucleate on two independent { 1111, planes respectively, and were formed in the way of filling in. The morphology of 1 1111, plate martensite found in this paper was similiar to that of thin plate martensite, bu it was mainly parallel and its substructure was partial twins.

Lath and Butterfly Composite Martensite Microstructure of a Medium-carbon Steel and its Quantitative Evaluation

“…6) Zhang et al reported such a microstructure and examined the finescale microstructure using step-quenched specimens. They also reported a similar morphology of butterfly and lath mixture in optical microscopy and TEM scales, 8) although the heat-treatment and etching conditions are different from those we report in the present study. Combined with the results in next paragraphs, the dark contrast corresponds to butterfly type coarse blocks, and the light contrast corresponds to surrounding lath type blocks.…”

“…Most microstructures of medium carbon steels are composed of dominant lath martensite and non-negligible butterfly martensite. [6][7][8] Lath martensite is composed of thin crystals having thickness of 0.2-0.5 μm, 9) but butterfly martensite appears as coarse plate-like grains. Both are filled with dense dislocation.…”