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Properties of high manganese Fe-Mn-Al-C alloys

Mazancova E., Rucka Z., Mazanec K.

Archives of Materials Science

2007

Vol. 28, nr 1-4

95-99

Acicular ferrite (AF) microstructure represents an excellent contribution of mechanical properties and the toughness level detected in low-alloy steel. Majority of neighbouring plates (laths) have mutual high-angle misorientation in contradistinction to upper bainite (B) microstructure. High-angle interfaces are only formed between B-packets consisting of low-angle plates (laths) set. The cleavage unit crack path (UCP) has been found to be a distance between two grains of high-angle ferrite regions (corresponding to the two crystallographic B-packets boundaries). In the AF the UCP value is defined as a distance between two neighbouring highly misorientated plates. It shows the UCP values are shorter what results in higher deviation frequency and consequently in limited (retarded) cleavage crack propagation. The nucleation AF conditions in austenite matrix after application of an optimized thermomechanically controlled process consisting of the consecutive straining processes realized in recrystallization and in non recrystallization regions have been determined. The applied nucleation mechanism (base on the nucleation process realized in structural matrix) represents the second variant resulting in the AF formation. The beneficial resistance of the AF particles to hydrogen embrittlement can be held for a very important property of this microstructure what demonstrates the valuable contribution of this microstructure to its engineering application. The AF microstructure is associated with effective combination of strength and toughness. The behaviour of this microstructure is compared with upper bainite properties. Following differences between the microstructural parameters are detected. The AF is nucleated on intragranular inclusions. In majority, plates show high-angle arrangement in comparison with upper bainite. Numerous low-angle interfaces are detected within crystallographic upper bainite packets. In AF microstructure the unit crack path is defined as a distance between two neighbouring highly misorientated plates. The AF microstructure contributes to the achievement of high steels resistance to hydrogen induced cracking due to special arrangement of its plates.