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Rationalisation of austenite transformation to upper or lower bainite in steels

Ławrynowicz Z.

Advances in Materials Science

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2014

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Vol. 14, nr 2(40)

14--23

EN

The paper presents an analytical evaluation of transition temperature from upper to lower bainite in Fe-C-Cr steel. The calculations was based on the model constructed by Matas and Hehemann which involves a comparison between the times needed to precipitate cementite within the bainitic ferrite plates (tθ), with the time required to decarburise supersaturated ferrite plates (td). The transition between upper and lower bainite is found to occur over a narrow range of temperatures (350-410°C) and depends on the thickness of bainitic ferrite laths and the volume fraction of precipitated cementite. On comparing the td and tθ times it was found that the transition temperature from upper to lower bainite reaction (LS) of about 350oC could be predicted if the thickness of bainitic ferrite laths is set as wo = 0.1 μm and volume fraction of cementite is set as ξ = 0.01.

2

Bainite Transformation in Experimental Fe-Cr-Mo-V-Ti-C Steel

Ławrynowicz Z.

Advances in Materials Science

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2013

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Vol. 13, nr 2(36)

12--18

EN

The mechanism and kinetics of bainite transformation have been studied in Fe-Cr-Mo-V-Ti-C steel using high speed dilatometry and transmission electron microscopy (TEM) backed by thermodynamic analysis. At any temperature investigated did not occur the complete transformation of austenite. Obtained results confirm the incomplete reaction phenomenon with the cessation of the bainite transformation well before paraequilibrium is achieved. These experimental data indicate that bainitic ferrite forms by a displacive transformation mechanism, but soon afterwards, excess of carbon is partitioned into the residual austenite.

3

An investigation of the mechanism of bainite transformation in experimental 0.2c-1v-2mn steel

Ławrynowicz Z.

Advances in Materials Science

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2012

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Vol. 12, nr 2(32)

19-30

EN

The paper presents an investigation of the mechanism of bainite transformation in 0.2C-1V-2Mn steel using high speed dilatometry and TEM backed by thermodynamic analysis. Obtained results confirm the incomplete reaction phenomenon with the cessation of the bainite transformation well before paraequilibrium is achieved. These experimental data indicate that bainitic ferrite forms by a displacive transformation mechanism, but soon afterwards, excess of carbon is partitioned into the residual austenite. The results are discussed in terms of the mechanism of bainite transformation.

4

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

Mazancova E., Rucka Z., Mazanec K.

Archives of Materials Science

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2007

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Vol. 28, nr 1-4

95-99

EN

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.