Morphology of bainite and widmanstätten ferrite

Ohmori, Yasuya; Ohtsubo, Hiroyuki; Jung, Yun Chul; Okaguchi, Shuji; Ohtani, Hiroo

doi:10.1007/bf02649046

Cited by 47 publications

(28 citation statements)

References 23 publications

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“…Furthermore, it has been suggested that the platelets of bainite tend to coalesce at low temperatures. This can be explained, theoretically, by an excess of available free energy during the transformation [12]. This coalescence counteracts the advantage of refining the bainite plates by their formation at low temperatures.…”

Section: Chemical Composition Modificationmentioning

confidence: 99%

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Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

García‐Mateo

Paul

Somani

et al. 2017

Metals

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Abstract:The major strengthening mechanisms in bainitic steels arise from the bainitic ferrite plate thickness rather than the length, which primarily determines the mean free slip distance. Both the strength of the austenite from where the bainite grows and the driving force of the transformation, are the two factors controlling the final scale of the bainitic microstructure. Usually, those two parameters can be tailored by means of selection of chemical composition and transformation temperature. However, there is also the possibility of introducing plastic deformation on austenite and prior to the bainitic transformation as a way to enhance both the austenite strength and the driving force for the transformation; the latter by introducing a mechanical component to the free energy change. This process, known as ausforming, has awoken a great deal of interest and it is the object of ongoing research with two clear aims. First, an acceleration of the sluggish bainitic transformation observed typically in high C steels (0.7-1 wt. %) transformed at relatively low temperatures. Second, to extend the concept of nanostructured bainite from those of high C steels to much lower C contents, 0.4-0.5 wt. %, keeping a wider range of applications in view.Keywords: bainite; ausforming; kinetics; plate thickness Structural Refinement of Bainitic Steels: General ConsiderationsBainitic steels can be designed on the basis of the theory that predicts the highest temperature at which bainite (Bs) and martensite (Ms) can start to form in a steel of a given composition. These two temperatures constitute the upper and lower limits at which the isothermal heat treatment can be performed to generate bainite.It has been reported that bainitic ferrite plate thickness depends primarily on three parameters, i.e., (1) the strength of the austenite at the transformation temperature, (2) the dislocation density in the austenite and (3) the chemical free energy change accompanying transformation [1][2][3]. In accord, a strong austenite possessing a high dislocation density and a large driving force results in finer plates. Austenite strength and dislocation density refine the structure by increasing the resistance to interface motion, and the thermodynamic driving force refines the structure by increasing the nucleation rate. All three factors-austenite strength, dislocation density and driving force-increase

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Section: Chemical Composition Modificationmentioning

confidence: 99%

“…This coalescence counteracts the advantage of refining the bainite plates by their formation at low temperatures. The consequences of this coalescence on toughness are detrimental, but the influence on other properties requires further investigation [12][13][14][15][16][17].…”

Section: Chemical Composition Modificationmentioning

confidence: 99%

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

García‐Mateo

Paul

Somani

et al. 2017

Metals

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“…Detailed examination by transmission electron microscopy revealed that Widmanstätten ferrite grows almost continuously from the allotriomorphic ferrite into one of the austenite grains and possesses a specific orientation relationship with this austenite grain such as that of K-S 29) or N. 30) It is interesting to note that the formation of Widmanstätten ferrite accompanies surface reliefs similar to those of upper bainite. 73) On the mechanism of Widmanstätten ferrite growth, there are some controversial arguments. The one is the diffusional formation model and the other is that of displacive growth.…”

Section: Widmanstätten Ferrite Formationmentioning

confidence: 99%

Advances in Physical Metallurgy and Processing of Steels. Microstructural Evolutions with Precipitation of Carbides in Steels.

Ohmori

2001

ISIJ International

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The studies on the crystallographic features of diffusional transformation of austenite in various steels have been reviewed. A primary ferrite forming at an austenite grain boundary is related to at least one of the austenite grains separated by the boundary with the Bain correspondence. In the case of the microstructures containing both ferrite and carbides such as pearlite, degenerate pearlite, interphase precipitation of alloy carbides, and upper and lower bainites, the crystallographic relationships between ferrite and carbides provide further information to elucidate the transformation mechanisms. In the present report, therefore, the orientation relationships between ferrite, carbide and austenite as well as the habit planes of platelike transformation products and the mechanisms of transformations are discussed.

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“…It was reported that B s can be over T 0 in high carbon content regions, in Fe-C, 5) Fe-2Mn-C, 7) and Fe-Cr-C 8) alloys; in middle carbon content regions, Tsuzaki et al 6) showed that B s does not change with increasing carbon content in Fe-9Ni-C alloys.…”

Section: Introductionmentioning

confidence: 99%

Effect of Carbon Content on Bainite Transformation Start Temperature in Middle–High Carbon Fe–9Ni–C Alloys

et al. 2018

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Bainite in steel is an industrially useful structure. However, the controlling factor of its transformation start point is not clearly known. In this study, we measured the bainite transformation start temperature (B s ) in Fe-9Ni-C alloys containing 0.3-0.9 mass%C via microstructure observation of the specimens held isothermally between 600 K and 798 K. B s existed between 758 K and 773 K in all alloys used, and was independent of carbon content. Especially, B s was higher than T 0 , at which fcc and bcc of the same composition have the same free energy, at more than 0.3 mass%C. This result was completely different from that of our previous study on low carbon Fe-9Ni-C alloys, in which B s decreased with the increase in carbon and kept the certain driving force of partitionless transformation from fcc to bcc. B s in middle-high carbon alloys corresponded to the temperature of the intersection point between T 0 ′, at which the driving force is 400 J/mol, and the γ/(γ + θ) phase boundary. This suggests that the nucleation and growth of bainitic ferrite in austenite containing solute carbon higher than T 0 ′ is promoted by the precipitation of cementite in austenite.

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Morphology of bainite and widmanstätten ferrite

Cited by 47 publications

References 23 publications

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Advances in Physical Metallurgy and Processing of Steels. Microstructural Evolutions with Precipitation of Carbides in Steels.

Effect of Carbon Content on Bainite Transformation Start Temperature in Middle–High Carbon Fe–9Ni–C Alloys

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