Promoting the coalescence of bainite platelets

Pak, Junhak; Suh, Dong‐Woo; Bhadeshia, H. K. D. H.

doi:10.1016/j.scriptamat.2012.02.041

Cited by 29 publications

(22 citation statements)

References 19 publications

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“…Figure 11a shows changes of growth planes through the crack growth. Those planes have a tendency to be orientated in a direction close to 45 degrees from longitudinal axis of the sample, in accordance with previous studies [28]. Likewise, in Figure 11b it is notable that in a bainitic microstructure, the crack growth process is through bainite grains (transgranular) and not along grain boundaries.…”

Section: Fractographic Analysis Of Toughnesssupporting

confidence: 90%

“…Figure 14 shows thin laths of bainite in different packets; where packets have a similar direction, wider coalesced laths appears, indicated by the arrows. According to another investigation [36], bainite laths in steel have a tendency to coalesce when the bainitic transformation is under conditions of stress; that is, because the number of crystallographic variables is reduced, that gives an orientation to the grains and sheaves of bainite, promoting the conditions for bainite coalescence. That directional tendency of the bainitic microstructure can be observed in bainitic steels of the current paper because of the applied stress due to strain in the thermo-mechanical bainitic transformation.…”

Section: Possible Interpretation Of Toughness Enhancementmentioning

confidence: 99%

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Influence of boron content on the fracture toughness and fatigue crack propagation kinetics of bainitic steels

Rodríguez-Galeano

Rodríguez-Baracaldo

Mestra-Rodriguez

et al. 2016

Theoretical and Applied Fracture Mechanics

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The relatively good combination of high strength and ductility makes bainitic steels a candidate to replace many other steels in industrial applications. However, in service, ductility and strength are not up to standard requirements. In many industrial components, toughness and fatigue performance are also very relevant. In the present study, bainitic steels with varying content of boron were fabricated, with the aim of analyzing the fracture toughness and changes in the fatigue life. The results show that a relatively small change in the boron content can cause a notable variation in the fracture toughness of bainitic steels. The maximum value obtained in fracture toughness was for the steel with the highest boron content. It was observed that the amount of interlath martensite constituents decreases in steels with the addition of boron, leading to the promotion of the presence of void coalescence and a remarkable rise in the toughness of bainitic steels. An increase on the fatigue life of the bainitic steels with an increase in the boron content was also observed, through analysis by means of Paris’ law. A comprehensive micrographic study was carried out in order to examine the mechanics of fatigue crack growth in the bainitic steels, revealing small longitudinal cracks in bainitic steels that lack boron. These cracks tend to disappear in bainitic steels that contain boron. To elucidate this behavior, micrographs of the surfaces generated by the crack growth process were taken, showing that several nano-cracks appeared between the bainite laths. It is finally argued that this high-energy consumption process of nano-crack nucleation and growth is the reason for the improved toughness and fatigue life observed in bainitic steels.Peer ReviewedPostprint (author's final draft

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Section: Fractographic Analysis Of Toughnesssupporting

confidence: 90%

Section: Possible Interpretation Of Toughness Enhancementmentioning

confidence: 99%

Influence of boron content on the fracture toughness and fatigue crack propagation kinetics of bainitic steels

Rodríguez-Galeano

Rodríguez-Baracaldo

Mestra-Rodriguez

et al. 2016

Theoretical and Applied Fracture Mechanics

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“…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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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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“…To the best knowledge of the authors, the effect of the stress on the thickness of the α B has not been assessed, although it would be expected that microstructures are refined because of the increase of driving force [146][147][148], σ Y (in the case σ Y is overcome) [146][147][148], and transformation kinetics [149]. It has been reported, though, that applying stress while a specimen is isothermally treated promoted the coalescence of α B plates [150]. Nevertheless, further research is needed.…”

Section: Stress or Strain Induced Bainite Transformationsmentioning

confidence: 99%

Future Trends on Displacive Stress and Strain Induced Transformations in Steels

Eres-Castellanos

García‐Mateo

Caballero

2021

Metals

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Displacive stress and strain induced transformations are those transformations that occur when the formation of martensite or bainitic ferrite is promoted by the application of stress or strain. These transformations have been shown to be one of the mechanisms by which the mechanical properties of a microstructure can be improved, as they lead to a better ductility and strength by the transformation induced plasticity effect. This review aims to summarize the fundamental knowledge about them, both in fully austenitic or in multiphase structures, pointing out the issues that—according to the authors’ opinion—need further research. Knowing the mechanisms that govern the stress and strain induced transformation could enable to optimize the thermomechanical treatments and improve the final microstructure properties.

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Promoting the coalescence of bainite platelets

Cited by 29 publications

References 19 publications

Influence of boron content on the fracture toughness and fatigue crack propagation kinetics of bainitic steels

Influence of boron content on the fracture toughness and fatigue crack propagation kinetics of bainitic steels

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Future Trends on Displacive Stress and Strain Induced Transformations in Steels

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