Analyses of Transformation Kinetics of Carbide-Free Bainite Above and Below the Athermal Martensite-Start Temperature

Yakubtsov, I.A.; Purdy, G.R.

doi:10.1007/s11661-011-0911-9

Cited by 27 publications

(14 citation statements)

References 32 publications

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“…Thus, finally the microstructure will consist of partially carbon-supersaturated plates of bainitic ferrite, α b , and C enriched austenite (γ + ), the latter as thin films between ferrite plates and submicron blocks between sheaves of bainite [5,6]. This microstructure has a high hardness and strength, owing to the combined influence of several types of obstacles to dislocation motion, such as interfaces and dislocations, and also to solid solution strengthening and to the interaction between carbon and defects [3,[7][8][9][10][11][12][13][14][15][16][17].…”

Section: Introductionmentioning

confidence: 99%

Quantitative Assessment of the Time to End Bainitic Transformation

Santajuana

Eres-Castellanos

Ruiz-Jimenez

et al. 2019

Metals

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Low temperature bainite consists of an intimate mixture of bainitic ferrite and retained austenite, usually obtained by isothermal treatments at temperatures close to the martensite start temperature and below the bainite start temperature. There is widespread belief regarding the extremely long heat treatments necessary to achieve such a microstructure, but still there are no unified and objective criteria to determine the end of the bainitic transformation that allow for meaningful results and its comparison. A very common way to track such a transformation is by means of a high-resolution dilatometer. The relative change in length associated with the bainitic transformation has a very characteristic sigmoidal shape, with low transformation rates at the beginning and at end of the transformation but rapid in between. The determination of the end of transformation is normally subjected to the ability and experience of the “operator” and is therefore subjective. What is more, in the case of very long heat treatments, like those needed for low temperature bainite (from hours to days), differences in the criteria used to determine the end of transformation might lead to differences that might not be assumable from an industrial point of view. This work reviews some of the most common procedures and attempts to establish a general criterion to determine the end of bainitic transformation, based on the differential change in length (transformation rate) derived from a single experiment. The proposed method has been validated by means of the complementary use of hardness measurements, X-ray diffraction and in situ high energy X-ray diffraction.

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Section: Introductionmentioning

confidence: 99%

Quantitative Assessment of the Time to End Bainitic Transformation

Santajuana

Eres-Castellanos

Ruiz-Jimenez

et al. 2019

Metals

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“…Microstructures are generally formed by a bainitic ferrite matrix with or without carbides, depending on the alloying elements and the isothermal holding time, and retained austenite, in the form of thin films or martensite-austenite (MA) islands. [7][8][9][10][11][12][13] The isothermally obtained phase product is generally called bainite. While the bainitic ferrite is free of carbides in upper bainite, lower bainitic ferrite can contain a fine dispersion of plate-like carbides, depending on the silicon content.…”

Section: Introductionmentioning

confidence: 99%

Effect of Prior Athermal Martensite on the Isothermal Transformation Kinetics Below M s in a Low-C High-Si Steel

Navarro-López

Sietsma

Santofimia

2015

Metall Mater Trans A

109

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A. NAVARRO-LÓ PEZ, J. SIETSMA, and M.J. SANTOFIMIA Thermomechanical processing of Advanced Multiphase High Strength Steels often includes isothermal treatments around the martensite start temperature (M s ). It has been reported that the presence of martensite formed prior to these isothermal treatments accelerates the kinetics of the subsequent transformation. This kinetic effect is commonly attributed to the creation of potential nucleation sites at martensite-austenite interfaces. The aim of this study is to determine qualitatively and quantitatively the effect of a small volume fraction of martensite on the nucleation kinetics of the subsequent transformation. For this purpose, dilatometry experiments were performed at different temperatures above and below the M s temperature for athermal martensite in a low-carbon high-silicon steel. Microstructural analysis led to the identification of the isothermal decomposition product formed above and below M s as bainitic ferrite. The analysis of the transformation processes demonstrated that the initial stage of formation of bainitic ferrite at heat treatments below M s is at least two orders of magnitude faster than above M s due to the presence of martensite.

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“…[4][5][6] At holding temperatures higher than M s in hypoutectoid steel specimens, the microstructure invariably consists of bainitic ferrite with or without carbides, depending on the alloying elements and austenite decomposition parameters (here, holding temperature and time), and RA in the form of thin interlath films, and/or coarse blocks. 7,8) The properties of these multiphase microstructures are comparable to those of carbide-free bainitic steels and quenched and partitioned (Q&P) steels that have a complex mechanical response to stress. [9][10][11] The novelty of this metallurgical processing lies in the acceleration of the bainite reaction through the presence of a small fraction of athermal initial martensite (IM) formed a priori on quenching to the desired temperature below M s .…”

Section: Structure-property Correlations Of a Medium C Steel Followinmentioning

confidence: 99%

Structure-Property Correlations of a Medium C Steel Following Quenching and Isothermal Holding above and below the M<sub>s</sub> Temperature

2021

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The processing of advanced multiphase high strength steels often includes isothermal treatments around the martensite start temperature (M s) for achieving a refined microstructure comprising bainiteaustenite and/or bainite-martensite-austenite phase constituents. The objective of this research work was to investigate the structure-property relationship for a medium carbon, high-silicon DIN 1.5025 steel (Fe-0.529C-1.67 Si-0.72Mn-0.12Cr (in wt.%)) following isothermal holding close to the M s temperature (~275°C) to enable low temperature austenite decomposition. For realizing multiphase microstructures, DIN 1.5025 steel samples were austenitized at 900°C for 5 min and then quenched to the isothermal holding temperatures 350 and 250°C for various times ranging from 5 to 3 600 s. Microstructural investigation corroborated the formation of multiphase microstructure comprising tempered martensite, bainite, retained austenite, and fresh martensite in both the samples isothermally held above (350°C) and below the M s (250°C) temperature. The sample isothermally held at 250°C showed a much more refined microstructure in comparison to that held at 350°C due to the presence of a fraction of initial martensite laths which acted as potential sites for bainite nucleation. Also, the evaluation of mechanical behaviour showed that the best tensile properties in terms of high tensile strength and good ductility were achieved in samples with high volume fractions of both interlath and blocky retained austenite, particularly those isothermally treated at 350°C for 200 s and at 250°C for 600 s, respectively.

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Analyses of Transformation Kinetics of Carbide-Free Bainite Above and Below the Athermal Martensite-Start Temperature

Cited by 27 publications

References 32 publications

Quantitative Assessment of the Time to End Bainitic Transformation

Quantitative Assessment of the Time to End Bainitic Transformation

Effect of Prior Athermal Martensite on the Isothermal Transformation Kinetics Below M s in a Low-C High-Si Steel

Structure-Property Correlations of a Medium C Steel Following Quenching and Isothermal Holding above and below the M<sub>s</sub> Temperature

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