Structural characterization of “carbide-free” bainite in a Fe–0.2C–1.5Si–2.5Mn steel

Hofer, Christina; Leitner, Harald; Winkelhofer, Florian; Clemens, Helmut; Primig, Sophie

doi:10.1016/j.matchar.2015.02.020

Cited by 50 publications

(29 citation statements)

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“…The applied process is schematically shown in Figure 4. Given that there exists the possibility of forming bainite by isothermal transformation at temperatures below the Ms [26][27][28][29][30][31][32][33], this is also becoming an attractive alternative, not only to accelerate the bainitic transformation, but also to obtain a finer microstructure in later stages of transformation. Figure 5 shows an example for 0.15 and 0.28 C steels transformed to bainite below the Ms; the author reports a decrease in the bainitic ferrite plate thickness of almost 40 nm in both cases, the final plate thickness being around 140-200 nm [26].…”

Section: Heat Treatment Variationsmentioning

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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Section: Heat Treatment Variationsmentioning

confidence: 99%

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

García‐Mateo

Paul

Somani

et al. 2017

Metals

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“…This lower C concentration and, therefore, the lower thermodynamic stability explains the transformation of these areas to mainly C‐rich martensite upon cooling to room temperature. The outer regions of the M‐A islands remain preferentially partly austenitic, reflecting the C enrichment at the bainitic ferrite‐austenite interface.…”

Section: Resultsmentioning

confidence: 99%

“…For light optical microscopy, metallographic samples were etched with Nital and the anodic surface layer etching LePera with the experimental details given in ref . The austenite phase fraction was determined magnetically by the Joch‐Isthmus method…”

Section: Methodsmentioning

confidence: 99%

“…However, the austenite stability is not only influenced by its chemical composition, but also the combination of size, distribution and morphology, as well as the microstructure of the surrounding matrix and the resulting strength mismatch affect the austenite stability. Depending on the stability of the austenite, parts of it may transform to fresh martensite during quenching to room temperature, forming the so‐called martensite‐austenite (M‐A) islands . During plastic straining, the moderately stable retained austenite present at room temperature can transform to martensite through the transformation‐induced plasticity (TRIP) effect leading to an increase in the deformability.…”

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confidence: 99%

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Influence of Heat Treatment on Microstructure Stability and Mechanical Properties of a Carbide‐Free Bainitic Steel

et al. 2017

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Carbide-free bainitic steels represent attractive candidates for the automotive industry to meet the conflicting requirements of increasing safety and decreasing vehicle weight. Their fine-grained multiphase microstructure enables a well-balanced combination of high strength and ductility. To identify and characterize all phases in dependence of the isothermal transformation temperature, metallographic and high-resolution techniques are applied. The stability of the individual phases is evaluated based on their C concentration determined by atom probe tomography and, subsequently, linked to the mechanical properties.

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“…Assim, o carbono será responsável por estabilizar a austenita retida e impedir quaisquer outras decomposições, assim como ocorre nos aços T&P[2] e que vai de encontro com à convencional têmpera com revenimento (T&R) em que, para a estabilização da microestrutura final, há um incentivo para que ocorra a decomposição da austenita não transformada. Dessa forma, é possível, por meio dos aços CFB, a obtenção de microestruturas muito refinadas formadas por ripas bainíticas com austenita retida na forma de filmes finos[71], que proporcionam propriedades muito atrativas[2][26][79][98]. Essa microestrutura é consequência da composição química que permite o aparecimento da morfologia supracitada, em que a bainita será responsável pela resistência e a austenita retida pela ductilidade: frear a propagação de trincas, aumentando a tenacidade; sofrer o efeito TRIP e aumentar a taxa de encruamento, o alongamento e até mesmo a formabilidade desses aços, como mencionado na seção 3.1.…”

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Têmpera e partição de aço baixa liga com pré-microestrutura bainítica

Mendonça¹

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Os aços temperados e particionados têm despertado interesse no que se refere ao equílibro entre resistência e ductilidade, comumente representado pelo índice PSE (produto da resistência pelo alongamento). A estabilização da austenita retida em baixas temperaturas pode provocar o fenômeno da plasticidade induzida por transformação (efeito TRIP). Com o surgimento da têmpera e partição (T&P), diversas variações desse tratamento têm sido propostas. Dentre elas, o efeito da pré-microestrutura tem se mostrado muito importante na melhoria das propriedades mecânicas. Dessa forma, no presente trabalho foi proposto um processo inédito de têmpera e partição a partir de pré-microestruturas bainíticas. O intuito dessa abordagem foi verificar, a partir de diferentes proporções e morfologias bainíticas, os diferentes níveis de estabilização da austenita retida. Além disso, a obtenção de microestruturas multifásicas têm sido relatada na literatura com boas características mecânicas. A caracterização microestrutural foi feita por meio de microscopias ópticas e eletrônica de varredura, e ensaios de dureza, tração e impacto. Os resultados encontrados foram de encontro aos relatados na literatura, indicando que uma homogênea distribuição de finos carbonetos pode gerar melhores resultados do que a tão aclamada austenita retida. Foram encontrados valores de limite de resistência à tração de 1.081 MPa com alongamento total de 23%, gerando valores de PSE de 24.8 GPa . % com resistência ao impacto de 62 J/cm². Os tratamentos térmicos realizados também mostraram-se mais eficazes que os tradicionais tratamentos de austêmpera e têmpera com revenimento para competir com os aços da nova geração. Palavras-chave: Bainita, Austenita retida, Têmpera e partição, Efeito TRIP, Prémicroestrutura. ABSTRACT MENDONÇA, R.R. Quenching and partitioning of a low alloy steel with bainitic prior microstructure. 2019. 175p. Tese (Doutorado) -Escola de Engenharia de São Carlos, Universidade de São Paulo, São Carlos, 2019. Quenched and partitioned (Q&P) steels have been of great interest to researchers due to its balance between strength and ductility, usually represented by the product of strength and elongation (PSE) index. Retained austenite, when stabilized at room temperature, can promote the transformation induced plasticity phenomenon (TRIP) effect. With the advent of quenching and partition heat treatment many variations to this process were made. Among them, the influence of prior microstructures has been shown to be very important in improving the mechanical properties. In this way, this research proposed a new processing route for the Q&P starting from a bainitic microstructure. The intention of this approach was to verify, by means of different bainite proportions, different levels of retained austenite stabilization. In addition, there have been positive results in obtaining of multiphase microstructures. Microstructural characterization was evaluated by both optical microscopy and scanning electron microscopy while mechanical properties were analyz...

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Structural characterization of “carbide-free” bainite in a Fe–0.2C–1.5Si–2.5Mn steel

Cited by 50 publications

References 23 publications

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Influence of Heat Treatment on Microstructure Stability and Mechanical Properties of a Carbide‐Free Bainitic Steel

Têmpera e partição de aço baixa liga com pré-microestrutura bainítica

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