The incomplete transformation phenomenon in steel

Aaronson, H.I.; Reynolds, W. T.; Purdy, G.R.

doi:10.1007/s11661-006-0116-9

Cited by 84 publications

(51 citation statements)

References 54 publications

(88 reference statements)

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“…The strength of the austenite from which the ferrite plates are formed is the main factor controlling the plate thickness. 9) As expected, the plates become thinner for the alloy with higher carbon content and with decreasing the transformation temperature due to the effect of both factors on enhancing the strength of the austenite. The observed thinner plate for alloy A1 annealed at 850°C in comparison with that annealed at 920°C is due to the carbon enrichment of the austenite phase.…”

Section: Structure Finenesssupporting

confidence: 57%

“…The incomplete transformation phenomenon, which limits the amount of bainite that can be formed at any temperature, is the reason for the existence of these large regions of untransformed austenite. 7,9) This paper is concerned with a further development of the ultra-fine bainitic steel by decreasing its carbon content aiming at both, lowering the retained austenite and accelerating the bainite transformation reaction. The consequence of lowering the carbon content below the eutectoid composition provides the possibility of intercritical annealing of the alloy which results in a wider window for designing mechanical properties.…”

Section: Introductionmentioning

confidence: 99%

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Ultra-fine Bainite Structure in Hypo-eutectoid Steels

Soliman

Palkowski

2007

ISIJ Int.

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Ultra-fine, carbide-free bainitic structure of plate thickness between 34 and 116 nm has been obtained by low temperature austempering process of two hypo-eutectoid steels with 0.42 and 0.57 % C. Decreasing the carbon content results in accelerating the bainite transformation reaction together with decreasing the retained austenite content, which is known to be detrimental to the mechanical properties. Furthermore, lowering the carbon content below the eutectoid composition allowed intercritical annealing of the material which resulted in a wider window for heat treatment parameters and consequently in a spread field for mechanical properties. Dilatometric measurements were used to design the suitable heat-treatment parameters including an estimation of the required time frames for the cessation of the bainitic reaction. The structure was characterized using light optical microscopy (LOM), scanning electron microscopy (SEM) and X-ray diffractometry. In order to investigate the effect of the microstructure parameters on the materials mechanical properties, compression tests had been conducted at room temperature.KEY WORDS: fine bainite; heat-treatment design; dilatometry; phase transformation; intercritical annealing. 1703© 2007 ISIJ Review Table 1. Chemical composition (wt%). num to prevent temper embrittlement due to phosphorus and silicon to prevent the precipitation of cementite during bainite formation. Note that cementite is a cleavage and void-initiating phase which is best eliminated from strong steels. Aluminium and cobalt had been added to accelerate the bainite transformation process. Dilatometry and Heat-treatmentDilatometric measurements were conducted on a Baehr dilatometer "DIL 805A/D", which has a resolution of 0.05 mm/0.05 K, using samples with 5 mm diameter and 10 mm length prepared from the homogenised material. The test specimens had been degreased with an acetone solvent. Sheathed type S "Pt/Pt-10 % Rh" thermocouples with a nominal diameter of 0.1 mm have been individually spot welded to the specimen's surface in central position to monitor temperature. Each sample was held between two quartz rods, with one of the rods fixed and the other one connected to a linear variable differential transducer (LVDT). A reference rod is also connected to the LVDT. The dimension variations of the specimens during the thermal cycle are transmitted via the moving quartz pushrod to the LVDT sensor. After placing the sample between the pushrods, the insulating sheaths on the thermocouple wires had been moved along the thermocouple wires until they contacted the specimen surface.10) The thermal cycles had been performed under vacuum of 5ϫ10 Ϫ5 mbar. Helium was used for cooling. The dilatometric curves had been recorded along the thermal cycle with the help of a computer-data acquisition system. The dilatometric results had been used for designing the appropriate heat treatment of the two alloys as will be described later.Using salt baths, the designed heat-treatment had been repeated on further mechanical testin...

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Section: Structure Finenesssupporting

confidence: 57%

Section: Introductionmentioning

confidence: 99%

Ultra-fine Bainite Structure in Hypo-eutectoid Steels

Soliman

Palkowski

2007

ISIJ Int.

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“…Assuming that the transformation features observed at the sample surface qualitatively reflect the features of the bainite formation in the bulk, the following conclusions are achieved: Given the composition of the samples studied, an incomplete bainite transformation phenomenon [32,58,[92][93][94] is expected to occur at the transformation temperatures imposed. As can be observed in Figures 8-11 (and in the Supplementary Information showing the original recordings of the isothermal transformation experiment at T = 723 K (450 • C)), the lengthening of the majority of bainite plates is independent of the nucleation time and location and terminates on geometrical obstacles of austenite grain boundaries or existing plates.…”

Section: Discussionmentioning

confidence: 99%

An In-Situ LSCM Study on Bainite Formation in a Fe-0.2C-1.5Mn-2.0Cr Alloy

Sainis

Farahani

Gamsjäger

et al. 2018

Metals

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Direct microscopic observation of the isothermal bainite evolution in terms of nucleation events, the location of the nuclei, as well as their growth is very valuable for the refinement of models predicting the kinetics of bainite transformation. To this aim, the microstructural evolution in a Fe-0.2C-1.5Mn-2.0Cr alloy during isothermal bainite formation at temperatures between 723 K and 923 K is monitored in situ using high temperature laser scanning confocal microscopy (LSCM). Both the nucleation and the growth kinetics of the bainitic plates are analyzed quantitatively. Bainitic plates are observed to nucleate on three different types of locations in the grain: at austenitic grain boundaries, on newly-formed bainite plates and at unspecific sites within the austenite grains. Grain boundary nucleation is observed to be the dominant nucleation mode at all transformation temperatures. The rate of nucleation is found to vary markedly between different austenite grains. The temperature dependence of the average bainite nucleation rate is in qualitative agreement with the classical nucleation theory. Analysis of plate growth reveals that also the lengthening rates of bainite plates differ strongly between different grains. However, the lengthening rates do not seem to be related to the type of nucleation site. Analysis of the temperature dependence of the growth rate shows that the lengthening rates at high temperatures are in line with a diffusional model when a growth barrier of 400 J mol −1 is considered.

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“…In high strength low alloyed steels Mo is well known to provide phase balance strengthening, via facilitating the bainite transformation [1][2][3][4][5], and solid solution strengthening [6][7][8][9]. It can decrease the rate of dynamic recrystallization of austenite [10][11][12], which may lead to grain refinement.…”

Section: Introductionmentioning

confidence: 99%

Comparative Effect of Mo and Cr on Microstructure and Mechanical Properties in NbV-Microalloyed Bainitic Steels

Kostryzhev

Singh

Chen

et al. 2018

Metals

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Steel product markets require the rolled stock with further increasing mechanical properties and simultaneously decreasing price. The steel cost can be reduced via decreasing the microalloying elements contents, although this decrease may undermine the mechanical properties. Multi-element microalloying with minor additions is the route to optimise steel composition and keep the properties high. However, this requires deep understanding of mutual effects of elements on each other's performance with respect to the development of microstructure and mechanical properties. This knowledge is insufficient at the moment. In the present work we investigate the microstructure and mechanical properties of bainitic steels microalloyed with Cr, Mo, Nb and V. Comparison of 0.2 wt. % Mo and Cr additions has shown a more pronounced effect of Mo on precipitation than on phase balance. Superior strength of the MoNbV-steel originated from the strong solid solution strengthening effect. Superior ductility of the CrNbV-steel corresponded to the more pronounced precipitation in this steel. Nature of these mechanisms is discussed.

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The incomplete transformation phenomenon in steel

Cited by 84 publications

References 54 publications

Ultra-fine Bainite Structure in Hypo-eutectoid Steels

Ultra-fine Bainite Structure in Hypo-eutectoid Steels

An In-Situ LSCM Study on Bainite Formation in a Fe-0.2C-1.5Mn-2.0Cr Alloy

Comparative Effect of Mo and Cr on Microstructure and Mechanical Properties in NbV-Microalloyed Bainitic Steels

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