On mixed upper bainite-martensite in an AISI 4330 steel exhibiting an uncommonly improved strength-toughness combination

Rao, T. Venkat Narayana; Dikshit, S. N.; Malakondaiah, G.; Rao, P. Rama

doi:10.1016/0956-716x(90)90350-p

Cited by 17 publications

(9 citation statements)

References 9 publications

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“…For example, previous TEM observations have revealed that, oil-quenched 300 M steel showed relatively lengthy martensitic laths in contrast to the isothermally transformed bainitic laths of the same material [36]. However, our findings differ from the mechanisms suggested by those previous studies, in which the martensite grain refining was attributed to partitioning of austenite grains by pre-formed bainitic ferrite [3,[11][12][13][14][15]37]. If such a mechanism were true in the current experiments, the steel would have shown lengthy bainitic ferrite laths or plates to partition the austenite grains.…”

Section: Hardening Mechanism In Austempering Treatmentcontrasting

confidence: 57%

“…When an austempering treatment is applied, wherein the austenised steel is firstly soaked for a short period in a salt bath of lower-bainite transformation and is subsequently quenched to room temperature, the resultant microstructure is a mixture of bainite, martensite and retained austenite [3,[12][13][14][15][16][17]. The new heat treatments were reported to bring about higher hardness and strength properties than the QT treatments, while the strengthening mechanisms have not been fully understood up to date.…”

Section: Introductionmentioning

confidence: 99%

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Effect of Austempering Time on the Microstructure and Carbon Partitioning of Ultrahigh Strength Steel 56NiCrMoV7

Luo

Kitchen

Abubakri

2017

Metals

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Ultrahigh strength steel 56NiCrMoV7 was austempered at 270 • C for different durations in order to investigate the microstructure evolution, carbon partitioning behaviour and hardness property. Detailed microstructure has been characterised using optical microscopy and field emission gun scanning electron microscopy. A newly developed X-ray diffraction method has been employed to dissolve the bainitic/martensitic ferrite phase as two sub-phases of different tetragonal ratios, which provides quantitative analyses of the carbon partitioning between the resultant ferrites and the retained austenite. The results show that, a short-term austempering treatment was in the incubation period of the bainite transformation, which resulted in maximum hardness being equivalent to the oil-quenching treatment. The associated microstructure comprises fine carbide-free martensitic and bainitic ferrites of supersaturated carbon contents as well as carbon-rich retained austenite. In particular, the short-term austempering treatment helped prevent the formation of lengthy martensitic laths as those being found in the microstructure of oil-quenched sample. When the austempering time was increased from 20 to 80 min, progressive decrease of the hardness was associated with the evolution of the microstructure, including progressive coarsening of bainitic ferrite, carbide precipitating inside high-carbon bainitic ferrite and its subsequent decarbonisation.

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Section: Hardening Mechanism In Austempering Treatmentcontrasting

confidence: 57%

Section: Introductionmentioning

confidence: 99%

Effect of Austempering Time on the Microstructure and Carbon Partitioning of Ultrahigh Strength Steel 56NiCrMoV7

Luo

Kitchen

Abubakri

2017

Metals

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“…On the other hand the mixed microstructure of upper bainite -martensite significantly lowers mechanical properties compared to the martensitic microstructures [12][13][14] . Contrary to the Tomita investigation, Rao et al 15 suggested that the presence of the upper bainite in the mixed microstructure in an AISI 4330V led to a significant improvement in toughness without affecting the strength of the fully martensitic microstructure. However no beneficial effect of the lower bainite on mechanical properties was observed in the lower bainite-martensite microstructure.…”

Section: Introductionmentioning

confidence: 77%

Effect of Austenitizing Temperature on Mechanical Properties of the Mixed Bainite - Martensite Microstructure in CrMoV Steel

2017

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The effect of austenitizing temperature on mechanical properties of the mixed bainite -martensite microstructure in CrMoV steel was studied in the present work. The result showed that at low austenitizing temperature (910°C -1000°C), the mixed microstructures containing 12-28% volume fraction of lower bainite showed higher yield and tensile strength than fully martensitic microstructure. The partitioning of the prior austenite grain by lower bainite was found to cause a refinement of the martensite packet size. In addition the strength of the lower bainite in the mixed microstructure is enhanced by plastic constraint induced by the surrounding stronger martensite. By increasing the austenitizing temperature from 1000°C to 1200°C (40min), the YS, UTS, %EL, %RA and CVN impact energy decreased for all samples. This is attributed mainly to the increase in austenite grain size and width of bainite sheaves.

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“…In a high-strength low-alloy steel conforming to AISI 4330 designation and additionally containing 0.12 wt.% vanadium, interaction processes involving V, P and N (V-P and V-N) 33 ( Fig. 5) were believed to be at least one of the explanations for the observed uncommon beneficial effect of upper bainite, leading to an improved strength-toughness combination in the upper bainite-martensite mixed microstructure.…”

Section: Effect Of Heat Treatment Microstructurementioning

confidence: 96%

Grain boundary segregation and fracture resistance of engineering steels

Misra

2001

Surface & Interface Analysis

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The developments in surface analytical techniques and experimental measurements have made it possible to understand grain boundary segregation processes and related fracture mechanisms. This article shows how experimental contributions have led to the concept of grain boundary segregation isotherms for a complete understanding of grain boundary segregation behavior, and its effect on material properties in terms of the kinetics of segregation, the interaction processes amongst trace and solute elements and the influence of solute elements, heat treatment practice and applied tensile stress on grain boundary segregation processes. The determining role of grain boundary chemistry in obtaining a marked improvement in toughness of engineering steels at the specified levels of strength is elucidated here.

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On mixed upper bainite-martensite in an AISI 4330 steel exhibiting an uncommonly improved strength-toughness combination

Cited by 17 publications

References 9 publications

Effect of Austempering Time on the Microstructure and Carbon Partitioning of Ultrahigh Strength Steel 56NiCrMoV7

Effect of Austempering Time on the Microstructure and Carbon Partitioning of Ultrahigh Strength Steel 56NiCrMoV7

Effect of Austenitizing Temperature on Mechanical Properties of the Mixed Bainite - Martensite Microstructure in CrMoV Steel

Grain boundary segregation and fracture resistance of engineering steels

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