Improvement of strength-toughness combination in austempered low carbon bainitic steel: The key role of refining prior austenite grain size

Lan, Huifang; Du, Linxiu; Li, Q.; Qiu, Chunlin; Li, J.P.; Misra, R.D.K.

doi:10.1016/j.jallcom.2017.03.024

Cited by 71 publications

(25 citation statements)

References 27 publications

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“…However, it decreases significantly but filmy RA increases with increasing time from 120 to 240 min (see Figure c,d). This result is similar to what have been reported by some researchers and our previous work …”

Section: Resultssupporting

confidence: 94%

“…It has been reported that the growth mode of ferrite is initially interface‐controlled followed by diffusion‐controlled mechanism, during which a diffusion field of carbon atoms gradually develops in the remaining austenite area adjacent to the ferrite/prior austenite interfaces . During the isothermal treatment (stage D in Figure ), the BF forms by paraequilibrium nucleation from austenite boundaries.…”

Section: Resultsmentioning

confidence: 99%

“…Upon straining, strain‐induced RA into martensite ahead of the crack tends to close the crack by absorbing relatively more energy . Lan and Chen et al have suggested that mechanical stability of RA is determined by its size and morphology . Blocky RA is mechanically less stable compared to filmy RA, thus it would be favorable to be replaced by filmy RA as much as possible for being effective in ductility .…”

Section: Resultsmentioning

confidence: 99%

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Role of Prior Martensite in a 2.0 GPa Multiple Phase Steel

Cui

Northwood

Liu

2018

steel research int.

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A 55Mn2SiCr steel is developed by a novel multiple-step process, which involves austenitizing at 900 C for 30 min, rapid quenching to 210 C, then holding at 170 C for 5 min, and isothermally holding at 250 C for different times, and finally cooling in air. The mixed microstructure consists of lenticular prior martensite (PM), fine needle bainitic ferrite (BF), and filmy retained austenite (RA). The results show that the highest tensile strength of 2030 MPa with a bending strength of 4000 MPa is achieved at 250 C for 120 min. This is attributed to a synergistic multiphase strengthening effect. The presence of martensite formed during the quenching process prior to the isothermal treatment, accelerates the kinetics of subsequent nano-scaled super bainitic transformation by bainitic laths nucleating quickly at the martensite-austenite interfaces. The product of austenite fraction and its carbon content is found to be another important factor for controlling the strength. In addition, the phase evolution as well as carbon partitioning mechanism during isothermal treatment is discussed.

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Section: Resultssupporting

confidence: 94%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Role of Prior Martensite in a 2.0 GPa Multiple Phase Steel

Cui

Northwood

Liu

2018

steel research int.

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“…Increases in the yield strength of martensite have been attributed to both finer PAGS [8] and effective grain size (block size) [46]. Moreover, increases in the yield and tensile strength of bainite have been attributed to the refinement of PAGS, packet size, and block size [47]. As suggested by Hutchinson et al [48], most of the strength of martensite is probably due to segregated carbon atoms.…”

Section: Changes In the Mechanical Properties Of Uhss Imentioning

confidence: 96%

The Effect of Electroslag Remelting on the Microstructure and Mechanical Properties of CrNiMoWMnV Ultrahigh-Strength Steels

Ali

Porter

Kömi

et al. 2020

Metals

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The effect of electroslag remelting (ESR) with CaF 2 -based synthetic slag on the microstructure and mechanical properties of three as-quenched martensitic/martensitic-bainitic ultrahigh-strength steels with tensile strengths in the range of 1250-2000 MPa was investigated. Ingots were produced both without ESR, using induction furnace melting and casting, and with subsequent ESR. The cast ingots were forged at temperatures between 1100 and 950 • C and air cooled. Final microstructures were investigated using laser scanning confocal microscopy, field emission scanning electron microscopy, electron backscatter diffraction, electron probe microanalysis, X-ray diffraction, color etching, and micro-hardness measurements. Mechanical properties were investigated through measurement of hardness, tensile properties and Charpy-V impact toughness. The microstructures of the investigated steels were mainly auto-tempered martensite in addition to small fractions of retained austenite and bainite. Due to the consequences of subtle modifications in chemical composition, ESR had a considerable impact on the final microstructural features: Prior austenite grain, effective martensite grain, and lath sizes were refined by up to 52%, 38%, and 28%, respectively. Moreover, the 95th percentiles in the cumulative size distribution of the precipitates decreased by up to 18%. However, ESR had little, if any, the effect on microsegregation. The variable effects of ESR on mechanical properties and how they depend on the initial steel composition are discussed.The prior austenite grains are divided into packets, which consist of blocks of laths with the same habit plane. These blocks contain sub-blocks in which the laths have similar crystal orientations. The boundaries between packets and blocks have high-angle misorientations while the boundaries between sub-blocks and laths have low-angle misorientations [8][9][10].Strength and toughness properties can be improved simultaneously by grain refining, which can be achieved, for example, by the pinning of the austenite grain boundaries by fine precipitates, which are stable at high temperatures [11]. A high number density of fine precipitates leads to enhanced pinning of the grain boundaries, which subsequently leads to a reduction of the final prior austenite grain, packet, and block sizes and a significant effect on the mechanical properties of the steel. It was observed by Wang et al. [12] that the yield strength of 17CrNiMo6 martensitic steel is increased by 235 MPa by reducing the prior austenite grain size (PAGS) by 33% and the Charpy U-notch impact energy at 77 K was enhanced more than eight-fold.In addition to the above strengthening factors, additional strengthening can be achieved through precipitation. In the case of dislocation bowing between precipitates, the Orowan relationship [13] shows that the yield strength increment from precipitation increases with increasing volume fraction and decreasing size of the precipitates. In the case of tempered martensite, the optimum combinati...

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“…After tempering at 200 • C, the microstructure was transformed into ferrite, pearlite and the granular bainite left behind after normalizing (Figure 11b). Low temperature tempering refined bainite grains and introduced more higher-angle boundaries, which helps to enhance the wear resistance of the substrate [36]. As can be seen from Figure 11f, the M 23 C 6 phase was precipitated again, the M 3 C 2 phase and M 6 C phase were precipitated at the same time.…”

Section: Initial Microstructure Under As-received State (Ar)mentioning

confidence: 97%

Effects of Microstructure Evolution on Fretting Wear Behaviors of 25CrNi2MoVE Steel under Different Tempering States

Lai

et al. 2020

Metals

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Increasing load requirements and harsh operating conditions have worsened the wear of drive shafts in special field vehicles. In this paper, the evolution of the microstructure and fretting wear behaviors of 25CrNi2MoVE torsion shaft steel and their influence on the wear mechanisms were investigated as a function of tempering temperature. The results showed that the coarse grain size, low matrix hardness and non-metallic inclusions in the as-received state lead to a high wear rate and serious adhesive wear. The grain refinement after normalizing and the formed M 5 C 2 carbide and bainite helped to improve the wear resistance and worn surface quality. Low temperature tempering is conducive to further improve the wear resistance of normalized samples, and the wear rate and worn surface roughness are increased gradually after tempering temperature increases. For quenching, although martensite structure can achieve a lower wear rate, the coefficient of friction is much higher; the wear mechanisms are primarily fatigue wear and adhesive wear. Although the adhesive wear degree and worn surface roughness were increased, the optimal anti-wear performances are obtained under tempering at 350 • C with good continuity of the surface oxide film. Excessive tempering temperature will make the softened matrix unable to form a beneficial "third-body wear".

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Improvement of strength-toughness combination in austempered low carbon bainitic steel: The key role of refining prior austenite grain size

Cited by 71 publications

References 27 publications

Role of Prior Martensite in a 2.0 GPa Multiple Phase Steel

Role of Prior Martensite in a 2.0 GPa Multiple Phase Steel

The Effect of Electroslag Remelting on the Microstructure and Mechanical Properties of CrNiMoWMnV Ultrahigh-Strength Steels

Effects of Microstructure Evolution on Fretting Wear Behaviors of 25CrNi2MoVE Steel under Different Tempering States

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