Notch-Fatigue Properties of Advanced TRIP-Aided Bainitic Ferrite Steels

Yoshikawa, Nobuo; Kobayashi, Junya; Sugimoto, Koh‐ichi

doi:10.1007/s11661-012-1246-x

Cited by 31 publications

(28 citation statements)

References 14 publications

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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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“…Fatigue tests were carried out using a multi-type fatigue testing machine at 25°C, with a sinusoidal wave of 80 Hz. The stress ratio, defined as the ratio of minimum stress (r min ) to maximum stress (r max ) was R = 0.1 [18,19]. The fatigue limit was defined by the maximum value of the stress amplitude (r R = r max -r min ) without failure up to 1.0 9 10 7 cycles.…”

Section: Materials and Experimental Proceduresmentioning

confidence: 99%

“…Because TM steel also possesses superior mechanical properties such as toughness [15][16][17], fatigue strength [18,19], and hydrogen embrittlement resistance [20] as compared to conventional structural steel (Fig. 2), application of TM steel to automotive drivetrain components such as gears, drive shafts, CV joints, clutch plates, etc.…”

Section: Introductionmentioning

confidence: 99%

Microstructure and Mechanical Properties of a TRIP-Aided Martensitic Steel

Sugimoto

Srivastava²

2015

Metallogr. Microstruct. Anal.

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This paper deals with the microstructural and mechanical properties of a transformation-induced plasticity-aided martensitic (TM) steel that is expected to serve as an advanced structural steel for automotive applications. The microstructure consisted of a wide lath-martensitestructured matrix and a mixture of narrow lath-martensite and metastable retained austenite of 2-5 vol% (MA-like phase). When 1%Cr and 1%Cr-0.2%Mo were added into 0.2%C-1.5%Si-1.5%Mn steel to enhance its hardenability, the resultant TM steels achieved a superior cold formability, toughness, fatigue strength, and delayed fracture strength as compared to conventional structural steel such as SCM420. These enhanced mechanical properties were found to be mainly caused by (1) plastic relaxation of the stress concentration, which resulted from expansion strain on the strain-induced transformation of the metastable retained austenite, and (2) the presence of a large quantity of a finely dispersed MA-like phase, which suppressed crack initiation or void formation and subsequent void coalescence.

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“…Figure 12(a) shows the notch-tensile strength ratios of steels A-D and steels F-H as a function of LEl/ TEl, with those of 0.2%C-1.5%Si-1.5%Mn-0-1.0%Cr-0-0.2%Mo-0-1.5%Ni-0.05%Nb TBF steels. 17) In the figure, these notch-tensile strength ratios tend to increase with increasing LEl/TEl, although steel A exhibits a lower notchtensile strength ratio. So, it is considered that the higher notch-tensile strength ratios of steels B-D are caused by larger LEl/TEl values under a constant stress triaxiality factor, similar to steels F-H and the TBF steels.…”

Section: Low Notch-tensile Strength Ratios Of Steels B-dmentioning

confidence: 88%

“…As a result, steels B-D exhibit high notch-tensile strength ratios (NSR = TSN/TS), except for steels A and E, although these notch-tensile strength ratios are somewhat lower than those of steels F-H. 17) 3.3. Notch-Fatigue Limit and Notch Sensitivity for Fatigue Figure 8 shows stress amplitude-number of cycles (S-N) curves of steels A-E. Figure 9 shows the fatigue limits of smooth and notched specimens and the "notch sensitivity factor q", 18) defined by the following equation: (2) where Kf and Kt are the fatigue-notch factor (= FL/FLN) and stress concentration factor (1.7 in this study), respectively.…”

Section: Vickers Hardness and Tensile Propertiesmentioning

confidence: 96%

Notch-fatigue Strength of Advanced TRIP-aided Martensitic Steels

2013

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The notch-fatigue limit and notch sensitivity of 0.1-0.6%C-1.5%Si-1.5%Mn transformation-induced plasticity (TRIP)-aided martensitic steels (TM steels) were investigated for use as common rails in nextgeneration automotive diesel engines. Also, these properties were related to the microstructural and retained austenite characteristics. When TM steels containing 0.2% to 0.4% C were subjected to heat treatment for isothermal transformation at 50°C and subsequent partitioning at 250°C, the steels achieved much higher notch-fatigue limits and lower notch sensitivities than those of conventional 0.2-0.4%C-1.0%Cr-0.2%Mo structural steels. This was principally associated with (i) plastic relaxation of localized stress concentration as a result of strain-induced transformation of 3-5 vol% metastable retained austenite and (ii) a large amount of finely dispersed martensite-austenite phase along prior austenitic, packet and block boundaries, as well as (iii) a small amount of carbide only in the wide lath-martensite structure, which may contribute to making fatigue crack initiation and/or propagation difficult.KEY WORDS: notch-fatigue strength; notch sensitivity; ultrahigh-strength steel; TRIP-aided steel; martensite; retained austenite; M-A constituent.

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Notch-Fatigue Properties of Advanced TRIP-Aided Bainitic Ferrite Steels

Cited by 31 publications

References 14 publications

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Transferring Nanoscale Bainite Concept to Lower C Contents: A Perspective

Microstructure and Mechanical Properties of a TRIP-Aided Martensitic Steel

Notch-fatigue Strength of Advanced TRIP-aided Martensitic Steels

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