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Oˇguz Gurkan Bilir - One of the best experts on this subject based on the ideXlab platform.

  • effect of annealing Temperature on Martensite Start Temperature in intercritical region
    2015
    Co-Authors: Ersoy Erisir, Oˇguz Gurkan Bilir
    Abstract:

    In dual-phase steels, partial austenization may result in different volume amount of austenite depending on intercritical annealing Temperature between Ac1 and Ac3 Temperatures. The annealing Temperature is also responsible of chemical composition and grain size of austenite. This study deals with the effect of intercritical annealing Temperature on Ms for dual-phase steels. In experimental studies using dilatometer, the specimens were first quenched to form Martensite from austenitizing Temperature of 1100°C. After martensitic transformation, specimens were annealed at different intercritical Temperatures and finally gas quenched again. Ms were measured with using dilatometer curves and compared to calculated values using empirical formulas. Grain sizes of final microstructures were also quantitatively analyzed. It was seen that Ms depend on the intercritical annealing Temperature. It is concluded that this double effect is attributed to intercritical annealing Temperature which is responsible for chemical composition and grain size of austenite.

  • effect of intercritical annealing Temperature on phase transformations in medium carbon dual phase steels
    Journal of Materials Engineering and Performance, 2014
    Co-Authors: Ersoy Erisir, Oˇguz Gurkan Bilir
    Abstract:

    This paper presents a study concerning phase transformations during quenching of a medium carbon dual phase steel using thermodynamic equilibrium calculations and dilatometry. Medium carbon steel was subjected to the intermediate quenching to produce a fine grained ferrite/Martensite dual phase steel. 4 samples quenched after intercritical annealing at 725, 730, 740, and 750 °C. Martensite-Start and bainite-Start Temperatures were calculated from dilatometric curves using plastodilotemeter. Experimental findings are supported by calculated phase diagrams and equilibrium phase compositions using ThermoCalc® and calculations from different empirical formulas. It is concluded that Martensite-Start Temperature depend on chemical composition and grain size of austenite.

Bruno C. Cooman - One of the best experts on this subject based on the ideXlab platform.

  • Influence of Martensite Fraction on the Stabilization of Austenite in Austenitic-Martensitic Stainless Steels
    'Springer Science and Business Media LLC', 2018
    Co-Authors: Ql Huang, Bruno C. Cooman, Biermann H, Mola J
    Abstract:

    The influence of Martensite fraction (f(alpha')) on the stabilization of austenite was studied by quench interruption below M-s Temperature of an Fe-13Cr-0.31C (mass pct) stainless steel. The interval between the quench interruption Temperature and the secondary Martensite Start Temperature, denoted as theta, was used to quantify the extent of austenite stabilization. In experiments with and without a reheating step subsequent to quench interruption, the variation of theta with f(alpha') showed a transition after transformation of almost half of the austenite. This trend was observed regardless of the solution annealing Temperature which influenced the Martensite Start Temperature. The transition in theta was ascribed to a change in the type of Martensite nucleation sites from austenite grain and twin boundaries at low f(alpha') to the faults near austenite-Martensite (A-M) boundaries at high f(alpha'). At low Temperatures, the local carbon enrichment of such boundaries was responsible for the enhanced stabilization at high f(alpha'). At high Temperatures, relevant to the quenching and partitioning processing, on the other hand, the pronounced stabilization at high f(alpha') was attributed to the uniform partitioning of the carbon stored at A-M boundaries into the austenite. Reduction in the fault density of austenite served as an auxiliary stabilization mechanism at high Temperatures. (C) The Minerals, Metals & Materials Society and ASM International 20161142sciescopu

  • influence of Martensite fraction on the stabilization of austenite in austenitic martensitic stainless steels
    Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2016
    Co-Authors: Qiuliang Huang, Horst Biermann, Bruno C. Cooman, Javad Mola
    Abstract:

    The influence of Martensite fraction (f α′) on the stabilization of austenite was studied by quench interruption below M s Temperature of an Fe-13Cr-0.31C (mass pct) stainless steel. The interval between the quench interruption Temperature and the secondary Martensite Start Temperature, denoted as θ, was used to quantify the extent of austenite stabilization. In experiments with and without a reheating step subsequent to quench interruption, the variation of θ with f α′ showed a transition after transformation of almost half of the austenite. This trend was observed regardless of the solution annealing Temperature which influenced the Martensite Start Temperature. The transition in θ was ascribed to a change in the type of Martensite nucleation sites from austenite grain and twin boundaries at low f α′ to the faults near austenite–Martensite (A–M) boundaries at high f α′. At low Temperatures, the local carbon enrichment of such boundaries was responsible for the enhanced stabilization at high f α′. At high Temperatures, relevant to the quenching and partitioning processing, on the other hand, the pronounced stabilization at high f α′ was attributed to the uniform partitioning of the carbon stored at A–M boundaries into the austenite. Reduction in the fault density of austenite served as an auxiliary stabilization mechanism at high Temperatures.

  • quenching and partitioning processing of transformable ferritic stainless steels
    Scripta Materialia, 2011
    Co-Authors: Javad Mola, Bruno C. Cooman
    Abstract:

    Austenite was stabilized at room Temperature in a low-interstitial 16%Cr transformable ferritic stainless steel by quenching from the intercritical annealing Temperature to below the Martensite Start Temperature of the intercritical austenite prior to the partitioning of interstitials C and N to the austenite at or above the quenching Temperature. Partitioning Temperatures as high as 500 °C were associated with austenite enrichment, indicating the suitability of quenching and partitioning processing for stainless steels.

Gunther Eggeler - One of the best experts on this subject based on the ideXlab platform.

  • Martensite aging avenue to new high Temperature shape memory alloys
    Acta Materialia, 2015
    Co-Authors: Thomas Niendorf, Gunther Eggeler, P Kroos, Christoph Somsen, Yuri Chumlyakov, H J Maier
    Abstract:

    Abstract High-Temperature shape memory alloys are attractive for efficient solid state actuation. A key criterion for shape memory alloys is the Martensite Start Temperature. The current study introduces a concept for increasing this Temperature of alloys initially not suited for high-Temperature actuation. Aging of stress-induced Martensite, referred to as SIM-aging in the current work, is able to increase the Martensite Start Temperature by about 130 °C as demonstrated in the present study for a Co–Ni–Ga shape memory alloy. The increase of transformation Temperatures can be explained based on the concept of symmetry-conforming short-range order. Following SIM-aging the Co–Ni–Ga alloy shows cyclic actuation stability at elevated Temperatures. While Martensite aging has always been viewed as detrimental in the past, it can actually be exploited to design new classes of high-Temperature shape memory alloys with excellent properties.

  • On the effect of alloy composition on Martensite Start Temperatures and latent heats in Ni–Ti-based shape memory alloys
    Acta Materialia, 2015
    Co-Authors: Jan Frenzel, I. Opahle, Burkhard Maas, Björn Maaß, André Wieczorek, Ralf Drautz, Anna Wieczorek, Gunther Eggeler
    Abstract:

    In the present work we explain the concentration dependence of the Martensite Start Temperature (MS) in Ni–Ti-based shape memory alloys (SMAs). We briefly review the present level of understanding and show that there is a need for further work. We then investigate the strong dependence of MS on alloy composition in binary Ni–Ti, ternary Ni–Ti–X (X=Cr, Cu, Hf, Pd, V, Zr) and quaternary Ni–Ti–Cu–Y (Y=Co, Pd) SMAs. For binary Ni–Ti, we combine differential scanning calorimetry experiments with insight gained through the application of the density functional theory (DFT) to show that heats of transformation ?H decrease as Ni concentrations increase from 50.0 to 51.2at.%. This causes a shift in the Gibbs free energy curves of austenite GA(T) and Martensite GM(T), which in turn results in a lower MS Temperature. Our DFT results suggest that the strong decrease of ?H is caused by a stabilization of the B2 phase by structural relaxations around Ni antisite atoms, together with a gradual destabilization of B19?. The Martensite Start Temperatures and the latent heats of transformation for binary, ternary and quaternary Ni–Ti-based SMAs are closely related. We observe smaller latent heats when the geometrical differences between the crystal structures of austenite and Martensite decrease.

H K D H Bhadeshia - One of the best experts on this subject based on the ideXlab platform.

  • powder metallurgical nanostructured medium carbon bainitic steel kinetics structure and in situ thermal stability studies
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2012
    Co-Authors: I Lonardelli, Mauro Bortolotti, W Van Beek, Luca Girardini, Mario Zadra, H K D H Bhadeshia
    Abstract:

    Abstract It has been possible to produce incredibly fine plates of bainitic ferrite separated by a percolating network of retained austenite in a medium carbon steel produced by mechanical alloying followed by spark plasma sintering and isothermal heat treatment. This is because the sintering process limits the growth of the austenite grains to such an extent that the Martensite-Start Temperature is suppressed in spite of the medium carbon concentration. Furthermore, the fine austenite grain size accelerates the bainite transformation, which can therefore be suppressed to low Temperatures to obtain a nanostructure. Microscopy and in situ synchrotron X-ray diffraction were used to investigate the morphology and the thermal stability of the retained austenite during continuous heating. These latter experiments revealed a gradient of carbon concentration in the retained austenite and a reduced thermal stability in high carbon film-austenite. It was also possible to correlate the evolution of defect density and carbon depletion in both retained austenite and bainitic ferrite during tempering.

  • uncertainties in dilatometric determination of Martensite Start Temperature
    Materials Science and Technology, 2007
    Co-Authors: Hong-seok Yang, H K D H Bhadeshia
    Abstract:

    AbstractMethods for the dilatometric determination of the Martensite Start Temperature of steels are discussed, with emphasis on noise in the experimental data. The methods are tested on a new set of experiments using a specially prepared steel. Relying on the first detection of expansion due to transformation is fraught with difficulties. Instead, an offset method is proposed which should enable independent investigators to reach the same conclusions given identical data. The technique at the same time preserves the notion that the early stages of Martensite formation correspond to the Start Temperature. The uncertainty in the Start Temperature deduced using this method is about ± 12°C, better than the reported values of noise in published data. The work underlines the need to state the sensitivity of the analysis technique when quoting transformation Temperatures.

Kohichi Sugimoto - One of the best experts on this subject based on the ideXlab platform.

  • formability of al nb bearing ultra high strength trip aided sheet steels with bainitic ferrite and or Martensite matrix
    Isij International, 2010
    Co-Authors: Kohichi Sugimoto, Muneo Murata, Sungmoo Song
    Abstract:

    Microstructure, tensile properties and stretch-flangeability of 980–1470 MPa grade Al or Al–Nb bearing TRIP-aided cold-rolled sheet steels with bainitic ferrite and/or Martensite matrix microstructure (TBF steels) were investigated for automotive applications such as impact member reinforcements, sheet flames and so on. In addition, these properties were related with the microstructure and the retained austenite characteristics. Complex additions of 0.5% Al and 0.05% Nb into a base steel with chemical composition of 0.2% C, 1.5% Si and 1.5% Mn (in mass%) significantly enhanced the total elongation and stretch-flangeability, especially when austempered at Temperatures below Martensite-Start Temperature. The excellent stretch-flangeability was primarily associated with (i) refined prior austenitic grain by NbC precipitates and (ii) uniform fine mixed matrix microstructure of bainitic ferrite and Martensite, as well as (iii) TRIP effect of metastable retained austenite.

  • ductility of 0 1 0 6c 1 5si 1 5mn ultra high strength trip aided sheet steels with bainitic ferrite matrix
    Isij International, 2004
    Co-Authors: Kohichi Sugimoto, Michitaka Tsunezawa, Tomohiko Hojo, Shushi Ikeda
    Abstract:

    The effects of heat treatment and forming conditions on retained austenite characteristics and ductility of 0.1-0.6C-1.5Si-1.5Mn, mass%, ultra high-strength TRIP-aided sheet steels with bainitic ferrite matrix were investigated. These steels possessed large total elongations of about 20-25 % in a tensile strength ranging from 700 to 1 300 MPa when austempered at Temperatures above Martensite-Start Temperature (M S ). The total elongations were enhanced by warm forming at two Temperatures, T P 1 and T P 2 . The first peak forming Temperatures T P 1 s were between 0°C and 75°C and were nearly constant regardless of carbon content of the steels. This was associated with the strain-induced Martensite transformation of a large amount of metastable retained austenite which suppressed a rapid fall of strain-hardening rate in an early strain range to resultantly increase the uniform and total elongations. On the other hand, the second peak forming Temperatures T P 2 s were between 200 and 300°C and further large total elongations beyond 30% were achieved in high carbon steels (0.4% C and 0.6% C steels) with tensile strength of 1 300-1 500 MPa. The large improvement was controlled by both the strain-induced bainite transformation and dynamic strain aging.

  • effects of retained austenite parameters on warm stretch flangeability in trip aided dual phase sheet steels
    Isij International, 1999
    Co-Authors: Kohichi Sugimoto, Mitsuyuki Kobayashi, Akihiko Nagasaka, Shunichi Hashimoto
    Abstract:

    Effects of volume fraction and carbon concentration of retained austenite on warm stretch-flangeability in high-strength TRIP-aided dual-phase (TDP) sheet steels with different silicon and manganese contents were investigated. A significant improvement of the stretch-flangeability was obtained by warm hole-punching at Temperatures between 150 and 200°C and the successive hole-expanding at Temperatures between 50 and 200°C, relating to Martensite-Start Temperature of the retained austenite. The warm stretch-flangeability was affected by carbon concentration of the retained austenite rather than by the volume fraction of retained austenite. Namely, the higher the carbon concentration of the retaind austenite, the larger the hole-expanding ratio of the steel. Such a large hole-expanding ratio was resulting from the following two reasons; (1) smaller surface damage and a large amount of retained austenite untransformed on hole-punching and (2) Iarge localized ductility due to the TRIP effect on hole-expanding.

  • effects of volume fraction and stability of retained austenite on ductility of trip aided dual phase steels
    Isij International, 1992
    Co-Authors: Kohichi Sugimoto, Noboru Usui, Mitsuyuki Kobayashi, Shunichi Hashimoto
    Abstract:

    The effects of silicon and manganese contents on volume fraction and stability of retained austenite particles in 0.2C-(1.0-2.5)Si-(1.0-2.5)Mn (mass%) TRIP-aided dual-phase steels were investigated. In addition, the relationships between above retained austenite parameters and ductility at room and moderate Temperatures were discussed through studies on strain-induced transformation behavior of retained austenite.As increasing the silicon and manganese contents except for 2.5 mass% manganese steel, the initial volume fraction of retained austenite increased with accompanied by reducing carbon concentration in retained austenite. It was found that the ductilities of these steels became maximum at a given Temperature between 23 and 175°C, i.e., a peak Temperature. The peak Temperature was concluded to agree well with the Temperature at which the strain-induced transformation of retained austenite was suppressed moderately for each steel. Moreover, the peak Temperature Tp (°C) was related to estimated Martensite-Start Temperature Ms (°C) of the retained austenite as Tp=3.04Ms+187. Strength-ductility balance, i.e., the product of tensile strength and total elongation, at the peak Temperature linearly increased with an increase in the initial volume fraction of retained austenite.