Добірка наукової літератури з теми "Nano-bainite"

ABSTRACTA commercial unalloyed ductile iron has been developed to produce a multiphase matrix microstructure consisting of lenticular prior martensite, feathery upper bainite and a nano-scaled super bainite of lath bainitic ferrite and carbon-enriched film retained austenite. Multi-step heat treatment composed of austenizing, rapidly quenching and isothermally holding at low temperature have been developed. A tensile strength of more than 1.6 GPa, a hardness higher than HRC 54, and an elongation in excess of 5%, are achieved. This is attributed to a synergistic multi-phase strengthening effect. The developed nano super bainite exhibits a good balance between strength and toughness. The presence of martensite formed during the quenching prior to the isothermal treatment, accelerates the kinetics of subsequent nano super bainitic transformation by bainitic laths nucleating quickly at the martensite-austenite interfaces.

The effect of composition and processing schedule on the microstructure of C-Mn-Si-Mo-(Al)-(Nb) steels containing nano-bainite was studied using transmission electron microscopy (TEM) and atom probe tomography (APT). The major phase formed in all steels was nano-bainite. However, the steels with lower carbon and alloying addition content subjected to TMP had better mechanical properties than high alloyed steel after isothermal treatment. The presence of ferrite in the microstructure can improve not only ductility but lead to the formation of retained austenite with optimum chemical stability.

This paper introduces the study and production on steel plate of Grade E550 in60mm thick by Thermo-mechanical processing (TMCP) and tempering for offshoreinfrastructure purpose. The steel plate obtained high strength and goodtoughness in both the base materials and the weld joints. Microstructure and precipitation characteristicswere examined in the plate is comprised of bainite with a small amount ofpre-eutectoid ferrite. Parallel arranged lath bainite characterizes thebainitic structure with high density of dislocations and precipitates. Carbides MxCy averaging 200nm indimension and MC type Carbonitrides in several nano to a dozen nano carbidesdistribute in the bainite ferrite and the bainte lath interfaces. Compatiblemechanical properties were achieved at welded joint. Microstructures were analysed in the welding heataffected zone (HAZ).

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.

Carbide free nano-structured bainitic steels typically have strength more than 2.0 GPa and impact toughness of 7 J or less. Most of this class of steels have sluggish kinetics and takes 16-72 h for complete bainitic transformation. The present work discusses key perspectives in developing carbide free nano-structured bainitic steel having strength more than 2.0 GPa and toughness more than 15 J. Further, ballistic evaluation of newly developed carbide free nano-structured bainitic steels having strength more than 2.0 GPa and high toughness of 20 J was carried out to understand the adaptability of these steels in combat vehicle applications. A comparison is made between newly developed high strength and tough carbide free nano-structured bainitic steels with typical martensite based ARMOX 500 class of armour steels. The developed nano-structured bainite showed ballistic performance much superior to ARMOX 500 steel. Monolithic plates of bainite provide complete protection against 7.62 AP projectiles at an areal density of 120 kgm-2. The ballistic efficiency of monolithic plates was further enhanced by using perforated geometrical configurations.

In order to match rapidly development of high strength low alloy steels, the new metal cored wire contained copper was designed. The multi-pass weld metals were obtained by gas metal arc welding. Results show that microstructure of weld metals is bainite with M-A constituents, and there are nano-9RCu precipitates after PWHT. The weld metal as welded achieves 737 MPa of yield strength (YS) and 1097 MPa of ultimate tensile strength (UTS), with an elongation 8%. After PWHT, the YS and UTS are 725 MPa and 968 MPa, respectively, and elongation increases to 16%. The lower dislocations density and reduction of Cu element at solid solution state hence for the decrease of YS. As well, the nano-Cu precipitates and reduction of effective grain size make it increase. The combined effect of these factors makes YS decrease by only 12 MPa after PWHT. Besides, a good impact performance (46.6 J) can be obtained after PWHT, while it is only 27.7 J of weld metals as welded. The improved toughness of weld metals after PWHT is obtained by the contributions of finer effective grain size, soft bainite, and dispersed nano-9RCu precipitation. The nano-Cu precipitates can improve strength without damage to toughness of weld metal after PWHT.

High-carbon nano bainitic steel is currently a hot research topic. The effect of the matrix’s carbon content and carbon atom distribution on the toughness of high-silicon, high-carbon bainitic steel is studied. The microstructure under an incomplete austenitization process consists of undissolved carbides, bainitic ferrite, and retained austenite. Using this process, the carbon content in bainitic ferrite is relatively low. Under the complete austenitization process, the carbon content in the bainite ferrite in the sample is high, and there is more retained austenite in the blocky type. The sample exhibits high impact toughness under an incomplete austenitization process, which is mainly affected by the low carbon content of bainite ferrite, high coordination ability of retained austenite, and high interface density of microstructure. The EBSD results show that the crack easily propagates between parallel bainite laths with low interface density compared with the high interface density perpendicular to the laths.

In situ neutron diffraction, transmission electron microscopy (TEM) and atom probe tomography (APT) have been used to study the early stages of bainite transformation in a 2 mass% Si nano-bainitic steel. It was observed that carbon redistribution between the bainitic ferrite and retained austenite at the early stages of the bainite transformation at low isothermal holding occurred in the following sequence: (i) formation of bainitic ferrite nuclei within carbon-depleted regions immediately after the beginning of isothermal treatment; (ii) carbon partitioning immediately after the formation of bainitic ferrite nuclei but substantial carbon diffusion only after 33 min of bainite isothermal holding; (iii) formation of the carbon-enriched remaining austenite in the vicinity of bainitic laths at the beginning of the transformation; (iv) segregation of carbon to the dislocations near the austenite/ferrite interface; and (v) homogeneous redistribution of carbon within the remaining austenite with the progress of the transformation and with the formation of bainitic ferrite colonies. Bainitic ferrite nucleated at internal defects or bainite/austenite interfaces as well as at the prior austenite grain boundary. Bainitic ferrite has been observed in the form of an individual layer, a colony of layers and a layer with sideplates at the early stages of transformation.

Les aciers nano-bainitiques représentent une nouvelle classe d’alliages, avec une microstructure bainitique nano-structurée obtenue à basse température avec une fraction importante d’austénite résiduelle conduisant à une limite d’élasticité et à une ductilité élevées. Récemment, il a été observé que l’addition d’éléments carburigènes comme le V et le Mo dans ces microstructures augmente la résistance à l’adoucissement ainsi que les propriétés mécaniques à moyennes températures. Ainsi l’étude de la précipitation des carbures secondaires dans les microstructures nano-bainitiques est nécessaire pour optimiser la microstructure et les traitements thermiques de revenu pour cette nouvelle classe d’aciers. Trois microstructures initiales ont été étudiées : martensite, martensite + austénite résiduelle et nano-bainite. L’étude de microstructures martensitiques plus conventionnelles a servi de base pour une meilleure compréhension des évolutions microstructurales dans la microstructure nano-bainitique. La précipitation pendant le revenu a été étudiée avec des techniques expérimentales : la dilatométrie, la diffraction des rayons X de haute énergie sur synchrotron et la microscopie électronique en transmission haute résolution. Il a été montré que les séquences de précipitation et de dissolution des carbures sont similaires pour les trois microstructures initiales. Les cinétiques sont similaires ainsi que la composition chimique de la cémentite et les distributions de taille de la cémentite et des carbures alliés. Nous avons aussi mis en évidence une importante stabilité de l’austénite résiduelle des trois microstructures. L’analyse des évolutions des paramètres de maille associé à des bilans de teneur en carbone ont permis de mieux comprendre sa distribution entre les carbures et les phases des matrices. Un modèle décrivant la germination et la croissance des précipités a été développé prenant en compte la précipitation des carbures secondaires, une cinétique de restauration basée sur les résultats expérimentaux et de nouvelles hypothèses comme le para-équilibre. Ce modèle prédit les cinétiques de précipitation mais aussi la densité de particules, les distributions de taille et la composition moyenne de la matrice et des carbures pour différentes conditions de revenu. Hormis la comparaison avec les résultats expérimentaux qui est discutée, le modèle a permis d’interpréter le comportement similaire des microstructures initiales au cours du revenu
Nano-bainitic steels represent a new class of alloys, whose microstructure consists of nanostructured bainitic ferrite formed at low temperature with a high amount of retained austenite leading to a high ductility and high tensile strength of the steel. Formation of nano-bainite has been studied thoroughly in literature as well as tempering of nano-bainitic steels. More recently it has been shown that adding carbide forming elements such as V and Mo increases the resistance to softening and hence the mechanical properties of nano-bainite at moderate temperature. Investigating the secondary carbide precipitation inside a nano-bainitic microstructure is thus necessary to optimize the thermal treatments for this promising new class of steels. Three initial microstructures of the same steel composition are investigated: martensite, martensite + retained austenite and nano-bainite. Studying the more conventional case of martensite has served as a basis to better understand the microstructure evolutions inside the nano-bainitic steel. The microstructural evolutions during the tempering were followed by complementary experimental techniques including dilatometry, in situ high energy synchrotron X-ray diffraction (HEXRD), conventional and high resolution TEM. The sequence of carbides precipitation and dissolution (transition-iron-carbides, cementite, and alloyed carbides) both during heating and holding is shown similar for the three initial microstructures. The kinetics are similar as well as cementite chemical composition and size distributions of cementite and alloyed carbides. It has been shown too that the three microstructures present a high retained austenite stability. Moreover, the analyses of the lattice parameters evolutions all along the tempering treatment associated with carbon mass balances have allowed to better understand the carbon distributions between carbides and matrix phases. The nucleation and growth model from a previous work was upgraded to take into account secondary precipitation and different new features (e.g. para-equilibrium interface condition for first stage of cementite growth, etc.). This model predicts the kinetics of precipitation, the particle densities and size distributions as well as matrix and carbides mean composition for different tempering conditions. Apart from the comparison with the experimental results that is discussed, it allowed to interpret the similar tempering behaviour for the three initial microstructures

Abstract The effect of different cooling processes on microstructure especially precipitation behavior and properties of high grade pipeline steel was investigated in this paper. The results showed that: By two stage controlled cooling process, ferrite-bainite phase was obtained in X80 pipeline steel with high deformability, the average size of Martensite and Austenitic islands (M/A) was approximately 1μm, the Uniform Elongation (UEl) was 10%. Whereas granular bainite was obtained after conventional laminar cooling process and fine M/A of 1∼2μm size dispersed in grains or along grain boundaries. Compared with conventional pipeline steel, high deformability pipeline steel through two stage controlled cooling process had higher Elongation (El), impact toughness and lower yield ratio, due to the existence of ferrie+bainite phase structure which reduced yield strength (YS) and enhanced deformation performance and fracture toughness. Additionally, the effect of precipitation strengthening of high grade pipeline steel produced by Ultra-fast cooling (UFC) process was superior to that produced by conventional laminar cooling process. A significant amount of nano particles precipitated with the size of 1nm to 5nm were obtained through the UFC process. The application of UFC made the precipitation process avoid the nose temperature of precipitation temperature time (PTT) curve, thus the size of nano particles were smaller and the distribution of precipitation was more dispersive.

To develop rails with higher hardness and thus better durability and longer life, alternative microstructures have been suggested, since conventional pearlitic rail steels have reached their hardness limit. Such a newly developed material has a fine bainite microstructure (coded J6 bainitic steel) and showed higher initial hardness but poorer on-site wear performance, compared to conventional pearlitic steels. This was explained by the fact that pearlitic steels show significant work hardening under severe stress conditions, even though their initial hardness was lower. In this work, the wear behavior of pearlitic and J6 bainitic rail steels was investigated at the micro/nano scale, using the nanoscratch technique. It was found that pearlitic steel shows better wear performance at the micro scale as well, in agreement with large scale rail field tests.