Showing papers on "Bainite published in 2016"

Effect of Prior Austenite Grain Size Refinement by Thermal Cycling on the Microstructural Features of As-Quenched Lath Martensite

Abstract: Current trends in steels are focusing on refined martensitic microstructures to obtain high strength and toughness. An interesting manner to reduce the size of martensitic substructure is by reducing the size of the prior austenite grain (PAG). This work analyzes the effect of PAGS refinement by thermal cycling on different microstructural features of as-quenched lath martensite in a 0.3C-1.6Si-3.5Mn (wt pct) steel. The application of thermal cycling is found to lead to a refinement of the martensitic microstructures and to an increase of the density of high misorientation angle boundaries after quenching; these are commonly discussed to be key structural parameters affecting strength. Moreover, results show that as the PAGS is reduced, the volume fraction of retained austenite increases, carbides are refined and the concentration of carbon in solid solution as well as the dislocation density in martensite increase. All these microstructural modifications are related with the manner in which martensite forms from different prior austenite conditions, influenced by the PAGS.

Mechanisms of Diffusional Phase Transformations in Metals and Alloys

Abstract: Applied Thermodynamics Free Energy-Composition Relationships for Binary Substitutional Solid Solutions Free Energy-Composition Diagram and Applications to Driving Force Calculations Thermodynamics of Interstitial Solid Solutions through Application to the Proeutectoid Ferrite Reaction in Fe-C Alloys Diffusional Nucleation in Solid-Solid Transformations Introduction through Qualitative General Statements Brief Comparative Survey of Nucleation in the Four Basic Types of Phase Transformation Outline of Approach for Development of Nucleation Theory Proof that the Equilibrium Concentration of Critical Nuclei Is Proportional to exp(_DG*=kT) Fictitious Equilibrium Nucleation Rate Derivation of Steady-State Nucleation Rate Estimation of b* Time-Dependent Nucleation Rate Feder et al.'s Treatment of t Time-Dependent Nucleation Rate for Homogeneous Nucleation with Isotropic g Ancillary Parameters Preliminary Consideration of the Approximation for f DGvpW Nonclassical Nucleation Theory Modifications of Homogeneous Nucleation Kinetics by Anisotropic Interfacial Energy Nucleation Kinetics at the Faces of Disordered Grain Boundaries Comparative Nucleation Kinetics at Grain Faces, Edges, and Corners Relative to Homogeneous Nucleation: Trade-Offs between N and DG* When gab Is Isotropic Nucleation at Dislocations Comparisons of Theory and Experiment Diffusional Growth Basic Differences between Diffusional Nucleation and Diffusional Growth A General Theory of Precipitate Morphology Disordered Interphase Boundaries Partially and Fully Coherent Interphase Boundaries Relative Growth Kinetics of Disordered and Partially Coherent Interphase Boundaries Precipitation Introduction Metastable Equilibrium Phase Boundaries GP Zones Transition Phases Nucleation Sites Successive Reactions Involving Different Phases Precipitate Free Zones Coarsening (Ostwald Ripening) Overall Evolution of the Microstructure Massive Transformation Definition and History Phase Diagrams Thermodynamics Overall Reaction Kinetics and the Existence Range Nucleation of Massive Transformation Growth Kinetics Interfacial Structure, Habit Planes, Orientation Relationships, and Growth Mechanisms Note on the Driving Force for Trans-Interphase Boundary Diffusion during Massive Transformation in a Two-Phase Field Cellular Reaction Definition and Introduction Systematics of Cellular Reactions Nucleation of Cellular Reactions Growth Kinetics of Cells Pearlite Reaction Systematics Crystallography, Nucleation, and Growth Mechanisms Edgewise Growth Kinetics of Pearlite Martensitic Transformations Definition Salient Characteristics (Described Briefly) Thermodynamics of Martensite Transformation Overall Kinetics of Martensite Transformation Nucleation of Martensite Crystallography and Growth (or Propagation) of Martensite Bainite Reaction and Role of Shear in Diffusional Phase Transformations Introduction Three Definitions of Bainite Upper Bainite vs. Lower Bainite, and Inverse Bainite Sources of Carbide Precipitation References Index

Effect of quenching temperature on martensite multi-level microstructures and properties of strength and toughness in 20CrNi2Mo steel

Abstract: The martensite multi-level microstructures of 20CrNi2Mo steel, which were quenched at the different temperatures of 900–1200 °C and tempered at 200 °C, were investigated by optical microscope (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM), and the relationship between the microstructures and properties of strength and toughness was discussed by the classic formula of Hall–Petch. The results show that the size of prior austenite grain ( d r ), martensite packet ( d p ) and block ( d b ) increase with increasing of the quenching temperature, while the martensite lath ( d l ) size is opposite. On another hand, the confusion degree of the martensite packets changes from disorder to order. The boundaries of prior austenite grain, packet, block and the martensite lath are high angle boundaries (HBs) and low angle boundaries (LBs), respectively, and the ratio of the low angle boundaries increase with the quenching temperature by calculating to the multi-level microstructure size with the mathematical model established by myself. In addition, the relationship between the packet/block and strength follows the classical formula of Hall–Petch, and the size of d b is far lower than the size of d p , d b is the effective control unit of the strength. Meanwhile, d l is the effective control unit of toughness because it strongly impacts the crack initiation and propagation and follows also the Hall-Petch with toughness in 20CrNi2Mo steel.

Iterative Determination of the Orientation Relationship Between Austenite and Martensite from a Large Amount of Grain Pair Misorientations

Abstract: An automatic, iterative method to determine the orientation relationship between parent austenite and martensite is described. The algorithm generates the orientation relationship from grain boundary misorientations through an iterative procedure based on correct symmetry operator assignment. The automatic method is demonstrated to work on both martensitic and bainitic steels and to provide comparable results to a manual grain selection method.

Effect of Prior Athermal Martensite on the Isothermal Transformation Kinetics Below M s in a Low-C High-Si Steel

Abstract: Thermomechanical processing of Advanced Multiphase High Strength Steels often includes isothermal treatments around the martensite start temperature (M s). It has been reported that the presence of martensite formed prior to these isothermal treatments accelerates the kinetics of the subsequent transformation. This kinetic effect is commonly attributed to the creation of potential nucleation sites at martensite-austenite interfaces. The aim of this study is to determine qualitatively and quantitatively the effect of a small volume fraction of martensite on the nucleation kinetics of the subsequent transformation. For this purpose, dilatometry experiments were performed at different temperatures above and below the M s temperature for athermal martensite in a low-carbon high-silicon steel. Microstructural analysis led to the identification of the isothermal decomposition product formed above and below M s as bainitic ferrite. The analysis of the transformation processes demonstrated that the initial stage of formation of bainitic ferrite at heat treatments below M s is at least two orders of magnitude faster than above M s due to the presence of martensite.

Acicular Ferrite Formation and Its Influencing Factors - A Review

Abstract: Acicular ferrite is a microstructure nucleating intergranularly on non-metallic inclusions and forming an arrangement of fine, interlocking grains. This structure is known to improve steel properties, especially steel toughness, essentially. The formation of acicular ferrite is mainly affected by steel composition, cooling rate, inclusion landscape and austenite grain size. In recent decades, extensive research has been conducted to investigate these factors. The present paper provides an overview of the impact of published results and the state of knowledge regarding acicular ferrite formation. Special attention is paid to the effect of carbon, manganese and titanium addition to steel, as well as the optimum size, number and composition of non-metallic inclusions. In addition, the reactions during the nucleation and growth of acicular ferrite needles are briefly addressed. Further, characteristics of acicular ferrite and bainite are summarized, which should help to distinguish these similar structures.

Effect of microstructural types on toughness and microstructural optimization of ultra-heavy steel plate: EBSD analysis and microscopic fracture mechanism

Abstract: The uniformity of toughness along thickness direction has long been the critical problem for producing ultra-heavy steel plate. To clarify the rule and mechanism of the influence of microstructures on toughness, different microstructures and their mixtures have been obtained through various heat treatment processes. The microstructures and substructures were characterized by means of optical microscope and transmission electron microscope. Furthermore, the correlations between misorientation, grain size, microscopic fracture propagation and toughness have been studied in detail using electron backscatter diffraction. Results suggest that after tempering, lath bainite can achieve better toughness property than martensite, whereas granular bainite is detrimental for toughness. Note that firstly generated lath bainite can effectively refine subsequent martensitic packets and blocks, and increase barriers for fracture propagation. Compared with single-phase martensite microstructure, the mixture of martensite+bainite obtains finer substructure and more percentage of large misorientation, which are favorable for hindering the propagation of microcrack, meanwhile it is an ideal microstructural type to achieve the optimal combination of toughness and strength. Besides, functional mechanism of packet boundaries and block boundaries for hindering crack propagation is different, as it is more difficult for crack propagation to bridge between different lattice planes than between different crystallographic orientations.

Ultrahigh strength-toughness combination in Bainitic rail steel: The determining role of austenite stability during tempering

Abstract: We elucidate here the impact of tempering on the stability of retained austenite (RA) in obtaining excellent combination of strength and toughness in an ultrahigh-strength low carbon air-cooled bainitic rail steels. The microstructure of rail steels comprises of bainite, martensite and martensite/austenite (M/A) constituents. The bainitic rail steels exhibit excellent strength and toughness combination after tempering at 280 °C (ultimate tensile strength: 1388 MPa; total elongation: 16%; U-notch impact toughness at 20 °C: 130 J/cm2). The filmy RA in the tempered microstructure has high mechanical stability, which ensures retention of superior mechanical properties of bainitic steels. The study underscores stability of filmy retained austenite obtained via tempering is the underlying reason for superior mechanical properties.

On the synergy of diffusible hydrogen content and hydrogen diffusivity in the mechanical degradation of laboratory cast Fe-C alloys

Abstract: The present work investigates the influence of hydrogen on the mechanical properties of generic Fe-C alloys by tensile tests on notched samples. Different microstructures, such as pearlite, bainite and martensite are generated in a 0.2%C Fe-C alloy by an appropriate heat treatment. “Pure” iron is used as a reference material and a variation in the carbon content up to 0.4% is established for the bainitic grade. The effect of hydrogen is demonstrated by mechanical tests on both in-situ hydrogen charged and uncharged specimens. At high cross-head deformation speed (5 mm/min), the results indicate a considerable, though variable hydrogen effect for different microstructures. The bainitic and martensitic materials both show ductility drops of about 20%, whereas the pearlitic and ferritic grades display a higher sensitivity to hydrogen embrittlement (HE) with a ductility loss of approximately 50%. In order to evaluate the role of the diffusible hydrogen, tensile tests are performed at a lower cross-head deformation speed (0.05 mm/min) as well. Next to the correlation between the amount of diffusible hydrogen and HE, the distance over which hydrogen can diffuse during a tensile test, determined by hydrogen diffusion coefficient, seems to play a crucial role as well.

Morphology and distribution of martensite in dual phase (DP980) steel and its relation to the multiscale mechanical behavior

Abstract: Among generations of advanced high-strength steel alloys, dual-phase steels exhibit a unique combination of strength and formability making them excellent candidates for use in the automotive industry. In this study, we seek to establish a relation between mechanical properties and microstructure of DP980. Electron backscatter diffraction (EBSD)and nanoindentation are used to identify and characterize martensite and ferrite phases. Spatial distributions of martensite and ferrite phases of subjected to various annealing treatments are found using a 2-point correlation function. Micro- and macro-mechanical properties are measured with nanoindentation, Vickers hardness and tensile tests and the results are used to determine the relation between martensite and ferrite phases and the strength of the metal. During the annealing/recovery process, the strength of the martensite phase decreases, the dislocation structure relaxes in the phase boundary region of the ferrite, and the martensite alignment along the rolling direction decreases resulting in the observed metal strength reduction. It is also shown that the higher the annealing temperature, the more homogeneous and equiaxed the distribution of martensite.

Characterization of the multi-pass weld metal and the impact of retained austenite obtained through intercritical heat treatment on low temperature toughness

Abstract: In this study, we explore the relationship between the microstructure and low temperature toughness of weld metal obtained from a real multi-pass weld joint (up to 55 mm) by submerged arc welding, which was developed for high strength (yield strength over 550 MPa) and heavy wall pipe fitting applications with composition of 0.1 wt% C, 2.0 wt% Mn and other micro-alloys. The study indicated that the necklace-type coarse martensite–austenite (M–A) constituent formed in the interlayer heat affected zone (IHAZ) of weld metal was responsible for low impact energy of 39 J at −40 °C. To enhance the toughness, conventional tempering and new intercritical heat treatment were designed. The results suggested that there was insignificant effect on toughness through conventional tempering, but obvious improvement through combination of quenching plus intercritical annealing and tempering. The impact energy was increased to ~98 J. The microstructure that benefit toughness primarily comprised of intercritical lath-like ferrite or acicular-type ferrite, bainite/martensite, and fine acicular retained austenite, with average size of ~0.3 μm. Retained austenite with volume fraction of ~6% was formed by the enrichment of Mn and Ni in reversed austenite during intercritical tempering process.

In-situ study of the deformation-induced rotation and transformation of retained austenite in a low-carbon steel treated by the quenching and partitioning process

Abstract: We report an in-situ study of the deformation-induced rotation and transformation of austenite grains in a low-carbon steel treated by the quenching and partitioning process using electron back-scattered diffraction and uniaxial tension experiments. It was found that retained austenite could be classified into four types according to different locations in the microstructure: retained austenite at triple edges, twinned austenite, retained austenite distributed between martensite and retained austenite embedded completely in a single ferrite. The results showed that at the early stage of deformation, the retained austenite at the triple edges and twinned austenite transformed easily, while the retained austenite at the boundaries between martensite and that embedded completely in a single ferrite rotated with no transformation; and did not transform until a large deformation was provided. This phenomenon implies that the retained austenite at the boundaries between martensite and that embedded completely in a single ferrite are more capable of resisting deformation. From the observations of Schmid factor maps and the texture of retained austenite, it can be concluded that the rotation of retained austenite followed a particular slip plane and slip direction. Moreover, the rotation of retained austenite could improve the ductility of the material. In comparison with the film-like retained austenite distributed between martensite, the retained austenite embedded completely in a single ferrite could resist a larger rotation angle, i.e. the latter could contribute more to the ductility of the steel. In addition, from the analysis of kernel average misorientation that the strain distribution mainly concentrated near the α − γ phase boundaries and in the interior of martensite, and the rotation angles and dislocation density of austenite increase with increasing strain.

In situ synchrotron X-ray diffraction studies of the effect of microstructure on tensile behavior and retained austenite stability of thermo-mechanically processed transformation induced plasticity steel

Abstract: Transmission electron microscopy and in situ synchrotron high-energy X-ray diffraction were used to investigate the martensitic transformation and lattice strains under uniaxial tensile loading of Fe-Mn-Si-C-Nb-Mo-Al Transformation Induced Plasticity (TRIP) steel subjected to different thermo-mechanical processing schedules. In contrast with most of the diffraction analysis of TRIP steels reported previously, the diffraction peaks from the martensite phase were separated from the peaks of the ferrite-bainite α-matrix. The volume fraction of retained γ-austenite, as well as the lattice strain, were determined from the diffraction patterns recorded during tensile deformation. Although significant austenite to martensite transformation starts around the macroscopic yield stress, some austenite grains had already experienced martensitic transformation. Hooke’s Law was used to calculate the phase stress of each phase from their lattice strain. The ferrite-bainite α-matrix was observed to yield earlier than austenite and martensite. The discrepancy between integrated phase stresses and experimental macroscopic stress is about 300 MPa. A small increase in carbon concentration in retained austenite at the early stage of deformation was detected, but with further straining a continuous slight decrease in carbon content occurred, indicating that mechanical stability factors, such as grain size, morphology and orientation of the retained austenite, played an important role during the retained austenite to martensite transformation.

A study on the relationship between microstructure and mechanical properties of acicular ferrite and upper bainite

Abstract: Acicular ferrite and bainite are microstructural constituents commonly found in ferritic weld metal and many other ferrous alloys. These often highly localised microstructural volumes are known to have a significant impact on the mechanical properties of the bulk material they constitute. It is well established that acicular ferrite and bainite have substantially different morphologies and features that can be directly linked to their intrinsic mechanical properties. However, in the bulk material they usually co-exist and interact with other microstructural constituents and features. Therefore, the individual contributions of acicular ferrite and bainite still remain unclear. This work utilises micro-testing techniques to evaluate and compare the mechanical properties of these two micro-constituents with an ultimate objective to understand their impact on bulk properties of ferrous alloys. Microscopic regions consisting of either acicular ferrite or upper bainite were first selected and then characterised using a high resolution Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD). Conventional nano-indentation and an advanced characterisation procedure were implemented to evaluate elastic modulus, yield strength, hardness and strain hardening exponent of both micro-constituents. The fracture resistance was estimated from micro-fracture tests that were conducted within the selected regions. The experimental studies have indicated that, at the micro-scale, despite their different microstructures, acicular ferrite and upper bainite have very similar mechanical properties. Nevertheless, the fracture resistance of upper bainite was more dominated by the contribution of plastic deformations.

Morphology change of retained austenite during austempering of carbide-free bainitic steel

Abstract: A change in the mechanical properties of a carbide-free bainitic steel was observed during prolonged holding at austempering temperature after termination of the bainitic transformation. To determine the origin of the property change, the microstructure was investigated by correlative electron microscopy. Although the retained austenite content remains the same during prolonged holding, its morphology changes from thin films separating the individual bainitic sub-units to a more globular structure. Since films of austenite contain a higher C concentration, the blocky austenite becomes gradually enriched in C during this morphology change. The more homogeneous distribution of the C after prolonged austempering leads to higher deformability as a result of a more pronounced TRIP effect.