In this work, we investigate the sensitivity of the microstructure and mechanical properties of an ultrafast heat treated low carbon-steel to the peak temperature. In all studied cases, the steel was heated within the intercritical temperature range (i.e. between the AC1 and AC3 temperatures). Both the peak temperature and soaking time were varied, and their effect on the size, the fraction of individual microstructural constituents and their tensile mechanical response were investigated. It is shown that the increasing peak temperature and soaking time promote austenite formation and recrystallization processes in the ferritic matrix. The highest nanohardness is shown by martensitic grains, while recovered ferrite demonstrated slightly higher nanohardness compared to recrystallized ferrite. The applied heat treatment parameters have strong effect on the nanohardness of martensite, whereas nanohardness of ferrite microconstituents is not sensitive to variation of the peak temperature and soaking time. The non-recrystallized ferrite is harder than its recrystallized counterpart due to the higher dislocation density of the former. Increasing peak temperatures promote strengthening in the material at the expense of its ductility mainly due to increased martensite fraction. The steel demonstrates enhanced strain hardening ability independently of the peak temperature. Analysis of the experimental results showed that the industrial processing window of +- 10 oC may lead to some heterogeneity of the local microstructure in the ultrafast heat treated sheets. However, the latter should not have any negative effect on the overall mechanical behavior of the ultrafast heat treated steel sheets on the macro-scale.
We introduce a microstructure informatics dataset focusing on complex, hierarchical structures found in a single Ultrahigh carbon steel under a range of heat treatments. Applying image representations from contemporary computer vision research to these microstructures, we discuss how both supervised and unsupervised machine learning techniques can be used to yield insight into microstructural trends and their relationship to processing conditions. We evaluate and compare keypoint-based and convolutional neural network representations by classifying microstructures according to their primary mi…
The article focuses on the effect of alloying and microstructure on formability of advanced high strength steels (AHSSs) processed via quenching and partitioning (Q&P). Three different Q&P steels with different combination of alloying elements and volume fraction of retained austenite are subjected to uniaxial tensile and Nakajima testing. Tensile mechanical properties are determined, and the forming limit diagrams (FLDs) are plotted. Microstructure of the tested samples is analyzed, and dramatic reduction of retained austenite fraction is detected. It is demonstrated that all steels are able …
This work focuses on the effect of soaking time on the microstructure during ultrafast heat treatment of a 50% cold rolled low carbon steel with initial ferritic-pearlitic microstructure. Dilatometry analysis was used to estimate the effect of heating rate on the phase transformation temperatures and to select an appropriate inter-critical temperature for final heat treatments. A thorough qualitative and quantitative microstructural characterization of the heat treated samples is performed using a wide range of characterization techniques. A complex multiphase, hierarchical microstructure cons…
The mechanism of the evolution of the deformed microstructure at the earliest stage of annealing where the existence of the lowest length scale substructure paves the way to the formation of the so-called subgrains, has been studied for the first time. The study has been performed at high temperature on heavily deformed Ti-modified austenitic stainless steel using X-ray diffraction technique. Significant changes were observed in the values of the domain size, both with time and temperature. Two different types of mechanism have been proposed to be involved during the microstructural evolution …
The superite phase and phase transition inducing multiscale solidification microstructures and segregations in steels
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences（中国科学院金属研究所）；Department of Materials Science and Engineering, University of Science and Technology Liaoning（辽宁科技大学材料科学与工程学院）；Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Advanced Research Institute, Chinese Academy of Sciences（中国科学院上海高等研究院）
The microstructural development in H13 tool steel upon nitriding by an ion beam process was investigated. The nitriding experiments were performed at a relatively low temperature of about 400°C and at constant ion beam energy (400 eV) of different doses in a high-vacuum preparation chamber; the ion source was fed with high purity nitrogen gas. The specimens were characterized by X-ray photoelectron spectroscopy (XPS), electron probe microanalysis (EPMA), scanning and transmission electron microscopy (SEM and TEM), and grazing incidence and Bragg-Brentano X-ray diffractometry. In particular, th…
A novel medium manganese steel named Novalloy with composition Fe-8.3Mn-3.8Al-1.8Si-0.5C-0.06V-0.05Sn was developed and thermomechanically processed through hot rolling and intercritical annealing. The steel possessed a yield strength of 1 GPa, tensile strength of 1.13 GPa and ductility of 41\%. In order to study the effect of cold rolling after intercritical annealing on subsequent tensile properties, the steel was further cold rolled up to 20\% reduction. After cold rolling, it was observed that the strain hardening rate increased continuously with increasing cold rolling reduction but witho…
We present an original approach for predicting the static recrystallization texture development during annealing of deformed crystalline materials. The microstructure is considered as a population of subgrains and grains whose sizes and boundary properties determine their growth rates. The model input parameters are measured directly on orientation maps maps of the deformed microstructure measured by electron backscattered diffraction. The anisotropy in subgrain properties then drives a competitive growth giving rise to the recrystallization texture development. The method is illustrated by a …
In this paper synchrotron microtomography on Plasma Sprayed Tungsten (PS-W) is presented and discussed. PS-W is a challenging material for microtomography since it exhibits a random porous network at different length-scales (from nanometers to micrometers) and is hardly penetrable by X-rays. Furthermore, inner porosity causes strong internal scattering. The key challenges were, firstly, to optimize the beam parameters considering the inherent trade-off between photon energy (penetration depth) and spatial resolution and, secondly, to develop effective signal filtering algorithms. Despite the l…
The induction hardening behavior of a new, hot-rolled 0.4 wt.% carbon steel with the two different starting microstructures of upper and lower bainite has been simulated using a Gleeble 3800. The effect of heating rate in the range 1 - 500 °C/s on austenite grain size distribution has been rationalized. Dilatometry together with Scanning Electron Microscopy combined with Electron Backscatter Diffraction analyses and thermodynamic simulations provide insight into the austenite formation mechanisms that operate at different heating rates. Two main mechanisms of austenite formation during re-aust…
A medium Mn steel of composition Fe-4.8Mn-2.8Al-1.5Si-0.51C (wt.\%) was processed to obtain two different microstructures representing two different approaches in the hot rolling mill, resulting in equiaxed vs. a mixed equiaxed and lamellar microstructures. Both were found to exhibit a simultaneous TWIP$+$TRIP plasticity enhancing mechanism where deformation twins and $α'$-martensite formed independently of twinning with strain. Interrupted tensile tests were conducted in order to investigate the differences in deformation structures between the two microstructures. A constitutive model was us…
Electron Backscattering Diffraction (EBSD) provides important information to discriminate phase transformation products in steels. This task is conventionally performed by an expert, who carries a high degree of subjectivity and requires time and effort. In this paper, we question if Convolutional Neural Networks (CNNs) are able to extract meaningful features from EBSD-based data in order to automatically classify the present phases within a steel microstructure. The selected case of study is ferrite-martensite discrimination and U-Net has been selected as the network architecture to work with…
Materials which can undergo extremely fast displacive transformations as well as very slow diffusive transformations are studied using a Ginzburg-Landau framework to understand the physics behind microstructure formation and time-temperature-transformation (TTT) diagrams. This simple model captures the essential features of alloys such as steels and predicts the formation of mixed microstructures by an interplay between diffusive and displacive mechanisms. The intrinsic volume changes associated with the transformations stabilize mixed microstructures such as martensite-retained austenite (res…
The present study evidences the role of ferrite grain size distributions on the occurrence of void initiation in a low carbon steel. Various thermomechanical treatments were done to create ultrafine, bimodal and coarse range of ferrite grain distributions. A two parameter characterization of probable void initiation sites is proposed; elastic modulus difference and difference in Schmid factor of the grains surrounding the void. All microstructures were categorized based on the ability to ease or resist void nucleation. For coarse grains, elastic modulus difference as well as the Schmid factor …
Experimentally validated process-microstructure-property relations of bainitic steels derived from phase-field simulations
Interdisciplinary Centre for Advanced Materials Simulation, Ruhr University Bochum（先进材料模拟跨学科中心，鲁尔大学波鸿分校）；Steel Institute, RWTH Aachen University（钢铁研究所，亚琛工业大学）
Fe- and steelmaking is the largest single industrial CO2 emitter, accounting for 6.5% of all CO2 emissions on the planet. This fact challenges the current technologies to achieve carbon-lean steel production and to align with the requirement of a drastic reduction of 80% in all CO2 emissions by around 2050. Thus, alternative reduction technologies have to be implemented for extracting iron from its ores. The H-based direct reduction has been explored as a sustainable route to mitigate CO2 emissions, where the reduction kinetics of the intermediate oxide product FexO wustite into Fe is the rate…
In the present work, experimental study has been carried out to expose the thermal, mechanical, and microstructural properties of low carbon steel as well as to inspects the influence of etchant concentration and etching time on its microstructure. Ultra-low carbon steel, in the form of a sheet, was collected from the Mughal Steel Industry, Peshawar, Pakistan. The sample was chemically etched, using Nital as an etchant, by two different methods: first, by changing the etching time while keeping the composition of etchant the same and second, by keeping the time constant while varying the etcha…
The evolution of deformation from plasticity to localization to damage is investigated in ferritic-pearlitic steel through nanometer-resolution microstructure-correlated SEM-DIC (u-DIC) strain mapping, enabled through highly accurate microstructure-to-strain alignment. We reveal the key plasticity mechanisms in ferrite and pearlite as well as their evolution into localization and damage and their relation to the microstructural arrangement. Notably, two contrasting mechanisms were identified that control whether damage initiation in pearlite occurs and, through connection of localization hotsp…
Models for the microstructure evolution during hot rolling are reviewed. The basic macroscopic phenomena related to recrystallization are summarized. Constitutive models based on semi empirical equations are compared to more sophisticated models based on cellular automata, vertex and Monte-Carlo-Potts methods. The applicability of each kind of model approach for online and offline process control in steel industry is discussed. While constitutive models are still state-of-the-art for online process control, mesoscale models with a spatial representation of the microstructure can provide better…
Data-efficient U-Net for Segmentation of Carbide Microstructures in SEM Images of Steel Alloys
Helmholtz AI Team Matter (FWCC-A)（海德堡人工智能团队物质（FWCC-A））；Helmholtz-Zentrum Dresden-Rossendorf（德罗斯登-罗斯托克研究中心）；Institute of Resource Ecology, Structural Materials Department（资源生态研究所，结构材料部门）
Since Reactor Pressure Vessel steels are ferromagnetic, they provide a convenient means to monitor changes in the mechanical properties of the material upon irradiation with high energy particles, by measuring their magnetic properties. Here, we discuss the correlation between these two properties (i.e. mechanical and magnetic properties) and microstructure, by studying the flux effect on the nuclear pressure vessel steel used in reactors currently under construction in Argentina. Charpy-V notched specimens of this steel were irradiated in the RA1 experimental reactor at 275°C with two lead fa…
Cu-Sn-Zn coatings have been widely used in industry for their unique properties, such as good conductivity, high corrosion resistance and excellent solderability. To further improve the mechanical performance of Cu-Sn-Zn coatings, powder-enhanced method was applied in the current study and Cu-Sn-Zn-TiO2 nano-composite coatings with different TiO2 concentration were fabricated. The microstructure of Cu-Sn-Zn-TiO2 nano-composite coatings were investigated by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The mechanical properties of coatings including microhardness and wear resi…
A versatile generic framework for parent grain reconstruction from fully or partially transformed child microstructures was integrated into the open-source crystallographic toolbox MTEX. The framework extends traditional parent grain reconstruction, phase transformation and variant analysis to all parent-child crystal symmetry combinations. The inherent versatility of the universally applicable parent grain reconstruction methods, and the ability to conduct in-depth variant analysis are showcased via example workflows that can be programmatically modified by users to suit their specific applic…
In this study, a post-heat treatment design for additively manufactured copper-bearing high-strength low-alloy (HSLA)-100 steel is performed by understanding the process-structure-property relationships. Hot isostatic pressing (HIP) is designed to reduce the porosity from 3% to less than 1% for the HSLA-100 steel processed by laser powder bed fusion (LPBF). Quenching dilatometry is employed to design the HIP parameters with the optimized cooling rate for the maximum amount of martensite transformed after HIP. Afterward, a post-heat treatment step with cyclic re-austenitization is introduced fo…
With 1.85 billion tons produced per year, steel is the most important material class in terms of volume and environmental impact. While steel is a sustainability enabler, for instance through lightweight design, magnetic devices, and efficient turbines, its primary production is not. For 3000 years, iron has been reduced from ores using carbon. Today 2.1 tons CO2 are produced per ton of steel, causing 30% of the global CO2 emissions in the manufacturing sector, which translates to 6.5% of the global CO2 emissions. These numbers qualify iron- and steel-making as the largest single industrial gr…
The unique sub-grain patterns have been found in some particular alloys (316L, Al-Si, Co-Cr-Mo) selective laser melting (SLM), the submicron-scale cellular, elongated cellular or even band structures are always coexisting inside one single macro-solidified grain. Furthermore, the cellular structures are symmetrical with hexagonal, pentagonal and square cellular patterns where the cellular size is only around 1μm. Single-layer and bulk 316L SLM experiments are presented that reveals the forming mechanism of these sub-grain cellular microstructures. Complex cellular sub-micron patterns were form…
Effective post-heat treatment is critical to achieve desired microstructure for high-performance in additively manufactured (AM) components. In this work, the influence of homogenization on microstructure-property relationship in 17-4PH steels has been investigated. Precipitation of NbC, oxides, and ε-Cu were observed in the as-built 17-4PH steels. To design an optimum post-heat treatment, homogenization was performed at 1050oC for different times followed by aging at 482oC for 1 hour. It was identified that homogenization for 1 hour followed by aging leads to the best combination of strength …
The onset of plasticity in quenched martensitic microstructures is characterized by a low initial yield stress followed by an extremely strong initial hardening response, and then a sudden hardening saturation. Literature attributes this behavior to residual stresses and dislocations inherited from the martensitic transformation, or to microstructural heterogeneities causing strength differences among the grains. Here, we argue that orientation-dependent yielding of lath martensite due to inter-lath sliding, which induces a substructure boundary sliding mechanism, may also contribute significa…
Combined effect of Carbon and Nitrogen levels and isothermal holding temperature on the microstructure, precipitation and the tensile properties of Vanadium microalloyed steels with 0.05 weight percent Vanadium were studied. Two different Vanadium steels, one having higher Carbon and lower Nitrogen content, HCLN steel, and the other having lower Carbon and higher Nitrogen content, LCHN steel, were prepared and subjected to isothermal holding treatment over a temperature range of 500 to 750 degree Celsius, after hot-deformation. Maximum precipitation strengthening from fine Vanadium carbonitrid…
In situ synchrotron X-ray diffraction study of flash austenitization and process design insights in medium-Manganese steels for energy applications
University of Kassel（卡塞尔大学）；Technische Universität Bergakademie Freiberg（弗莱贝格工业大学）；Karlsruhe University of Applied Sciences (HKA)（卡尔斯鲁厄应用科学大学）；Karlsruhe Institute of Technology (KIT)（卡尔斯鲁厄理工学院）
A comprehensive study of pulsed nitriding in AISI H13 tool steel at low temperature (400°C) is reported for several durations. X-ray diffraction results reveal that a nitrogen enriched compound (Epsilon-Fe2-3N, iron nitride) builds up on the surface within the first process hour despite the low process temperature. Beneath the surface, X-ray Wavelength Dispersive Spectroscopy (WDS) in a Scanning Electron Microscope (SEM) indicates relatively higher nitrogen concentrations (up to 12 at.%) within the diffusion layer while microscopic nitrides are not formed and existing carbides are not dissolve…
Electrical iron silicon steel is the most commonly used soft magnetic material in electrical energy conversion and transmission, and its demand is expected to increase with the need for electrification of the transportation sector and the transition to renewable energy to combat climate change. Although iron silicon steel has been used for more than 100 years, some fundamental relationships between microstructure and magnetic performance remain vague, especially with regard to the role of crystal defects such as grain boundaries and dislocations that are induced during the final cutting step o…
Investigating the difference in mechanical stability of retained austenite in bainitic and martensitic high-carbon bearing steels using in situ neutron diffraction and crystal plasticity modeling
The Timken Company（铁姆肯公司）；Chemical and Engineering Materials Division, Spallation Neutron Source, Oak Ridge National Laboratory（橡树岭国家实验室散裂中子源化学与材料工程部）
Max Planck Institute for Informatics（马克斯·普朗克信息学研究所）；Material Engineering Center Saarland（萨尔兰材料工程中心）；German Aerospace Center (DLR)（德国航空航天中心）；Saarland University（萨尔兰大学）
The inner structure of a material is called microstructure. It stores the genesis of a material and determines all its physical and chemical properties. While microstructural characterization is widely spread and well known, the microstructural classification is mostly done manually by human experts, which gives rise to uncertainties due to subjectivity. Since the microstructure could be a combination of different phases or constituents with complex substructures its automatic classification is very challenging and only a few prior studies exist. Prior works focused on designed and engineered …
The combined strengthening effects of grain refinement and high precipitated volume fraction (~6at.%) on the mechanical properties of FeSiTi alloy subjected to SPD processing prior to aging treatment were investigated by atom probe tomography and scanning transmission electron microscopy. It was shown that the refinement of the microstructure affects the precipitation kinetics and the spatial distribution of the secondary hardening intermetallic phase, which was observed to nucleate heterogeneously on dislocations and sub-grain boundaries. It was revealed that alloys successively subjected to …
For the first time, cyclic re-austenitization is carried out for additively manufactured high-strength low-alloy (HSLA) steels in order to refine the microstructure by reducing the prior austenite grain (PAG) size. In this work, HSLA-100 steels processed using laser powder bed fusion (LPBF) technique are subjected to several cycles of re-austenitization using quenching dilatometry. Microstructure characterization for every cycle revealed the presence of bainite, martensite and martensite/austenite (M/A) islands. From the analysis of the dilatometry curves and extensive microstructure character…
A systematic study of the sensitivities of simulation input on the computed stress distributions in two-phase microstructures is presented. The study supports a related investigation of the initiation and propagation of yielding in duplex stainless steel. Considered in the study are the identification of constitutive model parameters for the single-crystal elastic and plastic behaviors and the importance of including dominant phase and grain morphologies in the instantiation of virtual samples. Behaviors computed using a finite element formulation are evaluated against experimental data for th…
Effectiveness of galaxite (Al2O3MnO) and manganese oxide (MnO2) in nucleating acicular ferrite was demonstrated in a steel with composition of Fe-13.92Mn-4.53Al-1.28Si-0.11C. The hot rolled and quenched steel had a duplex microstructure of delta-ferrite and partially transformed austenite. Thermal treatments were used to manipulate the concentration and type of oxides and the ferrite plate density was found to correlate with inclusions of low misfit. Both bainitic and acicular ferrite were observed and the prior austenite had a grain diameter of 16.5 micrometers. Upon tensile testing the retai…
Fossil-free ironmaking is indispensable for reducing massive anthropogenic CO2 emissions in the steel industry. Hydrogen-based direct reduction (HyDR) is among the most attractive solutions for green ironmaking, with high technology readiness. The underlying mechanisms governing this process are characterized by a complex interaction of several chemical (phase transformations), physical (transport), and mechanical (stresses) phenomena. Their interplay leads to rich microstructures, characterized by a hierarchy of defects ranging across several orders of magnitude in length, including vacancies…
Laser surface hardening (LSH) is an effective process to produce hard and wear resistant steel surfaces. This work produced hardened surfaces using a fiber laser in a bare (B. uncoated) and carbon-coated (C) conditions. The obtained hardness and microstructure were compatible to martensite (B) and cementite (C) major constituents. A low coefficient of friction was obtained using C-condition. The treated surface also presented little damage during a reciprocating wear against a hard metal sphere.
Effects of mechanical stress, chemical potential, and coverage on hydrogen solubility during hydrogen-enhanced decohesion of ferritic steel grain boundaries: A first-principles study
Two finite element level-set (FE-LS) formulations are compared for the modeling of grain growth of 316L stainless steel in terms of grain size, mean values and histograms. Two kinds of microstructures are considered, some are generated statistically from EBSD maps and the others are generated by immersion of EBSD data in the FE formulation. Grain boundary (GB) mobility is heterogeneously defined as a function of the GB disorientation. On the other hand, GB energy is considered as heterogeneous or anisotropic, respectively defined as a function of the disorientation and both the GB misorientati…
