Evolution Of Steel Research Articles

Physical and Numerical Simulations for Predicting Distribution of Microstructural Features during Thermomechanical Processing of Steels.

The design of modern construction materials with heterogeneous microstructures requires a numerical model that can predict the distribution of microstructural features instead of average values. The accuracy and reliability of such models depend on the proper identification of the coefficients for a particular material. This work was motivated by the need for advanced experimental data to identify stochastic material models. Extensive experiments were performed to supply data to identify a model of austenite microstructure evolution in steels during hot deformation and during the interpass times between deformations. Two sets of tests were performed. The first set involved hot compressions with a nominal strain of 1. The second set involved hot compressions with lower nominal strains, followed by holding at the deformation temperature for different times. Histograms of austenite grain size after each test were measured and used in the identification procedure. The stochastic model, which was developed elsewhere, was identified. Inverse analysis with the objective function based on the distance between the measured and calculated histograms was applied. Validation of the model was performed for the experiments, which were not used in the identification. The distance between the measured and calculated histograms was determined for each test using the Bhattacharyya metric and very low values were obtained. As a case study, the model with the optimal coefficients was applied to the simulation of the selected industrial hot-forming process.

Friction stir processing as a strengthening technique for medium carbon low alloy steels

Friction stir processing (FSP) has shown its potential in the controlled microstructure evolution in steels. It is fact that the microstructure i.e. grain size, crystal orientation, type of phase and phase fraction, plays the most important role in the determination of mechanical properties. In this work, material gets strengthened by 1.5 times yield strength, 1.41 times ultimate strength as compared to base material after FSP. Ductility was observed to be decreased to 23% as compared to 32.5% of base material. The hardness of processed zone was observed to be 1.5 times that of the base material. The changes in the properties are studied in the context of microstructure resulted from the severe plastic deformation and thermomechanical effect of friction stir processing. The stir zone (SZ)/processed zone (PZ) is observed to have dual-phase structure, retained austenite along with nanosized carbide particles.

A Slow Positron Beam Study of Helium Clustering in Oxide Dispersion Strengthened Steels

The present work provides an innovative approach to the experimental study of the radiation tolerance of oxide dispersion strengthened (ODS) steel comparing to its non-ODS variant. Inhomogeneous radiation damage, introduced by helium irradiation, was probed in a depth profile manner by a variable energy positron beam. Although numerous studies reporting the use of slow positron beam (SPB) experiments on ion-implanted materials have been published in the literature, only a few of them address the continuous distribution of microstructural damage introduced by ion beam. In this paper, we use SPB to probe a wide range of radiation damage in a single sample of each material. By evaluating positron diffusion in different parts of the Bragg peak, we were able to track the evolution of radiation-induced damage from a high density of small defects such as helium-vacancy clusters to large bubbles grown by coalescence. Combining with the transmission electron microscopy results and slow positron Doppler broadening spectroscopy, we offer new insight into different stages of microstructural bubble evolution in steels exposed to harsh radiation conditions. The obtained results show that the higher density of vacancy-type defects in the oxide dispersion strengthened steel, which act as recombination centres, suppress the formation of large defect agglomerations. While the TEM provides an indication of this effect in high displacement damage and helium production rates, positron annihilation spectroscopy elucidates the origin of this behaviour, improving the understanding of the fundamental mechanism of helium embrittlement.

Availability of Opal Photonic Crystal Films for Visualizing Heterogeneous Strain Evolution in Steels: Example of Lüders Deformation

Availability of Opal Photonic Crystal Films for Visualizing Heterogeneous Strain Evolution in Steels: Example of Lüders Deformation

Tempforming as an Advanced Processing Method for Carbon Steels

The microstructural mechanisms providing delamination toughness in high-strength low-alloyed steels are briefly reviewed. Thermo-mechanical processing methods improving both the strength and impact toughness are described, with a close relation to the microstructures and textures developed. The effect of processing conditions on the microstructure evolution in steels with different carbon content is discussed. Particular attention is paid to tempforming treatment, which has been recently introduced as a promising processing method for high-strength low-alloyed steel semi-products with beneficial combination of strength and impact toughness. Tempforming consists of large strain warm rolling following tempering. In contrast to ausforming, the steels subjected to tempforming may exhibit an unusual increase in the impact toughness with a decrease in test temperature below room temperature. This phenomenon is attributed to the notch blunting owing to easy splitting (delamination) crosswise to the principle crack propagation. The relationships between the crack propagation mode, the delamination fracture, and the load-displacement curve are presented and discussed. Further perspectives of tempforming applications and promising research directions are outlined.

Availability of Opal Photonic Crystal Films for Visualizing Heterogeneous Strain Evolution in Steels: Example of Lüders Deformation

Availability of Opal Photonic Crystal Films for Visualizing Heterogeneous Strain Evolution in Steels: Example of Lüders Deformation

An Integrated Model for Predicting Hierarchical Microstructure Evolution of Steel during Hot Rolling

A robust model for calculating the necessary process variables such as strain, strain rate and temperature in hot rolling of a steel plate or strip is presented in this paper. The applied approach includes a well-constructed velocity function in the deformation model that is validated using a commercial finite element solver. The developed microstructure model is then integrated into the process model and includes the most essential descriptions of restoration and austenite grain structure evolution phenomena during hot rolling. Furthermore, the concept of hierarchical grain structure evolution is described as a method track the evolution of un-recrystallized and recrystallized features of the microstructure throughout a given pass schedule. The most important outcome of this approach is that each grain size component is modelled separately based on its prior thermomechanical history. A computer implementation of these models called MICDEL is used together with a simulation example to demonstrate its capability of predicting process variables and austenite grain structure evolution in hot strip rolling of steel.

Ductile fracture of advanced pipeline steels: study of stress states and energies in dynamic impact specimens - CVN and DWTT

The development of robust protocols for assessing the structural integrity of gas pipelines is of paramount relevance, since failures can lead to financial and human losses. In this scenario, the material’s ability to slow down the propagation of a running crack (crack arrest) becomes a design requirement. Several empirical models and criteria, calibrated by real pipeline burst tests, have been developed, being the Battele Two Curve Method (BTCM) one example of technique widely employed during decades. With the evolution of steels, there was a significant increase of ductility and toughness, in a way that such semi-empirical models usually based on the energy absorbed in the Charpy impact test (ISO 148-1, ASTM E-23) began to present unsatisfactory predictions. This may be explained by the fact that in current high-ductility and high-toughness materials (e.g.: API-5L X65, X80, X100), the dominant mechanism of fracture propagation is plastic collapse. Consequently, the energies involved in deforming and fracturing a laboratory specimen are remarkably altered and transferability to pipelines by means of the aforementioned models can be lost. Therefore, for a better phenomenological comprehension of the ductile fracture process under such circumstances, this work investigates Charpy and DWTT (ASTM E-436) dynamic tests assessing stress fields and respective energies involved in deformation and fracture. It is of great interest to evaluate the energy associated to steady state ductile fracture and thus try to characterize the energy available to slow down an ongoing fracture. Pipelines are references for the developments and support assumptions and some conclusions. Based on these golas, numerical analyses including damage models (XFEM and GTN) were implemented, including parameters’ calibration and sensitivity analyses. The methodology closely reproduced available experimental results. Besides that, stress fields and energies could be quantified for the studied geometries and such analyses indicated the potential and limitations of Charpy and DWTT specimens to characterize the energies required to describe steady state ductile crack propagation and crack arrestability. Results support further developments related to pipeline integrity assessments.

Simulation of precipitate ensemble evolution in steels with V and Nb

A model for describing the evolution of ensembles of carbonitride precipitates of several compositions in steel at the stages of growth, dissolution, and coagulation is developed in the scope of the mean field approximation. Calculations of the growth and dissolution kinetics of carbonitrides in the Fe-Nb-V-C-N systems are performed based on the numerical implementations.

Simulation of Precipitates Evolution in Steels with V and Nb at Annealing

Based on the mean field approximation, a model has been worked out for the description of evolution of carbonitride precipitate ensemble with various composition in steels at the stages of their growth, dissolution and coarsening. Based on the numerical realization of this model, the calculations of growth and dissolution kinetics of carbonitrides in a Fe-Nb-V-C-N system have been carried out.

Tournemire argillite/carbon steel evolution under bacterial influence after 10 years of in situ interaction

The purpose of the present study was to characterise the physicochemical evolution of argillite/carbon steel coupons after 10 years of interaction inside the Tournemire Underground Research Laboratory (France). Carbon steel coupons were introduced in boreholes drilled in the geological formation in contact with the argillite, and were overcored 10 years later to study the interfaces. The iron release by steel corrosion induced mineralogical dissolutions (Ca phases) and precipitations (iron oxides and hydroxides), leading to a partial clogging of the argillite porosity. A sulphur enrichment was observed in the corrosion area, linked to a potential microbial activity. The biodiversities differed depending on the steel type, indicating the influence of iron–clay interactions. This analysis has shown the presence of bacteria known for their involvement in corrosion processes and isolates able to develop at elevated temperatures.

Study on Fractal Characteristics of Acoustic Emission in Circular Double-Layer Stirrup Confined Concrete

Vertical bearing capacity experiments were conducted with circular double-layer stirrup confined concrete columns as study objects, data acquisition was carried out using acoustic emission (AE) equipment and the AE parameters and graphs acquired during the experiments were analyzed to obtain the damage evolution of steel reinforced concrete columns under compression. The correlation between fractal dimension of AE graphs and curve was studied using the fractal theory, and the results show that the AE parameter graphs have fractal characteristics and the box dimension of each AE parameter graph constantly increases with the increase in stress, with a positive correlation exhibited between the fractal dimension and stress level. The loss variable expressed with fractal dimension was defined to establish actual equivalent stresses and the equivalent stress curves and experimental curves were compared; the results show that the fractal dimension of acoustic emission parameter graph can characterize the damage laws of the concrete member.

Inclusion Development in Steel During Ladle Metallurgical Treatment ‐ A Process Simulation Model – Part: Industrial Validation

Steel cleanliness strongly affects the properties of the products. There is an ever increasing requirement of strict control of inclusion composition and amount during ladle treatment. A comprehensive model for inclusion development in gas stirred ladles developed by the authors is validated in the industrial ladle treatment processes. It can be used for the process simulation and optimization. The important factors, like stirring intensity, reaction between steel and slag as well as refractory material, and the conditions needed for separation and floatation of non‐metallic inclusions are taken into account. FactSage and SimuSage packages are employed as the thermodynamics database and simulation tools, respectively. The evolution of steel and slag composition along with the amount and composition of inclusions during the ladle treatment are calculated and compared with results from the industrial process. The comparison between model predictions and actual plant data shows good agreement as well for 210 t heats as for 30 t heats and different steel grades. The present model is a useful tool for simulation and optimization of ladle metallurgical treatments in industry.

Use of Advanced Austenitic and Duplex Stainless Steels for Applications in Oil & Gas and Process Industry

Stainless steels are widely used in the Oil & Gas and chemical process industry. This group of materials is today available in a large variety of alloy compositions, and practically all product forms needed for a construction are available. A historical view and application examples are given on the stainless steel evolution, from the standard grades used in chemical processes to todays most advanced applications in the chemical and oil & gas industry, where demands on reliable and long lasting solutions are necessary. The influence of alloying elements on the properties and manufacturability is described. The chemical industry is a very wide definition of a large group of industries with very different products, from plastics and organic acids to fertilizers, drugs and pesticides. Applications of stainless steels within the chemical industry are described. The first example is organic acids, where the use of high alloyed duplex stainless steels such as UNS S32205 and UNS S32750 have been successful. Another example is phosphoric acid applications, where the aggressiveness of the process solution depends very much on the fluoride and chloride content of the rock phosphate. In sulfuric acid, the material of construction is very much dependent on the acid concentration and temperature. Nitric acid is another common fertilizer acid which is highly oxidizing, and thereby demands stainless steels with high chromium content but with low molybdenum contents. The Oil & Gas industry uses very high quantities of carbon steel and stainless steel in their constructions. The oil wells are defined as sweet when the well contains carbon dioxide and no substantial amounts of hydrogen sulfide, when there is hydrogen sulfide present in the well, the wells are defined as sour. An overview on materials depending on the application is given. In subsea applications, hydraulic control lines (umbilicals) are used for control of valves and for methanol injection in subsea platforms. UNS S32750 is a high strength duplex stainless steel which today is the first choice for umbilicals and has been chosen for a very large amount of umbilical projects worldwide. The newly developed hyper duplex stainless steels with a combination of even higher corrosion resistance and strength are introduced for applications in oil-gas industry. The possibilities with stainless steels are endless, and new alloys are constantly being developed to meet industrial challenges of today and in the future. By choosing the right stainless steel grade, it is possible to find a solution to almost all challenges in the Oil & Gas and Process industry.

Three-dimensional phase-field modeling of martensitic microstructure evolution in steels

In the present work a 3-D elastoplastic phase-field (PF) model is developed, based on the PF microelasticity theory proposed by A.G. Khachaturyan and by including plastic deformation as well as anisotropic elastic properties, for modeling the martensitic transformation (MT) by using the finite-element method. PF simulations in 3D are performed by considering different cases of MT occurring in an elastic material, with and without dilatation, and in an elastic perfectly plastic material with dilatation having isotropic as well as anisotropic elastic properties. As input data for the simulations the thermodynamic parameters corresponding to an Fe–0.3%C alloy as well as the physical parameters corresponding to steels acquired from experimental results are considered. The simulation results clearly show autocatalysis and morphological mirror image formation, which are some of the typical characteristics of a martensitic microstructure. The results indicate that elastic strain energy, anisotropic elastic properties, plasticity and the external clamping conditions affect MT as well as the microstructure.

Progress in mesoscopic modeling of microstructure evolution in steels

The mesoscopic modeling developed rapidly in the past three decades is a promising tool for predicting and understanding the microstructure evolution at grain scale. In this paper, the recent development of mesoscopic modeling and its application to microstructure evolution in steels is reviewed. Firstly, some representative computational models are briefly introduced, e.g., the phase field model, the cellular automaton model and the Monte Carlo model. Then, the emphasis is put on the application of mesoscopic modeling of the complex features of microstructure evolution, including solidification, solid-state phase transformation, recrystallization and grain growth. Finally, some issues in the present mesoscopic modeling and its perspective are discussed.

3D Phase Field Modeling of Martensitic Microstructure Evolution in Steels

Complex martensitic microstructure evolution in steels generates enormous curiosity among the materials scientists and especially among the Phase Field (PF) modeling enthusiasts. In the present work PF Microelasticity theory proposed by A.G. Khachaturyan coupled with plasticity is applied for modeling the Martensitic Transformation (MT) by using Finite Element Method (FEM). PF simulations in 3D are performed by considering different cases of MT occurring in a clamped system, i.e. simulation domain with fixed boundaries, of (a) pure elastic material with dilatation (b) pure elastic material without dilatation (c) elastic perfectly plastic material with dilatation having (i) isotropic as well as (ii) anisotropic elastic properties. As input data for the simulations the thermodynamic parameters corresponding to Fe - 0.3% C alloy as well as the physical parameters corresponding to steels acquired from experimental results are considered. The results indicate that elastic strain energy, dilatation and plasticity affect MT whereas anisotropy affects the microstructure.

In-Situ Observations of Martensitic Transformation in Blast-Resistant Steel

A hybrid in-situ characterization system, which couples the laser scanning confocal microscopy (LSCM) with the time-resolved X-ray diffraction (TRXRD) measurement with synchrotron radiation, was used to characterize the microstructure evolution during heat-affected zone (HAZ) thermal cycling of high-strength and blast-resistant steel. The combined technique has a time resolution of 0.3 seconds that allows for high-fidelity measurements of transformation kinetics, lattice parameters, and morphological features. The measurements showed a significant reduction in the martensite start transformation temperature with a decrease in the prior austenite grain size. In addition, the LSCM images confirmed the concurrent refinement of martensite packet size with smaller austenite grain sizes. This is consistent with dilatometric observations. The austenite grain size also influenced the rate of transformation (df m /dT); however, the measurements from the hybrid (surface) and dilatometric (volume) measurements were inconsistent. Challenges and future directions of adopting this technique for comprehensive tracking of microstructure evolution in steels are discussed.

Phase field modeling of microstructure evolution in steels

This article provides an overview on the application of phase field models to describe microstructure evolution in steels. The focus will be on phase field modeling of the austenite–ferrite transformation as this has emerged as a particularly active area of research in the past few years. Phase field models are powerful tools to deal with the complex morphologies, e.g. Widmanstätten ferrite, that may result from these transformations. Even though much progress has been attained there is still significant work to be done in applying these models to processing of advanced steels with complex multi-phase microstructures. In particular, the phase field approach promises to have significant impact on modeling of bainite formation and the microstructure evolution in the heat affected zone of welds.

Experimental and numerical characterization of damage evolution in steels

This paper presents an experimental and numerical characterization of ductile damage evolution in steels subjected to large plastic deformations. To this end, a set of tensile tests combining load–unload tensile cycles is firstly carried out in order to evaluate the deterioration exhibited by the Young’s modulus for increasing levels of plastic deformation. This task allows, in turn, to derive the characteristic parameters involved in a well-established evolution equation for the isotropic damage variable. In this context, a new damage identification procedure is presented. Different aspects of it are particularly addressed. The obtained material parameters are the basic data to be considered in the simulations that are performed afterwards: the analysis of the tensile test mainly aimed at assessing the proposed characterization and, in addition, the modelling of the flattening process of a cylinder studied to discuss the performance of the constitutive model in the prediction of damage evolution.