Roumen Petrov | Ghent University (original) (raw)

Papers by Roumen Petrov

Metallurgical and Materials Transactions A

European Journal of Materials

WIT Transactions on The Built Environment, 2016

Surface structural changes, formed during wheel/rail contact, in R260Mn grade rails were investig... more Surface structural changes, formed during wheel/rail contact, in R260Mn grade rails were investigated by electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) in scanning electron microscope (SEM) and automatic crystal orientation mapping in transmission electron microscope (ACOM-TEM). Grain fragmentation and refinement of ferrite are characterized by all applied methods as well as grain alignment towards traffic direction. Substructures, having misorientation lower than 5°, are identified by kernel average misorientation (KAM). Detection of retained austenite indicates martensitic nature of the structural change observed in the heavily deformed surface layer.

Materials Science and Engineering: A, 2021

The article focuses on the effect of alloying and microstructure on formability of advanced high ... more 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 to accumulate much higher amount of plastic strain when tested using Nakajima method. The observed phenomenon is related to the multiaxial stress state and strain gradients

Materials Science and Technology, 2020

Materials Science and Engineering: A, 2020

Physica Scripta, 2020

The effect of neutron irradiation on the fracture toughness of two commercially pure tungsten mat... more The effect of neutron irradiation on the fracture toughness of two commercially pure tungsten materials processed according to ITER specifications has been investigated for three doses: 0.08 dpa, 0.44 dpa, and 0.67 dpa at 600 °C. The choice of this temperature was motivated by its technological importance due to the risk of irradiation-induced embrittlement. The temperature of 600 °C is below the void swelling peak temperature (~800 °C) and, at the same time, well above the ductile to brittle transition temperature (DBTT) of the reference material (~300 °C). Neutron irradiation was performed in the BR2 material test reactor inside the fuel channel in order to limit the transmutation of rhenium and osmium close to the rates expected in a fusion environment. The results of the mechanical tests performed up to 600 °C show that the fracture toughness decreases with the increase in the irradiation dose for both tungsten products. The fracture surfaces of the non- and irradiated specimens were systematically analysed to determine the evolution of the failure mechanisms.

Metallurgical and Materials Transactions A, 2020

Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The ... more Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The microstructure was characterized by means of optical and scanning electron microscopy and X-ray diffraction measurements. The results showed that the main microstructural components are tempered martensite, bainite, and retained austenite. The thermodynamic analysis showed that the constrained carbon equilibrium in the presence of cementite CCE θ model fits the experimental data for the isothermal heat treatment at a temperature well below the M S . Neither of the existing models for predicting the equilibrium between martensite and austenite ( i.e. , constrained carbon equilibrium CCE and CCE θ ) seems to predict the measured austenitic carbon content nearby the M S , which might indicate that local equilibrium (LE) conditions during bainitic transformation govern the final stage of both single and two-phase heat treatments.

Materials Characterization, 2019

Metallurgical and Materials Transactions A, 2018

Metallurgical and Materials Transactions A, 2015

Electron backscatter diffraction was used to investigate the softening effect in low-carbon steel... more Electron backscatter diffraction was used to investigate the softening effect in low-carbon steel [Fe-0.051C-0.002Si-0.224Mn-0.045Al (wt pct)] during tensile deformation with in situ ultrasonic treatment. A bimodal grain size distribution is observed with relatively small equiaxed grains with an average diameter of 10 lm at the grain boundaries of large elongated grains. The formation of these relatively small equiaxed grains is interpreted in terms of dynamic recrystallization by lattice and sub-grain rotation.

Metallurgical and Materials Transactions A, 2013

ABSTRACT In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched... more ABSTRACT In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)) structural steel during continuous cooling under different mechanical loading conditions has been studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the phase fractions and lattice parameters of the phases during heating and cooling cycles under different loading conditions. In addition to the thermal expansion behavior, the effects of the applied stress on the elastic strains during the formation of bainite from austenite and the effect of carbon on the lattice parameter of bainitic ferrite were calculated. The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation. The start temperature for the bainitic transformation decreases upon increasing the applied tensile stress. The elastic strains increase with increase in the applied tensile stress.

Scripta Materialia, 2012

ABSTRACT The occurrence of shear bands in the rotated Goss ({1 1 0}〈1 1 0〉) orientation of Fe–1.2... more ABSTRACT The occurrence of shear bands in the rotated Goss ({1 1 0}〈1 1 0〉) orientation of Fe–1.2 wt.% Si alloy was investigated by the hypothesis of the geometric softening mechanism. The reduction of Taylor factor and the orientation evolution of in-band crystals were calculated by applying the full-constraints Taylor crystal plasticity model. The presence of in-grain shear bands and particularly the Cube-oriented crystal volumes in shear bands was predicted by the models and confirmed by experimental electron backscattered diffraction observation.

IOP Conference Series: Materials Science and Engineering, 2015

Due to progressive deformation, the dislocation densities in crystals are accumulated and the res... more Due to progressive deformation, the dislocation densities in crystals are accumulated and the resistance of grains to further deformation increases. Homogeneous deformation becomes energetically less favorable, which may result for some orientations in strain localization. In-grain shear banding, a typical kind of localized deformation in metals with BCC crystal structure, has been accounted for by the geometric softening of crystals. In this study, the occurrence of shear bands in rotated Goss ({110}<110>) orientations of Fe-Si steel is predicted by crystal plasticity simulations and validated by EBSD measurements. It was observed and confirmed by crystal plasticity modeling that such shear bands exhibit stable cube orientations The orientation evolution of crystals in shear bands and its impact on annealing texture of materials are also described.

Metals

Damage in bearings is closely associated with the presence of microstructural alterations, known ... more Damage in bearings is closely associated with the presence of microstructural alterations, known as white etching areas (WEAs) and white etching cracks (WECs). One of the main reasons for the creation of these microstructural alterations is the presence of defects in the material, such as non-metallic inclusions. Manganese sulfides and aluminum oxides are widely reported in the literature as the most common types of non-metallic inclusions found in bearing steels. This study classifies 280 non-metallic inclusions in an investigated bearing steel according to several criteria: bonded/debonded with the matrix, size, shape, orientation angle, depth below the raceway surface, and chemical composition. Contrary to the findings in the literature, this investigation reports that the chemical composition of the inclusion (MnS + Al2O3) is of secondary importance when considering factors for damage initiation. The orientation of the microstructural alterations is observed to coincide with the...

Modeling of plasticity is often hampered by the difficulty in accurately characterizing dislocati... more Modeling of plasticity is often hampered by the difficulty in accurately characterizing dislocation density on the microscale for real samples. It is particularly difficult to resolve measured dislocation content onto individual dislocation types at the length scales most commonly of interest in plasticity studies. Methods have been developed to analyze dislocation content at the continuum level using the Nye tensor and Kr€ oner's fundamental relation of continuum dislocation theory to interpret the local strain gradients obtained from high resolution electron backscatter diffraction (HREBSD). This paper assesses an implementation of the NyeeKr€ onereBilby method for resolving measured geometrically necessary dislocation content onto individual dislocation types via a novel simulation of distortion fields around continuum fields of dislocation density based on classical continuum elasticity equations. It is then applied to HREBSD data for a micro-indented nickel single crystal.

Volume 3: Operations, Monitoring, and Maintenance; Materials and Joining, 2018

The results of in-depth analysis of microstructure and crystallographic texture of several indust... more The results of in-depth analysis of microstructure and crystallographic texture of several industrial batches of line pipes with known full-scale burst test results are presented. Several microstructural features promoting splitting are highlighted and quantified using electron back-scattered diffraction (EBSD) data. The actual splitting intensity is evaluated by means of Charpy tests in the direction of pipe wall thickness (Z-direction) and correlates with the microstructural parameters determined by EBSD analysis. This knowledge can be used in the design of thermo-mechanical controlled processing (TMCP) dedicated to production of splitting-free steel.

Procedia Structural Integrity, 2016

During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.

Materials Characterization, 2018

High-resolution characterization of the martensite-austenite constituent in a carbide-free bainit... more High-resolution characterization of the martensite-austenite constituent in a carbide-free bainitic steel

steel research international, 2017

A comparative study on the microstructural changes after conventional (20 8C s À1) and ultrafast ... more A comparative study on the microstructural changes after conventional (20 8C s À1) and ultrafast (300 8C s À1) heating is performed on a medium carbon steel in the soft annealed condition. Continuous-heating dilatometry experiments are carried out. The phase transformations and the volume phase fraction of austenite are determined among other microstructural changes. The microstructure is first observed using Optical Microscopy (OM), further characterized by Scanning (SEM), and detailed analyzed by Transmission Electron Microscopy (TEM). The effect of heating rate on the kinetics of cementite dissolution and austenite formation is rationalized. The experimental results are compared with Dictra calculations, and the possible effects on the kinetics of diffusion-controlled austenite formation are rationalized as well. Metallographic observations indirectly suggest the enhanced nucleation of austenite above A m .

Metallurgical and Materials Transactions A

European Journal of Materials

WIT Transactions on The Built Environment, 2016

Surface structural changes, formed during wheel/rail contact, in R260Mn grade rails were investig... more Surface structural changes, formed during wheel/rail contact, in R260Mn grade rails were investigated by electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) in scanning electron microscope (SEM) and automatic crystal orientation mapping in transmission electron microscope (ACOM-TEM). Grain fragmentation and refinement of ferrite are characterized by all applied methods as well as grain alignment towards traffic direction. Substructures, having misorientation lower than 5°, are identified by kernel average misorientation (KAM). Detection of retained austenite indicates martensitic nature of the structural change observed in the heavily deformed surface layer.

Materials Science and Engineering: A, 2021

The article focuses on the effect of alloying and microstructure on formability of advanced high ... more 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 to accumulate much higher amount of plastic strain when tested using Nakajima method. The observed phenomenon is related to the multiaxial stress state and strain gradients

Materials Science and Technology, 2020

Materials Science and Engineering: A, 2020

Physica Scripta, 2020

The effect of neutron irradiation on the fracture toughness of two commercially pure tungsten mat... more The effect of neutron irradiation on the fracture toughness of two commercially pure tungsten materials processed according to ITER specifications has been investigated for three doses: 0.08 dpa, 0.44 dpa, and 0.67 dpa at 600 °C. The choice of this temperature was motivated by its technological importance due to the risk of irradiation-induced embrittlement. The temperature of 600 °C is below the void swelling peak temperature (~800 °C) and, at the same time, well above the ductile to brittle transition temperature (DBTT) of the reference material (~300 °C). Neutron irradiation was performed in the BR2 material test reactor inside the fuel channel in order to limit the transmutation of rhenium and osmium close to the rates expected in a fusion environment. The results of the mechanical tests performed up to 600 °C show that the fracture toughness decreases with the increase in the irradiation dose for both tungsten products. The fracture surfaces of the non- and irradiated specimens were systematically analysed to determine the evolution of the failure mechanisms.

Metallurgical and Materials Transactions A, 2020

Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The ... more Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The microstructure was characterized by means of optical and scanning electron microscopy and X-ray diffraction measurements. The results showed that the main microstructural components are tempered martensite, bainite, and retained austenite. The thermodynamic analysis showed that the constrained carbon equilibrium in the presence of cementite CCE θ model fits the experimental data for the isothermal heat treatment at a temperature well below the M S . Neither of the existing models for predicting the equilibrium between martensite and austenite ( i.e. , constrained carbon equilibrium CCE and CCE θ ) seems to predict the measured austenitic carbon content nearby the M S , which might indicate that local equilibrium (LE) conditions during bainitic transformation govern the final stage of both single and two-phase heat treatments.

Materials Characterization, 2019

Metallurgical and Materials Transactions A, 2018

Metallurgical and Materials Transactions A, 2015

Electron backscatter diffraction was used to investigate the softening effect in low-carbon steel... more Electron backscatter diffraction was used to investigate the softening effect in low-carbon steel [Fe-0.051C-0.002Si-0.224Mn-0.045Al (wt pct)] during tensile deformation with in situ ultrasonic treatment. A bimodal grain size distribution is observed with relatively small equiaxed grains with an average diameter of 10 lm at the grain boundaries of large elongated grains. The formation of these relatively small equiaxed grains is interpreted in terms of dynamic recrystallization by lattice and sub-grain rotation.

Metallurgical and Materials Transactions A, 2013

ABSTRACT In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched... more ABSTRACT In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)) structural steel during continuous cooling under different mechanical loading conditions has been studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the phase fractions and lattice parameters of the phases during heating and cooling cycles under different loading conditions. In addition to the thermal expansion behavior, the effects of the applied stress on the elastic strains during the formation of bainite from austenite and the effect of carbon on the lattice parameter of bainitic ferrite were calculated. The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation. The start temperature for the bainitic transformation decreases upon increasing the applied tensile stress. The elastic strains increase with increase in the applied tensile stress.

Scripta Materialia, 2012

ABSTRACT The occurrence of shear bands in the rotated Goss ({1 1 0}〈1 1 0〉) orientation of Fe–1.2... more ABSTRACT The occurrence of shear bands in the rotated Goss ({1 1 0}〈1 1 0〉) orientation of Fe–1.2 wt.% Si alloy was investigated by the hypothesis of the geometric softening mechanism. The reduction of Taylor factor and the orientation evolution of in-band crystals were calculated by applying the full-constraints Taylor crystal plasticity model. The presence of in-grain shear bands and particularly the Cube-oriented crystal volumes in shear bands was predicted by the models and confirmed by experimental electron backscattered diffraction observation.

IOP Conference Series: Materials Science and Engineering, 2015

Due to progressive deformation, the dislocation densities in crystals are accumulated and the res... more Due to progressive deformation, the dislocation densities in crystals are accumulated and the resistance of grains to further deformation increases. Homogeneous deformation becomes energetically less favorable, which may result for some orientations in strain localization. In-grain shear banding, a typical kind of localized deformation in metals with BCC crystal structure, has been accounted for by the geometric softening of crystals. In this study, the occurrence of shear bands in rotated Goss ({110}<110>) orientations of Fe-Si steel is predicted by crystal plasticity simulations and validated by EBSD measurements. It was observed and confirmed by crystal plasticity modeling that such shear bands exhibit stable cube orientations The orientation evolution of crystals in shear bands and its impact on annealing texture of materials are also described.

Metals

Damage in bearings is closely associated with the presence of microstructural alterations, known ... more Damage in bearings is closely associated with the presence of microstructural alterations, known as white etching areas (WEAs) and white etching cracks (WECs). One of the main reasons for the creation of these microstructural alterations is the presence of defects in the material, such as non-metallic inclusions. Manganese sulfides and aluminum oxides are widely reported in the literature as the most common types of non-metallic inclusions found in bearing steels. This study classifies 280 non-metallic inclusions in an investigated bearing steel according to several criteria: bonded/debonded with the matrix, size, shape, orientation angle, depth below the raceway surface, and chemical composition. Contrary to the findings in the literature, this investigation reports that the chemical composition of the inclusion (MnS + Al2O3) is of secondary importance when considering factors for damage initiation. The orientation of the microstructural alterations is observed to coincide with the...

Modeling of plasticity is often hampered by the difficulty in accurately characterizing dislocati... more Modeling of plasticity is often hampered by the difficulty in accurately characterizing dislocation density on the microscale for real samples. It is particularly difficult to resolve measured dislocation content onto individual dislocation types at the length scales most commonly of interest in plasticity studies. Methods have been developed to analyze dislocation content at the continuum level using the Nye tensor and Kr€ oner's fundamental relation of continuum dislocation theory to interpret the local strain gradients obtained from high resolution electron backscatter diffraction (HREBSD). This paper assesses an implementation of the NyeeKr€ onereBilby method for resolving measured geometrically necessary dislocation content onto individual dislocation types via a novel simulation of distortion fields around continuum fields of dislocation density based on classical continuum elasticity equations. It is then applied to HREBSD data for a micro-indented nickel single crystal.

Volume 3: Operations, Monitoring, and Maintenance; Materials and Joining, 2018

The results of in-depth analysis of microstructure and crystallographic texture of several indust... more The results of in-depth analysis of microstructure and crystallographic texture of several industrial batches of line pipes with known full-scale burst test results are presented. Several microstructural features promoting splitting are highlighted and quantified using electron back-scattered diffraction (EBSD) data. The actual splitting intensity is evaluated by means of Charpy tests in the direction of pipe wall thickness (Z-direction) and correlates with the microstructural parameters determined by EBSD analysis. This knowledge can be used in the design of thermo-mechanical controlled processing (TMCP) dedicated to production of splitting-free steel.

Procedia Structural Integrity, 2016

During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.

Materials Characterization, 2018

High-resolution characterization of the martensite-austenite constituent in a carbide-free bainit... more High-resolution characterization of the martensite-austenite constituent in a carbide-free bainitic steel

steel research international, 2017

A comparative study on the microstructural changes after conventional (20 8C s À1) and ultrafast ... more A comparative study on the microstructural changes after conventional (20 8C s À1) and ultrafast (300 8C s À1) heating is performed on a medium carbon steel in the soft annealed condition. Continuous-heating dilatometry experiments are carried out. The phase transformations and the volume phase fraction of austenite are determined among other microstructural changes. The microstructure is first observed using Optical Microscopy (OM), further characterized by Scanning (SEM), and detailed analyzed by Transmission Electron Microscopy (TEM). The effect of heating rate on the kinetics of cementite dissolution and austenite formation is rationalized. The experimental results are compared with Dictra calculations, and the possible effects on the kinetics of diffusion-controlled austenite formation are rationalized as well. Metallographic observations indirectly suggest the enhanced nucleation of austenite above A m .