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Youn Hee Kang

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Domenico Gentile

Università del Molise

Frattura Integrità Strutturale

Andrew Ruggiero

University of Cassino and Southern Latium

Université de Technologie de Troyes (UTT)

Milan Veljkovic

Miroslav Urbánek

Fernando Souto Suárez

Université de Technologie de Belfort-Montbéliard (UTBM)

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Papers by Youn Hee Kang

A Comparative Study of Ductile Damage Models Approaches for Joint Strength Prediction in Hot Shear Joining Process

Procedia Engineering, 2017

Ductile fracture prediction is often considered as a challenging task in complex stress state app... more Ductile fracture prediction is often considered as a challenging task in complex stress state applications, such as metal joining process. A systematic method for predicting the joint strength in the hot shear joining process is presented in this paper by considering two different ductile damage models, namely Johnson-Cook and modified Lemaitre damage models. Since the crack formation in metalworking is considered to be a combination of both load conditions and metallurgical properties, the microstructure evolution is taken into account by characterizing the considered material, AISI-1045, at different temperatures and for different strain rates and by testing different specimen shapes, namely round bar, joint bar, flat and notched specimens. The derived material properties have been inversely calibrated in order to be used in the developed numerical model. A shear joining process simulation has been implemented in DEFORM and have been validate with shear joining experiments carried out at 950°C, allowing to validate the accuracy of the proposed model in simulating the hot shear joining, both in terms of joining strength and identification of the damage locations. Thanks to a precise material characterization, and by utilizing the proposed numerical model, both the joining tool design and the process condition can be studied and improved.

The effect of alloy compositions on the joint strength of carbon steels welded by the extended hot-rolling process

Materials Science and Engineering: A, 2012

In this study, we investigated the factors that determine the joint strength of carbon steels wel... more In this study, we investigated the factors that determine the joint strength of carbon steels welded by an extended joining process in air. Our main focus was on the effects of alloying elements (e.g., C, Si, Cr, and Mn, which are more reactive with O than with Fe) on scale adhesion, joint microstructure, and, ultimately, joint strength. Five carbon steels with different compositions of the same set of alloying elements are selected for this investigation. The model steels are hot-rolled, and surface descaling is performed several times during processing. The hot-rolled plates are welded with 1000% deformation at approximately 1000 1C. The joint strengths are evaluated by uniaxial tension testing. The joint microstructures and fractured surfaces are investigated with field emission scanning electron microscopy (FE-SEM). In addition, chemical analysis for phase identification is performed by electron probe X-ray micro-analysis (EPMA) and transmission electron microscopy (TEM). The results of our analyses reveal that Si increases the scale content at the joint by forming Fe 2 SiO 4 along the interfaces between the Fe oxides and alloys and that this phase anchors the scale to the substrate. When both the C and Si contents are increased, Fe 2 SiO 4 constitutes a larger portion of the scale-substrate interface, and the density of the internal oxide particles in the substrate near the joint is markedly increased. The tension test results demonstrate that the joint strength is decreased with increasing contents of C and Si because the joint failure is initiated by brittle fracture of the scale at the joint and the void nucleation sites around the internal oxides.

Development of a New Solid-State Joining Process for Endless Hot Rolling

Iron Steel Technology, 2009

POSCO has developed a new solid-state joining process that satifies three prerequisites for endle... more

굴곡측정법을 이용한 극후판 용접부 잔류응력분포 정량분석

Journal of Welding and Joining, 2015

Residual stresses arising from the materials processing such as welding and joining affect signif... more Residual stresses arising from the materials processing such as welding and joining affect significantly the structural integrity depending on the external loading condition. The quantitative measurement of the residual stresses is of great importance in order to characterize the effects of the residual stresses on the structural safety. In this paper, we introduce a newly devised destructive technique, the contour method (CM), which is applied for the measurements of the residual stress distributions through the thickness of a 80 mm thick steel weld. Residual stresses are evaluated from the contour, which is the normal displacement on a cut surface produced by the relaxation of residual stresses, using a finite element model. The CM provides a two-dimensional map of the residual stresses normal to the cut surface. The CM developed in the present study was validated in comparison with the residual stress distribution determined by a wellestablished neutron-diffraction residual stress instrument (RSI) instrumented in HANARO neutron research reactor.

철강 용접부 열해석 정도 향상에 관한 연구

Journal of the Korean Welding and Joining Society, 2007

This paper is the first part of the study on the accuracy improvement of numerical analysis of st... more This paper is the first part of the study on the accuracy improvement of numerical analysis of steel welds. The aim of this paper is to raise the accuracy of thermal analysis results, such as the shape and size of the weld cross section and the hardness distribution in HAZ(Heat-Affected Zone). It is known that the factors affecting on the accuracy are thermal properties, metallurgical properties and welding heat source model. It was found that the arbitrary distributed heat source model should be used to predict practical weld cross section shape and size. Also, in order to improve the prediction accuracy of HAZ hardness distribution, it was essential to consider 2 CCT(Continuous Cooling Transformation) diagrams in calculating volume fraction of transformed phases. One is the peak temperature being around melting temperature. The other is the peak temperature being around metallurgical transformation temperature.

The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels

Materials Science and Engineering: A, 2012

We present the structure-property relationships of oxide scales and interfaces formed on carbon s... more We present the structure-property relationships of oxide scales and interfaces formed on carbon steels. We select four different steels having the same alloying elements, but different compositions. Using thermodynamic calculations, the phases of the scales formed at a fixed temperature are predicted as a function of oxygen partial pressures (p O2 ). The model steels are oxidized under the same condition to the calculations for a fixed time. The scale microstructures are analyzed by electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The mechanical strength and fracture patterns of the scales are analyzed by tension test. According to the analysis and test results, the adhesion strength of the scales are determined by the contents of reactive elements such as Si and Cr that form continuous oxide layers along the interfaces. The overall structure of the scales and the cohesion strength depend on O concentration in the scales which is controlled by C content. p O2 gradient in the layers of mostly Fe oxides becomes greater with more C for a fixed Si concentration increasing the fracture resistance. The predicted phases agree well with the microstructural analysis results.

Development of a magnetic abrasive jet machining system for precision internal polishing of circular tubes

Journal of Materials Processing Technology, 1997

A recently developed finishing process using a rotating magnetic field is known to be very effici... more

The mechanical property of the oxide scale on Fe–Cr alloy steels

Journal of Alloys and Compounds, 2013

In this study, we investigated the influence of the Cr content on the microstructure and interfac... more In this study, we investigated the influence of the Cr content on the microstructure and interfacial adhesion of the oxide scale formed on steel alloys. The four different alloys selected for this study are stainless steels and low-and medium-carbon steels with different Cr and Si contents. The model steels were oxidized at 1100-1200 °C for 4-5 h. The types of oxide phases in the scale were analyzed by X-ray diffraction (XRD), and the results were compared with the phases predicted by thermodynamic calculations. The cross-sectional scale microstructures were analyzed by electron probe micro-analysis (EPMA). The interfacial adhesion strength of the scale with the model alloys were analyzed by a uniaxial tension test. According to our results, continuous Cr-oxide layers are formed along the interfaces; the thickness of these layers proportionally increases with Cr, while the total thickness of the scale is inversely proportional to the Cr content. The thick Cr-oxide layers seem to hinder the interdiffusion of Fe and O between the scale and the substrate, which decreases the growth rate of Fe-oxides on the top surface. The predicted phases in the scale and the minimum oxygen partial pressure at which each oxide phase is formed agree well with the microstructural analysis results. Our results also revealed that the Cr-oxide layers survived the tension test for up to 5% of the strain, whereas the scale on top of the Cr-oxides cracked and delaminated. Based on these results, the Cr content determines the adhesion strength of the scale.

Development of discontinuous grinding wheel with multi-porous grooves

International Journal of Machine Tools and Manufacture, 1997

Conventionally, grinding of aluminium, copper and stainless steel is difficult owing to the mecha... more Conventionally, grinding of aluminium, copper and stainless steel is difficult owing to the mechanical properties such as low hardness and high toughness which result in the wheel loading and poor surface finish. In order to grind these materials easily, a discontinuous grinding wheel with multi-porous grooves has been recently developed. The multi-porous grooves were formed during wheel manufacturing. The discontinuous grinding wheel dramatically increases the grinding performance. It is desirable to use discontinuous grinding wheel when grinding materials with high efficiency and accuracy which is impossible using conventional wheels. In this paper, the construction and manufacturing method of discontinuous grinding wheel with multi-porous grooves are explained. The grinding characteristics of the discontinuous grinding wheel is also illustrated.

Study on the characteristics of Magneto-Electrolytic-Abrasive Polishing by using the newly developed nonwoven-abrasive pads

International Journal of Machine Tools and Manufacture, 1998

... 4. Compared with other ultra-precision polishing materials, the newly developed nonwoven abra... more

High-spend machining of aluminium using diamond endmills

International Journal of Machine Tools and Manufacture, 1997

Milling experiments on aluminum alloy were carded out using a laboratory-designed diamond endmill... more Milling experiments on aluminum alloy were carded out using a laboratory-designed diamond endmill. The machining of aluminium alloy using a conventional tool at a conventional cutting speed generally results in short tool-life, poor surface quality and poor edge finishes because of the formation of built-up edges and burrs. The machining technology of aluminum alloy surfaces with a good surface finish and edge finishing is very important and required in many industries. The experimental results reveal the possibility of machining a mirror-like aluminum alloy without built-up edges and burrs using the designed diamond endmill at high speed.

이형관의 내면정밀가공용 순환식 자기입자분사가공 시스템 개발 = Development of magnetic abrasive jet machining for precision internal polishing of complex shaped tubes

A Comparative Study of Ductile Damage Models Approaches for Joint Strength Prediction in Hot Shear Joining Process

Procedia Engineering, 2017

Ductile fracture prediction is often considered as a challenging task in complex stress state app... more Ductile fracture prediction is often considered as a challenging task in complex stress state applications, such as metal joining process. A systematic method for predicting the joint strength in the hot shear joining process is presented in this paper by considering two different ductile damage models, namely Johnson-Cook and modified Lemaitre damage models. Since the crack formation in metalworking is considered to be a combination of both load conditions and metallurgical properties, the microstructure evolution is taken into account by characterizing the considered material, AISI-1045, at different temperatures and for different strain rates and by testing different specimen shapes, namely round bar, joint bar, flat and notched specimens. The derived material properties have been inversely calibrated in order to be used in the developed numerical model. A shear joining process simulation has been implemented in DEFORM and have been validate with shear joining experiments carried out at 950°C, allowing to validate the accuracy of the proposed model in simulating the hot shear joining, both in terms of joining strength and identification of the damage locations. Thanks to a precise material characterization, and by utilizing the proposed numerical model, both the joining tool design and the process condition can be studied and improved.

The effect of alloy compositions on the joint strength of carbon steels welded by the extended hot-rolling process

Materials Science and Engineering: A, 2012

In this study, we investigated the factors that determine the joint strength of carbon steels wel... more In this study, we investigated the factors that determine the joint strength of carbon steels welded by an extended joining process in air. Our main focus was on the effects of alloying elements (e.g., C, Si, Cr, and Mn, which are more reactive with O than with Fe) on scale adhesion, joint microstructure, and, ultimately, joint strength. Five carbon steels with different compositions of the same set of alloying elements are selected for this investigation. The model steels are hot-rolled, and surface descaling is performed several times during processing. The hot-rolled plates are welded with 1000% deformation at approximately 1000 1C. The joint strengths are evaluated by uniaxial tension testing. The joint microstructures and fractured surfaces are investigated with field emission scanning electron microscopy (FE-SEM). In addition, chemical analysis for phase identification is performed by electron probe X-ray micro-analysis (EPMA) and transmission electron microscopy (TEM). The results of our analyses reveal that Si increases the scale content at the joint by forming Fe 2 SiO 4 along the interfaces between the Fe oxides and alloys and that this phase anchors the scale to the substrate. When both the C and Si contents are increased, Fe 2 SiO 4 constitutes a larger portion of the scale-substrate interface, and the density of the internal oxide particles in the substrate near the joint is markedly increased. The tension test results demonstrate that the joint strength is decreased with increasing contents of C and Si because the joint failure is initiated by brittle fracture of the scale at the joint and the void nucleation sites around the internal oxides.

Development of a New Solid-State Joining Process for Endless Hot Rolling

Iron Steel Technology, 2009

POSCO has developed a new solid-state joining process that satifies three prerequisites for endle... more

굴곡측정법을 이용한 극후판 용접부 잔류응력분포 정량분석

Journal of Welding and Joining, 2015

Residual stresses arising from the materials processing such as welding and joining affect signif... more Residual stresses arising from the materials processing such as welding and joining affect significantly the structural integrity depending on the external loading condition. The quantitative measurement of the residual stresses is of great importance in order to characterize the effects of the residual stresses on the structural safety. In this paper, we introduce a newly devised destructive technique, the contour method (CM), which is applied for the measurements of the residual stress distributions through the thickness of a 80 mm thick steel weld. Residual stresses are evaluated from the contour, which is the normal displacement on a cut surface produced by the relaxation of residual stresses, using a finite element model. The CM provides a two-dimensional map of the residual stresses normal to the cut surface. The CM developed in the present study was validated in comparison with the residual stress distribution determined by a wellestablished neutron-diffraction residual stress instrument (RSI) instrumented in HANARO neutron research reactor.

철강 용접부 열해석 정도 향상에 관한 연구

Journal of the Korean Welding and Joining Society, 2007

This paper is the first part of the study on the accuracy improvement of numerical analysis of st... more This paper is the first part of the study on the accuracy improvement of numerical analysis of steel welds. The aim of this paper is to raise the accuracy of thermal analysis results, such as the shape and size of the weld cross section and the hardness distribution in HAZ(Heat-Affected Zone). It is known that the factors affecting on the accuracy are thermal properties, metallurgical properties and welding heat source model. It was found that the arbitrary distributed heat source model should be used to predict practical weld cross section shape and size. Also, in order to improve the prediction accuracy of HAZ hardness distribution, it was essential to consider 2 CCT(Continuous Cooling Transformation) diagrams in calculating volume fraction of transformed phases. One is the peak temperature being around melting temperature. The other is the peak temperature being around metallurgical transformation temperature.

The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels

Materials Science and Engineering: A, 2012

We present the structure-property relationships of oxide scales and interfaces formed on carbon s... more We present the structure-property relationships of oxide scales and interfaces formed on carbon steels. We select four different steels having the same alloying elements, but different compositions. Using thermodynamic calculations, the phases of the scales formed at a fixed temperature are predicted as a function of oxygen partial pressures (p O2 ). The model steels are oxidized under the same condition to the calculations for a fixed time. The scale microstructures are analyzed by electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The mechanical strength and fracture patterns of the scales are analyzed by tension test. According to the analysis and test results, the adhesion strength of the scales are determined by the contents of reactive elements such as Si and Cr that form continuous oxide layers along the interfaces. The overall structure of the scales and the cohesion strength depend on O concentration in the scales which is controlled by C content. p O2 gradient in the layers of mostly Fe oxides becomes greater with more C for a fixed Si concentration increasing the fracture resistance. The predicted phases agree well with the microstructural analysis results.

Development of a magnetic abrasive jet machining system for precision internal polishing of circular tubes

Journal of Materials Processing Technology, 1997

A recently developed finishing process using a rotating magnetic field is known to be very effici... more

The mechanical property of the oxide scale on Fe–Cr alloy steels

Journal of Alloys and Compounds, 2013

In this study, we investigated the influence of the Cr content on the microstructure and interfac... more In this study, we investigated the influence of the Cr content on the microstructure and interfacial adhesion of the oxide scale formed on steel alloys. The four different alloys selected for this study are stainless steels and low-and medium-carbon steels with different Cr and Si contents. The model steels were oxidized at 1100-1200 °C for 4-5 h. The types of oxide phases in the scale were analyzed by X-ray diffraction (XRD), and the results were compared with the phases predicted by thermodynamic calculations. The cross-sectional scale microstructures were analyzed by electron probe micro-analysis (EPMA). The interfacial adhesion strength of the scale with the model alloys were analyzed by a uniaxial tension test. According to our results, continuous Cr-oxide layers are formed along the interfaces; the thickness of these layers proportionally increases with Cr, while the total thickness of the scale is inversely proportional to the Cr content. The thick Cr-oxide layers seem to hinder the interdiffusion of Fe and O between the scale and the substrate, which decreases the growth rate of Fe-oxides on the top surface. The predicted phases in the scale and the minimum oxygen partial pressure at which each oxide phase is formed agree well with the microstructural analysis results. Our results also revealed that the Cr-oxide layers survived the tension test for up to 5% of the strain, whereas the scale on top of the Cr-oxides cracked and delaminated. Based on these results, the Cr content determines the adhesion strength of the scale.

Development of discontinuous grinding wheel with multi-porous grooves

International Journal of Machine Tools and Manufacture, 1997

Conventionally, grinding of aluminium, copper and stainless steel is difficult owing to the mecha... more Conventionally, grinding of aluminium, copper and stainless steel is difficult owing to the mechanical properties such as low hardness and high toughness which result in the wheel loading and poor surface finish. In order to grind these materials easily, a discontinuous grinding wheel with multi-porous grooves has been recently developed. The multi-porous grooves were formed during wheel manufacturing. The discontinuous grinding wheel dramatically increases the grinding performance. It is desirable to use discontinuous grinding wheel when grinding materials with high efficiency and accuracy which is impossible using conventional wheels. In this paper, the construction and manufacturing method of discontinuous grinding wheel with multi-porous grooves are explained. The grinding characteristics of the discontinuous grinding wheel is also illustrated.

Study on the characteristics of Magneto-Electrolytic-Abrasive Polishing by using the newly developed nonwoven-abrasive pads

International Journal of Machine Tools and Manufacture, 1998

... 4. Compared with other ultra-precision polishing materials, the newly developed nonwoven abra... more

High-spend machining of aluminium using diamond endmills

International Journal of Machine Tools and Manufacture, 1997

Milling experiments on aluminum alloy were carded out using a laboratory-designed diamond endmill... more Milling experiments on aluminum alloy were carded out using a laboratory-designed diamond endmill. The machining of aluminium alloy using a conventional tool at a conventional cutting speed generally results in short tool-life, poor surface quality and poor edge finishes because of the formation of built-up edges and burrs. The machining technology of aluminum alloy surfaces with a good surface finish and edge finishing is very important and required in many industries. The experimental results reveal the possibility of machining a mirror-like aluminum alloy without built-up edges and burrs using the designed diamond endmill at high speed.

이형관의 내면정밀가공용 순환식 자기입자분사가공 시스템 개발 = Development of magnetic abrasive jet machining for precision internal polishing of complex shaped tubes