Marcell Gaspar | Miskolc University (original) (raw)

Papers by Marcell Gaspar

Welding in the World

The appropriate matching of base and filler material is a complex task, where yield strength matc... more The appropriate matching of base and filler material is a complex task, where yield strength matching is the most general aspect. As the strength properties of structural steels have significantly improved in the past decades, the matching problem has become more relevant today. The mismatch phenomenon significantly affects the behavior of welded joints under dynamic and cyclic loading. Among cyclic loading, low-cycle fatigue (LCF) often occurs in welded steel constructions; furthermore, the LCF resistance of these advanced steels and their welded joints is limitedly known. In this paper, welding experiments are presented for the analysis of the LCF behavior of differently matched butt-welded joints made from two grades of quenched and tempered (Q + T) high-strength steels. For S690QL steel, matched and overmatched consumables were applied, while for S960QL steel, matched and undermatched filler materials were used. Material tests were performed to determine the mechanical propertie...

Welding in the World

In recent decades the toughness of welded structures becomes more and more important. This trend ... more In recent decades the toughness of welded structures becomes more and more important. This trend is particularly valid for the advanced high-strength steels (AHSS), which have reduced toughness comparing to the low-strength structural steels. Dynamic characteristics of the welded joints of the newly developed high-strength steel sheets required by the automotive industry have been neglected, with welding procedures being optimized on the static joint properties, mainly on the tensile-shear force. Up to the present time, testing of dynamic properties of the spot-welded joints has been performed by increased testing speed during tensile testing. The authors have developed a new dynamic testing method and designed new testing equipment for impact bending, which can give a numeric result to characterize the resistance of spot-welded joints against dynamic load. In this paper, this method will be used to evaluate resistance spot-welded joints made on DP600 and DP800 steels with three dif...

Lecture Notes in Mechanical Engineering, 2018

In the automotive industry there is a growing tendency for the application of high strength alumi... more In the automotive industry there is a growing tendency for the application of high strength aluminium alloys. In spite of their significant role in weight reduction there are still obstacles for their wider use due to their limited formability and weldability. Hot forming and in-die quenching (HFQ) process was recently developed for the forming of car body sheets. During the HFQ technology the sheet metal forming should be performed in a solution annealed condition. In the solution annealed condition the aluminium alloys have lower strength and better formability properties. The forming process is followed by a precipitation hardening which is generally connected with the painting of body parts (bake hardening). Besides the formability the implementation of HFQ has an effect on the weldability properties, too. HFQ must have an effect on the resistance spot welding (RSW) of aluminium sheets since the weld nuggets are produced after the HFQ, in the assembly part of the production chain, when the aluminium alloy is in a solution annealed and formed condition. The final properties of the welded joints are determined by the precipitation hardening which is the final step of the whole production process. The present research work aims to investigate the effect of the HFQ process on the weldability of AA6082-T6 aluminium alloy. The properties of the RSW joints are examined in different conditions (T6 delivery condition, solution annealed, precipitation aged). The materials tests include conventional macro testing, hardness tests and tensileshear tests extended with EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscatter Diffraction) tests in order to characterize the distribution of alloying elements and to analyze the grain structure.

Ferrous alloys are still the most widely used metallic materials. Basic design goal for engineers... more Ferrous alloys are still the most widely used metallic materials. Basic design goal for engineers is to decrease the size and weight of structure, which can be achieved by steels using various strength increasing methods. However, the method applied for strength increasing has a significant effect on the latter production, especially welding challenges. In order to preserve the unique strength and toughness properties after welding, precisely controlled linear energy, limited interpass temperature and the conscious selection of filler metal is needed by considering the deep investigation of the so-called matching question.

Multidiszciplináris Tudományok, Mar 25, 2013

There is a growing demand for the wider application of high strength steels in automotive industr... more There is a growing demand for the wider application of high strength steels in automotive industry. Since these steels are mostly used in welded structures, therefore welding engineers should have deep knowledge about the welding of high strength steels, requiring technological solutions different from mild steels. When the weldability of these steels is investigated, the selection of the appropriate filler metal and the effect of welding parameters on heat affected zone should be focused on. This paper aims to present the properties of HAZ of (Q+T) high strength steel joints by the use of physical simulation, while the effect of welding parameters (t8,5/5 cooling times) on the different HAZ zones is also investigated.

The highest strength structural steels belong to the quenched and tempered group (Q+T). By increa... more The highest strength structural steels belong to the quenched and tempered group (Q+T). By increasing the strength of structural steels, the welding lobe becomes narrow. During the planning of the welding technology for a quenched and tempered high strength steel an extremely narrow welding lobe should be defined. The welding technology can be successfully planned on the basis of the optimal t8.5/5 cooling time range. In this paper welding experiments are presented to describe the effect of technological parameters (preheating/interpass temperature, linear energy) on joint properties, whilst the new development tendencies of high strength structural steels are also highlighted.

The publications of the MultiScience - XXX. MicroCAD International Scientific Conference, 2016

Lecture Notes in Mechanical Engineering, 2017

Based on the welding heat cycle models physical simulators are capable for the creation of critic... more Based on the welding heat cycle models physical simulators are capable for the creation of critical heat-affected zones (HAZ). The simulated HAZ areas can be examined by various material testing methods (e. g. Charpy V-notch impact test) due to their increased homogeneous volume compared to their extension in real welding experiments. In our research work relevant technological variants (t 8.5/5 = 2.5…30 s) for gas metal arc welding technology were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the coarse-grained HAZ was analysed. In thermo-mechanical simulators the achievable cooling rate is always the function of specimen geometry and the presence of external cooling. Therefore a special drilled specimen with external cooling was developed for performing a shorter (t 8.5/5 = 2.5 s) cooling than 5 s, which cannot be realized on the conventional Gleeble specimen. Heat cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ) zone were investigated by scanning electron microscope, hardness test and Charpy V-notch impact test.

Lecture Notes in Mechanical Engineering, 2017

High cycle fatigue tests were performed on two strength categories of high strength steels, on qu... more High cycle fatigue tests were performed on two strength categories of high strength steels, on quenched and tempered (S690 and S960) and thermomechanical (S960) types, on base materials and their gas metal arc welded joints, and on different matching conditions. The planning and optimization of welding technologies based on investigations under cyclic loading conditions were built upon a large number of investigations and statistical evaluation of the test results. Statistical approach was already applied during the preparation of the investigations, which have been allowed the expansion of the scope of the results and the increasing of their reliability. The article demonstrates and evaluates the results comparing with each other and with literary data.

Advanced Materials Research, 2014

There are different prescriptions containing fatigue crack propagation limit curves and rules for... more There are different prescriptions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed (i) to determine fatigue crack propagation limit curves for high strength steels and their welded joints, based on the Paris-Erdogan law; (ii) to use the determined limit curves for engineering critical assessment (ECA) calculations. Experiments were performed on different high strength steels and their welded joints; and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed by ΔK-decreasing and constant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the limit curves. Engineering critical assessment calculations were performed on a welded structural element having crack like defects.

Design, Fabrication and Economy of Metal Structures, 2013

In order to guarantee the outstanding mechanical properties of high strength steel joints during ... more In order to guarantee the outstanding mechanical properties of high strength steel joints during the welding, preheating, strictly limited linear heat input and interpass temperature should be applied. These parameters together define a narrow optimal t8.5/5 cooling range, and result low productivity. Since significant rise cannot be achieved in the productivity by increasing the heat input due to the metallographycal limitations, steel producers cooperating with filler metal producers have developed flux cored wires for the improving of productivity. These tubular wires can be suitable for raising the melting rate; however, their application may have risks regarding the weld quality (slag inclusions). In this article, welding experiments for AHSS (Advanced High Strength Steel) will be demonstrated for the comparison of two flux cored wires (basic and metal powder) to a traditional solid wire.

Based on the welding heat cycle models physical simulators are capable for the creation of critic... more Based on the welding heat cycle models physical simulators are capable for the creation of critical heat-affected zones (HAZ). The simulated HAZ areas can be examined by various material testing methods (e. g. Charpy V-notch impact test) due to their increased homogeneous volume compared to their extension in real welding experiments. In our research work relevant technological variants (t8.5/5 = 2.5…30 s) for gas metal arc welding technology were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the coarse-grained HAZ was analysed. In thermo-mechanical simulators the achievable cooling rate is always the function of specimen geometry and the presence of external cooling. Therefore a special drilled specimen with external cooling was developed for performing a shorter (t8.5/5 = 2.5 s) cooling than 5 s, which cannot be realized on the conventional Gleeble specimen. Heat cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ) zone were investigated by scanning electron microscope, hardness test and Charpy V-notch impact test.

High cycle fatigue tests were performed on two strength categories of high strength steels, on qu... more High cycle fatigue tests were performed on two strength categories of high strength steels, on quenched and tempered (S690 and S960) and thermomechanical (S960) types, on base materials and their gas metal arc welded joints, and on different matching conditions. The planning and optimization of welding technologies based on investigations under cyclic loading conditions were built upon a large number of investigations and statistical evaluation of the test results. Statistical approach was already applied during the preparation of the investigations, which have been allowed the expansion of the scope of the results and the increasing of their reliability. The article demonstrates and evaluates the results comparing with each other and with literary data.

Materials Science Forum, 2015

Physical simulation aided process optimisation aimed sufficient HAZ toughness for quenched and tempered AHSS

The outstanding mechanical properties of quenched and tempered high strength steels cannot be ade... more The outstanding mechanical properties of quenched and tempered high strength steels cannot be adequately preserved during the welding due to the irreversible microstructural changes in the heat affected zone (HAZ). There is a general statement that the area of weldability lobe decreases by the application of higher strength categories. According to numerous experimental results, by the application of low heat input welding the disadvantageous microstructural changes, and therefore the decrease of strength and toughness properties can be minimized, although other welding difficulties can be noticed (e.g. cold cracking). HAZ properties can be limitedly analysed by conventional material tests, therefore physical simulators (i.e. Gleeble) were developed for the examination of different HAZ areas. In our research work three technological variants 5, 15 and 30 s t8.5/5 were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the critical HAZ areas of single and multipass welded joints was analysed. Heat cycles were determined according to Rykalin 3D model, where the whole heat cycle, including heated part as well, was described by time-temperature points instead of the automatic software settings. The properties of the selected coarse grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse grained (ICCGHAZ) zones were investigated by scanning electron microscopes, macro and micro hardness tests and instrumented Charpy V-notch impact tests. The industrial utilization possibilities of experimental results are also presented by an industrial partner in this research work.

There is a growing demand for the wider application of high strength steels in automotive industr... more There is a growing demand for the wider application of high strength steels in automotive
industry. Since these steels are mostly used in welded structures, therefore welding engineers
should have deep knowledge about the welding of high strength steels, requiring
technological solutions different from mild steels. When the weldability of these steels is
investigated, the selection of the appropriate filler metal and the effect of welding parameters
on heat affected zone should be focused on. This paper aims to present the properties
of HAZ of (Q+T) high strength steel joints by the use of physical simulation, while the effect
of welding parameters (t8,5/5 cooling times) on the different HAZ zones is also investigated.

Structural inhomogeneity and the chemical composition are one of the core issues of weld-ing with... more Structural inhomogeneity and the chemical composition are one of the core issues of weld-ing with filler metal. The most significant factors influencing the extent of inhomogeneity are the joint geometry, welding technology and the selection of filler metal. The appropri-ate matching of base and filler metal is a complex work, where strength matching is the most general aspect. As the strength properties of structural steels have significantly im-proved in the past decades, the role of matching problem has been more relevant today. Nowadays there are advanced high strength steels (AHSS) that cannot be welded by match-ing filler metal according to our knowledge. However, there are special cases and steel categories when undermatched electrodes can be advantageous. In present paper welding experiments are presented for the analysis of mismatch behaviour. Welded joints were made from one of the highest category of quenched and tempered high strength steels by a matched and an undermatched welding consumable. Tensile tests were analysed by a spe-cial four-camera recording system including a so-called Auto Grid strain analysis system.

Quenched and tempered steels belong to the highest strength categories of structural steels. Thei... more Quenched and tempered steels belong to the highest strength categories of structural steels. Their weldability is more complicated than mild steels due to the risk of cold cracking and the decrease of toughness and strength properties in heat affected zones. Since the out-standing strength properties can be primarily exploited in mobile structures where signifi-cant energy saving can be achieved with their application, cyclic loading can often occur among the various loading conditions. Due to the above mentioned reasons the analysis of fatigue crack growth is relevant in terms of this steel group. There are different prescrip-tions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed to determine fatigue crack propagation limit curves for S960QL and its welded joint, based on the Paris-Erdogan law. Experiments were performed on specimens cut from gas metal arc welded joints and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed by ΔK-decreasing and con-stant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the fatigue crack propagation limit curves.

Welding in the World

The appropriate matching of base and filler material is a complex task, where yield strength matc... more The appropriate matching of base and filler material is a complex task, where yield strength matching is the most general aspect. As the strength properties of structural steels have significantly improved in the past decades, the matching problem has become more relevant today. The mismatch phenomenon significantly affects the behavior of welded joints under dynamic and cyclic loading. Among cyclic loading, low-cycle fatigue (LCF) often occurs in welded steel constructions; furthermore, the LCF resistance of these advanced steels and their welded joints is limitedly known. In this paper, welding experiments are presented for the analysis of the LCF behavior of differently matched butt-welded joints made from two grades of quenched and tempered (Q + T) high-strength steels. For S690QL steel, matched and overmatched consumables were applied, while for S960QL steel, matched and undermatched filler materials were used. Material tests were performed to determine the mechanical propertie...

Welding in the World

In recent decades the toughness of welded structures becomes more and more important. This trend ... more In recent decades the toughness of welded structures becomes more and more important. This trend is particularly valid for the advanced high-strength steels (AHSS), which have reduced toughness comparing to the low-strength structural steels. Dynamic characteristics of the welded joints of the newly developed high-strength steel sheets required by the automotive industry have been neglected, with welding procedures being optimized on the static joint properties, mainly on the tensile-shear force. Up to the present time, testing of dynamic properties of the spot-welded joints has been performed by increased testing speed during tensile testing. The authors have developed a new dynamic testing method and designed new testing equipment for impact bending, which can give a numeric result to characterize the resistance of spot-welded joints against dynamic load. In this paper, this method will be used to evaluate resistance spot-welded joints made on DP600 and DP800 steels with three dif...

Lecture Notes in Mechanical Engineering, 2018

In the automotive industry there is a growing tendency for the application of high strength alumi... more In the automotive industry there is a growing tendency for the application of high strength aluminium alloys. In spite of their significant role in weight reduction there are still obstacles for their wider use due to their limited formability and weldability. Hot forming and in-die quenching (HFQ) process was recently developed for the forming of car body sheets. During the HFQ technology the sheet metal forming should be performed in a solution annealed condition. In the solution annealed condition the aluminium alloys have lower strength and better formability properties. The forming process is followed by a precipitation hardening which is generally connected with the painting of body parts (bake hardening). Besides the formability the implementation of HFQ has an effect on the weldability properties, too. HFQ must have an effect on the resistance spot welding (RSW) of aluminium sheets since the weld nuggets are produced after the HFQ, in the assembly part of the production chain, when the aluminium alloy is in a solution annealed and formed condition. The final properties of the welded joints are determined by the precipitation hardening which is the final step of the whole production process. The present research work aims to investigate the effect of the HFQ process on the weldability of AA6082-T6 aluminium alloy. The properties of the RSW joints are examined in different conditions (T6 delivery condition, solution annealed, precipitation aged). The materials tests include conventional macro testing, hardness tests and tensileshear tests extended with EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscatter Diffraction) tests in order to characterize the distribution of alloying elements and to analyze the grain structure.

Ferrous alloys are still the most widely used metallic materials. Basic design goal for engineers... more Ferrous alloys are still the most widely used metallic materials. Basic design goal for engineers is to decrease the size and weight of structure, which can be achieved by steels using various strength increasing methods. However, the method applied for strength increasing has a significant effect on the latter production, especially welding challenges. In order to preserve the unique strength and toughness properties after welding, precisely controlled linear energy, limited interpass temperature and the conscious selection of filler metal is needed by considering the deep investigation of the so-called matching question.

Multidiszciplináris Tudományok, Mar 25, 2013

There is a growing demand for the wider application of high strength steels in automotive industr... more There is a growing demand for the wider application of high strength steels in automotive industry. Since these steels are mostly used in welded structures, therefore welding engineers should have deep knowledge about the welding of high strength steels, requiring technological solutions different from mild steels. When the weldability of these steels is investigated, the selection of the appropriate filler metal and the effect of welding parameters on heat affected zone should be focused on. This paper aims to present the properties of HAZ of (Q+T) high strength steel joints by the use of physical simulation, while the effect of welding parameters (t8,5/5 cooling times) on the different HAZ zones is also investigated.

The highest strength structural steels belong to the quenched and tempered group (Q+T). By increa... more The highest strength structural steels belong to the quenched and tempered group (Q+T). By increasing the strength of structural steels, the welding lobe becomes narrow. During the planning of the welding technology for a quenched and tempered high strength steel an extremely narrow welding lobe should be defined. The welding technology can be successfully planned on the basis of the optimal t8.5/5 cooling time range. In this paper welding experiments are presented to describe the effect of technological parameters (preheating/interpass temperature, linear energy) on joint properties, whilst the new development tendencies of high strength structural steels are also highlighted.

The publications of the MultiScience - XXX. MicroCAD International Scientific Conference, 2016

Lecture Notes in Mechanical Engineering, 2017

Based on the welding heat cycle models physical simulators are capable for the creation of critic... more Based on the welding heat cycle models physical simulators are capable for the creation of critical heat-affected zones (HAZ). The simulated HAZ areas can be examined by various material testing methods (e. g. Charpy V-notch impact test) due to their increased homogeneous volume compared to their extension in real welding experiments. In our research work relevant technological variants (t 8.5/5 = 2.5…30 s) for gas metal arc welding technology were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the coarse-grained HAZ was analysed. In thermo-mechanical simulators the achievable cooling rate is always the function of specimen geometry and the presence of external cooling. Therefore a special drilled specimen with external cooling was developed for performing a shorter (t 8.5/5 = 2.5 s) cooling than 5 s, which cannot be realized on the conventional Gleeble specimen. Heat cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ) zone were investigated by scanning electron microscope, hardness test and Charpy V-notch impact test.

Lecture Notes in Mechanical Engineering, 2017

High cycle fatigue tests were performed on two strength categories of high strength steels, on qu... more High cycle fatigue tests were performed on two strength categories of high strength steels, on quenched and tempered (S690 and S960) and thermomechanical (S960) types, on base materials and their gas metal arc welded joints, and on different matching conditions. The planning and optimization of welding technologies based on investigations under cyclic loading conditions were built upon a large number of investigations and statistical evaluation of the test results. Statistical approach was already applied during the preparation of the investigations, which have been allowed the expansion of the scope of the results and the increasing of their reliability. The article demonstrates and evaluates the results comparing with each other and with literary data.

Advanced Materials Research, 2014

There are different prescriptions containing fatigue crack propagation limit curves and rules for... more There are different prescriptions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed (i) to determine fatigue crack propagation limit curves for high strength steels and their welded joints, based on the Paris-Erdogan law; (ii) to use the determined limit curves for engineering critical assessment (ECA) calculations. Experiments were performed on different high strength steels and their welded joints; and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed by ΔK-decreasing and constant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the limit curves. Engineering critical assessment calculations were performed on a welded structural element having crack like defects.

Design, Fabrication and Economy of Metal Structures, 2013

In order to guarantee the outstanding mechanical properties of high strength steel joints during ... more In order to guarantee the outstanding mechanical properties of high strength steel joints during the welding, preheating, strictly limited linear heat input and interpass temperature should be applied. These parameters together define a narrow optimal t8.5/5 cooling range, and result low productivity. Since significant rise cannot be achieved in the productivity by increasing the heat input due to the metallographycal limitations, steel producers cooperating with filler metal producers have developed flux cored wires for the improving of productivity. These tubular wires can be suitable for raising the melting rate; however, their application may have risks regarding the weld quality (slag inclusions). In this article, welding experiments for AHSS (Advanced High Strength Steel) will be demonstrated for the comparison of two flux cored wires (basic and metal powder) to a traditional solid wire.

Based on the welding heat cycle models physical simulators are capable for the creation of critic... more Based on the welding heat cycle models physical simulators are capable for the creation of critical heat-affected zones (HAZ). The simulated HAZ areas can be examined by various material testing methods (e. g. Charpy V-notch impact test) due to their increased homogeneous volume compared to their extension in real welding experiments. In our research work relevant technological variants (t8.5/5 = 2.5…30 s) for gas metal arc welding technology were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the coarse-grained HAZ was analysed. In thermo-mechanical simulators the achievable cooling rate is always the function of specimen geometry and the presence of external cooling. Therefore a special drilled specimen with external cooling was developed for performing a shorter (t8.5/5 = 2.5 s) cooling than 5 s, which cannot be realized on the conventional Gleeble specimen. Heat cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ) zone were investigated by scanning electron microscope, hardness test and Charpy V-notch impact test.

High cycle fatigue tests were performed on two strength categories of high strength steels, on qu... more High cycle fatigue tests were performed on two strength categories of high strength steels, on quenched and tempered (S690 and S960) and thermomechanical (S960) types, on base materials and their gas metal arc welded joints, and on different matching conditions. The planning and optimization of welding technologies based on investigations under cyclic loading conditions were built upon a large number of investigations and statistical evaluation of the test results. Statistical approach was already applied during the preparation of the investigations, which have been allowed the expansion of the scope of the results and the increasing of their reliability. The article demonstrates and evaluates the results comparing with each other and with literary data.

Materials Science Forum, 2015

Physical simulation aided process optimisation aimed sufficient HAZ toughness for quenched and tempered AHSS

The outstanding mechanical properties of quenched and tempered high strength steels cannot be ade... more The outstanding mechanical properties of quenched and tempered high strength steels cannot be adequately preserved during the welding due to the irreversible microstructural changes in the heat affected zone (HAZ). There is a general statement that the area of weldability lobe decreases by the application of higher strength categories. According to numerous experimental results, by the application of low heat input welding the disadvantageous microstructural changes, and therefore the decrease of strength and toughness properties can be minimized, although other welding difficulties can be noticed (e.g. cold cracking). HAZ properties can be limitedly analysed by conventional material tests, therefore physical simulators (i.e. Gleeble) were developed for the examination of different HAZ areas. In our research work three technological variants 5, 15 and 30 s t8.5/5 were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the critical HAZ areas of single and multipass welded joints was analysed. Heat cycles were determined according to Rykalin 3D model, where the whole heat cycle, including heated part as well, was described by time-temperature points instead of the automatic software settings. The properties of the selected coarse grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse grained (ICCGHAZ) zones were investigated by scanning electron microscopes, macro and micro hardness tests and instrumented Charpy V-notch impact tests. The industrial utilization possibilities of experimental results are also presented by an industrial partner in this research work.

There is a growing demand for the wider application of high strength steels in automotive industr... more There is a growing demand for the wider application of high strength steels in automotive
industry. Since these steels are mostly used in welded structures, therefore welding engineers
should have deep knowledge about the welding of high strength steels, requiring
technological solutions different from mild steels. When the weldability of these steels is
investigated, the selection of the appropriate filler metal and the effect of welding parameters
on heat affected zone should be focused on. This paper aims to present the properties
of HAZ of (Q+T) high strength steel joints by the use of physical simulation, while the effect
of welding parameters (t8,5/5 cooling times) on the different HAZ zones is also investigated.

Structural inhomogeneity and the chemical composition are one of the core issues of weld-ing with... more Structural inhomogeneity and the chemical composition are one of the core issues of weld-ing with filler metal. The most significant factors influencing the extent of inhomogeneity are the joint geometry, welding technology and the selection of filler metal. The appropri-ate matching of base and filler metal is a complex work, where strength matching is the most general aspect. As the strength properties of structural steels have significantly im-proved in the past decades, the role of matching problem has been more relevant today. Nowadays there are advanced high strength steels (AHSS) that cannot be welded by match-ing filler metal according to our knowledge. However, there are special cases and steel categories when undermatched electrodes can be advantageous. In present paper welding experiments are presented for the analysis of mismatch behaviour. Welded joints were made from one of the highest category of quenched and tempered high strength steels by a matched and an undermatched welding consumable. Tensile tests were analysed by a spe-cial four-camera recording system including a so-called Auto Grid strain analysis system.

Quenched and tempered steels belong to the highest strength categories of structural steels. Thei... more Quenched and tempered steels belong to the highest strength categories of structural steels. Their weldability is more complicated than mild steels due to the risk of cold cracking and the decrease of toughness and strength properties in heat affected zones. Since the out-standing strength properties can be primarily exploited in mobile structures where signifi-cant energy saving can be achieved with their application, cyclic loading can often occur among the various loading conditions. Due to the above mentioned reasons the analysis of fatigue crack growth is relevant in terms of this steel group. There are different prescrip-tions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed to determine fatigue crack propagation limit curves for S960QL and its welded joint, based on the Paris-Erdogan law. Experiments were performed on specimens cut from gas metal arc welded joints and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed by ΔK-decreasing and con-stant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the fatigue crack propagation limit curves.