Metallkundliche Untersuchungen zum Hochtemperaturverformungsverhalten strahlgeschweißter hochwarmfester Werkstoffe

Due to the more productive manufacturing processes and the achievable cost reduction in the manufacture of high-quality components, laser and electron beam welding are becoming increasingly important for high-temperature materials. This is contrasted by the fact that, to date, there is almost no information available regarding the creep and fatigue properties of beam-welded connections made of heat-resistant materials in the range from room temperature to the operating temperature. The aim of this research program is to provide the designer with data on the static and cyclical dimensioning of beam-welded components made of selected high-temperature alloys in the temperature range up to 900 ° C on the basis of metallurgical studies. The results obtained should serve as a model for similar alloys in this group of materials. Since the influence of welding processes on the microstructural changes and on the mechanical-technological characteristics of high-temperature resistant materials has not yet been fully clarified, the answer to this question, along with studies on the influence of post-heat treatment after welding, is the focus of the planned research project.

Research area: Material Science

Contact: Dr.-Ing. Thomas Spirowski

Mikrostrukturelle Veränderungen in der Zylinderlaufbahn von PKW Dieselmotoren aus Grauguss und mittels thermischer Spritzverfahren hergestellter Stahlschichten

Research area: Material Science

Contact: Dr.-Ing. Marieke Hahn

Possible Pathways of Particle Formation in CoCrMo Sliding Wear

This study belongs to the areas of tribology and orthopaedic research. It deals with the tribological system of metal-on-metal hip joints made from CoCrMo alloys. Such hip joints operate within the ultra-low sliding wear regime, but yet can be subjected to failure due to particle formation and resulting adverse tissue reactions. In order to gain a better understanding of complex processes within the tribological interface leading to the formation of wear particles the focus is set on four aspects: microstructure, surface alterations, subsurface alterations and wear particle structure and composition. Therefore high resolution techniques like SEM, TEM, AFM and FIB are utilized. The findings of this work contribute to a better understanding of current clinical issues with some metal-on-metal hip devices, but can also be applied to other tribological applications by explaining tribologically induced processes enabling low wear rates and the formation of nanometer size wear debris.

Research area: Material Science

Contact: Dr.-Ing. Robin Pourzal

Einfluss der Kaltverformung auf das Ermüdungsverhalten von austenitischen hochinterstitiell legierten Stählen

The combination of already known and new knowledge makes it easier to understand and design functional materials like austenitic high interstitial steels (AHIS). AHIS are functional metals because of their advantageous mechanical, chemical and physical material properties. These properties are the main reason, why AHIS are used for diverse applications in different industrial domains.

Especially mechanical properties of materials play an essential role for technical design of applications. In solution annealed state AHIS show better monotonic and cyclic strength than conventional FeCrNi-steels. In cold worked state only the monotonic strength is better than FeCrNi-steels. The endurance limit doesn’t increase in the same ratio as the yield strength in cold worked AHIS.

In this study, the influence of cold working to the fatigue behaviour of AHIS with C+N-content of 0.85 – 1.07 wt-% was investigated. The specimens were cold worked by a defined strain rate. After cold working the specimens were fatigued by total strain controlled fatigue tests at room temperature. Monotonic and cyclic mechanical properties e.g. tensile strength and endurance limit of AHIS were measured by means of mechanical testing. After mechanical testing the samples were investigated using light microscopy, SEM including EBSD and TEM. The monotonic strength of AHIS increases with cold working and increasing content of interstitial alloying elements C+N. Cold working and an increasing content of interstitial alloying elements C+N have an opposite effect on the endurance limit of AHIS. With increasing content of interstitial alloying elements C+N and increasing cold working state the cyclic strength e.g. endurance limit decreases in AHIS. The distinct ability of dislocation hardening and solid solution hardening of AHIS promotes the formation of secondary cracks at the surface and in the bulk of the specimens. Most secondary cracks were found along persistent Lueders bands. The advantage of cold worked AHIS is the higher robustness against damage. A high amount of secondary cracks leads to relaxation within material and permits crack branching. Thus, the lifetime of the AHIS material and component can be extended.

Research area: Material Science / Fatigue

Contact: Dr.-Ing. Sedat Güler

Mn and N Influence on the Crack Propagation Mechanisms in Austenitic Steels

In conventional Ni-alloyed austenitic steels cold hardening leads to an increase of the number of cycles to failure in both, the finite and the infinite life fatigue strength, whereas in high-N-alloyed austenitic steels the number of cycles to failure increases just for the finite life fatigue strength. The reason for these discrepancies may be found in the deformation mechanisms (wavy vs. planary slip) during cyclic loading, which differ significantly and are influenced by the stacking fault energy, near field effects between substituted and interstitial atoms in the solid solution and the density of free electrons in the sliding planes.

The aim of this project is the investigation of these effects and their correlation with the cycling mechanical properties. Therefore austenitic CrNiMoC-, MnC-, CrNiMoN-, and CrMnMoN-steels will be investigated as to their cyclic mechanical properties (crack initiation and propagation) in the solution annealed state, followed up by the microstructural analysis of the load and strain dependent microstructural changes by means of SEM incl. EBSD and TEM.

Research area: Material Science / Fatigue

Contact: Dr.-Ing. Michael Schymura

The Divergent Pathways and Mechanisms of Energy Dissipation at the Interfaces of Martensitic Tribocouples

The requirements for technical systems subjected to friction and wear become more demanding, therefore the components are exposed to increasing stresses. Besides possible safety matters, failure of tribologically loaded systems cause tremendous maintenance costs. Due to the lack of reliable wear prediction models, tribometer tests are used in order to investigate wear behavior of materials and lubrication conditions. For well-aimed optimizations of tribological contacts a comprehensive understanding of wear processes is necessary. However, the transferability of many studies into technical applications is arguable due to applied loads, lubrication conditions and material selection. In this study specimens with different topographies and subsurface structures were investigated prior to and after tribological testing. The tests have been carried out under application related conditions regarding material properties, lubrication and loading conditions. The analyses of surface and subsurface characteristics were performed using complementary microscopy techniques, such as EBSD and TEM. Findings from microscopic analyses were linked to the frictional and wear behavior in order to gain information about energy dissipation and dissipative mechanisms within the respective system. The presence of a grain-refined layer appears to have a beneficial influence on the adaptation of the counterfaces and enhances the robustness of the tribosystems. In addition wear debris was analyzed and different mechanisms of particle generation were discussed.

Dissertation: Dr.-Ing. Priska Stemmer

The influence of surface finish on the localized dissipation of frictional power at ultra-mild wear

Tribological systems are subjected to a steady decrease of friction and wear due to ecological and economical requirements. These guidelines can change the tribological loads and, therefore, result in more severe conditions. The need for maintaining wear as low as possible towards ultra-mild wear rates an integral approach is needed, which has to regard the contact conditions, surface topography, near and sub-surface physical properties. The ultra-mild sliding wear rates, in the order of some nanometers per hour, are desired for example for gears of wind turbines, valve and drive train components, and artificial hip joints to maintain or increase service life time and sustainability. These small wear rates imply a non-linear characteristic of wear, because the amount of wear per load cycle falls below the inter-atomic distances of (technical) materials and consequently can not be a continuous process anymore. Here highly localized effects of dissipated friction energy govern the acting wear mechanisms and alterations of tribosystems, which are still not quantified on the micro- and nano-scale. However today mostly empirically determined wear factors are used to calculate the amount of wear for a given tribological load and hence material failure on those scales can not be predict. This technical matter is complicated by the fact that classical investigations of wear like weighing and micro structural analysis are difficult at the scale and extent of occurring wear appearances within the ultra-mild wear regime. Combined wear tests, micro structural analysis and numerical calculations are presented for the individual analysis of the material response to tribological loads. If a quantification of the failure sequence succeeds on those scales, new design guidelines could be developed, in order to further increase the service life time and predict failure modes more precisely.

Dissertation: Dr.-Ing. Daniel Stickel

Microstructure and Properties of Coatings produced by Friction Surfacing

Friction Surfacing is up to now a rather unknown solid-state joining technique, by which metallic coatings of several hundred micrometer thickness can be produced. During the process, coating materials temporarily reach temperatures just below the melting point and undergo high degrees of deformation at high strain rates. Under such conditions, unique material properties of the coatings can be observed, which are, amongst others, due to homogenisation and grain refinement. The mechanisms, which lead to this particular state of microstructure (e.g. dynamic recristallisation) are, in the case of friction surfacing, not fully understood and differ significantly for different materials (aluminum, copper, steel...).

In the scope of this P.h.D. thesis, interrelations between microstructure and material properties of coatings produced by friction surfacing are investigated using a combination of mechanical testing (e.g. wear tests) and microscopic examinations (e.g. Electron-Backscatter-Diffraction to look at the degree of recristallisation).

Research area: Tooling

Dissertation: Dr.-Ing. Stefanie Hanke

Wear of Steel-on-Steel Pairings Lubricated with Diesel Fuel under Multidirectional Reciprocating Sliding

The wear phenomena and wear characteristics of reciprocating sliding wear with superimposed lateral vibrations were investigated using a ball-on-disc tribometer. Additionally, the possibility of rinsing the contact area with the lubricant, in order to remove wear particles, was implemented. The disc surfaces were unidirectionally ground to different roughness values and were tested against polished balls.

Without removing the wear particles by rinsing, two wear regimes - mild (tribochemically dominated) and severe (mechanically dominated) - were found. The transition between mild and severe wear was found to take place at a specific amplitude of the lateral "disturbance" vibration. With rinsing, no transition, but a continuous change of wear mechanisms takes place. A wear model was developed to describe the correlation between surface topography, wear particle size and the maximum allowable disturbance vibration amplitude, to stay within the mild wear regime.

Dissertation: Dr.-Ing. Ingo Samerski

Microstructure, Sliding Wear and Corrosion Properties of High Nitrogen Martensitic Steel

Keywords: Martensitic High Nitrogen Steel, Microstructure, Tribology, Corrosion, Retained Austenite, Heat Treatment

This study is addressed to engineers and scientists who deal with material science. The main aim of this work was basically the determination of wear behavior and mechanisms as well as corrosion properties of high nitrogen martensitic steel. The material and its microstructure have been examined in different states of heat treatment. The selected parameters differ in hardening and tempering temperature which resulted in a distinct amount of retained austenite. The presence of retained austenite played a crucial role in the change of microstructure which in consequence had influence on corrosion and wear. The reciprocating sliding wear test carried out on each heat treated state has been studied with respect to wear mechanisms, corrosion and changes in microstructure. By means of transmission electron microscopy and a special cross section preparation technique it was possible to investigate the surface and the subsurface of the unworn and worn materials and to obtain the knowledge about type and crystal structure of the precipitates and their correlation to the matrix.

Dissertation: Dr.-Ing. Anna Runiewicz

Influence of Oligocrystallinity on the Deformation Behaviour of Materials and Structures in Biomedical Technique

Keywords: High Nitrogen Steels, Oligocrystals, Microstructure, Microtexture, Fatigue, Electron Microscopy, EBSD-Analysis, Orientation Mapping, Coronary Stents

The aim of this study was the characterisation of microstructure and microtexture of special steels used in biomedical technique. The influence of nitrogen content on the microstructure development under static and cyclic load was investigated comparing selected austenitic steels. These materials were investigated with focus on their use in oligocrystalline state as coronary stents. The dimension of these types of hardware is so small, that grain size approaches the dimension of the specimen itself. But in crystalline samples with oligocrystalline structure a size effect of mechanical properties can be observed. The deformation behavior of oligocrystalline structures has been investigated with focus on the development of microstructure and microtexture during deformation. Typical size effects like differences in the inner and outer part of the structure, grain rotations and anisotropic deformation behavior of differently oriented gains were observed. For microstructure characterisation scanning electron microscopy as well as transmission electron microscopy and single grain orientation determination were used.

Habilitation: Priv.-Doz. Dr.Ing.habil. Sabine Weiß

Mechanical Properties of Oligocrystalline Structures

Keywords: Coronary Stents, Oligocrystal, Mechanical Properties, Dilation, Fatigue, Wire, Tension Test, Grain Size - Diameter - Relation

The aim of this study was the characterization of mechanical properties oligocrystalline structures using balloon expandable coronary stents and thin wires as examples. The dilation behavior of different coated and uncoated stents was investigated in air and in Ringer solution. A modified procedure of fatigue tests of coronary stents was proposed. The microstructure of these types of hardware was investigated with its influence on the mechanical properties. The processes of characterization of implants were discussed critically with focus on their suitability. If possible, improvements were proposed. In a second part of this study quasistatic mechanical test of thin wires of different materials and thicknesses between 100 mm and 300 mm were carried out. Different grain size - diameter - relations were analyzed with focus on the transition from polycrystalline to oligocrystalline deformation behavior. The experimental procedures as well as the wire properties in relation to the microstructure were discussed.

Dissertation: Dr.-Ing. Andreas Meißner

Project: Cyclic Deformation Behaviour of the High Nitrogen Alloyed Austenitic Steel X13CrMnMoN18-14-3 (1.4452)

Within this work the fatigue behaviour of the austenitic steel X13CrMnMoN18-14-3 is described in the solution annealed and cold hardened state with and without simulated physiological conditions. The investigated material is a Ni-free high-N-containing austenitic stainless steel. The interrelation of it's excellent properties regarding strength, ductility, non- magnetizability and corrosion resistance makes it an cost effective alternative to Ni-containing austenitic steels as well as Ti- and Co-base implant materials.

The investigation matrix contains testing to failure as well as to specified load cycles including the micro structural analysis of the investigated state. Axial tension-compression and cyclic bending behaviour is investigated in air and in Ringer's solution. The micro structural analysis contains light-, scanning- and transmission electron microscopy.

Dissertation: Dr.-Ing. Ilya Tikhovskiy

Gefügeumwandlung und Partikelbildung in künstlichen Metall/Metall-Hüftgelenken

Hüftendoprothesen aus CoCrMo Guss- und Schmiedlegierungen weisen seit vielen Jahren sehr hohe Lebensdauern und geringen Verschleiß auf. Dies ist aufgrund der artgleichen Paarung und vorliegenden Grenz- bzw. Mischreibung nicht zu erwarten.

Im Rahmen dieser Arbeit wurden die genau Mechanismen die zur Bildung eine nano-kristallinen Oberflächenschicht und der Zusammenhang zu kleinen Verschleißraten untersucht. Hierzu wurden mittels TEM unterschiedlich oberflächennahe Bereiche charakterisiert. Es stellte sich heraus, dass die Größe von Verschleißpartikeln der Korngröße innerhalb der in-situ gebildeten nano-kristalllinen Schicht entspricht. Aufgrund der vorliegenden Verschleißraten und der niedrigen Stapelfehlerenrgie von CoCrMo wurde so gezeigt, dass Verschleißpartikel nur innerhalb dieser Schicht entstehen.

Dissertation: Dr.-Ing. Robin Büscher

Wear and Fatigue of High Nitrogen Cold Work Steels after Laser Beam Hardening

Martensitic high nitrogen steels (MHNS) reveal compared to conventional cold work steels an enhanced corrosion resistance and better wear characteristic as well as good low- and high-cycle fatigue strength. Nitrogen has influence on these characteristics directly but is also related to a fine distribution of precipitates.

The material X30CrMoN15-1 has been used since years for applications in aerospace industry. The initial condition for further optimization of corrosion and wear resistance, as well as low- and high-cycle fatigue was a high annealed (620°C) cold work steel with good ductility. Short-time tempering of the surface by means of a laser beam leads to anew hardening of the surface zone which particularly increases the wear resistance in this area. Due to the anew heat treatment residual compressive stresses occur in the surface zone which allow for enhanced fatigue properties. Besides this effects in an alteration of precipitation where nitrides partially dissolve and the amount of solved nitrogen and chromium increases. Thus the corrosion resistance is enhanced.

Dissertation: Dr.-Ing. Martin Heitkemper

Mechanical Properties of Coronary Stents

Keywords: Coronary Stents, Dilation, Compression, Fatigue, Bending Test, Microstructure, Dislocations, Corrosion, Metal Ions

Coronary heart disease has become the most common source for death in western industrial countries. Since 1986 a metal vessel scaffold (stent) is inserted to prevent the vessel wall from collapsing. Nevertheless, the success of stent implantation is limited by the amount of restenosis. The reasons for restenosis were discussed controversy, but with focus on biological and medical fields. This study is a discussion of mechanical properties of coronary stents in combination with chemical load. The metallurgical influences of different balloon expandable coronary stents on the occurrence and development of restenosis were investigated with focus on microscopic analyses. The changes of microstructure due to dilatin were discussed. In addition, the biological processes were explained to increase the communication cardiologists, materials engineers, material scientists and producers of stents as a basis for ongoing development in stent optimization.

Dissertation: Dr.-Ing. Holger Brauer

Fatigue behaviour of the cold hardened austenitic Implant Material X2CrNiMo18-15-3 - 1.4441

Within this work the fatigue behaviour of the austenitic steel X2CrNiMo18-15-3, which is used for orthopaedic implants, is described in the solution annealed and cold hardened state with and without physiological media (to simulate conditions within the human body). Therefore the material is investigated in the shape of fatigue samples under axial tension-compression and cyclic bending conditions. The investigation matrix contains testing to failure as well as to specified load cycles including the micro structural analysis of the investigated state.

Axial tension-compression and cyclic bending behaviour is investigated in air and in Ringer's solution. Additionally axial experiments with superimposed tribological loading are performed. The micro structural analysis contains light-, scanning- and transmission electron microscopy.

Dissertation: Dr.-Ing. Peter Göbbeler

Cyclic deformation behavior and microstructure of sintered iron and selected sintered steels

Powder metallurgy becomes more and more important in engineering applications because complex components can be produced in only a few manufacturing steps - powder production, pressing and sintering - close to or even in the given tolerance. Expensive machining can be avoided,mostly, the consumption of material decreases, production times are shorten and the production line is reduced. First of all cyclic properties of sintered steel formed parts differ distincly from properties of melting metallurgically generated parts because the microstructure shows additional inhomogeneities such as pores, oxide inclusions and significant concentration gradients of alloying elements. Hence, it is technically necessary to determine the high cycle deformation behaviour based upon microstructural characteristics. With this knowledge the development of new improved materials and the identification of important process parameters are possible.

Dissertation: Dr.-Ing. Gerhard Biallas

Isothermal High Temperature Fatigue Properties of X22CrMoV12-1 Steel

The industrial, technical and civilising development is closely connected available energy sources and in particular with the supply of electric energy. Besides energy supply aspects of economic and ecological nature gain in importance today. Possible savings can be obtained by increasing the degree of efficiency in power plants. Here an increase of the main steam temperature is thermodynamically most effective. Increasing the temperature by 20 - 30°C (from 570 or 580 to 600°C) increases the degree of efficiency about 6-8%. Thermally and mechanically high stressed components in conventional power plants are made of high temperature ferritic chrome steels (9-12wt%) with endurance strength and scaling resistance as well as corrosion resistance. In current developments these steels tend to be applied up to 600°C. Within the present work an established basis for a reliable assessment of the isothermal fatigue behavior in a temperature field ranging from 20°C <= T <= 650°C of the steel X22CrMoV12-1 is given. A characterisation of the cyclic deformation behavior and the description of the microstructural changes that come about during cyclic stressing are demonstrated within this work.

Dissertation: Dr.-Ing. Thomas Petersmeier

Verschleiß von kraftstogeschmierten Stahl-Stahl-Paarungen unter multidirektionaler reversierender Gleitbewegung

Contact: Dr. -Ing. Ingo Samerski

Selbstschärfende Schneidwerkzeuge für abrasive Schnittgüter : eine bionische Entwicklung

The dissertation documents how the natural tribological principle of cutting can be applied to technical applications. Role models are teeth that are exposed to heavy wear due to abrasive food. Biological principles were abstracted for the transfer to the cutting technology and the tool concept valid today was fundamentally revised: Wear is controlled and no longer avoided - with astonishing consequences for the energy and material efficiency of technical cutting systems.

Environmentally and economically motivated regulations for CO2 emissions of passenger cars lead to broad activities in automotive engineering to further reduce fuel consumption. Downsizing concepts in combination with increasing ignition pressures in future combustion engines demand improved mechanical and tribological properties for cylinder liner and piston ring materials. Thermally sprayed, low-alloyed Fe-base coatings as cylinder liner surfaces now form the focus of future crankcase developments. In addition to mild steels, high-alloyed ones are also of interest for developing cylinder liner coatings with enhanced tribological properties. Another challenge for modern combustion engines are fuel qualities with elevated levels of sulfur or of biological origin. Sulfur reaches the cylinder liner surface by exhaust gas recirculation as sulfurous and sulfuric acid if the water dew point is undershot. Biofuels are hygroscopic due to their increased ethanol content and known to harm metals such as aluminum and copper by alcoholate corrosion. In addition to an increased wear resistance, newly developed spray coatings should also withstand corrosive gas condensates of sulfur-containing fuel and the effects of biofuels. In this work, different grades of stainless steel wires were sprayed by means of Twin Wire Arc Spraying (TWAS) onto an aluminum and cast iron substrate with different spray distances. The microstructure of the spray coatings was investigated using suitable methods. Selected coatings were honed in two variations with different applied forces in the finishing process. The influence of honing was discussed by electron microscopy, X-ray diffraction and instrumented indentation testing. The tribological behavior of these coatings was investigated by means of a reciprocating sliding wear apparatus. The counterbodies were chromium-plated piston ring segments reinforced with embedded microdiamonds. The test parameters were chosen to mimic the conditions of top and bottom dead center of a passenger car combustion engine with respect to temperature, sliding speed, and state of lubrication. The corrosion resistance of the sprayed coatings has been investigated by means of potentiodynamic polarization and was compared to bulk material. A discussion of the wear and corrosion mechanisms should demonstrate the influence of microstructure and defect state of the spray coatings. An evaluation of friction was carried out in a rotational friction tribometer and by drag measurements.

Contact: Dr. -Ing. Benjamin Happek

Neuartiger Ansatz zur Untersuchung des tribologischen Systems Ventilspindel/Sitzring für Großgasmotoren

An Investigation on the Structure/Property Relationships of Solid State Welding Processes in a Titanium Matrix Composite Alloy (Ti6Al4V + 10wt.% of TiC)

TiC particulate reinforced Ti6Al4V metal matrix composites have a high strength-to-weight ratio even at elevated temperatures. Still, the successful application of these composites is hindered by the lack of reliable joining techniques to produce high-quality joints. Poor fusion welding performance is usually observed, due to the high reactivity of titanium and the resulting interaction with the reinforcing material. To avoid these difficulties, solid-state diffusion bonding and rotary friction welding are more suitable.

In the present work, microstructure and mechanical properties, as well as their interaction, of composite Ti materials joined by diffusion bonding and rotary friction welding are investigated. Metallographic examinations of base materials and joints, as well as tensile, microflat tensile and fracture toughness tests were conducted for this purpose.

Research partner: Geesthacht, GKSS-Forschungszentrum

Dissertation: Dr.-Ing. Antonio Augusto Monaco da Silva

Production and Evaluation of Formability of Friction Stir Welded Tailored Blanks from Aluminum Alloys

The demand to reduce the weight of vehicles in order to reduce fuel consumption led to the introduction of Tailored Welded Blanks (TWB) in automotive industry. Replacing steel materials by aluminum could bring about even more weight savings. However, difficulties in welding aluminum prevent the successful application of Al-TWBs in automotive industry. With Friction Stir Welding (FSW) some of these difficulties can be avoided e.g. due to its excellent reproducibility and robustness. Therefore, it offers an attractive alternative to conventional fusion welding processes. TWBs from the aluminum alloys 5754-H22 and 6181-T4 with different gauges (1 to 2 mm) are produced with welding speeds up to 10 m/min, and tested in the scope of this thesis. Test methods applied are uniaxial and notched tensile test, as well as deep drawing and Nakazima/Hasek tests.

Research partner: Geesthacht, GKSS-Forschungszentrum

Dissertation: Dr.-Ing.Shahram Sheikhi