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Induction heat treatment facilities have a wide application range for heat treatment of cylindrically shaped materials in the steel processing industry due to their reduced process-time and high throughput. The adjustment of the heat treatment process usually aims at reaching a desired hardness. However, the question arises whether the full potential of the applied material is actually exploited. Therefore, this work systematically investigates the influence of the primary microstructure, austenitisation and tempering conditions to the resulting notch impact energy and flow behaviour of a 50CrMo4 quenched and tempered steel, with normalised and soft-annealed prior microstructures. The heat treatments, performed with a laboratory induction heat treatment facility, show that low austenitising temperatures lead to a distinct yield point with reduced strain hardening, while increasing the tempering heating rate results in the precipitation of smaller carbides and a significant increase in tensile strength. Austenitising needs to be adjusted to the primary microstructure to reach an optimum solution state to exploit the hardness and notch impact energy potential.

WC-Co hardmetals are utilized as tool materials in metal cutting applications in which they are exposed to high mechanical cyclic loads and elevated temperatures. A better understanding of the failure mechanisms of WC-Co hardmetals under these application conditions and the ability to diagnose the damage evolution state are key factors to understand the limits of endurable cyclic load at a certain temperature. The aim of the current work was the experimental determination of stress-strain-hysteresis loops for the investigation of damage indicators in uniaxial cyclic tests at a stress ratio of R = σmin/σmax = −1 for two WC-10 wt% Co hardmetals at 700 °C and 800 °C in vacuum. An increase in the stress-strain-hysteresis loop area and tension-compression-strain asymmetry was recorded with increasing number of load cycles at 800 °C, with earlier failure than at 700 °C. The relationship between the stress-strain-hysteresis loop parameters and the damage evolution state at the microstructure level, as well as the deformation behavior of WC- and Co-phases with increasing number of load cycles, were analyzed. To this end, the microstructure for one WC-Co hardmetal grade was analyzed by scanning electron microscopy and electron backscatter diffraction after cyclic testing up to defined numbers of load cycles at 800 °C. It was observed that the hysteresis loop area and strain asymmetry coincide with the formation of nanopores at WC/WC interfaces and WC/Co phase boundaries, which enlarge to form larger cavities with increasing number of load cycles. Additionally, electron backscatter diffraction data showed that the fcc Co-phase partially transformed into hcp Co under cyclic loading. All specimens, in which an increase in the stress-strain hysteresis loop area or strain asymmetry was observed, ultimately failed when a sufficiently high number of load cycles was applied. Thus, these results indicate that the investigated parameters are reliable indicators for bulk material damage.

The aim of the current work was the experimental determination of limit stresses in the uniaxial cyclic compression test that lead to advancing ratcheting in different WC-Co hardmetal grades at elevated temperature. At stresses below the limit stress the plastic strain per cycle reduces and plastic strain accumulation stops after a characteristic number of load cycles. Special attention was paid to the microstructural influence on the onset of advancing ratcheting and the associated damage development at the microstructure level. WC-Co hardmetals are used in various areas such as forming and forging tools, where they are exposed to high temperatures and pressure loads. Their good high-temperature properties allow them to be used under these conditions, but these properties are influenced by the microstructure. Investigations of the mechanical properties of hardmetals had been carried out under monotonously increasing loads and cyclic tests at room temperature and elevated temperatures. In these tests, the effect of different stress ratios R = σmin/σmax on the fatigue behaviour of hardmetals were studied, However, no studies are known for WC-Co hardmetals and their limit stresses in relation to strain ratcheting under cyclic compressive loading at elevated temperature. Hence, in the current work the influence of increasing stresses as a function of microstructure and their effect on the evolution of the strain of six different WC-Co hardmetal grades are discussed. For this purpose, the materials were investigated by uniaxial cyclic compression tests at a stress ratio of R = −∞ at 700 °C and 800 °C in vacuum. The investigated hardmetal grades differ on the one hand in their WC grain size, which varies between 0.4 μm and 2.0 μm, and on the other hand in their Co-content, which varies between 6 wt% and 12 wt%. The residual strain value eres at zero applied stress was observed to stabilize with increasing number of load cycles at low applied stress ranges. Strain ratcheting occurred above a critical stress range, referred to as the limit stress for strain ratcheting. Strain ratcheting is the accumulation of plastic strain with increasing number of load cycles in which no strain stabilization occurs. Further, for all investigated hardmetal grades, the limit stresses were observed to decrease with increasing temperature. In the following, the microstructure of one hardmetal grade was analysed after loading below and above the limit stress by scanning electron microscopy and electron backscatter diffraction (EBSD). The influence of strain stabilization and ratcheting was analysed with regard to damage development and deformation behaviour of the WC and Co-phases. Strain ratcheting was observed to result in the formation of cavities and nanopores at phase boundary triple points and WC/Co interfaces. Additionally, the EBSD data showed that the fcc Co phase was transformed into hcp Co. Therefore, it is assumed that on the one hand, a certain strain value needs to be exceeded for strain ratcheting to occur and, on the other hand, that besides dislocation movement, microdefect formation and phase transformation significantly contribute to the increase in strain.

The concept of quenching and partitioning (Q&P) is a tool to generate steels with high strength and high ductility resulting from a relatively high amount of martensite and austenite. As the parameters of the Q&P treatment influence the strength and ductility properties, the goal of this work was to analyze the effects of varying quenching and partitioning temperatures on the phase evolution, on the retained austenite phase fraction and on the mechanical properties. The phase evolution during heat treatment was analyzed by dilatometer and the microstructure after processing was characterized by light microscopy in combination with color etching according to Klemm. The austenite phase fraction and its carbon content were determined by X-Ray diffraction and the mechanical properties were evaluated by tensile tests. It is shown that the quenching temperature has a minor impact on the phase fraction of austenite as well as its stabilization by carbon and thus, also the mechanical properties. Furthermore, austenite transformation during the partitioning treatment is illustrated.

The bainitic phase transformation during continuous cooling in low alloyed steels is a complex process since many different reactions are taking place simultaneously. Since all of them are rather sensitive to the applied cooling rates, the present study investigates the mechanisms taking place during the bainitic phase transformation by using a comprehensive set of methods including in-situ high energy X-ray diffraction combined with dilatometry, ex-situ microstructure characterization by means of scanning and transmission electron microscopy including electron back scatter diffraction. The results demonstrate that the phase transformation characteristics changes from a continuous to a two-stage behaviour with decreasing cooling rate. The reason for this change is discussed and explained on basis of an adapted T0′-limit concept taking an additionally required strain energy between about 950 and 1100 J/mol in the energy balance into account. Apart from the observed transformation characteristics, it is also shown that the amount of blocky instead of film like retained austenite increases with decreasing cooling rate. Keywords: Bainite, Continuous cooling, Strain energy

At elevated temperatures, the material behaviour of WC-Co hardmetals shows differences under similar loading conditions depending on WC grain size and Co-content. Variations in the chemical composition and microstructure in hardmetals cause different material properties such as strength or creep resistance. In the current work, the influence of WC grain size on creep mechanism and creep resistance was investigated for WC-12 wt% Co hardmetals with 0.4 μm, 0.7 μm and 2.0 μm average WC grain size. Specimens were tested in uniaxial tensile and compression step-loading creep tests at 700 °C and 800 °C under vacuum conditions. Time-dependent creep behaviour with steady-state secondary creep was observed for all hardmetal grades investigated, with specimens creeping faster under tensile than under compressive loading. At 700 °C, the medium-grained hardmetal grade exhibited the highest minimal creep rates e min compared to the submicron and ultrafine-grained grades. In contrast, the ultrafine-grained hardmetal grade showed higher e min at low stresses and 800 °C, because of the high amount of grain boundary area per unit volume, which is advantageous for vacancy diffusion at grain boundaries. Therefore, the e min of the medium-grained hardmetal grade was less affected by temperature than that of the finer-grained grade. Also two stress exponent n-ranges were observed at 700 °C and 800 °C: At low stress levels, n was in the range of about 1. Above a critical stress level, n reached values between about 4 and 6. Beside the influence of the WC grain size on the creep mechanism and creep resistance, damage evolution with increasing stress levels was analysed for the ultrafine-grained grade at 800 °C. The microstructures of three compression step-loading creep tested specimens were examined after maximum stress levels of −350 MPa, −950 MPa and −1350 MPa. Microstructural investigations performed via scanning electron microscopy showed that more and larger cavities had formed at WC/WC interfaces and WC/Co phase boundaries in the specimen tested up to −1350 MPa compared to the ones tested up to −350 MPa and −950 MPa.

This work investigates the effect of different initial microstructures on the electromagnetic magnetisation curves/hysteresis of 50CrMo4 steel and technically pure iron introducing a novel measurement method. Surprisingly little is known about the relationship between microstructure and the magnetic properties, although this is of great importance for a better understanding and multi-physics modelling of induction heat treatment. Analysis of the magnetic hysteresis and Distortion Analysis of Magnetic Excitation (DAME) are important techniques to characterise magnetic behaviour of materials. To perform these measurements, a specific measurement setup based on excitation circuit/yoke with secondary coils was implemented. Using these methods, the characterization of the effect of mechanical properties (i.e.: hardness) and different microstructures on electromagnetic magnetisation curves/hysteresis is possible with high precision. The effect of the eddy-currents on the B-H hysteresis shows a systematic behaviour, which opens the possibility for a new kind of categorisation of these effects. DAME shows a closely linear relationship with the assumed precipitation size allowing indirect analysis of the microstructure, also at elevated temperatures.

Kurzfassung Das induktive Anlassen bekommt aufgrund der kurzen Prozesszeiten und der dadurch wirtschaftlicheren Produktionsmöglichkeiten vor allem bei Stabmaterial eine immer größere Bedeutung. Allerdings kommt es durch die schnelle Erwärmung und die kurzen Haltedauern zu einer veränderten Ausscheidungskinetik und dadurch zu einem unterschiedlichen Härte-Anlass-Verhalten gegenüber einer konventionellen Wärmebehandlung. Des Weiteren tritt bei der induktiven Erwärmung das Problem des sogenannten “Skin-Effekts“ auf, welcher bei schlechter Prozesssteuerung zu einer überhöhten Randtemperatur, verglichen mit der im Kern, führt. Diese prozessbedingten Charakteristika erschweren das Verständnis der Auswirkung des Prozesses auf die Mikrostruktur des untersuchten Vergütungsstahles. Diese Arbeit möchte daher mithilfe eines kombinierten Ansatzes aus Experimenten an einer Laborinduktionsanlage und Simulationen mithilfe der Finite-Elemente-Methode (FEM) die Optimierung bzw. individuelle Feineinstellung von induktiven Anlassvorgängen für Stab-Geometrien aus dem betrachteten Werkstoff aufzeigen. Dazu wurde mittels FEM-Simulationen die zeitliche Entwicklung des Temperaturfeldes im Werkstück berechnet und diese mit experimentellen Resultaten zu Mikrostruktur, Härte und Zähigkeit in Verbindung gebracht.

WC-Co hard metals show defect-controlled fracture behaviour. This means that the fracture stress under tensile loading conditions is directly related to the size of the largest defect within the loaded volume. The size distribution of these defects relevant to failure under static and cyclic loading conditions can be quantitatively described by Weibull statistics. Prior studies on the fracture behaviour of hard metals and the nature and size of material inhomogeneities, i.e. defects, do not attain their data from one single batch of material. To avoid uncertainties from possible variations in material composition the current work studies the fracture behaviour of a single material batch as a function of the effectively loaded volume V eff . A variety of specimen geometries and loading arrangements was applied in order to vary V eff over five orders of magnitude. Fracture stress values are found to increase from about 1500 MPa to 4200 MPa while decreasing the effectively loaded volume from about 100 mm 3 down to about 0.01 mm 3 . The observed volume effect of strength shows the expected qualitative trends with quantitative deviations from Weibull-like strength behaviour. The presence of two different kinds of defects in the form of pores and single enlarged WC grains on the attained fracture surfaces, as found by a fractographic analysis in the SEM, is a possible explanation for this observation. Information provided by the current study is the basis for the judgment of the relevance of volume defects regarding the failure of tools or structural components made of WC-Co hard metals under certain loading conditions.

This study focuses on the influence of a fast heat treatment, as occurring with induction heating processes, on the microstructural features, and their effect on the mechanical properties of a 50CrMo4 steel. To this end, three different heating rates (1 K/s, 10 K/s, and 100 K/s) and three different austenitizing temperatures (950 °C, 1000 °C, 1050 °C) were applied to a ferritic-pearlitic steel. After quenching, the resulting martensitic microstructures were investigated and compared to a conventional furnace heat treating sequence using electron back scattered diffraction (EBSD). Thus, the effect of variable heating rates during austenitization on the resulting martensitic microstructure, including the evaluation of packet and block sizes and orientations, was analyzed. Through the investigations of this work, it could be shown, that the adaption of the Kurdjumov-Sachs (K-S) variants is changing with the prior heat treatment. The occurrence of all 24 K-S variants in the same intensity increases and the misorientation angle across a martensite block decrease with higher austenitizing temperatures, despite fast heating rates. Therefore, it is suggested, that the austenitizing temperature has more impact on the martensitic structure than the heating rate.

Decreasing processing time of a quench and temper heat treatment is of high interest for industry due to the possibility of cost reduction. One option to reduce processing time is to shorten the austenitizing cycle by applying high heating rates and minimum holding times. However, due to the high heating rates, the analysis of their influences on the formation kinetics of austenite and its crystallographic parameters is challenging. Thus, this work concentrates on the in situ analysis of the austenitization process by means of high-energy X-ray diffraction to study a range of heating rates applied to ferritic–pearlitic and soft annealed initial microstructures. The transformation kinetics from ferrite/pearlite and soft annealed state to austenite, the cementite dissolution behavior and the homogeneity of the freshly formed austenite were analyzed. The results indicate three distinct steps of austenite formation independent of initial microstructure and heating rate: (1) nucleation of carbon-rich austenite at cementite–ferrite interfaces, (2) growth of austenite phase fraction accompanied by a reduction of the carbon content, until reaching the mean carbon content of the steel, followed by growth of the austenite grain size, (3) regarding austenite homogeneity, the combination of austenitization temperature and initial microstructure are the main influencing factors.

WC-Co hardmetals are popular tool materials, which are used in applications such as metal milling or turning. In these applications, elevated temperatures occur in the tools during the machining process, although they are also cooled. This results in a complex interaction of thermal and mechanical loads in the tools. Within this current work, a strain asymmetry of a WC-10 wt% Co hardmetal after tensile and compression uniaxial step-loading creep tests is described. Two types of tests were performed: Firstly, specimens were deformed to certain strain limits at 700 °C and 800 °C. Strain asymmetry was observed for tensile and compression stresses above 600 MPa at 700 °C and above 250 MPa at 800 °C. In the second type of test, the specimens were stepwise loaded up to a stress of 300 MPa under tensile and compressive load at 800 °C. The aim of test 2 was to identify the physical reason for the strain asymmetry from the first tests at 800 °C. The material's microstructure was analyzed for the specimens from test 2 by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The influence of the loading type was analyzed with regard to damage development and deformation behaviour of the WC- and Co-phase. SEM images showed that the faster increase in strain over time under tensile than under compressive loading was caused by the formation of cavities at WC-WC interfaces and at WC-Co phase boundaries. Due to the larger number of observed microdefects under tension than under compression, it was assumed that this was the physical reason for the strain asymmetry. In addition, EBSD data showed that during the compression and tension creep tests, the fcc Co-phase was partially transformed into the hcp Co-phase.

It is well known that high Co-Ni steels exhibit excellent toughness. Since the good toughness in these steels is supposed to be related to thin layers of austenite between martensite crystals, this work presents an experimental study corroborated with diffusional calculations to characterize the evolution of reverted austenite. Atom probe measurements were conducted for analyzing the element distribution in austenite and martensite during tempering. These results were correlated with crystallographic information, which was obtained by using transmission electron microscopy investigations. Additionally, the experimental findings were compared with kinetic calculations with DICTRA™. The investigations reveal that reverted austenite formation during tempering is connected with a redistribution of Ni, Co, Cr, and Mo atoms. The austenite undergoes a Ni and Cr enrichment and a Co depletion, while in the neighboring martensite, a zone of Ni and Cr depletion and Co enrichment is formed. The changes in the chemical composition of austenite during tempering affect the stability of the austenite against phase transformation to martensite during plastic deformation and have thus decisive influence on the toughness of the material.

Understanding of the load situation and consequently the lifetime of cutting tools made of WC–Co hard metal requires quantitative data for thermo-mechanical properties. For the elevated temperatures present in application, these data are currently rather rare. The present work does discuss elastic material properties up to 1100 °C and compressive yield strength up to 900 °C, both as a function of Co content. The fracture toughness was determined as a function of the WC grain size and Co content up to 800 °C. Young's modulus and yield strength decrease with increasing temperature. A significant rise in fracture toughness was observed at 800 °C with increasing Co content and decreasing WC grain size. A possible reason for this increase is an increase in the plastic zone size at elevated temperatures.

In coated hard metal milling inserts the main damage mechanisms are thermal fatigue induced by interrupted tool–workpiece contact and wear. Dependent on the magnitudes of thermal and mechanical loads in two applied test setups, either wear or thermal fatigue in the form of combcracks is induced. The evolution of residual stress and damage in the used milling inserts was documented over their complete lifetime. In a region of interest on the tool rake face a significant buildup of tensile residual stress was observed via a synchrotron based technique. A special preparation technique enabled position resolved measurements in this area by in-house X-ray diffraction facilities to study the evolution of residual stress over the entire tool lifetime. The onset of cracking was observed to happen in this region of interest by means of focused ion beam milling and scanning electron microscopy. The residual stress levels observed are comparable in used inserts at early stages of application, independent of the different cutting conditions and the applied characterization technique. At the end of tool life wear damage dominated inserts showed tensile residual stress, whereas thermal fatigue as the dominant damage mechanism resulted in compressive residual stresses.

Through process modelling of surface densified gears produced by powder metallurgy (PM) was established by coupling the modelling of the manufacturing processes surface densification, carburization, and heat treatment. The complete model allows the prediction of the local microstructure and hardness in the gear as well as the appearance and direction of residual stresses in the final part. The structural integrity of the part is governed, on the one hand, by the local material properties and residual stresses and, on the other hand, by the load stresses calculated for typical operating conditions. A simplified hardness dependent Haigh diagram was used to calculate the maximum allowable cyclic stresses for the gear tooth and to derive a local utilization ratio as target entity for optimization of the individual steps of the production chain.

The material requirements for the frog of a turnout are quite complex. The crossing nose region of the frog represents a discontinuity in the track as the wheel needs to change from one rail to the other. This causes increased dynamic contact forces and slip between wheel and frog. The high contact pressures and slip can cause damage such as wear, rolling contact fatigue and severe plastic deformation in the frog. The material from which the frog is constructed plays a crucial role in the development of damage. On the one hand, a plastic material response can reduce the contact pressures by an adaption of the frog’s geometry. On the other hand, materials have a different resistance to wear, crack initiation and growth. To clarify the requirements for materials for the manufacture of frogs, five different steel grades used to create frogs that are in service in various railway turnouts were investigated. The chosen materials were ‘Hadfield steel’ (an Mn-13, abrasion-resistant grain-refined steel with 400 HV), bainitic railway steel, tempered steel 51CrV4 (DIN 1.8159) and maraging steel. They were investigated in the same conditions as they would experience in service, covering a variety of steel microstructures and macroscopic hardness values. To compare the materials’ responses to the high impact forces, their impact fracture toughness was measured and selected fracture surfaces were investigated with scanning electron microscopy. To compare their response to the cyclic loading, the materials were tested with respect to their monotonic tensile strength as well as fatigue strength by means of low cycle fatigue tests. Measurements of the fracture toughness KIC were performed on all materials to compare their fracture resistance. The achieved results can be used as input data for finite-element-based calculations of the local loading conditions in railway turnouts. Additionally, the comparison can help the choice of the best material for specific loading conditions, i.e. specific crossing geometries and track loading conditions.

Pressure casting dies are exposed to harsh service conditions consisting of cyclic thermal and mechanical loading and thus undergo thermo-mechanical fatigue. Due to cyclic plastic deformation of the material near the surface of the dies the loading conditions gradually change because of the formation of tensile residual stresses which add to the stress field from external loading. This change in the stress field influences the nucleation and the growth of cracks. Typically after a few thousand casting cycles a network of heat checks forms. In such a network crack shielding has a big influence on the evolution of the crack array. Firstly, it influences the propagation rates of the cracks and secondly it may change the propagation direction compared to the case where no neighbors are present. The crack growth rate and the length at which the cracks stop growing are also influenced by the thermo-physical and mechanical properties of the die material. It was found that the shielding effects of neighboring cracks are of equal importance. Crack deflection caused by the presence of neighboring cracks can lead to break-outs at the surface ensued by fast degradation eventually necessitating the replacement of the die. Consequently, the focus in this work is put on the investigation of the interaction of cracks in a network and their effect on the fatigue life. The problem is tackled by means of an automated strategy based on the finite element method.

The fracture behaviour of an ultrafine grained WC–Co hard metal was investigated in tensile and bending tests using different specimen sizes and test arrangements in order to study the size effect on the tensile strength, by varying the effectively tested volume over a range of roughly 10 orders of magnitude. Mechanical testing of centimetre sized specimens was performed by means of tensile tests using an hour glass shaped specimen. Millimetre sized specimens were tested in four point and three point bending test set-ups. Micrometre sized specimens, rectangular beams produced via focused ion beam milling, were loaded in situ in a scanning electron microscope utilizing a piezo-electrically controlled cube corner micro-indenter. The resulting fracture surfaces were examined in order to identify crack origins. The main result of the present work is that strength values are found to increase from about 2500 to about 6000 MPa when the size of the effectively loaded volume is varied from about 100 to about 10−8 mm3. This kind of behaviour is typical for brittle materials in which strength is defect controlled and can be explained by a size effect according to Weibull theory. In the case of the micrometre sized specimens no defects were found on the fracture surfaces. Estimations of critical defect sizes in these specimens based on linear elastic fracture mechanics give values in the order of magnitude of the submicron sized tungsten carbide particles. It is therefore expected that the high strength values found in these specimens are close to the inherent material strength.

Due to the inherent hardness and brittleness of WC-Co hard metals the determination of mechanical properties such as yield strength, ultimate strength and fatigue strength is a difficult task. A testing setup to determine stress-strain curves of hard metals under static and cyclic uniaxial compression and tension loading was developed. The investigated hard metal grades varied in WC grain size and Co-binder content ranging from 0.2 to 1.3 μm and 6 to 12 wt. %, respectively. The static stress-strain behaviour was analysed in uniaxial compression and tensile tests and specific material data like Young’s modulus and yield strength were determined. The cyclic stress-strain response along with the ratchetting behaviour as well as subcritical fatigue crack growth were investigated under uniaxial loading conditions. Fractographic studies of the fracture surfaces were conducted using scanning electron microscopy to identify the nature of crack origins. Based on Murakami’s concept the threshold of the stress intensity factor range at which fatigue crack growth starts from inhomogeneities was determined to be about 4.3, 6.2 and 9 MPa √ m at stress ratios of 0.1, −1 and −3 respectively.

Induction heating processes are of rising interest within the heat treating industry. Using inductive tempering, a lot of production time can be saved compared to a conventional tempering treatment. However, it is not completely understood how fast inductive processes influence the quenched and tempered microstructure and the corresponding mechanical properties. The aim of this work is to highlight differences between inductive and conventional tempering processes and to suggest a possible processing route which results in optimized microstructures, as well as desirable mechanical properties. Therefore, the present work evaluates the influencing factors of high heating rates to tempering temperatures on the microstructure as well as hardness and Charpy impact energy. To this end, after quenching a 50CrMo4 steel three different induction tempering processes are carried out and the resulting properties are subsequently compared to a conventional tempering process. The results indicate that notch impact energy raises with increasing heating rates to tempering when realizing the same hardness of the samples. The positive effect of high heating rate on toughness is traced back to smaller carbide sizes, as well as smaller carbide spacing and more uniform carbide distribution over the sample.

Kurzfassung Hochcarbidhaltige Nickelbasislegierungen sind vielversprechende Werkstoffe für Anwendungen, bei denen eine hohe Verschleißbeständigkeit bei erhöhten Temperaturen gefordert ist. Anhand zweier aluminium- und carbidhaltiger Nickelbasislegierungen wurde studiert, wie sich Wärmebehandlungen auf die Mikrostruktur auswirken. Dazu wurden die Legierungen nach jedem der nacheinander durchgeführten Herstellungsschritte Schmelzverdüsen, heiß-isostatisches Pressen, Weichglühen und Auslagerungsglühen mit Hilfe metallographischer und röntgenographischer Methoden untersucht. Es wurde beobachtet, dass sich die eutektisch erstarrten M2C-Carbide in M6C-Carbide umwandeln. Zudem kommt es zu einer Ausscheidung von Carbidphasen des Typs M2 3C6 entlang von Seigerungszonen der γ-Matrix. Beide Carbidreaktionen beeinflussen das Ausscheidungsverhalten der für die Aushärtung der Matrix verantwortlichen intermetallischen Ni3Al-Phase.

The microstructure of the Cr/CrN and Ti/TiN multilayers coatings deposited with the pulsed laser deposition (PLD) technique was studied using transmission electron microscope Philips CM20 TWIN (200 kV). It was found that the metallic buffer layers are characterized by the smallest columnar crystallite size and the highest defect density; this might rise their hardness but compromise the coating adhesion. The intermediate metallic layers showed a larger and less defected columnar structure as compared with the nitride layers, what should improve the coatings toughness. The scratch test performed with a Rockwell penetrator of 200 μm diameter loaded from 0.03 N up to 20 N indicated that the switching from a single layer to a multilayered metal/nitride coating of the same thickness improved the resistance to scratch test in case of Ti/TiN from 16.4 N for a TiN single layered to 22.9 N for a 32 layered coating and Cr/CrN from 4.4 N for a CrN single layered to 6.6 for a 32 layered ones.

Oxygen is usually found as a contaminating species in chromium (Cr) and chromium nitride (CrN x ) coatings grown by various PVD techniques (e.g. sputtering, arc evaporation). The current work shows that the high energetic pulsed plasma conditions in the Pulsed Laser Deposition (PLD) technique intensify the oxygen trapping in Cr and CrN x coatings and strongly influence their microstructure. Cr-based coatings were deposited by an industrially designed 4-beam PLD evaporation system at room temperature (25 °C) by using Nd:YAG laser radiation (wavelength: 1064 nm) for Cr target ablation in N 2 /Ar gas mixtures. Due to the decrease of the oxygen content in films of higher thickness the source of O 2 was found to be the rest gas atmosphere after evacuation to pressures lower than 2×10 −3 Pa. Coatings deposited from the pure metallic Cr targets in Ar rich atmospheres possess a very fine-grained α-Cr phase structure with strongly increased lattice parameters resulting from the exchange of Cr by O atoms on bcc lattice sites. In contrast, high N 2 contents in the deposition atmosphere result in the predominant formation of rhombohedral Cr 2 O 3 mixed with fcc CrN.

Silicon nitride (SiNx) thin films of various stoichiometries (x) were prepared on Si (100) substrates applying the Nd:YAG (λ=1064 nm) pulsed laser deposition (PLD) process from pure Si targets in the “shaded off-axis” technique at room temperature. The specific arrangement of this technique with perpendicular target and substrate surfaces and a metallic screen in between guarantees very low particulate (droplet) deposition and, thus, excellent surface qualities. The about 80- to 100-nm-thick silicon nitride films have very smooth surfaces (∼0.5–1.5 nm roughness) and dense structures. The N2 partial pressure strongly influences the nitrogen content and the silicon bonding structure of the films analyzed by means of secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS), resp. As a consequence, the optical properties examined by spectroscopic ellipsometry are tailorable in a wide spectral range between 250 and 1200 nm.

Fatigue due to thermal cycling is one of the main reasons for the damage of tools used in die casting processes. In order to investigate the thermal fatigue behaviour of tool steels a thermal fatigue testing facility was designed and built up. For better understanding of the mechanisms of thermal fatigue finite element-simulations were carried out. To specify the cyclic material behaviour push-pull-tests at different temperatures were performed. The Chaboche-model was used to describe the kinematic material response. Isotropic softening is also taken into account. Depending on the arising accumulated plastic strain stable cyclic deformation or continuous softening occurs. The results are consistent with the accomplished thermal fatigue tests on different hot work tool steels.

The aim of the present work was the improvement of titanium–aluminium nitride (TiAlN) coating by the solid-solution hardening with carbon atoms leading to titanium–aluminium carbonitride (Ti,Al)CxN1−x coatings with varying carbon (x) and nitrogen contents. The request of low deposition temperatures which are indispensable for the coating of heat-sensitive materials like tool steels of high hardness and polymers was reached by the application of the room temperature pulsed laser deposition (PLD) technique. A Nd:YAG laser of 1064 nm wavelength operated at two different laser pulse energies was used in the ablation experiments of pure TiAl targets (50 at.% Al) in various C2H2–N2 gas mixtures. The results reveal a strong dependency of the gas mixture on the C and N content of the coatings. Furthermore, the different pulse energies change the ratio of Ti/Al atoms in the grown coatings. The crystallization was influenced too by the pulse energy and, thus, the energy and distribution of the atoms, ions and clusters. For the hardness and elastic moduli, two distinct stages in dependency on the crystallinity were found: High carbon contents lead due to the formation of pure carbon precipitations, found in the Raman investigations, to the destruction of the crystallin structure and, thus, to up to 60% lower hardnesses and elastic moduli. A low friction behaviour was found for coatings of the higher Al contents deposited at lower laser pulse energies due to the formation of carbon rich transfer layers on both the coatings and the Al2O3 counterparts.

Titanium carbonitride (TiC x N 1− x ) hard coatings possess excellent tribological behavior in metal punching and forming as well as high biocompatibility. Because almost all PVD and CVD coating techniques for TiC x N 1− x coatings require substrate temperatures higher than 150–200 °C for reaching sufficient adhesion strength to the substrate surface, there is high demand in the development of large-area and high-rate low-temperature vacuum deposition processes. In the present work, this call was fulfilled by the application of the room temperature pulsed laser deposition (PLD) for TiC x N 1− x coating with various carbon ( x ) and nitrogen contents. A pulsed Nd:YAG laser (wavelength: 1064 nm) was used for the vaporization of pure titanium targets in low-pressure N 2 /C 2 H 2 atmospheres. The highly ionized metal vapor was deposited onto polished substrates (molybdenum, AISI D2 steel), forming a nearly particulate-free, very smooth and dense film structures, which can be hardly reached by using other PVD techniques. The variation of the gas flow during deposition causes a change of the chemical composition and the microstructure of the nearly particulate-free coatings, and, consequently, of the mechanical and tribological properties. The solid solution hardening of the fcc TiN lattice by low contents of carbon increases the hardness and elastic modulus, while higher carbon contents decrease this effect. Furthermore, the maximum in hardness and elastic modulus correspond with the minima in the friction coefficients (∼0.2) and wear rates of both the coated disc and the counterparts (AISI 52100 (DIN 100Cr6) ball-bearing steel balls).

The high hardness and the low-wear rates characterize the outstanding tribological behaviour of titanium nitride (TiN) making it to the most widespread material for hard coatings, which were deposited industrially in recent years by employing various deposition methods. For coating of heat sensitive materials and machine parts at low temperatures down to room temperature, only a few deposition techniques can be applied. One of these is the pulsed laser deposition (PLD) method. In this work a multi-spot PLD system with four Nd:YAG laser beams (1064-nm wavelength) was applied to vaporize titanium targets in low-pressure N 2 atmospheres at room temperature. For the deposition of the TiN coatings, high speed tool steel (AISI M2) and corrosion resistant steel (AISI 630 HT) substrates were used. To investigate the differences of film growth in dependency of the target–substrate arrangement, the substrate surfaces were situated parallel (on-axis) and rectangular (off-axis) to the target. The coatings were examined by light-microscopy, scanning electron microscopy, X-ray diffraction, nanoindentation and pin-on-disc tests against ball-bearing steel (DIN 100Cr6/AISI 52100) and alumina (Al 2 O 3 ) counterparts. The results indicate a high influence of the target–substrate arrangement on the textures, residual stresses and hardnesses of the TiN coatings. In spite of these differences all coatings on both on-axis and off-axis placed substrates possess excellent adhesion and high-wear resistance.

Titanium oxide (TiOx) coatings exhibit extraordinary optical, electrical and tribological properties, but very less is known about coatings deposited at room temperature. In the present work this gap was closed by employing the room temperature pulsed laser deposition (PLD) technique. In order to simulate different deposition conditions, the substrate (AISI M2, molybdenum) surfaces were situated parallel (‘on-axis’ geometry) and perpendicular (‘off-axis’) to the pure titanium targets ablated by the Nd:YAG lasers with a wavelength of 1064 nm. The combination of a four-beam PLD evaporator with a suitable movement of the substrates resulted in a high-rate film growth on large surfaces. The coating structure investigated by X-ray diffraction consists of a mixture of monoclinic β-TiO2 and amorphous TiOx phase. The ratio of the amount of the phases, the hardness and the elastic constants strongly depend on the O2 gas pressure and the target–substrate arrangement applied for deposition. Owing to the very high adhesion and their very smooth surfaces, the TiOx coatings exhibit friction coefficients of approximately 0.55 and wear rates of lower than 10−15 m3 N−1 m−1, turning out that titanium oxide coatings deposited by means of PLD are very promising candidates for wear protection applications.

Titanium nitride thin layers were fabricated by pulsed laser deposition (PLD) using a Nd:YAG laser on both metallic (ferritic steel, pure titanium) and non-metallic (polyurethane) substrates by ablation of pure titanium in nitrogen environment. On-axis and off-axis geometry of deposition was applied. Residual stresses were measured in the TiN phase showing the compressive values in the range of −6 to −8 GPa for the on-axis growth while of approximately −2.8 GPa for the off-axis position. Texture examinations revealed the {110} 〈112〉 main texture component in the substrate, while differences were stated in the TiN phase in respect to the geometry of deposition. In the case of on-axis growth, the mostly axial texture with plane {110} parallel to surface with tendency to {110} 〈011〉 was observed, while in the case of the off-axis growth the very pronounced {112} 〈223〉 dominant orientation was stated. Examinations of crystalline size and lattice strain were carried out on the basis of diffraction line broadening. Deposition of the TiN at room temperature on the polyurethane substrate revealed uniform thin layers. Residual stresses showed compressive stresses of approximately −1500 MPa. The measured texture was stated to be close to random. Morphology of deposited layers, examined by means of AFM, revealed similarity to the three-dimensional growth mechanism.

Titanium–aluminium based nitride (Ti,Al)N and carbonitride (Ti,Al)(C,N) hard coating systems possess excellent tribological behaviour in metal cutting and polymer forming contacts. In the present work (Ti,Al)N and (Ti,Al)(C,N) coatings were deposited by employing the pulsed laser deposition (PLD) technique. A pulsed Nd:YAG laser with 1064 nm wavelength was used for the vaporization of TiAl targets in low-pressure N2 or N2/C2H2, atmospheres at room temperature. The highly ionized metal vapour was deposited onto polished substrates (molybdenum, AISI D2). The coatings were characterized by light-microscopy, scanning electron microscopy, X-ray diffraction and hardness tests. The variation of the deposition parameters causes a change of the chemical composition, the texture and crystallinity of the coatings and, consequently, the mechanical properties and tribological behaviour. The latter was characterized in pin-on-disc tests at room temperature by using coated discs and uncoated AISI 52100 (DIN 100Cr6) steel and alumina pins as counterparts. The results demonstrate the excellent industrial applicability of these coatings for cold-forming operations: very low-wear rates were found for the (Ti,Al)N coatings. In contrast, the (Ti,Al)(C,N) coatings possess low-friction coefficients of approximately 0.2. As an outstanding advantage of these coatings, which were deposited at the room temperature by the PLD process, their excellent adhesion to the substrate can be pointed out, reaching the highest level (HF 1) in the Rockwell indentation test.

Amorphous, hydrogenated carbon thin films with varying silicon content (a-Si(1−x)Cx:H) for optical and tribological applications were deposited by means of pulsed laser deposition (PLD) from silicon targets with a high power pulsed Nd:YAG laser of 1064-nm wavelength in argon and C2H2 containing atmospheres. To minimize the deposition of particulates and droplets originating in the laser ablation process of the pure silicon targets, a perpendicular arrangement of the target and substrate surface as well as a screen between them was applied (‘shaded off-axis’ geometry). The chemical composition was investigated employing X-ray photoelectron spectroscopy, revealing carbon contents between 80 and 97 at.% and a mainly CC4 bonded structure. A distinct effect of the carbon content on the hardness and elastic modulus of the films determined by nanoindentation was observed. In contrast, the optical behavior of the films investigated employing spectroscopic ellipsometry was found to be nearly independent on the carbon content. A comparison of the optical properties for all films with literature data revealed a hydrogenated diamond-like carbon bonding structure.

The aim of the present work is the investigation of the structural, mechanical and tribological properties of low-wear diamond-like carbon (DLC) coatings for industrial applications. Amorphous hydrogen-free (a-C) and hydrogenated (a-C:H) DLC coatings were coated onto various steel substrates (AISI 1045H, B7, H13, D2, M2) with hardness levels varying from 28 to 66 HRC, by employing the pulsed laser deposition (PLD) method. Therefore, graphite targets were ablated with the 1064 nm wavelength of an Nd:YAG laser in argon and C 2 H 2 atmospheres. The high mean laser power of the applied PLD equipment guarantees deposition rates competitive to other physical vapour deposition (PVD) techniques. Because of the specific process conditions and the use of pure titanium adhesive interface layers, coatings with high adhesion to the substrates were produced at room temperature. The investigations of the coatings by means of light and scanning electron microscopy reveals the high surface quality and extremely dense coating structures. XRD measurements indicated the amorphous structure of the coatings. The nature of the chemical bonding was examined by XPS, indicating different amounts of sp 3 carbon bonds. Pin-on-disc tests against 100Cr6 ball-bearing steel balls as counterparts show an excellent wear resistance of a-C and a-C:H DLC coatings on all different steel substrates. These results, demonstrated the applicability of PLD–DLC coatings for wear protection of high precision components in the field of tools and mechanical components.

Pulsed laser deposition (PLD) is a physical vapour deposition coating technique for the production of thin films with complex chemical compositions. One of the main advantages of PLD is that excellent coating properties can be achieved even at low deposition temperatures. However, particulate defects in the growing films resulting from the evaporation process are often mentioned as the most important disadvantages of the PLD process. Unfavourable optical, thermo-physical and mechanical properties of the target material evaporated by laser radiation promote the formation of particulate defects. This paper presents some results on silicon-based PLD-films with reduced density of particulates. Silicon, SiO x and SiO 2 thin films were deposited by laser ablation from silicon targets with a high power pulsed Nd:YAG laser of 1064 nm wavelength in argon and oxygen containing atmospheres. The substrates were arranged in shaded off-axis geometry. The chemical composition and structure of the films were investigated employing transmission electron microscopy (TEM), secondary ion mass spectroscopy, X-ray photoelectron spectroscopy and ellipsometry. The results demonstrate the capability of PLD for the deposition of SiO x films with varying composition (0⩽ x ⩽2) by shaded off-axis PLD. The results of TEM and spectroscopic ellipsometry are indicating amorphous film structures in all cases.

Purpose of this work is to investigate the effects of reverted and retained austenite on the fracture toughness of high Co–Ni steels. Different cryogenic and tempering treatments are applied to a high Co–Ni steel in order to alter the austenite content, its chemical composition and hence its stability against transformation to martensite. Austenite stability is analyzed by applying thermal cycles in the dilatometerand by X-ray diffraction. For each heat treatment, the fracture toughness is characterized by J-integral and fractographic crack growth resistance measurements. The results indicate that cryogenic and two-step tempering treatments lead to a higher austenite stability and impede austenite transformation during cooling after tempering. Consequently, the toughness is considerably improved.

The possibility of obtaining similar mechanical properties with faster heating processes than the conventional ones has been of interest for several years. In the present study, investigations were performed in terms of the influences of such fast heat-treatments on the microstructure and mechanical properties of the material. This investigation compares an inductive with a conventional furnace heat treating process of a 50CrMo4 steel, however only the austenitizing treatment was changed and subsequent quenching and tempering was done in the same way. To this end experiments with a middle frequency generator, using different heating rates and austenitizing temperatures, were conducted and followed by oil quenching of the workpieces. The resulting structures were characterized regarding their microstructures and mechanical properties in order to gather a better understanding of the differences between the inductive and the conventional heat treating process. As a main result it was found, that the fast austenitized samples exhibited worse ductility than the conventional treated material.

The Materials Center Leoben Forschung GmbH (MCL) and the Austrian Society for Metallurgy and Materials (ASMET) have jointly organized the "First International Conference on Materials, Processing and Product Engineering - MPPE" held in the Congress Center of Leoben from Nov. 3rd to 5 th, 2015. The main topic of the conference was to present the state of the art in fundamental knowledge and technological expertise enabling and stimulating the development of knowledge based innovations across the entire product value chain starting from the material synthesis, proceeding with the processing of products and concluding with their in-service behaviour until the end of the product life time. Hence, the International Conference on Materials, Processing and Product Engineering MPPE 2015 focused on the core regions of industrial production chains, covering topics such as • Development and characterization of materials; • Materials synthesis and processing; • Dimensioning of components including integrated materials and process modelling; • Behaviour of materials and components during service. The scientific and technological goal was to present the state of the art of theoretical, experimental and numerical techniques and their combinations that are capable of improving the competitiveness of modern production facilities.

For high Co-Ni steels sub-zero treatments are conducted to reduce the retained austenite phase fraction for obtaining excellent fracture toughness properties, but in general, cryogenic treatment has a great impact on the microstructural evolution of steels during tempering. Hence, the aim of this work was to analyze the influence of cryogenic treatments on the microstructural evolution of high Co-Ni steels, including carbide precipitation kinetics and austenite phase fraction evolution, during heating to elevated temperatures. In order to study the formation properties of carbides, the heating processes of cryogenically and non- cryogenically treated specimens were analyzed by dilatometer measurements. Furthermore, for determining the evolution of austenite phase fraction and hardness due to tempering, dilatometer investigations were combined with X-ray diffraction analyses and hardness measurements. It is revealed that sub-zero treated samples exhibit much stronger carbide precipitation signals. This was ascribed to the lower phase fraction of retained austenite, as more carbon is available for carbide precipitation.

The Materials Center Leoben (MCL) is a Competence Center in the field of “Integrated Research in Materials, Process, and Product Engineering”. The MCL provides the manufacturing industry with advanced fundamentals, simulation methods, and experimental techniques in order to promote materials-enabled innovations and to strengthen competitiveness of this sector.

A low-friction and wear resistant TiAlN/VN multilayer coating with TiAlN/VN bilayer thickness 3 nm has been grown by using the combined cathodic arc etching and unbalanced magnetron sputtering deposition on high speed steel tools for dry cutting of aluminium alloys. In this paper, in-lab and industrial high speed milling tests have been performed on an aerospace aluminium alloy 7010-T7651. The results show that the TiAlN/VN coated tools achieved lower cutting forces, lower metal surface roughness, and significantly longer tool lifetime by three times over the uncoated tools as a result of the low friction and eliminated tool-metal adhesion. Under the same conditions, a TiAlN based multicomponent coating TiAlCrYN also increased the tool lifetime by up to 100% despite the high cutting forces measured.

The material requirements on aircraft engine mainshaft bearings increase due to an elevated speed index (bearing bore diameter multiplied by rotational shaft speed) and slip ratios [1]. The formation of reaction layers on surfaces in mechanical contact is strongly affected by the tribological loading conditions, the materials used, the lubricant, and the service temperature. An appropriate reactivity between material and lubricant in tribological systems decreases wear and friction and increases the durability [2,3]. Goal of the paper is to compare wear and friction properties of standard aerospace bearing steel AMS 6491 (M50) with that of the high strength stainless steel grade AMS 5898. The nominal chemical compositions are 0.82C-4.1Cr-1V-4.2Mo (wt%) and 0.3C-0.4N-15.2Cr-1Mo (wt%), respectively. In order to characterize the material behavior under pure sliding conditions, ball on disc (BOD) experiments were performed with a contact pressure of 1GPa and a sliding speed of 10 cm/s at room temperature and at 150°C. As lubricant the jet engine oil Mobil Jet II was used. It is assumed that the reaction layer formation depends on the material composition and is also effected by the counterpart and the lubricant additives. Thus, the experiments were performed with two different ball materials. The first ball material was the same standard aerospace bearing steel (M50) as mentioned above and the second was a ceramic (Si3N4). The homogeneity and the distribution of the reaction layers as well as the wear rate were determined in the contact zone by means of optical profilometry, scanning electron microscopy (SEM) and Fourier transformation infrared spectroscopy (FTIR). The study showed that wear is significantly higher on the stainless steel grade compared to M50 (Fig. 1a and Fig. 1b). The dark areas in Fig. 2 are phosphorus rich regions on the wear track of M50. This reaction layer is mainly built up of phosphates, which result from the TCP lubricant additive. In the FT-IR spectra (Fig. 3) absorption bands at 1130 cm−1 (room temperature-BOD test) and at 1160 cm−1 (150°C-BOD test) are visible, which result from the P=O stretching [4]. The shift of the absorption band to lower wave numbers with decreasing test temperature is probably due to hydrogen bonding [5]. Contrary to AMS 6491 no measurable reaction layer was found on AMS 5898 after testing at room temperature (Fig. 1b). The friction coefficients of the two steels against Si3N4 balls determined in the BOD tests are compared in Fig. 4. AMS 5898 shows an abrupt increase of the friction coefficient after a sliding distance of 3.5 m. The reason for that is a material transfer of disc material to the ceramic ball as can be seen in Fig. 5. This transfer material causes a plowing of the disc and thus, an increased wear can be observed. The different Cr-contents and consequently oxide layers of AMS 5898 and AMS 6491, which react differently with TCP, might explain this behavior. AMS 5898 does not sufficiently react with TCP at room temperature to form a protective layer. Consequently, material transfer and increased wear occurs. In case of AMS 6491 an increase of the operating temperature cause a change of the reaction layer (see Fig. 3) and to an increase in the wear rate.