Gefügemäßige Bewertung der Schweißeignung von Stahlgusssorten unter Berücksichtigung der Kaltrissbeständigkeit

Microstructural evaluation of weldability of cast steels considering the sensitivity to cold cracking The behaviour of different types of cast steel at welding of structures is widely determined by the specialities of the microstructure in the heat affected zone (HAZ). The identification of the different areas of microstructure depends on the alloying system of the cast material. At the evaluation of the weldability of cast steels with high or low carbon contents high attention should be drawn to the sensitivity to cold cracking, which correlates with the chemical composition as well as with the inhomogenieties of the initial microstructure.     

Schäden wälzbeanspruchter DLC‐Schichten auf Stahlsubstraten unterschiedlicher Härte

Damages of slip‐rolling tested DLC coatings on steel substrates of different hardness Extremely hard diamond coatings on hard SSiC substrates, various hard DLC coatings on 100Cr6 substrates (HRC60) as well as selected DLC coatings on unhardened steel substrates (HRC20) were tested under slip‐rolling conditions. Unadditivated paraffin oil was used as a lubricant. The tests were carried out in an Amsler type twin disc tester at initial maximum pressures of P₀=2.3 GPa according to Hertz. The tests were terminated after n=1.000.000 revolutions (endurance tests: n=10.000.000 revolutions) or if a coherent damaged area of A>1 mm² occurred. The slip‐rolling tests showed that the SSiC had a supportive influence on the diamond coatings which, however, failed due to fractures in the substrate. At least two of the DLC coatings on 100Cr6 substrates (HRC60) withstood the slip‐rolling test for up to n=10.000.000 revolutions with nearly no visible damage. These coatings deposited onto a soft, nitrogen alloyed steel (HRC20) were able to adjust to the deformation of the substrate without major damaged areas (A>1 mm²).     

Slip‐rolling resistance of ta‐C and a‐C coatings up to 3,000 MPa of maximum Hertzian contact pressure

The slip‐rolling resistances of hard and stiff thin films under high Hertzian contact pressures can be improved by optimizing the “coating/substrate systems”. It is known from former investigations that the so‐called “egg‐shell” effect is no general hindrance for high slip‐rolling resistance of thin hard coatings. The coating stability depends more on specific deposition process and coating/substrate interface design. In this article it is experimentally shown, that pure amorphous carbon thin films with hardness between 15 and 63 GPa can be slip‐rolling resistant several million load cycles under a maximum Hertzian contact pressures of up to 3.0 GPa. Whereas all coatings were stable up to 10 million load cycles in paraffin oil at room temperature, reduced coating lifetime was found in SAE 0W‐30 engine oil at 120°C. It was shown how the coating hardness and the initial coating surface roughness influence the running‐in process and coating lifetime. No clear correlation between coating hardness and coating lifetime could be observed, but friction coefficients seem to be reduced with higher coating hardness. Very low friction down to ?0.03 in unmodified engine oils was found for the hardest ta‐C film.     

Konzept zur Verbesserung der Meerwasserbeständigkeit von Wälzlagerstahl

To develop corrosion‐resistant hardenable bearing steels for seawater applications the contents of Mo and Cr were raised in respect to X65Cr14 and the carbon content was lowered by making use of the soluble N content. The steels X51CrN(14)14 and X51CrMoN(14)15‐3, to be molten under normal pressure, and steel X33CrMoN(32)15‐3, to be processed under an N₂ pressure of = 4.7 bar, promise a higher resistance to pitting corrosion and a lower liability of coarse eutectic carbides in segregated areas without discernible disadvantages in respect to hardness and retained austenite. The study has led to a pre‐selection of promising new steels just by thermodynamic calculations and empirical equations without time‐consuming and costly experiments.     

Eigenschaften langzeitgealterter,artgleicher Schweißnähte der Ni‐Basislegierung alloy 602 CA

Properties of long‐time aged matching joints of Ni‐base alloy 602 CA The paper presents results about the influence of a 2‐year exposure in the production process of ammonia synthesis on the microstructure and the mechano‐technological properties of matching TIG‐ and e‐manual welds of the high‐temperature resistant Ni‐based alloy 602 CA (NiCr25FeAlY, Material‐No. 2.4633 / UNS N 06025). These results show, that Ni‐base alloy 602 CA possesses a very good resistance against metal dusting. Furthermore typical metallurgical characteristics of the welding process of this extremely hot‐crack sensitive alloy are presented and recommendations for a hot‐crack safe manufacturing of components made of this alloy are given.     

Über adiabatic shearbands und die Entstehung der „Steilen Weißen Bänder”︁ in Wälzlagern

About adiabatic shear bands and the generation of “high‐angle white bands” in roller bearings During the rolling motion of roller components structural changes are generated by over‐elastic distressing below the bearing face. Micro‐ and macro‐plastic distortions of the microstructure due to the three‐axial distressing accompany the process of rolling contact fatigue with the start from the first load cycle. They determine the life‐duration of the roller component. Further structural changes in ball bearings besides the plastic deformations are the so called “butterflies” and the so called low‐ and high‐angle “white bands”. The “white bands” you only can detect later, after an extended number of rolling actions. They are inclined to average circumferential angles at ≈ 30 ° respectively ≈ 80 °. Butterflies and white bands develop obviously due to a two‐axial material stressing. An influence on the life duration is not proved. The structure and the mechanism of their generation are under discussion. There exist phenomena of the microstructure which are similar to the white bands, the so called adiabatic shear bands. These generate after a local macro‐shear process of the microstructure, caused by a single, rapid shock‐shear stressing. Flash‐temperatures near the melting point generate in the shear zone of martensitic hardened materials the formations of new hardened zones; these microstructures cannot be etched too. The paper contributes to the question, if there exists a stress‐constellation which causes the adiabatic formation of butterflies and of white bands during the steady and long during process of plastic deformation. If one considers the latest knowledge about rolling contact fatigue and stressing by EHD (elasto‐hydro‐dynamic) flow, he can come to the conclusion, that at least the “high‐angle white bands” are adiabatic shear bands.     

Large‐Pore Mesoporous CeO2–ZrO2 Solid Solutions with In‐Pore Confined Pt Nanoparticles for Enhanced CO Oxidation

Active and stable catalysts are highly desired for converting harmful substances (e.g., CO, NO_x) in exhaust gases of vehicles into safe gases at low exhaust temperatures. Here, a solvent evaporation–induced co‐assembly process is employed to design ordered mesoporous Ce_xZr_1?xO₂ (0 ≤ x ≤ 1) solid solutions by using high‐molecular‐weight poly(ethylene oxide)‐block‐polystyrene as the template. The obtained mesoporous Ce_xZr_1?xO₂ possesses high surface area (60–100 m² g^?1) and large pore size (12–15 nm), enabling its great capacity in stably immobilizing Pt nanoparticles (4.0 nm) without blocking pore channels. The obtained mesoporous Pt/Ce_0.8Zr_0.2O₂ catalyst exhibits superior CO oxidation activity with a very low T₁₀₀ value of 130 °C (temperature of 100% CO conversion) and excellent stability due to the rich lattice oxygen vacancies in the Ce_0.8Zr_0.2O₂ framework. The simulated catalytic evaluations of CO oxidation combined with various characterizations reveal that the intrinsic high surface oxygen mobility and well‐interconnected pore structure of the mesoporous Pt/Ce_0.8Zr_0.2O₂ catalyst are responsible for the remarkable catalytic efficiency. Additionally, compared with mesoporous Pt/Ce_xZr_1?xO₂‐s with small pore size (3.8 nm), ordered mesoporous Pt/Ce_xZr_1?xO₂ not only facilitates the mass diffusion of reactants and products, but also provides abundant anchoring sites for Pt nanoparticles and numerous exposed catalytically active interfaces for efficient heterogeneous catalysis.     

Tribologische Modelluntersuchungen an metallischen Werkstoffen für Formeinsätze zum Mikro‐Pulverspritzgießen mit ZrO2‐Formmasse

Tribological studies on metallic materials used as mould inserts in micro powder injection moulding (microPIM) with zirconia feedstock Wear behaviour of steel X38CrMoV5‐1 and C 45E, electroplated nickel and brass Cu63Zn37 used as mould inserts in micro powder injection moulding (microPIM) was studied in two different laboratory tribometers. Using an abrasive wheel test with 220 mesh flint the volumetric wear increased with decreasing hardness from steel to nickel and brass. Experiments using a laboratory tester simulating powder injection moulding with zirconia feedstock at 170 and 190°C showed contrary results. Volumetric wear of the softer materials nickel and brass was significantly lower than that of steel X38CrMoV5‐1 and C 45E. The presented results indicate that in micro powder injection moulding wear behaviour can depend more on microstructural parameters like homogeneity or the ability of work‐hardening and deformation but on hardness of the materials.     

Niedrigemittierende Außenoberflächen bei Wärmedämmverbundsystemen – ein Studie zur Vermeidung von Algenwuchs

Algae growth on the outside surface of thermal insulating composite systems (TICS) occurs because of long‐lasting wetting. The present techniques for its avoidance are without long‐term effect. Especially, dew and frost are an essential reason for its occurrence because of the highly thermal insulation of the TICSs. In the following report, it will be investigated by an algorithm on the basis of a worst case scenario for outside condensation whether outside condensation on TICSs can be counteracted by a low‐emissivity outside surface as it is well‐proven on highly insulating window glazing. The calculations showed that outside condensation on TICSs with heat transfer coefficients (U) values from 0.1 to 0.2 W/(m² K) cannot be avoided theoretically by thermal emissivity values ε in the range of 0.05 till 0.1. However, because the most critical scenario conditions for outside condensation are very stringent and, in addition, the calculations have shown that outside condensation is only possible for relative outdoor humidity rH_o ≥ 95%, it can be deduced that the occurrence of outside condensation will be very improbably in praxis for TICSs with U values from 0.1 to 0.2 W/(m² K) and ε values from 0.05 to 0.1. A weathering and soiling resistant surface with ε ≤ 0.1 can be realized by aluminum which may be additionally passivated by a titanium dioxide coating. Because of the hydrophilic property, this coating shows also self‐cleaning effect and quickly drying of the surface after rain. The latter may counteract algae growth, too.     

Die Werkstoffbeanspruchung im Wälzkontakt bei hoher Flächenpressung,ermittelt nach Hertz und nach der EHD‐Strömungshypothese

Material stressing at rolling contact with high specific load determined by HERTZian‐ and elastohydrodynamic‐ (EHD) flow hypothesis For the calculation of the material stressing at rolling contact HERTZian distribution of the normal stresses and the tangential stresses within the contact area, which are proportional to the normal stresses, are assumed. This numerical explication is based upon the calculation of the contact fatigue endurance. Taking in consideration the real stress distribution of forces at EHD contact, it can be explained by the EHD flow hypothesis, that at special load constellations the materials stressing leads to higher values.     

The effect of heat treatment on superconductivity of Zn‐doped YBa2Cu3‐xZnxO7‐y

YBa₂Cu_3‐xZn_xO_7‐y compounds with x = 0, 0.05, 0.15, and 0.30 have been synthesized by standard solid state reaction method. The crystal structure, lattice parameters, and oxygen content are not changed by the substitution of Zn for Cu since both valence state and ionic radius are almost identical for Zn and Cu elements in YBa₂Cu_3‐xZn_xO_7‐y. However, the superconducting transition temperature T_c decreases with the increase of Zn content, reflecting the T_c‐suppression effect of Zn substitution. Heat treatment experiments indicate that the heat treatment at low temperature is beneficial to improve the superconductivity of the sample. But T_c decreases with the increase of annealing temperature when the treatment temperature is above 300°C, and finally the superconductivity disappears at approximately 920°C, 700°C and 550°C for the samples with x = 0.0, 0.05 and 0.15, respectively. Our experiments indicate that the superconductivity of the sample with higher Zn content is more sensitive to the oxygen content, and a small decrease in the oxygen content can lead to a considerable decrease of T_c.     

Untersuchung des Einflusses von Prozessparametern in der Fertigstrasse auf die Tertiärzunderausbildung

Investigation of the influence of processing parameters during hot rolling on tertiary scale formation The permanent demand for a cost saving production of flat products of steel as well as the technical progress in the engineering established the basis for a substitution of cold rolled products by hot rolled products. Precondition is a systematic control of the inevitable oxidation of the strip during the entire hot rolling process. With the generation of appropriate scale properties at hot rolled strip, taking into account the required mechanical properties, a direct processing of hot strip without prior pickling, cold rolling and annealing could be possible. The knowledge of the influence of the processing conditions during hot rolling on the properties of tertiary scale, especially the thickness, the constitution and adherence is incomplete till now. That’s why influences on the oxidation and scale properties respectively during the hot rolling process starting from the finishing line to the coiling as well as consequences for the downstream process are discussed in this paper based on the results of laboratory hot rolling trials to the simulation of considerably processing parameters in the finishing line.     

Über die Beanspruchung oberflächennaher Werkstoffbereiche von Zahnflanken bei Voll‐ und Mangelschmierung

Strengthening of surface‐near‐materials‐zones of gear flanks with plain and deficient lubrication Couplings of gears, as strengthened over long duration, show often ruptures on the flanks. Their distribution and orientation among the surface can be explained by the kinematics of rolling. The cracks show an angle inclined towards the surface between 20 ° and 40 °. They are opposite to the occurring tangential stresses. Only in negative slip zones the ruptures grow to secondary cracks, rupturing occurs mainly at this point. The commonly used calculation of the materials strengthening in surface near zones by the HERTZian distressing conditions applied for tangential stresses does not give any hint to a preferred area of the flank stressed with negatives slip. The various failure phenomena can be explained by the elastohydrodynamic (EHD) – hypothesis. This hypothesis points out that the lubricants flow causes additional strengthening which influences the fatigue endurance and lowers the value determined after HERTZ.     

Charakterisierung von Randentkohlungsvorgängen bei der Austenitisierung des Wälzlagerstahls 100Cr6. Teil 2: Modellierung des Kohlenstoff‐Tiefenverlaufs mit Hilfe der Methode der Finiten Elemente

Characterization of Decarburisation Processes During Austenitising of the Rolling Bearing Steel 100Cr6. Part 2: Modelling of the Carbon Concentration Profile by Means of the Finite Element Method The quantitative measurement of carbon concentration‐distance curves serves as fundamental prerequisite for the evaluation of rim zone properties connected with decarburisation processes in material science. This was shown in part 1 of the present work with two samples from through‐hardenable rolling bearing steel 100Cr6 (SAE 52100) austenitised in different oxidising atmospheres by position dependent determination of hardness, residual stresses, and X‐ray line broadening ({211} α’‐Fe diffraction line). In practice, it is important to predict carbon concentration‐distance curves under prevailing heat treatment conditions or to conclude conversely from profile measurements. Based on a refined kinetics model of a diffusion‐controlled process, part 2 therefore presents a simulation tool developed by means of the finite element method (FEM). Apart from the concentration dependence of the diffusion coefficient, it also considers the decarburisation induced austenite‐ferrite phase transformation, the time dependent influence of scaling, and variable atmosphere conditions. The interpretation of the carbon concentration‐distance curves, measured very accurately in the rim zone of both 100Cr6 samples by secondary ion mass spectrometry (SIMS), confirms the possibilities of application of the new numerical tool.     

Pulvermetallurgische Erzeugung von SiC‐ und Al2O3‐verstärkten Al‐Cu‐Legierungen

Powder metallurgical fabrication of SiC and Al₂O₃ reinforced Al‐Cu alloys Based on metallographic studies the states of composite powder formation during high‐energy ball milling will be discussed. Spherical powder of aluminium alloy AA2017 was used as feedstock material for the matrix. SiC and Al₂O₃ powders of submicron and micron grain size (<2 μm) were chosen as reinforcement particles with contents of 5 and 15 vol.‐% respectively. The milling duration amounted to a maximum of 4 hours. The abrasion of the surface of the steel balls, the rotor and the vessel is indicated by the content of ferrous particles in the powder. High‐energy ball milling leads to satisfying particle dispersion for both types of reinforcement particles. Further improvements are intended. The microstructure of compact material obtained by hot isostatic pressing and extrusion was studied in detail by scanning and transmission electron microscopy. For both types of reinforcement the microstructure of composites is similar. The microporosity is low. The interface between reinforcement particles and matrix is free of brittle phases and microcracks. In the case of SiC reinforcement particles, a small interface interaction is detectable which implies a good embedding of reinforcement particles. High‐energy ball milling under air‐atmosphere leads to the formation of the spinel phase MgAl₂O₄ during the subsequent powder‐metallurgical processing. Because of the size, rate and dispersion of the spinel particles, an additional reinforcement effect is expected.     

Ermüdungslebensdauerbewertung von Warmumformwerkzeugen – Ein Überblick über den Stand der Forschung und Anwendung

Hot work tools are subjected to complex thermal and mechanical loads during hot forming processes. Locally, the stresses can exceed the material's yield strength in highly loaded areas. During mass production, this leads to cyclic plastic deformations and thermomechanical fatigue of the tools, which can be a major lifetime limiting factor. However, established concepts for thermomechanical fatigue life assessment of hot work tools do not exist, since this aspect first reached attention in the last years with the needs for higher resource and energy efficiency as well as optimized manufacturing processes (e. g. in the frame of Industrie 4.0). Hence, in this paper, the contemporary industrially used concepts for dimensioning hot forming tools regarding the tooling fatigue life are presented. Furthermore an overview of existing plasticity and lifetime models is given. The models are divided in phenomenological and mechanism based models. The review shows that further research is essential in this field.