Spanende Bearbeitung von Hartlegierungen – Drehen und Schleifen. Teil III: Schleifen von Hartlegierungen

Metal Cutting of Hard Alloys – Turning and Grinding. Part III: Grinding of Hard Alloys Straight surface plunge grinding tests were performed at a peripheral wheel velocity of v_c = 15 m/s on exemplary iron based alloys FeCr12C2.1, FeCr13Nb9MoTiC2.3 and FeCr14Mo5WVC4.2 both in soft annealed and hardened and slightly relieved microstructural condition. Vitrified bonded wheels of type SC 60 3/4 (SiC) and 2B252 M6 V240 (CBN) with a diameter of D_S = 300 mm were used as abrasive tools. When selecting the machining parameters due consideration was given to materials related aspects and an acceptable volumetric removal rate per unit width of Q_w′ = 4 mm³/mms was taken into account. Subsequent examinations focused on the analysis of the thermomechanical load imposed on the microstructure. The grinding of hard alloys by means of conventional abrasives is significantly influenced by the content and type of the hard phases present. Due to their excessive hardness primarily solidified M₇C₃ are highly resistant to the ingressing abrasive grains. Accordingly, maximum grinding normal forces are encountered with the FeCr14Mo5WVC4.2 alloy. However, the normal forces are also found to be higher in case of a hardened metal matrix, when SiC is used as abrasive or a wet grinding process is applied. As regards tangential forces they also show a tendency of being influenced by structure-specific hard phases. Process temperature and stress condition ahead of the grain cutting edges are of great significance for the behaviour of the hard phases during grinding. If the mechanical component is predominant, accumulations of near-surface eutectic carbides are destroyed. Individual stalk-like carbides often break in the phase center. However, in the event of thermal load an eutectic M₇C₃ may also be deformed plastically. Through the cleavage fracture of coarse primary M₇C₃ phases microcracks are initiated that in the end will grow into macrocracks. Carbide fragments broken off will impair the surface quality. In particular the metal matrix reacts strongly to the process heat generated. In case of dry grinding using SiC a rehardening zone has been detected near the surface. The alloy-specific austenizing temperature of approximated 1000 °C was exceeded. As the distance to the surface increased a tempered area with hardness figures below those of the basic structure was found. No rehardening will occur if CBN is used as abrasive. The residual surface stresses determined correlate with the extent of thermal and mechanical load imposed. Whereas an extensive crack network is evident after dry grinding when SiC has been used as abrasive, no surface cracks were detected when employing the CBN abrasive. Due to the excellent thermal conductivity characteristics of this grinding medium a thermal damage during dry grinding can be avoided. While the use of grinding fluid will improve the surface roughness, cracks may form due to the abrupt quenching effect, especially if hardened material is involved. Increasing the workpiece velocity will also contribute to reduce the risk of crack development, but, on the other hand, leads to a surface quality deterioration that cannot be accepted.     

Spanende Bearbeitung von Hartlegierungen – Drehen und Schleifen. Teil I: Gefüge und Eigenschaften von Hartlegierungen

Metal Cutting of Hard Alloys – Turning and Grinding. Part I: Structure and Properties of Hard Alloys Hard alloys count among the materials that contain hard phases. This involves primary and/or eutectic hard phases embedded in a metallic matrix. The characteristics of the individual microstructural constituents may be combined to form a material featuring excellent wear resistance and a high resistance to fracture. For that reason, the material can be widely used in all applications where the wear resistance to abrasion is essential. In the event low operating temperatures are involved the component costs to service life ratio speaks for hard alloys on Fe basis. Above 600 °C heat resistant Ni and Co matrices are to be given preference. Carbides and borides of the transition metals are specially suited as hard phases. They attach well to the surrounding matrix. Nowadays, alloys of the FeCrC system are primarily employed for economic reasons. As nickel-based material the NiCrSiB alloying system is frequently employed. Hard alloys on cobalt basis usually belong to the CoCrWC (stellite) system. In many fields of application components of this material group require a largearea metal cutting technique (eg for barrel extruders, crushing rollers, valve seats). However, problems may be encountered during machining due to the high hardness and excellent wear resistance of this material. The structural difference between hard phases and metallic matrix causes different reactions to stresses exerted during the machining process. Process-related changes of the microgeometric surface characteristics and the physical condition of the surface zone of a material are paraphrased by the term “surface integrity”. To create a basis for assessing the machining influence on the multiphase component surface layer, the first part of the paper discusses manufacturing techniques, constitution of the microstructure and main properties of the individual structural components.     

Drehen und Fräsen pulvermetallurgischer Hartlegierungen für warmgehende Werkzeuge

Turning and Milling of Powder Metallurgical Hard Alloys for Tools in Hot Working Applications Hard metals are high wear resistant materials. The microstructure of these composites consists of hard phases which are embedded in a metal matrix. The high hardness and the high content of the hard phases lead to a difficult machining of these materials. The present study investigates the turning and milling of D3 cold work steel (X210 Cr 12) and the powder metallurgical Fe‐based alloys ASP60 and ASP23 + WC/W₂C. The cutting tool materials were polycrystalline cubic boron nitrides (CBN) and ceramic inserts. The machining process could be judged by means of tool wear and machining quality (surface roughness and changes in the surface near zone). The investigations illustrate that the machinability of the different hard metals depends on the cutting speed and the cutting tool material.     

Spanende Bearbeitung von Bauteilen aus Al‐Matrix‐Verbundwerkstoffen

Machining of Components of Al Matrix Composites The microstructure of metal matrix composites consists of hard reinforcements which are embedded in a metal matrix. The high hardness of the reinforcements leads to a difficult processing of these materials. The present paper demonstrates the machining of components of Al matrix composites for the automotive and the aircraft industry. The components are SiC particle reinforced brake drums, cylinder blocks with local Si particle and Al₂O₃ short fiber reinforced cylinder liners and TiB₂ particle reinforced extrusion molding profiles. The investigations illustrate that good results can be achieved when machining these components by turning, boring, drilling and milling with polycrystalline diamond (PCD) or CVD diamond thick‐film cutting tool materials.     

Prüfung von Flanschverbindungen in GFK-Rohrleitungen NW 200 durch Kurz- und Langzeitbeanspruchung – Teil I: Dichtverhalten

Testing of flanged joints in GRP pipelines of 200 mm nominal diameter by means of short-term and long-term stressing. Part I: Sealing behaviour . The first part of this report deals with the sealing behaviour of joints with fixed and loose flanges in GRP pipelines of 200 mm nominal diameter. Tests were made with seven different flanges from three manufacturers and four different types of sealing rings. At room temperature the spring rate of the GRP flanges amounted to between 8% and 18% of that of welded steel flanges and at a temperature of 80°C within the pipe it was 5% to 16% of that of the steel flanges. Of the sealing rings tested a bulged soft rubber ring with steel reinforcement was found to have the flattest resilience characteristic and required the least bolt force to produce a tight joint under operational conditions. Its relaxation rate was low after low bolt force had been applied and increased rapidly when the initial bolt force was increased to higher values. Because of their lower spring rates, the GRP flanges sealed with this ring required 2.5 to 3.5 times as much initial bolt force as welded steel flanges. In an 1100-hour creep test at 80°C internal temperature and 10 bar internal pressure, a joint consisting of loose steel flanges and GRP shoulders exhibited considerably less loss of initial force than a fixed GRP flanged joint. Because losses of initial force increase at a disproportionately high rate as the force is raised, high initial force would not seem to increase remarkably the reliability of the seal in the long term. Since rigid flanges require less initial force than less rigid ones, high spring rate appears desirable. Apparently it could best be achieved by loose steel flanges with GRP shoulders. The results of the strength tests will be presented in the second part of this report.     

Röntgenographische Untersuchung von Spannungszuständen in Werkstoffen. Teil II. Fortsetzung von Matwiss. und Werktoffechn. Heft 3/1995, S. 148–160

X-ray Investigation of Stress States in Materials X-ray stress analyses on crystalline or partially crystalline materials are based on the determination of elastic lattice strains which are converted to stresses by means of theory of elasticity. The development of the sin² ψ-method of X-ray stress analysis and of diffractometers substituting film chambers during the 1960s initiated an enormous progress in X-ray stress analysis during the following three decades both in respect of the knowledge of the underlying principles and in respect of the practical application. This report sketches the historical development of X-ray stress analyses and describes the actual state of the art of this important tool for materials science and engineering. Besides some important elements of X-ray physics and theory of elasticity, experimental aspects of practical applications are outlined. Standard measuring procedures and special measuring problems are described, and hints for practical solutions are given. In particular, examples of destructive and non-destructive depth profiling of residual stresses, of residual stress analyses in thin coatings, in multilayer structures of thin coatings and in chemically graded coatings, of residual stress analyses in presence of textures, of residual and loading stress analyses in heterogeneous materials, in coarse grained, and in single crystalline materials are presented. The methods established up to now are explained and possible future developments are pointed out.     

Vakuumwaagen – Teil 2: Mikrowaagen

For sorption measurements and for thermogravimetric investigations vacuum beam balances were developed with resolution down to the nanogram region. In corrosive atmosphere or under very clean conditions the magnetic suspension balance should be favoured. The sample tube is separated from the balance and can be hermetically sealed from the environment. It contains only the pan with the sample connected to a permanent magnet, suspended at a stable position in the field of a controlled electromagnet. The spring balance is an inexpensive alternative, if a minor relative sensitivity can be accepted. Quartz resonators are used to control evaporating and sputtering processes. Resonator systems of any type need no gravitational field and can be applied, therefore, in space technology.     

Untersuchungen Zum Temperaturwechselverhalten von Gußeisen - Teil II: Werkstoffwissenschaftliche und metallphysikalische Interpretation

Test examing the temperature shift behaviour of cast iron - Part II: Interpretation according to materials science and metallphysics The first part of this article discusses the most important influences on temperature shock behaviour of cast iron materials and experimental results of the examinations. The presented second article offers an interpretation of theoretical as well as practical examination results according to materials scientific views. A method will be presented determining the lifetime of thermal shock stressed components using activation-energy-stress diagrams and statisticall significant regression-relationships.     

Simulation des Mikrorisswachstums unter Schwingbeanspruchung. Teil 2: Simulationsergebnisse – Einfluss des Spannungszustandes und Reihenfolgeeffekte

Simulation of fatigue micro crack growth. Part 2: Results of simulation – influence of stress state and sequence effects In part one the modelling of micro crack growth due to alternating loading has been presented. Simulation results for tension/compression, torsion and proportional multiaxial loading, the scatter of the simulated lifetimes to a macroscopic crack length of 500 μm as well as the influence of the density of crack seeds and the grain size have been presented. In part two the influence of the stress state under proportional and non‐proportional loading is examined. Additionally the sequence effect of High‐Low and Low‐High as well as consecutive load sequences will be discussed. The comparison of the simulation results to experimental results shows that the influences of multiaxial loading and sequences can be simulated qualitatively correctly. The simplifications of the modelling have been to be considered. If the length of the maximum crack is interpreted as a measure of the damage, it can be concluded that the damage accumulation is non linear and non continuous. The main proportion of lifetime from the crack seed to the macro crack is contributed in the phase in which the length of the maximum micro crack comes close to the size of the grain size.     

Röntgenographische Untersuchung von Spannungszuständen in Werkstoffen. Teil III. Fortsetzung von Matwiss. und Werkstofftechn. Heft 3/1995, S. 148–160 und Heft 4/1995, S. 199–216

X-Ray Investigation of Stress States in materials X-ray stress analyses on crystalline or partially crystalline materials are based on the determination of elastic lattice strains which are converted to stresses by means of theory of elasticity. The development of the sin²Ψ-method of X-ray stress analysis and of diffractometers substituting film chambers during the 1960s initiated an enormcus progress in X-ray stress analysis during the following three decades both in respect of the knowledge of the underlying principles and in respect of the practical application This report sketches the historical development of X-ray stress analyses and describes the actual state of the art of this important tool for materials science and engineering. Besides some important elements of X-ray physics and theory of elasticity, experimental aspects of practical applications are outlined. Standard measuring procedures and special measuring problems are described and hints for practical solutions are given. In particular, examples of destructive and non-destructive depth profiling of residual stresses, of residual stress analyses in thin coatings, in multilayer structures of thin coatings and in chemically graded coatings, of residual stress analyses in presence of textures, of residual and loading stress analyses in heterogeneous materials, in coarse grained, and in single crystalline materials are presented. The methods established up to now are explained and possible future developments are pointed out.