Entwicklung mit Hilfe des Warmpreßverfahrens hergestellter Sinterstähle mit optimaler Warmstreckgrenze,Dauerfestigkeit und Kerbschlagzähigkeit – Teil 1

Development of Hot-Forged Sintered Steels with Optimum Yield Point at Higher Temperature, Fatigue and Impact Strength . The material characteristic values of hot-forged sintered steels are reported. Besides the usual properties, (ultimate tensile strength, yield strength and elongation at room temperature). The behaviour of materials at higher temperature is of great importance. Further, fatigue strength and impact strength must be considered. The influence of heat treatment on these properties were investigated and brought to a correlation with the formation of microstructures. The hot forging P/M preform requires a special attention on alloying elements and alloying techniques. Optimum properties results when using completely alloyed powders or mixed powders by employing special master alloys. This and further problems involving alloying techniques and particularly the behaviour of carbon as an alloying element is reported in details. The correlation between mechanical properties and fracture behaviour is discussed.     

Vergütungssinterstahl aus mit Nickel und Molybdän legiertem Stahlpulver

Hardenable Sintered Nickel-Molybdenum Alloy Steel. From all the posible alloying elements to be used for high-strength Powder-Metal-Steels, Nickel and Molybdenum should be preferred, particularly when heat treatment is required. As an example a P/M-Steel with 2,0% Ni and 0,5% Mo is being presented. The pros and cons of blends and prealloyed powders are being displayed as well as the physical properties as-sintered and as-heat treated. In general, high-strength sintered steels loose their ability to be calibrated because of their high yield strength and poor deformability. It is therefore very difficult to calibrate them to close tolerances, hence they cannot very often compete with other metal working procedures which would permit to achieve the required tolerance. The author describes the careful selection of the carbon concentration. Despite a tensile strength figure in the order of 80–90 kp/mm² the material retained its ability to be manufactured to very close tolerances. The method by which the samples were prepared are described. The materials obtained from blends and prealloyed powders exhibit different structures; their influences on the physical properties are being explained.     

Effect of partial mechanical alloying on the self-propagating high-temperature synthesis of Ni3Si

Self-propagating high-temperature synthesis (SHS) is a new method for economical processing of intermetallic compounds and ceramic materials, as well as composites based on them. On the other hand, mechanical alloying is an effective method for producing highly metastable and, therefore, reactive metal powders. In this paper an overview of partial mechanical alloying is given. The effect of partial mechanical alloying on the self-propagating high-temperature synthesis of Ni₃Si-compounds is studied. The influence of alloying time on powder characteristics, e.g. particle size distribution, is given. The effect of alloying time on the properties of Ni–Si composite powders and on the characteristics of the SHS process, e.g. propagation rate, is reported. Ni₃Si was chosen as the object for this study because of its corrosion and high-temperature oxidation resistance. Like other L1₂-type compounds, the strength of Ni₃Si shows an anomalous behaviour as a function of temperature, therefore, it has potential for high-temperature applications.     

Möglichkeiten zur Steigerung der Oberflächenfestigkeit bei Aluminiumlegierungen mit Plasma-Pulver-Schweißverfahren

Improving of the Wear Resistance of Aluminium Alloys by Plasma Transferred Arc Welding Aluminium alloys are extensively used materials which can be found in a wide range of industrial applications. They have distinctive advantages such as a high strength/weight ratio, an excellent workability and good corrosion behaviour. However, aluminium alloys possess wear resistance properties which limitate their usage in heavy wear applications. Besides many technologies applying thin film wear resistant layers on aluminium alloys there is hardly a technology which can produce strengthened surfaces in a range of several millimeters. Plasma Transferred Arc (PTA) Welding using a modified polarity DCCP (Direct Current Combined Polarity) was used for enhancing the wear properties of wrought and cast aluminium alloys by alloying and dispersing. Hardness and wear resistance of the plasma welded surfaces could be improved significantly.     

Oberflächenlegieren von Werkzeugstählen mit Hartkarbiden durch Laser/Elektronenstrahlschmelzen

Surface Alloying of Tool Steels with Carbides by Laser or Electron Beam Melting. It is shown how the properties of tool steels can be improved by laser or electron beam melting and alloying of the surface. Various techniques can be employed depending on the base material and intended application. Crack free surface treated layers 1-1.5 mm thick can be produced. The hardness profiles, structures and oxidation resistance of these layers are compared for the different techniques and materials.     

Verformungsverhalten von Aluminium-Verbunddrähten mit Chloridzusätzen

Deformation of aluminium composite wires with admixtures of silver-alkali-chlorides . The deformability, the formation of the fibre texture and the tensile strength properties of aluminium compound wires with 5 Vol% saline components of the phase diagramm NaCl-AgCl are investigated. The systematic change of the extended plasticity of the AgCl ionic crystal by alloying with NaCl to the pure NaCl component effects a a penetrating influence of the deformation mechanisms of the aluminium matrix. This fact is obvious by the meaningful change of the fibre texture (110) (111) to complete random orientation of the metallic matrix with increase NaCl portion of the saline solution phase. The excellent plasticity of the pure AgCl-compound produces a fibre structure and a maximum of tensile strength of the fibre like compound wires by fibre reinforcement. With increasing solid solution hardening of the second phase the fibre like shape of the components diminishes. The increment of the strength by pure NaCl compound is caused by dispersion hardening of the aluminium matrix.     

Europäische Beiträge zur Entwicklung der Legierungstechnik bei der Herstellung von Sinterformteilen

European Contribution towards Development of Alloying Technique for Manufacturing Sintered Structural Parts . In Europe in the last decade the consumption of iron, steel and non-ferrous metal powders for the production of filters, bearings and structural parts has trippled. This positive development has been largely influenced by the progresses made in the p/m alloying techniques. Among the most important European contributions in this scope belongs the development of the monophasic iron-copper and iron-copper-nicle-alloys. The progressing development and introduction in production of the iron-phosphor alloys is a further important contribution. An important progress is represented by the development of the diffusion-alloyed iron powders with copper, nicle and molybdenum as alloying components. Recent and specially promissing developments are carbon-, manganese- and chromium containing sintered steels in which an oxidising resistent pre-alloy is used as alloying partner which contains the alloying elements in highly concentrated form.     

Entwicklungstendenzen bei hochfesten schweißbaren Kessel- und Druckbehälterstählen

Development Trends in High Tensile Weldable Boiler and Pressure Vessel Steels. Increase of dimensions, pressure and temperature of boilers and pressure vessels necessitate the development of high tensile steels. To improve the tensile properties of steels water quenching and tempering (Q. T.) is applied more and more beside the addition of alloying elements. Results of tests on Q. T. micro- and low alloyed pressure vessel steels have been described. In addition to tensile and brittle fracture properties the behaviour of Q. T. Steels in creep tests, fatigue tests and intercrystalline corrosion tests has been reported on. Water quenching and tempering of boiler and pressure vessel steels offers technologically and economically favourable means of development.     

Das Kriechverhalten von stickstofflegierten,austenitischen Stählen mit erhöhter Festigkeit bei Raumtemperatur

The Creep Behaviour at Room Temperature of Higher Strength Austenitic Steels containing Nitrogen. The mechanical properties of special austenitic steels with increased Cr and Ni contents and N₂ content up to 0,35 % have been investigated. In the as-received condition i.e. annealed and water quenched, the 0,2 % offset yield strength of these steels ranges from 400 to 500 N/mm². However, regardless of these increased yield strengths, the stress-strain characteristics are the same as for conventional austenitic steels. Since first plastic deformations correspond to an applied stress equal to 50 % of the 0,2 % yield strength, in practical applications the creep behaviour of these steels may be significant. Therefore, the creep behaviour at room temperature of the base metals and weldments has been investigated to determine the influence of thermal and mechanical treatments after annealing. The elastic limit of airquenched steels is significantly higher than of water-quenched, even though the 0,2 % yield strengths are essentially the same. This different stress-strain dependence influences the creep behaviour of the steel. The cold working of these steels by plastic straining in both tension and compression results in a useful strain hardening effect only when the direction of applied stress is the same as the stress during cold working. When the direction of stressing was reversed, a considerable Bauschinger-effect was observed.     

Einfluss der Schweißparameter auf das Ermüdungsverhalten hochfester Baustähle

High strength low alloy steels are used in many different engineering areas. A commonly used joining technique for those steels is fusion welding. Generally, these components have to withstand fatigue due to dynamic loading. Using thermal joining techniques affect the mechanical properties of the steel. This study focuses on the influence of the heat input on the microstructure of high strength low alloy steels (S690). Furthermore, the fatigue behaviour with special regard to crack initiation and crack propagation is characterized.     

Optimierung warmgepreßter Sinterstähle–Mechanische Eigenschaften,Bruchverhalten und Wirtschaftlichkeit

Optimization of Hot Forged Sintered Steels – Mechanical Properties, Fracture Mechanism and Economy . The hot forging technique of powder metal preforms is a new technology for manufacturing structural parts with close tolerances and better surface roughness than conventional forging. A particular significance is gained by the development of high strength materials, whereby the alloying systems and alloying content have to be adjusted in an optimum way. The present work deals with a systematic investigation of static and dynamic properties of H/F steels. By using a new developed alloying technique, the processing of the high oxygen affinity elements, manganese, chromium and vanadium results in H/F steels with minimum oxygen content. It was established that these materials fulfil certain correlations between the individual mechanical properties and the reduction of area. These relations allow to optimize the materials as well as compare them with normal wrought steels. The investigation of fracture mechanism with the help of scanning-electron-microscope gave valuable information of the material behaviour.     

粉末冶金TiVNbTa难熔高熵合金的组织和力学性能

将机械合金化(MA)与放电等离子烧结(SPS)相结合制备了难熔TiVNbTa高熵合金,研究了这种合金的机械合金化过程、相组成和显微组织,以及烧结温度和O、N含量对其力学性能的影响。结果表明:机械合金化后高熵合金粉末为BCC结构,放电等离子烧结成的块体高熵合金由BCC基体和FCC析出相组成,其析出相为TiN+TiC+TiO的复合物。烧结温度为1100℃的高熵合金具有良好的综合力学性能,压缩屈服强度达到1506.3 MPa,塑性应变为33.2%。随着烧结温度的提高,合金发生了从准脆性到塑性再到脆性断裂的转变。O和N含量的提高对高熵合金强度的影响较小,但是使其塑性显著降低。     

Weldability of Al4C3–Al composites via diffusion welding technique

In this study, Al–Al₄C₃ composites, produced by powder metallurgy in situ techniques, were joined by diffusion welding technique at 250 MPa pressure with various welding temperatures and durations. Microstructures and shear strengths of the joined areas were determined. Al powders were mixed with 2% carbon black and milled in a high energy ball mill (mechanical alloying) for up to 20 h. In order to obtain cylindrical blanks with 10 mm in diameter and 15 mm in height, powders were compacted in a single action press at 1000 MPa. Samples were sintered in Ar atmosphere at 650 °C and metal matrix composite (MMC) containing 8% Al₄C₃ particles were produced. Products were then joined to each other by using diffusion welding techniques. Scanning electron microscopy examination was carried out on the welded interfaces and shear tests were conducted to the sample interfaces to find out the effect of welding temperatures and duration on the weldability properties. It was found that high welding temperatures resulted in increase of both joined strength and shear properties. However, increase in welding duration did not make any detectable changes. Results indicated that MMC could be joined by diffusion welding technique successfully with the 88% strength of base material.     

Dualphasen-Stähle mit erhöhter Festigkeit und Verformbarkeit

Dual-Phase Steels with Improved Strength and Ductility. The term “dual-phase steel” refers to a new class of steels with a special ferritic-martensitic microstructure, which offers a superior combination of good formability, particularly concerning deep-drawing, and of high tensile strenght. This paper describes the alloys and the thermomechanical treatment to process a dual-phase microstructure. Comparing tensile tests of dual-phase structures and other ferritic-martensitic structures, factors are analysed which determine the typical mechanical properties of dual-phase structures. Their optimum mechanical properties of dual-phase structures. Their optimum mechanical properties, which are found to be better than those of conventional high-strength steels are due to the topology of the microstructure and to the addition of the unusual alloying element phosphorus. These new steels are of interest to all automobile manufactures because weight reduction of auto-body parts can be realized. The future perspectives for the application of dual-phase steels are discussed.     

Beitrag zum Lebensdauerverhalten und zur zerstörungsfreien Bewertung geschweißter und fehlerbehafteter ultrafester Stähle

Relationship between fatigue behaviour and non-destructive assessment of weld defects of TIG-welded ultra-high strength steels With respect to the influence of welding defects on the fatigue properties of welded high strength special steels and on the dynamic load carrying capacity of joints in welded constructions, a minimum of information is available. For this reason, the preparation of an assessment standard for classifying weld defects was considered practical, whereby the character of the defects was to be determined using non-destructive testing methods. The investigations were concerned with a special method of producing specimens containing defined and reproducable defects, with the characterization of the defects using X-ray and ultrasonic techniques, and with the determination of the influence of the weld defects on the fatigue strength of TIG-welded joints. As a result of the investigations, a relationship between allowable and critical defect size with respect to an assessment standard has been sought. By introducing weighting factors for various types and sizes of defects occurring in welds the efficiency of non-destructive testing methods for the assessment of weld defects is considerably increased.     

Amorphization behaviour in mechanically alloyed Ni—Ta powders

Amorphization behaviour of NixTa100–x alloy powders synthesized by mechanical alloying mixtures of pure crystalline Ni and Ta powders with a Spex high energy ball mill was studied. The mechanically alloyed powders were amorphous for the composition range between Ni10Ta90 and Ni80Ta20. This range is larger than amorphous alloys prepared by the rapid-quenching process or by electron-gun deposition technique. A supersaturated nickel solid solution formed for Ni-rich composition. The thermal stability has been investigated by differential thermal analysis. The crystallization temperature of amorphous Ni—Ta powders was proportional to the Ta content, and the activation energy of amorphous Ni—Ta powders exhibited a maximum near the eutectic composition. It is found that the amorphization rate at the early stage of the mechanical alloying process was faster in the intermediate compositions than those at both Ni- and Ta-rich compositions.     

ECAP‐Umformung mittel‐ und hochfester ausscheidungshärtbarer Aluminiumknetlegierungen

Equal‐channel angular pressing of medium‐ to high‐strength precipitation hardening aluminium wrought alloys The study deals with the optimisation of medium‐ to high‐strength aluminium wrought alloys. The goal is to define processing routes in order to improve the mechanical properties if compared to their commercial counterparts. It is shown for the Al‐Mg‐Si and the Al‐Cu‐Mg‐Si system that the application of ECAP enables a significant increase in strength. The strengthening as well as the grain size reduction respectively, benefit from increasing alloying as well as from the degree of aging. It is also shown that the presence of a considerably fine particulate reinforcement hardens the material tremendously during ECAP. The combination of a pre‐ or post‐ECAP heat treatment enables the improvement of the workability on the one hand, reducing the loads on the die, and also gives a better ductility on the other hand. This positive effect is particularly pronounced for low alloying contents and high aging temperatures and can be attributed to the interaction of deformation induced defects and the precipitation activity. The combination of an appropriate set of ECAP parameters (heat treatment condition, ECAP‐strain, ‐temperature, ‐backpressure) enables the efficient production of outstanding properties. Due to the low workability of AA7075 (Al‐Zn‐Mg‐Cu system) no significant improvement in properties was achieved.     

Werkstoffverhalten und Mikrostrukturentwicklung hochfester Mn‐Al‐Si‐Leichtbaustähle unter Zugbelastung

Material behaviour and microstructure evolution of high‐strength Mn‐Al‐Si‐light‐weight steels under tensile loading Because of their extraordinary combination of high strength and superior ductility high‐strength high Mn‐steels with reduced density and additions of aluminium and silicon are interesting candidates for structural applications. The initial microstructure consisting of stable austenite or austenite and ε‐ and α'‐martensite was achieved by alloying. During plastic deformation intense strain induced martensitic transformation and / or mechanical twinning was observed. These deformation mechanisms are used to extend the limited forming capability and contribute to a high energy absorption (in impact tests) up to very high strain rates. Tensile tests reveal that the properties are maintained up to strain rates of about 1000 1/s. The flow stress behaviour is strongly influenced by the initial microstructure and their evolution during deformation processes is determined by the rates of martensite and twin formation, respectively.     

Characterization of mechanically alloyed ternary Fe–Ti–Al powders

The effects of Ti-substitution for Fe in the Fe₃Al system on the mechanical alloying process were investigated. For this purpose, blended elemental powders with the following nominal compositions (at.%): Fe₇₅Al₂₅, Fe₇₀Ti₅Al₂₅, Fe₆₅Ti₁₀Al₂₅, Fe₆₀Ti₁₅Al₂₅, were mechanically alloyed in a high energy attritor-type ball milling system for up to 100 h. The structural evolution in these powders was characterized by scanning electron microscopy, differential thermal analysis and X-ray diffraction techniques. It was found that elemental powders were progressively transformed into nanocrystalline solid solutions during mechanical alloying. The addition of Ti in the powders shortened the milling time for solid solution formation. With increasing Ti content, the grain size of the solid solutions decreased, but the lattice parameter increased. Upon heating, the milled powders were transformed into ordered (Fe,Ti)₃Al intermetallic compounds in an extended range of temperature (from 350 to 500°C). Ti addition enhanced the occurrence of DO₃ ordering in heated powders.     

Nichtrostende Stähle mit TRIP/TWIP/SBIP‐Effekt

Stainless steels with TRIP/TWIP/SBIP effect Economic austenitic steels with high energy absorption capability are in the focus of worldwide research activities, whereby the steels which show TRIP, TWIP and/or SBIP effects play a crucial role. New austenitic or austenitic‐martensitic stainless steels with a high cold workability and energy absorption capability are currently developed and tested in laboratory scale at the Institute of Iron and Steel Technology at the Technical University Bergakademie Freiberg. The mechanical properties of these steels are essentially influenced by the TRIP, TWIP and SBIP effect, becoming evident in hot formed and solution annealed steels as well as in as‐cast steels. The TRIP/TWIP/SBIP effects have a significant impact on the toughness and the strength of stainless steels consisting of metastable austenite. The TRIP effect owns a paramount position since it serves for a simultaneous increase of toughness and strength. The influences of alloying elements like manganese or nickel on the TRIP effect are in the centre of the investigations at the Institute of Iron and Steel Technology. These austenitic or austenitc‐martensitic stainless steels provide the ability for new applications fields due to their excellent mechanical properties. Exemplary, in the Collaborative Research Centre SFB 799 “TRIP‐Matrix‐Composites”, financed through the Deutsche Forschungsgemeinschaft DFG, the suitability of this new class of steels for cast components in ductile and transformation strengthened high performance (metal) ceramic composite materials will be investigated.