Bruchverhalten der Stähle

Fracture Behaviour of Steels Crack formation during production or by monotonic, cyclic, chemical and thermal loading during service. Stable and unstable crack propagation. Clivage, dimple, intercristalline and fatigue fracture. Influence of temperature, loading conditions and microstructure on the transition from tough to brittle fracture behaviour.     

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.     

Schichtcharakterisierung nitrocarburierter Stähle

Coating characterization of nitrocarburized steels In this investigation, a systematic characterization of the diffusion coatings was carried out. These coatings, developed by different nitrocarburizing processes, namely salt-bath, gas and plasma nitrocarburizing, were conducted on various steel substrates. The tested substrates were St. 52-3, Ck 45, 42 CrMo 4, 30 CrNiMo 8 and X 20 Cr 13. The thickness of compound layers and their porosity were measured using optical microscopy. The structure of compound layers was characterised using an X-ray diffractometer and their surface roughness by a stylus profilometer. The surface hardness measurements and hardness profiles were carried out according to the Vickers principle.     

Aussagen der instrumentierten Schwingprüfung zur Dauerfestigkeit unlegierter Stähle

Evidence of the Instrumented Fatigue Test for Fatigue Strength of Structural Steel In the instrumented fatigue test the measurement of the frequency of loading and the power absorption of the resonance endurancetesting machine with electromagnetic energization discovery the rate of damage of the specimen. The energy and the energy gradient per cycle are especially suitable for characterize the fatigue life of the specimen. Rapid methods for evaluation fatigue strength are developed constructive on the results of experiments of fatigue tests in one load stage. The rapid methods are tested with cylindrical specimen (St 52-3) and spot weldings (St 1203). The results of measurement indicate a certain possibility of evaluation fatigue strength.     

Entwicklung einer niedrigschmelzenden Legierung und deren Applikation zum Korrosionsschutz hochfester Stähle

Development of low‐temperature galvanizing and its application for corrosion protection of high‐strength steels Apart from reliability and quality, vehicle safety and cost efficiency are the decisive criteria for automobile manufacturers. Corrosion protection plays a decisive role because it increases the service life. The ultra‐high‐strength steels are materials which exhibit high lightweight potential as well as a very good energy absorption capacity because of their mechanical properties. In connection with the possibility of hot forming, they are predestined for the fabrication of complicated, load‐compatible shapes in the crash‐relevant frame and body construction. The application of these steel qualities has been carried out in structural parts which are protected from corrosion by a hot‐dip coat of FeAl7 – the so‐called Usibor. However, at the moment there is no ready‐for‐production solution for later corrosion protection of already hot‐formed parts. Therefore, a corrosion protection system on the basis of conventional low‐temperature galvanizing processes has been developed and utilized. First, the softening behavior of the highly‐resistant 22MnB5 substrate was analyzed. Afterwards, a galvanizing system was developed and applied. The corrosion protection coatings were characterized with regard to their structure and corrosion protection potential. As a result, a significant improvement of the corrosion behaviour has occurred.     

Einfluß der Zunderung auf das Zeitstandverhalten hitzebeständiger Stähle

Influence of scaling on creep rupture behaviour of heat resistant steel Interrupted and uninterrupted creep-rupture tests and annealing tests on 4 heat resistant steels, performed mainly at 1000 and partly at 1100°C. Determination of the metal loss due to scaling. Validation of an improved method for correcting time dependent strength properties due to metal loss. Determination of further influences.     

Aktivierte Randaufstickung nichtrostender Stähle

Activated Solution Nitriding of Stainless Steels The solution nitriding of the stainless steels can be optimized by a two stage process. The first stage involves an surface activation and an enrichment of nitrogen in the case due to internal nitriding. After this step at temperatures between 1070 °C and 1150 °C follows the dissolution of the chromium nitrides and a solution nitriding. Investigations of ferritic, martensitic and austenitic steels showed that this technology is superior compared to the one stage technology. The treatment time for an given layer thickness in the high temperature stage is cut in halve. The case concentration of nitrogen can be controlled by a material specific choice of the treatment temperature and the partial pressure of nitrogen. For the investigated steels the desired microstructure of the case could be achieved by partial pressures of nitrogen between 0,35 an 1 bar. The solution nitriding of ferritic-martensitic steels eneables the production of martensitic cases with a hardnesses up to 700 HV 0,3. An austenitic case with higher hardness and stability of the austenit can be produced by enrichment the surface of austenitic and ferritic-austenitic stainless steels with nitrogen.     

Vorbeanspruchungsabhängige Schadensakkumulation bei der Kriechermüdung warmfester Stähle

Influenced prior loading on the creep fatigue damage accumulation of heat resistant steels On two heat resistant power plant steels the influence of prior strain cycling on the creep rupture behaviour and the influence of prior creep loading on the strain cycling behaviour is investigated. These influences concern the number of cycles to failure and the rupture time being the reference values of the generalized damage accumulation rule and they are used for a creep fatigue analysis of the results of long term service-type strain cycling tests.     

Abrasiver Verschleiß nitrocarburierter Stähle

Abrasive wear of nitrocarburized steels Diffusion coatings have been used increasingly to improve surface properties of several machine components for tribological applications. In this investigation, the abrasive wear of various diffusion coatings has been studied. These coatings developed by three different nitrocarburizing processes, namely salt-bath, gas and plasma nitrocarburizing, were conducted on five various steel substrates. The substrates were St 52-3, Ck 45, 42 CrMo 4, 30 CrNiMo 8 and X 20 Cr 13. The thickness of the compound layers and their porosity were measured using optical microscopy. The structure of compound layers was characterized using an X-ray diffractometer, and their surface roughness by a stylus profilometer. The surface hardness measurements and the hardness profile were made according to the Vickers principle. The abrasion test was carried out using a pin-on-table machine. The predominant wear mechanisms have been evaluated using scanning electron microscopy. It was found that the employed nitrocarburizing processes have improved the abrasive wear resistance of all tested coating-substrate-combinations. In general, the abrasive wear rate decreases with an increase in the hardness of the compound layer as well as its thickness. The best coating-substrate-combinations to improve abrasion resistance were salt-bath and gas nitrocarburized specimens of the substrate X 20 Cr 13. Plasma nitrocarburized specimens of the substrate St 52-3 and Ck 45 were the worst.     

Die Prüfung der Temperaturwechselbeständigkeit hitzebeständiger Stähle und Legierungen

Testing the Resistance to Thermal Shocks of Heat-Resisting Steels and Alloys Scaling resistance of heat-resisting stells and alloys is impaired to a large extent by a reiterating change in temperature and is, in particular, a function of the temperature interval and the rate at which the change in temperature occurs. A test stand for thermal cycling is described which permits quenching upto 80 specimens at a time from temperatures as high as 1100 °C in air, oil or water. As many as 30 temperature cycles are possible in one day. Repeated quenching in water causes considerable changes in specimen shape: length fo austenitic specimens of size 10 mm diam. × 90 mm length increased by about 30%, with cracks forming before specimens broke. Ferritic steel specimens are shortened, in some cases to half their initial length. In steels with a mixed austenitic and ferritic structure length increase is particularly marked. Change in length s a function of the rate of change in temperature, as can be seen by comparing the modes of quenching in water, oil and air.     

Beitrag zum Einfluß von Druckwasserstoff auf die mechanischen Eigenschaften von Stählen

Contribution to the Influence of Pressurized Hydrogen on the Mechanical Properties of Steels The investigation method of using hollow specimens in tensile tests to examinate steels for hydrogen service is described. Reduction of area after fracture, Z, resulting from these tensile tests with constant strain rates shows not only a significant but also a constant difference between hydrogen and nitrogen when being plotted versus strain rate. Therefore a newly introduced term named “Index of embrittlement” and characterized by the relation of Z with nitrogen and with hydrogen was found to be independent of strain rate. With strain rate ε = 10^?6 s^?1 chosen for a “critical” one because of lacking in a maximum index of embrittlement several European steels for hydrogen service were tested to find out their individual index of embrittlement. Assisted by light optical microscopy it was found that there is a special correlation between microstructure, index of embrittlement and tensile strength of a given steel grade.     

Mehrstufige betriebsähnliche Dehnwechselbeanspruchung warmfester Stähle

Multi-stage service-type strain cycling of heat resistant steels The creep-fatigue behaviour of three heat resistant steels is investigated by service-type three-stage creep-fatigue tests up to 8000 h duration. Creep-fatigue damage is calculated on the basis of the generalized damage accumulation rule. Different cycle counting methods are taken into consideration.     

Zähigkeitsanforderungen an Stähle in tieftemperaturbeanspruchten Bauteilen

Toughness Requirements for Steels in Low-Temperature Components In order to exclude the possibility of low-ductility fractures (brittle fractures) in components with safety relevance (e. g. pressurized containments) the materials used have to have an adequate toughness. The toughness requirements for materials are laid down in sets of regulations or in specifications. If the guidelines are compared, however it becomes clear that opinions vary considerably concerning minimum toughness to be guaranted and procedure to be evaluated. In this paper the regulations are discussed.     

Einfluß der Versuchsparameter auf die Abschreckgeschwindigkeiten beim Laseroberflächenumschmelzen am Beispiel ledeburitischer Stähle

Influence of the Process Parameters on Quenching Rates in Laser Melting of Ledeburitic Steels On quenching, surface melted ledeburitic steels solidify dendritically. The secondary arm separation can be used to estimate the cooling rate and hence enables the effect of the operating conditions on the cooling rate to be assessed. In general, the residual melt solidifies as an eutectic or as chromium carbide or a mixture of chromium carbide and iron. The resultant carbide net and the resultant grain refinement change the hardness of the surface. The hardness can be controlled by varying the quenching rate in surface melting and if necessary a subsequent heat treatment.     

Kriechermüdungsverhalten warmfester Stähle bei betriebsähnlicher Hochtemperaturdehnwechselbeanspruchung

Creep fatigue behaviour of heat resistant steels under service- type strain cycling at high temperature On three typical heat resistant steels the creep fatigue behaviour is investigated up to about 10 000 h test duration using a service-type strain cycle. In a creep fatigue life analysis the cyclic deformation behaviour and the applicability of the generalized damage accumulation rule are investigated and possibilities of long-term creep fatigue prediction are studied.     

Beständigkeit verschiedener Stähle und Nickelbasiswerkstoffe unter dem Einfluß aufnitrierend wirkender Gase in der Ammoniak-Synthese

Resistance of Various Steel Types and Nickel-Base Alloys Under the Influence of Nitriding Gases in Ammonia Syntheses Plants The paper describes test results obtained during 5 years of practical trials with heat-resisting steels, steels for hydrogen service, austenitic steels and nickel-base alloys under the nitriding operation conditions found in an ammonia plant at temperatures of more than about 400°C. During the test, nitriding rates and depths at high temperatures and under stresses were established, as well as changes in the mechanical properties as a result of nitriding. Based on the test results and the knowledge of the subject matter, recommendations and service condition limits for material selection are given. The superior behaviour of austenitic materials, especially with increasing nickel contents, could be confirmed. An attempt is made to find an explanation for the reduced resistance to hydrogen attack under the influence of nitridation.     

Langzeitverhalten einiger warmfester Stähle unter betriebsähnlicher Kriechermüdungsbeanspruchung

Long–term behaviour of some heat resistant steels under service–type creep–fatigue loading The long-term creep-fatigue behaviour of four heat resistant steels is investigated by service-type strain cycling with test durations up to 30 000 h. The creep-fatigue life is analysed on the basis of the generalized damage accumulation rule. The influences of prior service-type strain cycling on the basic creep and fatigue properties are taken into account.     

Rost‐ und säurebeständige Stähle unter Komplexbeanspruchung

Rust and acid resistant steels under complex loading With corrosion fatigue investigations under different loadings (R = 0,27, ‐1 and 10) and with inner pressure tests a duplex steel (1.4462) and a super austenitic steel (1.3974) are compared. The investigations were carried out in 10 % HNO₃, artificial seawater according to DIN 50905 and in two very important liquids for application (artificial seawater according to DIN 50905 with H₂S aerated and water with chloride, fluoride, bromide, iodide). For comparison the same tests under only mechanical loads were done in acid free oil. The results show that the super austenitic steel has an significantly higher resistance against corrosion fatigue as the duplex steel under one‐ and three dimensional loads up to 10⁷ cycles. The proportion between the results in the corrosive liquids and the oil can be described by a factor. In artificial seawater factors between 0,67 and 0,92 and in 10 % HNO₃ between 0,67 and 0,83 were determined depending from the loading for the duplex steel. With the super austenitic steel factors between 0,86 and 1 in artificial seawater and between 0,7 and 1,0 in 10 % HNO₃ were received.