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.     

Struktur der Randschicht nichtrostender Stähle nach Tieftemperatur‐Nitrierung – eine Mößbauerstudie

Case structure of stainless steel after low temperature nitriding – a Mössbauer study Due to the nitriding of stainless steel at temperatures between 300 °C and 400 °C cases of high hardness and nitrogen contents ranges between 8wt.% and 12 wt.% could be prepared. In the present work the nitriding was performed by gas and plasmanitriding. The phase generation was investigated by use of the Conversion‐Mössbauer‐specrtoscopy (CEMS and XCMS) and the X‐ray diffraction. The chemical composition was determined by GDOS and the hardness test using the Martens‐hardness under load. In spite of different principles of action no significant differences between the structure and properties of the gas and plasmanitrited samples could be observed within the bounds of the used test methods. Clear differences were found in the nitriding behaviour of different steels.     

Schnellbewertung von Dispersionsgefüge und mechanischen Eigenschaften karbidbildender Stähle

Fast Evaluation of dispersed Microstructure and mechanical Properties of Carbide forming Steels A possibility of a fast evaluation concerning the dispersed microstructure and the properties of C-, Cr- and Mn-alloy steels is presented. The dispersed microstructure was adjusted by precipitation and coarsening from the martensitic initial condition. The quick recording coercive field strength H_C and microstructural parameters obtained from scanning electronmicroscopic investigations were used in the relationship H_C-particle diameter d_p relative to a microstructure fast evaluation. Due to the repeated nucleation the M₃C carbides grow and coarse parallel and in particle generations. Thus always a particle size spectrum is obtained. A fast evaluation of properties is realized by the connection H_C – yield strength R_p0.2 / tensile strength R_m / hardness HV. Since H_C reacts on the size of precipitated particles, and the course H_C – time t / d_p / particle volume (d_p/2)³ encloses two maxima, which both separate the precipitation (A) and coarsening stage (V), at any H_C-value one of the attaching possibilities (A or V) must be excluded. This can be reached by considering the obtained temperature and time limiting values. The magnetic measuring method with the structure-sensitive quantity H_C yields to an assertion relative to the microstructual condition within one minute. The error amounts to ≤0.5%. The application of the H_C-measurement is time- and cost-economic. A special sample preparing is not necessary. The condition of the surface does not enter into the measuring result. The yielded connections H_C – R_p0.2 / R_m / HV can be used like a standard referred to the tested grades of steel.     

Struktur und Eigenschaften einer hochfesten sprühkompaktierten Aluminiumlegierung

Structure and Properties of a High Strength Spray-Deposited Aluminum Alloy A 7XXX series high strength aluminium alloy was produced by spray-deposition via the Osprey process. This alloy, designated A1Zn11MgCu, exhibits an excellent combination of fracture toughness and strength in both the peak-aged T6 temper and the overaged T7 temper. The yield strength and fracture toughness at room temperature are in the T6 temper 700 MPa and about 40 MPa√m, and in the T7 temper 500 MPa and 75 MPa√m, respectively. The exceptional high fracture toughness is attributed to the absence of coarse constituents and oxides. The microstructure is homogeneous, free of segregation and has a fine grain size. The hardening precipitates in the T6 temper are mainly η′, and the T7 temper they are η (MgZn₂), resulting in a different fracture behavior: in the T6 temper the fracture mode is micro-shearing and in the T7 temper fracture mode is ductile failure.     

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.     

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.     

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.     

Eigenschaften und Verwendungsgebiete von niedriglegierten warmfesten Stählen

Properties and applications of low-alloy steels for elevated temperatures. Demands on steels for elevated temperatures. Tensile and toughness data on Manganese Nickel Molybdenum Vanadium steels with ferritic-bainitic structures in dependence of the chemical composition and the temperature. Influence of pulsating stresses. Examples of application.     

Zeitstandverhalten warmfester Stähle bei zyklischer und bei intermittierender Beanspruchung

Creep and Creep Rupture Behaviour of Heat Resistant Steels under Cyclic and Intermittent Loading Conditions On typical heat resistant steels creep and creep rupture tests under rectangular cyclical conditions of tension stress or/and temperature were continued. Comparable creep and creep rupture tests with tension and compression phases and with intermittent loading conditions were started. Besides this single-stage loading conditions, double-stage loading conditions were investigated. The test results are analysed with the modified life fraction rule. Concepts of relative life are improved and partly new established.     

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.     

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.     

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.     

Kriech- und Bruchverhalten warmfester Stähle in kennzeichnenden Fällen veränderlicher Zeitstandbeanspruchung

Creep and Creep Rupture Behaviour of Heat Resistant Steels under Variable Loading Conditions The creep and creep rupture behaviour under rectangular cyclical conditions was measured on 8 typical heat resistant steels up to 35 000 h test duration. The results are described by the modified life fraction rule. The main dependencies of the characteristic of that rule, the relative life, are investigated. Concepts for the evaluation of the relative life are presented in regard to design and supervision of components.     

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.     

Korrosionsuntersuchungen an drei Stählen und einer Nickellegierung in Mono- und Trichloressigsäure

Investigations Into the Corrosion of Three Steels and one Nickel Alloy in Mono- and Trichloroacetic Acid The corrosion resistance of three steels and one nickel alloy was investigated in aqueous and nonaqueous monochloroacetic acid and in nonaqueous trichloroacetic acid at various temperatures. There is no difference in the corrosion behaviour of the steels whereas the resistance of the nickel alloy is better in all cases by a factor 10. In aqueous monochloroacetic acid the corrosion attack is due to the hydrolytic formation of hydrochloric acid. Nonaqueous monochloroacetic acid behaves more corrosive than nonaqueous trichloroacetic acid.     

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.