Entwicklungstendenzen bei korrosionsbeständigen hochwarmfesten Werkstoffen und ihre schweißtechnische Verarbeitung

Trends of corrosion resistant high-temperature resistant materials and their welding processing High-temperature resistant materials and their joints have to show sufficient mechanical characteristics and good corrosion resistance at temperatures above 500°C during a longer period. This applies also to the Ni-Fe-Cr-alloy Nicrofer 3718 – alloy 330 (material No. 1.4864) which is used primarily in furnace building in connection with the filler metal Thermanit Nicro 82 (material No. 2.4806). Occurring damages on such welded joints show that it is insufficient to determine the seam characteristics only immediately after welding. Rather it is necessary to consider the later processing and operating conditions of the component and to simulate them in an appropriate way at the welding seams during the investigations.     

Randschichteigenspannungen durch mechanisches Schleifen bei hochfesten Federstählen

Oil tempered steel, like the high strength spring steel 54SiCr6 (1.7102), is mainly used for helical springs. To enlarge the contact area between spring and adjacent parts, the spring tail is often flat‐grinded. Due to wire‐drawing process residual stress is already existing before spring manufacturing. The spring manufacturing process contains several sequenced steps like winding, even grinding and shot peening. All of these steps cause additional or influence the existing residual stress in wire. The knowledge of these mechanisms and the level of residual stress is mandatory for design of application and its operation. Grinding processes in general has major influence to the residual stress level. In fact the choice of grinding parameter is essential for the stress height. In general: thermal effects while grinding cause basically tensile stress, mechanical effects cause basically pressure stress. Presented investigation shows the qualitative influence of several grinding parameters like infeed, conventional or synchronized grinding operation mode and the optional usage of cooling lubricant. The superposition of grinding‐based residual stress with different residual stress depends not only on the stress‐height but also on the characteristic. Therefore universally statements are not available.     

Vorhersage der Lebensdauer und Dauerfestigkeit von hochfesten,durchhärtenden Stählen

Fatigue Lifetime and Endurance Limit Prediction for High‐Strength Steels Smooth and notched specimens of the bearing steel 100Cr6 (SAE 52100) in a bainitic condition were used to determine the S‐N curves under tensile, torsional and combined in‐ and out‐of‐phase loading. In the area of high‐cycle fatigue, crack initiation was most likely caused by inclusions like Titanium Carbonnitrides or Aluminium Oxides. A micro mechanical model for the crack initiation by inclusions was developed. Another model was developed to describe the influence of these inclusions on the lifetime. A weakest‐link model, using the statistical distribution of inclusions and surface flaws, was used to describe the endurance limit.     

Konstruieren mit neuen hochfesten Stählen mit niedrigem Ausdehnungskoeffizienten

Designing with new high-strength low-expansion steels The potential of high-strength low-expansion steels has been illustrated by a series of specific design examples through comparing performance against that of alternative materials. The new products have advantages where large temperature excursions are encountered in piping and pressure vessel applications. The benefits to be gained through using the alloys in the design of labyrinth seals are also demonstrated. Finally the position with respect to diesel engine combustion chamber components is examined, and the alloys are shown to be better than alternatives. Apart from the different design requirements in the above applications, other possible applications are suggested, based on this work which has shown one of their main design merits to be low expansivity coupled with low moduls resulting in very low thermal stresses.     

Theoretisch-numerische Beanspruchungsanalyse und Versagensbeurteilung von Druckbehältern

Theoretic-numerical Stress Analysis and Failure Assessment of Pressure Vessels An extensive research program has been conducted to technically evaluate the burst behaviour of model vessels with nozzles as a function of the dimensions and material toughness. As a supplement of this program, the stresses in the vicinity of the nozzle under internal pressure were analysed using the Finite Element Method. The ratio of the wall thickness of the vessel and nozzle were varied. By applying different failure hypotheses, the trend of the experimental burst pressure with temperature could be established.     

Haftfestigkeit von Metallschichten auf Stählen bei statischer und wechselnder Beanspruchung

Influences of Static and Dynamic Load on the Adhesive Strength of Thermally Sprayed Metal Coatings on Steels It were the aims of this work to produce metal-sprayed specimens and workpieces with high resistance to static and dynamic load, to prove these properties by appropriate measurements and to give advices for practical application. For this purpose, two different substrate materials were metal-coated with five materials by use of three spraying techniques. As a result of the investigations, it can be summarized that specimens, which were shot-peened prior to spray-coating reached the same fatigue limit as only shot-peened specimens, independent on the spraying technique applied.     

Rißfortschritt in ultrafesten Stählen und deren Schweißverbindungen bei dynamischer Beanspruchung

Crack propagation in ultra-high-strength steels and their welded joints under dynamic loading . Reported are results of investigation into the propagation of cracks in the base metal and weld metal of an ultra-high-strength steel. The material used in the investigations was a Ni? Co? Mo? alloy maraging steel with a yield point of 170 kp/mm². The steel was arc welded and TIG welded. The joints exhibited a drop of static strength in the range of 5 to 8 percent related to the base metal. Under zero-to-tension stress cycles the fatigue strength corresponded that of other high-strength steels, under tension-compression stress cycles the steel exhibited a higher fatigue strength. It was possible to show striations with the aid of scanning microscopy. Comparing the track propagation calculated in the microscopic range with the results obtained from the crack growth curves produced approximate agreement.     

Einflüsse auf die Kriechporenbildung und deren Nachweis bei warmfesten niedriglegierten CrMoV-Stählen

Effects on creep cavitation and determination of creep cavities on heat resistant low alloy CrMoV-steels The aim of this work is to describe the evolution of damage due to creep cavities under long-term creep conditions as well as to quantify it by metallographic methods. The chosen materials comprised the high temperature steels 12 CrMov 4 3 and 15 CrMoV 5 10. The investigations are made under consideration of unaxial stress, temperature, time, life-fraction, creep-strain and the initial microstructure.     

Karbidpartikelbildung und Diffusion in Stählen

Carbide Particle Formation and Diffusion in Steels The adjustment of dispersed microstructures in Cr and Mn alloyed steels by precipitation of carbide particles M₃C was carried out from the martensitic initial condition by annealing in the temperature range T = 500°…700°C about the time t = 1…4290 min. In this connection it was already shown that the nucleation, the particle growth and coarsening run off parallel, where the coarsening is the dominating process. With the increase of the particle volume according to r³ α t (r – particle radius) final particle sizes were obtained asymptotically. The increase of the particle volume in the coarsening stage can be only maintained, if small particles will be dissolved again. This process can be written well with the Ostwald-ripening relationships. Permanent concentration changes at the nucleation sites resp. in the decompositing matrix are necessary. By means of the Ostwald-ripening law the effective chemical diffusion coefficients D_eff of the alloying elements C, Mn, Si Mo and Cr in the α-Fe-matrix could be evaluated. D_eff changes with the decomposition of the matrix by a few orders of magnitude. The diffusibility of the elementes depends on the alloying content in the matrix. C has as expected the greatest mobility, Cr shows the smallest one. The lattice bond seems to be of significance. Moreover the values of activation energy of carbide particle precipitation Q and the frequency factors D₀ in the Arrhenius-relationship were evaluated applied to the steels investigated. With this the necessary diffusion data relative to short till to longtime thermal treatment for the adjustment of heat-treated microstructures are available. The lower and the upper limiting values of the carbide particle size spectra can be taken from the plots referred to the Mn and Cr alloyed steels.     

Die Werkstoffdämpfung von Stählen bei hohen Dehnungsamplituden

Damping of Steels at High Strain Amplitudes The amplitude dependent damping has been investigated in two martensitic chromium steels (X 22 CrMoV 12 1, X 20 Cr 13) and a cold worked austenitic steel (X 12 CrNiWTi 16 13) at room temperature. Due to the magneto-elastic effects, the martensitic steels show a strong amplitude dependent damping. It cound be shown that the damping is reduced remarkably by a reduction of the annealing temperature. The austenitic material shows a very high damping in the cold worked condition. Tempering at relatively low temperatures (above 100 °C) reduces damping to the low values usually expected for austenitic materials. This effect was attributed to the pinning of dislocations during tempering.     

Zusammenwirken von Spannungsrißkorrosion und Ermüdung bei der Schwingungsrißkorrosion eines hochfesten Stahles

Conjoint Action of Stress Corrosion Cracking and Fatigue on Corrosion Fatigue of a High Strength Steel The corrosion fatigue characteristics of a high strength, martensitic steel in 0.5 n NaCl solution is investigated with regard to the fatigue and stress corrosion cracking behaviour of the material. Test parameters are stress ratio and frequency, testing is carried out with fracture mechanics methods, the crack surfaces are examined fractographically. An analysis of the results reveals that corrosion fatigue in high strength steel is caused by fatigue or by stress corrosion cracking, depending on the kinetics of the two processes. Fatigue and stress corrosion cracking do not act cumulative or additive. Instead, the kinetically faster process causes crack advance. The crack growth characteristics are interpreted with respect to the fractographic appearance of the crack surfaces. Corrosion fatigue cracks propagate either intergranular relative to the prior austenite grain boundaries as stress corrosion cracks do or transgranular like fatigue cracks, depending on the crack growth rates of the two processes. Fatigue and stress corrosion cracking do not interact, at least in a measurable degree, because of the different crack path of the two fracture processes. Results can be assessed quantitatively with the “process competition model”.     

Beschreibung und Beurteilung gebräuchlicher Verfahren zur Ermittlung von JIc-Werten für Reaktordruckbehälterstähle

Characterization and Evaluation of Common Methods of Measuring Jic-Values for Nuclear Pressure Vessels Steels A J_Ic test procedure using a single deeply cracked specimen was proposed. The crack extension is measured by partially unloading the specimen to determine the elastic compliance. J_Ic tests were made using ferritic steels. No size effect was found. The errors due to simple formulation of J_I calculation, periodic partial unloading, and simplified analysis for the extension of deep cracks in compact specimens are explored. The measurement point of crack extension for establishing J_Ic is discussed. Moreover, a proposal of characterising J_Ic by means of a resistance curve technique was developed. J-values from CT-specimens containing a deep precrack are plotted as a function of crack extension measured from the specimen fracture surface. Using these plots a technique is suggested for establishing a J_Ic measurement point. Finally, a step-by-step procedure for measuring J_Ic was outlined.     

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.     

Bruchverhalten von Polypropylen bei dynamischer Schneid‐ und Scherbeanspruchung in Schneidmühlen

Fracture behaviour of polypropylene under dynamic cutting and shearing action in granulators The fracture behaviour of polypropylene has been examined in a granulator‐like cutting test bench at rotor tangential velocities up to 40 m/s and rotor knife wedge angles of 30°, 55° and 80°. Depending on the wedge angle, the failure is either more due to tensile stresses or rather due to shear stresses. This has a significant influence onto the necessary cutting energy. The fracture surface shows different zones of failure, which can be related to the actual cutting energy.     

Der Einfluß von Festigkeit und Druckeigenspannungen auf die Wechselfestigkeit von Stählen bei höheren Temperaturen

Influence of Hardness and Comprehensive Residual Stresses on the Fatigue Limit of Steels under Elevated Temperatures The room temperature fatigue behaviour under alternating stresses of bcc steels is characterized by two stationary states existing below two different reversed stress limits: Below the one no crack initiation occurs, below the other no crack propagation can be found even after an unlimited number of stress cycles. The fatigue endurance of unnotched parts is determined by the crack initiation conditions that can be improved by higher hardness of the material. The fatigue strength of notched parts under reversed stresses is decidently determined by the minimum stress required to propagate a crack; this stress can be raised by compressive residual stresses. – At elevated temperatures these two stationary states are not further existent and there are no alternating stress amplitudes that can be endured either without crack initiation or without crack propagation to fracture. Both influencing parameters hardness and compressive residual stresses are dependend on the temperature, this can be explained by Snoek's and by Cottrell's mechanisms and with the temperature depending release of residual stresses corresponding to the temperature sensitive yield point.     

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.     

Auswirkungen der Schädigungsanisotropie auf die Lebensdauer von Stählen bei Zeitstandbeanspruchung

The effect of anisotropic damage on the rupture-time of steel under creep conditions The aim of the investigation is the prediction of the influence of creep history, e. g. pre-damage and pre-loading, on the further creep behaviour after changing the loading direction. The changing of the loading direction has a significant influence on the rupture-time. A model has been developed which describes the dependence of rupture-time from the anisotropic effect of damage.     

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.     

Stand der Entwicklung und Anwendung des Kaltpreßschweißens

Recent Progress in cold pressure welding technology and its application. A definition of cold pressure welding is given and different forming processes applied for cold pressure welding are described. The factors influencing the quality of joints are discussed and several examples for the practical use of the process are given.     

Trockener Gleitverschleiß an Ingenieurkeramiken und Stählen durch mineralische Stoffe

Dry Sliding Wear of Engineering Ceramics and Steels in Contact with Minerals Dry and unlubricated sliding wear against flint, corundum and silicon carbide was studied on oxide and non-oxide ceramics by using two different model systems at room temperature.-Wear behaviour of the ceramics was compared with that of steels and hard chromium coating. Type anß size of the abrasive minerals and the load were systematically varied. The results showed that wear resistance of the ceramics can be greater or smaller than that of the steels depending on the experimental conditions. Conclusions may be drawn for applications of modern engineering ceramics.