Untersuchungen zur Schwingungsrißkorrosion von unlegierten Stählen in wäßrigen Medien

Although a lot of work is done up to now in investigating corrosion fatigue many questions about the corrosion fatigue process of steel are still open. This applies for instance to the quantitative proportion at the complete damaging process which the corrosion strain component will assume and it applies moreover to special details of the crack initiation stage. The damaging process of corrosion fatigue of steel in the active state is generally supposed to proceed in the following three stages: “formation of pits”, “crack initiation” and “crack propagation”. The objective of the present study was at first to get by statistical means a quantitative assumption about the influence of the corrosive medium and secondly to get information about the contribution of pitting to the crack initiation process. Tension-compression experiments were conducted with specimen from steels Ck 15 and Ck 35 in the normalized state both without a corrosive influence (inert spindle oil) and with corrosive influence (desalted water, 3% NaCl-solution). It was found that the damaging process in its principal course is not affected by the kind of the corrosive medium but is hardly influenced by the extent of the load-amplitude at a given load-frequency.     

Erfahrungen mit Warmwasserbereitern aus nichtrostenden Stählen

Experience with Water Heaters of Stainless Steel . Different stainless steels and their importance in the construction of domestic water heaters. Types of corrosion and their importance in water heating practice. Important aspects for manufacture. Experience gained with stainless steel water heaters.     

Schwingungsrißkorrosion beschichteter Verdichterschaufel-Werkstoffe

Corrosion Fatigue of Coated Compressor Blade Materials Damage of compressor blades due to corrosion fatigue constitutes a substantial fraction of the observed stationary gas turbine failures. Dependent on environmental conditions and filtering, these blades in addition may be exposed to mechanical erosion. For corrosion fatigue protection, coatings have been used since many years. However, recently the earlier used NiCd coatings have been substituted to an increasing extent by inorganic coatings containing dispersed aluminum particles as manufactured e.g. by SermeTel. Such coatings are designed to warrant improved corrosion resistance. Within the scope of a joint research project of the German gas turbine manufacturers AEG KANIS, BBC and KWU, some of these protective coatings were tested under simulated operating conditions in different corrosive media with respect to their corrosion fatigue protection capabilities. As a result, it was shown that the aluminum dispersed coatings offer good protection against corrosion in aqueous neutral media at room temperature. One of the coatings tested guarantees in strongly acidious media only a time-limited corrosion protection. At elevated temperatures and also after erosive pre-damage, the fatigue strengths are reduced heavily. In comparison with uncoated specimens, galvanically or chemically deposited nickel-coatings, which were tested concurrently, improved the corrosion resistance only unessentially.     

Elektronenstrahl‐Randschichtbehandlung für die Herstellung verschleißbeständiger Auftragschichten auf nichtrostenden Stählen

Stainless steel components exposed to mechanical stresses are subjected not only to corrosion, but to abrasive wear. There are several possibilities for enhancing the wear resistance of stainless steels; however, such processes are very often associated with a reduction in corrosion resistance. This paper presents an electron beam surface treatment technology to significantly improve the wear resistance of austenitic steels (e.g. X6CrNiMoTi17‐12‐2) and duplex steels (e.g. X2CrNiMoN22‐5‐3), without a negative influence on the corrosion behavior. Fe‐ and Co‐additive wires were deposited thermally by electron beam cladding. The cladding layers produced were free of defects such as cracks and pores, and were well metallurgical bonded to the base materials. Microstructural analysis, hardness measurements, wear tests and corrosion tests were carried out. The wear rate k was reduced by a factor of 100 compared to the base materials for electron beam cladding with Fe‐based wire and by a factor of 10 with Co‐based wire. Corrosion resistance was preserved for the Fe‐based cladding layers and slightly increased (by a factor of 3) for the Co‐based cladding layers.     

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.     

Verhalten von Stählen unter Hohlladungsbeschuß

Behaviour of Steels under Shaped Charges Bombardement. There is at present not too much known about the behaviour of steel under the very high pressures of some 100 kbars as they will be produced by the hydrodynamic jet force stream of a shaped charge. In a short revue the hithero existing knowledge of the formation of a jet force stream is given as the phenomens in the penetration process of a material. It is recognized by bombardement tests on iron and steels of different compositions and of different heat treatments that there are some changes of the structure near the boundaries of the penetration channels. There is given some information by light- and electronmicroscopical investigations about the occurence during the hydrodynamic penetration process of the jet force stream and about the behaviour of the different structures under local concentrated pressures of shock waves. Microprobe-analyses and scanning electron microscope micrographs supply the understanding.     

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.     

Verzug von Stählen infolge Wärmebehandlung

Distortion of Steels Due to Heat Treatment Distortion comprises size and shape distortion. Size distortion is brought about by thermal stresses and structural changes, mainly by hardening and tempering. Thermal stresses cause size-dependant, structural changes size-in dependent distortion. Size-distortion is a system property. Tempering at room temperature over a long period of time influences the size stability. Material anisotropy leads to directional distortion. Shape distortion or warpage originates in residual stresses and differences within the material. Further a nonsymmetrical temperature distribution or geometry of the part have to be considered.     

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”.     

Aktivierungsenergie der wiederholten Karbidausscheidung in Stählen

Activation energy of repeated carbide precipitation processes in steels Dispersion microstructures in steels can be formed from the martensitic condition by repeated precipitation and coarsening of M₃C (or M₆C) particles. The precipitation stage comprehends the nucleation and the growth of one particle generation according to r ∝ t^1/2 (r – particle radius, t – time), and already after some minutes it turns into the dominating coarsening according to r ∝ t^1/3 (OSTWALD-ripening). The coarsening of the first precipitated particle generation is superposed with the following precipitation of further generations. They can be proved by the coercive field strength, the yield strength, microstructural mean values from scanning electron micrographs and by hardness tests. The occurring maxima of precipitation allow the evaluation of the activation energy for the effective volume diffusion in the α – Fe phase. By including the electronegativity (after L. Pauling ) the covalent bond in the M₃C phase is considered. With that is possible to attach the estimated activation energy to the main alloying (and diffusing) element and also to the total composition of the steel.     

Untersuchungen des Gleit- und Furchungsverschleißes an thermochemisch behandelten Stählen

Sliding and sliding abrasion tests of thermochemically treated steels The results of research work during several years are presented summarily. The results were obtained jointly by the Bundesanstalt für Materialprüfung (BAM), Berlin, and Stiftung Institut für Härterei-Technik (IHT), Bremen. The wear tests included hardened, casehardened, nitrided, nitrocarburized, chromized, chromvanadized and vanadized steels and were performed with dry friction in air and in vacuum, and with mixed lubrication. Under these conditions adhesion, abrasion and tribo-oxidation were the main wear mechanisms. The results reveal that the wear evaluation order of the differently heat treated steels depends on the type of wear and the operating wear mechanisms.     

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.     

Kurzzeitschwingfestigkeit von duktilen Stählen unter mehrachsiger Beanspruchung

Low-Cycle Fatigue of Ductile Steels under Multiaxial Deformations To investigate the fatigue behaviour of cyclically softening and hardening steels under multiaxial elastic-plastic strains, axial strain and shear strain controlled fatigue tests under constant amplitude loading were carried out. S-N curves under axial strain and torsional pure shear as well as under combined axial strain and shear, in and out of phase, were obtained for the cyclically softening tempered steel 30 CrNiMo 8 (similar to AlSI-Type 4340) and the cyclically hardening quenched stainless steel X 10 CrNiTi 189 (AISI-Type 321) in the region of low-cycle fatigue. For both steels, used in the design of vessels, pipings, shafts, etc. the fatigue life to crack initiation is reduced by an out of phase (δ = 90°) shearing of the strained specimens in comparison to the in phase loading. The decrease of fatigue life under out of phase strains is caused by changing direction of principal strains resulting in an interaction of the deformations in all directions of the surface. This interaction is taken into account by a calculation procedure deriving an equivalent strain and predicting the fatigue life under combined strain on the base of S-N curves for unaxial strain.     

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.     

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.     

Beitrag zur Vermeidung von chloridinduzierter transkristalliner Spannungsrißkorrosion bei höher legierten Chrom–Nickel–Stählen durch Inhibitoren

Avoidance of Chloride Induced Transgranular Stress Corrosion Cracking of Stainless Steels by Inhibitors The possibility to avoide chloride induced transgranular SCC of stainless steels with higher contents of chromium and nickel with the aid of organic inhibitors is shown. This method is successful as well in boiling aqueous solutions with high chloride concentration (open system) as in closed systems (higher temperature and pressure) with low chloride contents.     

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.