Eine Probeneinspann- und Heizvorrichtung für Dauerschwingversuche im Temperaturbereich 20 °C < T < 600 °C

Grip °and Specimen Heating-System for Fatigue Tests at the Temperature-Range 20 °C < T < 600 °C A grip and specimen heating system for fatigue tests is presented. The equipment is easily to handle, long-time stable and available up to a test-temperature of 600 °C also with inert environment. Specimens can be fixed without bending moments.     

Das Spannungs-Dehnungs-Verhalten von PTFE-Schälfolie im Temperaturbereich zwischen +25°C und −80°C und dessen Beeinflussung durch Kerben

Stress/strain properties of PTFE skived film over a temperature range of +25 °C to ?80 °C and the problem of notch effect Chemical plant under severe corrosive stress can be protected with “made-to-measure” liners welded up from PTFE skived film. For correct manufacture, handling and use of these chemical plant linings, a thorough knowledge of the semi-finished product and particularly of its load/deformation properties over a wide temperature range is required. The present paper deals with questions of strength and ductility and with the problem of notch effect at temperatures below + 25 °C. It is based on tensile tests carried out on PTFE skived film. Defined punched and cut notches were used to obtain information on notch sensitivity below room temperature. As the tests relate to two very different skived films, the reasons for these differences are analysed. The part played by the degree of crystallinity in determining many property differences is examined more closely with special reference to its effect on ultimate tensile strength, ductility and notch sensitivity. The aim of this study is not to determine material constants for PTFE but to identify tendencies in the behaviour of the semi-finished product, PTFE skived film, and so to gain useful information for its use in chemical plant lining.     

Eine Cermet-Glasschmiermittelkombination für Reibpaarungen bei Arbeitstemperaturen von 650–1100°C

A cermet/lubricating glass-combination for friction pairs at working temperatures 650–1100°C The function and the efficency of technical plants are limited by the used materials and their behaviour under working conditions. At elevated temperatures, the corrosion and wear resistance and the thermal shock behaviour are as necessary as mechanical stability at high operating temperatures. Metals and ceramics often cannot meet the conditions. Dynamic applications may require lubricatings, metals surfaces may be too soft and not very oxidation-resistant. Metal-ceramic materials, -cerments-, use a combination of the properties of their microstructural parts. This paper shows a combination of a cermet and a lubricating glass with interesting aspects for high temperature operation.     

Relaxationsverhalten des Werkstoffs X6 CrNi 18 11 (DIN 1.4948) für den Temperaturbereich 450–650 °C

Relaxation Behaviour of the Austenitic Stainless Steel X6 CrNi 18 11 (DIN 1.4948) in the Temperature Range 450 °C to 650 °C Based upon a set of actual data from relaxation tests after tensile loading in the temperature range 450–650 °C and with test times up to 100 h, an average stress vs. time relation for relaxation behaviour is derived. The actual dataset at the temperature of 550 °C is compared with the average relation. Relaxation behaviour is numerically computed from creep data of X6 CrNi 18 11 using different hardening theories and compared with the average relation derived from actual data.     

Heißkorrosion im Brennergas bei 1200 °C

Hot Corrosion in Burner Gas at 1200°C . The lifetime of a material at high temperatures is controlled by its hot corrosion behaviour against the components (ashes, salts, sulfur) of the burner gas. In this paper Na₂SO₄, Na₂CO₃, NaCl, NaF and V₂O₅ as well as H₂S, SO₂ and HCl were used for the experiments. It could be shown that the hot corrosion behaviour of MoSi₂ coated Nb-and Ta-alloy is excellent against acid additives (NaCl, NaF, V₂O₅) to the burner gas while alcalic melts (Na₂SO₄, Na₂CO₅) corrode the silizide. The solubility of the MoSi₂ is caused by the transport rate of molecular oxygen through the glasses formed by oxidation on the surface of the materials. It can be shown, that the lifetime of sintered MoSi₂ coatings on refractory metals is more than 10 times longer as by NiAl coated superalloys in hot corrosion test.     

The mechanism of the low-temperature hot corrosion of pure iron and manganese at 600–800°C

The corrosion of pure iron and manganese in simulated combustion gases at 600–800°C in the presence of sodium sulfate deposits is faster than the same reaction without salt deposits. This effect is due to the formation of a liquid solution between sodium sulfate and the sulfates of the corroded metals. The nature of these liquid solutions is examined together with the possible mechanisms for the transport of matter through the liquid and the dissolution-reprecipitation of solid metal compounds in the liquid salt. Finally, the reasons for the formation of sulfides in mixture with oxides in the inner solid region of the scale and the effects produced by the presence of sulfides on the overall reaction rate are considered.     

Kriech-Ermüdungsverhalten der Titanlegierung IMI 834 bei 600 °C

Creep-Fatigue Behaviour of the Titanium Alloy IMI 834 at 600 °C In the present study the creep-fatigue behaviour of the Titanium alloy IMI 834 at 600°C was investigated. A comparison of the crack initiation life behaviour and of the crack propagation as caused by different types of complex creep-fatigue cycles (with hold times into tension and/or into compression direction and with different loading rates into tension and/or into compression direction) showed, that a slow increase of the loadings into tension reduced the life and increased the crack velocity more than hold times at the maximum load. Furthermore, there existed environmental influences. On the basis of the experimental investigations the prediction capability of convenient crack initiation life prediction methods was evaluated. It turned out that the prediction capability of the Strain Range Partitioning Method could be improved if it was frequency modified. The prediction capability of the Frequency Modification Method could also be improved, if mean stresses in the cycles were explicitly accounted for. In the short and long crack stage the propagation behaviour could be correlated well if the effective cyclic J-Integral was used. This is of importance for damage tolerance considerations. Because the strains and the stresses at the crack tip are most important for the crack propagation behaviour, they were analysed on the basis of the Finite Element Method. It was found that the strains and stresses differed for different types of creep-fatigue cycles.     

Micromechanism of cyclic plastic deformation of alloy IN 718 at 600 °C

In the present investigation, an attempt was made to understand the cyclic deformation micromechanism of gas turbine alloy Inconel 718 at 600 °C (i) by conducting low cycle fatigue and creep–fatigue interaction tests and (ii) by studying the microstructure evolution in the material during fatigue tests through extensive electron microscopy. Bilinear slope was obtained in the Coffin–Manson plot for all low cycle fatigue tests, and it was confirmed through transmission electron microscopic examination that microtwinning was the predominant mode of deformation at low plastic strain values, whereas slip and shearing of γ″ precipitates were the predominant mode of deformation at higher plastic strain values. Fatigue life was adversely affected when hold time was introduced at peak tensile strain during creep–fatigue interaction tests. Formation of stepped interface at microtwin boundaries and coarsening of niobium carbide precipitates were observed to be the major microsturctural changes during creep–fatigue interaction tests.     

Ermittlung des Versagensverhaltens thermisch gespritzter MCrAlY/IN617-Verbundwerkstoffe im Zugversuch bei Temperaturen bis 800 °C im Rasterelektronenmikroskop

Fracture mechanisms of Thermally Sprayed MCrAlY/lN617 Composite Materials during Tensile Tests up to 800 °C Tensile strength of thermally sprayed MCrAlY/IN617 composite materials has been prooved at temperatures up to 800 °C in a SEM. For these investigations two different qualities of composite materials have been developed. One sort of composite material had been produced with a good quality, the other one was produced with defects in the coating layer. The tensile strength of the good homogeneous material shows no dependence with increasing temperature, whereas the inhomogeneous composite materials develop better strength values with increasing temperature. The inhomogeneous coating always breaks in the near of the defect.     

About thaumasite formation in Portland-limestone cement pastes and mortars––effect of heat treatment at 95 °C and storage at 5 °C

Owing to the presence of finely divided calcite, mortars and concretes made with Portland-limestone cements are particularly susceptible to damaging thaumasite formation during sulfate attack at lower temperatures. This work reports the results of investigations on mortars made according to DIN/EN 196 and pastes (w/c ratio of 0.5) with CEM I 42,5 R, as well as with mixtures of cement with limestone filler. Some of the samples were heat-treated at 95 °C. The length changes and resonant frequencies of the samples were measured during long-term water-storage at 20 and 5 °C. There was no evidence from X-ray diffraction data of thaumasite formation in the samples. Only for pastes containing 30 wt.% limestone filler were small areas found by SEM and X-ray microanalysis whose chemical analysis matched thaumasite or a thaumasite–ettringite solid solution.     

Zeitstandprüfung von Polypropylen bei 120 °C

Long Term Creep Tests of Polypropylene at 120 °C The mechanism of damage in the pipe wall during the internal pressure creep test of polypropylene-pipes at a test temperature of 120° is described. The resulting consequences from the investigations relating to the test method are shown and a test station with automatic recording of the time of damage is presented. Further on solutions are explained to eliminate the security risks during the tests of big pipes. A practical performance is described. On the basis of the time-temperature-correlation of the long term behavior of polypropylene the mathematical basis to interpolate minimum requirements for quality control are presented.     

Creep deformation of Alloys 617 and 276 at 750–950 °C

Alloys 617 and 276 were subjected to time-dependent deformation at elevated temperatures under sustained loading of different magnitudes. The results indicate that Alloy 617 did not exhibit strains exceeding 1 percent (%) in 1000 h at 750, 850 and 950 °C when loaded to 10% of its yield strength (YS) values at these temperatures. However, this alloy was not capable of sustaining higher stresses (0.25YS and 0.35YS) for 1000 h at 850 and 950 °C without excessive deformation. Interestingly, Alloy 617 showed insignificant steady-state creep rate at 750 °C irrespective of the applied stress levels. Alloy 276 almost met the maximum creep deformation criterion when tested at 51 MPa–750 °C. Severe creep deformation of both alloys at 950 °C could be attributed to the dissolution of carbides and intermetallic phases remaining after solution annealing or precipitated during quenching.     

Reduced temperature (22–100 °C) ageing of an Mg–Zn alloy

Heat treatment at intermediate temperatures (70–100 °C) doubles the age hardening response of the binary Mg–2.8 at.% Zn alloy when compared to the conventional T6 heat treatment and ambient temperature ageing. The maximal hardening produced at 70 °C is associated with the highest number density of the homogeneously distributed precipitates. At least six different types of coherent and semicoherent precipitates were simultaneously present in the microstructure aged at 70 °C: [0 0 01]_Mg rods and laths, Guinier Preston (GP) zones, GP1 zones {0 0 0 1}_Mg plates and prismatic precipitates containing 19–26 at.% Zn. Artificially aged alloy is strengthened mainly by sparsely distributed and precipitates and occasional GP zones. Strengthening in the naturally aged condition is produced by the combination of GP1 zones, prismatic precipitates and clusters containing about 20 Zn atoms.     

Wechselverformungskurven und Mikrostruktur bei spannungskontrollierter Zug-Druck-Wechselbeanspruchung von Ck 45 im Temperaturbereich 295 K ≤ T ≤ 873 K

Cyclic deformation curves and microstructure of SAE 1045 after stress-controlled push-pull loading in the temperature range 295 K ≤ T ≤ 873 K Stress-controlled cyclic deformation tests were performed with normalized SAE 1045 in the temperature range 295 K ≤ T ≤ 873 K. From the measured mechanical hysteresis loops cyclic deformation curves were determined, which are characterized by temperature dependent cyclic softening and hardening processes. Due to the cyclic deformation processes at any temperature distinct dislocation structures are formed depending on the stress amplitudes and the number of cycles chosen. In the whole temperature range and for all stress amplitudes the plastic strain amplitude was proved to be a suitable parameter to describe the actual fatigue state.     

Embrittlement of a superduplex stainless steel in the range of 550–700 °C

Duplex stainless steels are very attractive materials, combining high mechanical properties with improved corrosion resistance. However, these steels present technical limitations because they can experience embrittlement as a consequence of thermal cycles. Moreover, the higher level of alloying elements (Cr and Mo) in the new duplex generation, called superduplex, accelerates the precipitation kinetics of harmful intermetallic phases, which are the responsible of embrittlement. This fact raises the question of the influence of these cycles on the actual performance of duplex components and structures and on its possible failure during service.

In order to investigate this question, heat treatments in the range of 550–700 °C, with different exposure times and cooling rates, have been made on a superduplex stainless steel type EN 1.4507. The evolution of mechanical properties has been followed by means of hardness measurements, impact and fracture toughness tests, whereas microstructural changes have been identified by using optical and transmission electron microscopy analysis. A correlation between the degree of embrittlement and the different types of precipitates has been established.     

Physical mechanisms of dielectric breakdown in SiO2 for the range of −150°C to 150°C

This paper explains the physical nature of breakdown for 10 nm oxide in the range of −150°C to 150°C for N- and P-channel devices (NFETs and PFETs). At 30°C, two mechanisms occur: trap creation and impact ionization; at lower temperatures, the trap creation is significantly suppressed with a corresponding decrease in activation energy. Using the physical model, the reliability of NFETs is compared with what the PFETs relative to processing conditions.