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.     

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.     

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.     

PREDICTION AND EVALUATION OF LONG-TERM CREEP-FATIGUE LIFE

Ordinary short-term creep fatigue tests were carried out on a servo-hydraulic testing machine for various heat resistant steels. From these results a life prediction method in the long-term creep-fatigue regime is proposed using creep rupture ductility data up to 10⁵ h. To verify the predicted life the authors have developed a new type of testing machine driven by thermo-actuator which can evaluate the long-term creep-fatigue behaviour of materials beyond 10⁴ h.     

Verhalten von Gasturbinenschaufelwerkstoffen unter langzeitiger Kriechermüdungsbeanspruchung

Behaviour of gas turbine blade materials under long-term creep fatigue loading The long-term creep fatigue behaviour of the gas turbine blade material IN-738 LC HIP is investigated by a service-type strain cycle up to a test duration of about 12000 h. Further, it is reported about shorter tests of the same type on alloy IN-100 HIP. The cyclic deformation of alloy IN-738 LC HIP in the long-term region is analysed and the creep fatigue crack initiation is described by the generalized damage accumulation rule using two different methods.     

Zum Einfluss des Schweißens auf das Kriechverhalten moderner Stähle für die thermische Energieerzeugung

The influence of welding on creep behaviour of modern steels for thermal power generation Un‐ and low alloyed ferritic/bainitic Chromium steels as well as high alloyed ferritic/martensitic 9–12 % Chromium steels are widely used for high temperature components in thermal power generation. Welding in all its variety is the major repair and joining technology for such components. The weld thermal cycle has significant influence on the base material microstructure and its properties. The Heat Affected Zone is often regarded as the weakest link during high temperature service. While weldments of un‐ and low alloyed ferritic Chromium steels can show significant susceptibility to Reheat Cracking in the coarse grained heat affected zone, weldments of high alloyed ferritic Chromium steels generally fail by Type IV Cracking in the fine grained heat affected zone during long term service. In this paper the influence of the weld thermal cycle on the base material microstructure is described. Long‐term creep behaviour of weldments is directly related to the main failure mechanisms in creep exposed ferritic weldments and implications for industries using heat resistant ferritic steels are shown.     

High temperature low cycle fatigue

Fatigue at high temperature is a complex phenomenon as it is influenced by a number of time-dependent processes which become important at elevated temperatures. These processes include creep, oxidation, phase instabilities and dynamic strain ageing (DSA), acting either independently or synergistically influence fatigue behaviour, often lowering the fatigue life. Current design approaches employ linear summation of fatigue and creep damage with suitable factors on permissible damage to take care of uncertainties in interaction between cyclic and time-dependent processes. It is, therefore, important to develop a deeper understanding of the processes that occur during high temperature fatigue so that realistic life predictions could be made. Results on the high temperature fatigue behaviour of austenitic stainless steels, ferritic steels and nickel base alloys are presented here. The important mechanisms of interaction of high temperature time-dependent processes with fatigue under various conditions are discussed in detail. Emphasis is placed on cyclic stress response, fatigue life, deformation substructure and fracture behaviour. This is followed by a review of important life prediction techniques under combined creep-fatigue loading conditions. Life prediction techniques considered here include linear damage summation, strain range partitioning, ductility exhaustion approach, frequency modified and frequency separation methods, techniques based on hysteresis energy and damage rate models, and methods based on crack-cavitation interation models.     

Beschreibung des Kriechverhaltens einiger warmfester stähle

The creep behaviour of important heat resistant steels is examined to establish creep equations. For two well tested materials, equations are proposed, which are valid in the whole range of temperature, stress and time.     

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.     

Creep-fatigue interaction in eutectic lead-tin solder alloy

A test protocol pivoting about stress cycling where the load waves were trapazoidal and the cycling frequency controlled by balanced time on load and off load was used to determine frequency, mean tensile stress, and compressive stress effects on the creep-fatigue behaviour of the Pb–Sn eutectic solder alloy at ambient temperature. It is consistently found that the minimum creep (or cyclic creep) rate decreases as frequency increases, that is, as hold time decreases. Both the number of cycles to failure as well as the time to failure increases as frequency increases. The cyclic creep rate increases drastically and the number of cycles to failure decreases drastically as the mean applied stress is increased. These results are consistent with anelastic strain recovery mechanism for creep-fatigue interaction. Similar results are also found in butt-joint solder junction configured specimens.     

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.     

CONTINUOUS SEM OBSERVATIONS OF CREEP-FATIGUE DAMAGE PROCESSES

In order to examine the relation between damage evolution and changes in microstructure, e.g. from creep cavities, surface micro-cracks and dislocation structures at high temperature, strain controlled creep-fatigue tests were performed and interrupted at several damage levels on Types 304 and 316 stainless steels. The creep-fatigue tests on Type 304 stainless steel at a low strain level were conducted in a high-temperature fatigue testing machine combined with a scanning electron microscope, and the micro-crack initiation and growth behaviour were continuously observed to clarify the damage extension mechanism. It was found that even though many cavities were initiated and grew on the internal grain boundaries of the specimens during the strain-controlled tests, the failure life was governed by the propagation of surface cracks. On the other hand, micro-cracks of about the order of one grain size were initiated mainly along grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the micro-cracks was slow and random due to the nature of the microstructures. The micro-cracks gradually opened in the loading direction with increasing number of cycles and coalescence contributed to growth.     

Charakterisierung des Ermüdungsverhaltens hochbeanspruchter Eisenbahnvollräder für den Hochgeschwindigkeitspersonenverkehr bei betriebsrelevanten Temperaturen

Fatigue assessment of highly loaded railway monobloc‐wheels for application in high‐speed passenger‐traffic trains under thermal service conditions Railway wheel steels rank among the most loaded components of railway vehicles. Whereas increasing driving speeds increase in particular the cyclic mechanical loading, increasing acceleration and braking torques lead to enormous thermal loads. Hence, a detailed knowledge about the microstructure‐related characterisation of the fatigue behaviour of railway wheel steels under thermal service conditions is the fundamental precondition for a save and economic operation of railway systems. Current investigations concentrate on the fatigue behaviour of actual UIC‐conform high‐speed passenger‐traffic railway wheel steels, which were provided in industrial relevant heat treatment conditions. Microstructural gradients are a consequence of the industrial heat treatment and the size of the component. Specimens were machined from defined depth positions of the wheel rim from original monobloc wheels. Plastic strain amplitude, change in temperature and change in electrical resistance measurement techniques were used to characterise the fatigue behaviour in detail. The specimen position and therefore the local microstructure influence the fatigue behaviour in a characteristic manner.     

High-temperature fatigue: Example of creep lifetime prediction for grade 2 alloy 800 at 550°C

Summary Experimental data on the material characteristics of structures subjected to thermal and mechanical cycling are needed for designing structural parts for creep and creep-fatigue interaction. Moreover, high-temperature low-cycle fatigue data are not sufficient to predict the fatigue creep lifetime.In order to check the reliability of steam generators, tests on pipe materials are conducted under cyclic thermal loading. The tests have been performed on an iron-nickel chromium alloy (alloy 800) in grade I condition. Isothermal low-cycle fatigue tests have been conducted at 350°C. For creep-fatigue tests, the strain was held constant during a variable relaxation period. To be representative of the service conditions, the duration of the hold time was equal to 168 hours with a total strain amplitude of 1.5%. Two damage processes are examined: fatigue and creep. The data are combined in three different models given by Cailletaud, Chaboche and Bul-Quoc. These models are used to predict fatigue-creep lifetime for very long hold periods. They were verified with the hold times of 1 week. It appears that the quality of the fatigue-creep lifetime predictions depends strongly on the quality of the creep relaxation phenomenon model. Comparison between prediction and experimental data Is quite satisfactory.Translated from Published in Problemy Prochnosti, No. 4, pp. 36–42, April, 1994.     

Ratcheting‐fatigue behaviour of bainite 2.25Cr1MoV steel with tensile and compressed hold loading at 455°C

The ratcheting behaviour of a bainite 2.25Cr1MoV steel was studied with various hold periods at 455°C. Particular attention was paid to the effect of stress hold on whole‐life ratcheting deformation, fatigue life, and failure mechanism. Results indicate that longer peak hold periods stimulate a faster accumulation of ratcheting strain by contribution of creep strain, while double hold at peak and valley stress has an even stronger influence. Creep strains produced in peak and valley hold periods are noticeable and result in higher cyclic strain amplitudes. Dimples and acquired defects are found in failed specimen by microstructure observation, and their number and size increase under creep‐fatigue loading. Enlarged cyclic strain amplitude and material deterioration caused by creep lead to fatigue life reduction under creep‐fatigue loading. A life prediction model suitable for asymmetric cycling is proposed based on the linear damage summation rule.     

PREDICTION OF BENDING FATIGUE BEHAVIOUR BY THE REFERENCE STRESS APPROACH

Abstract— A technique is presented for prediction of low-cycle fatigue behaviour under bending conditions from the knowledge of uniaxial fatigue data. The technique utilizes the reference stress approach used in the analysis of creeping components. The results enable determination of nominal elastic bending stress in terms of the observed saturation stress amplitude in an uniaxial fatigue test conducted at a reference value of strain amplitude. The reference parameters are determined for beams with both rectangular and circular cross-sections. The predictions of fatigue bending behaviour are in excellent agreement with closed form solutions reported recently. The merits of this alternative approach lie in its potential applicability to other structural situations once their reference uniaxial parameters are determined. Since the reference parameters are nearly the same for both creep and fatigue, the approach can be a useful experimental tool for relating creep-fatigue interaction effects in structural components with uniaxial creep-fatigue tests conducted under the reference conditions.     

LIFE PREDICTION FOR ADVANCED FERRITIC STEELS SUBJECT TO THERMAL FATIGUE

Abstract— Thermal fatigue is a well recognised source of damage in headers and steam piping of thermoelectric power plant. This topic has been extensively examined in the past for low alloy ferritic steels typically used in such applications. Experimental evidence obtained in low cycle fatigue testing with tensile hold times on Modified 9Cr1Mo and E911 steels suggests that the Linear Damage Summation rule conventionally used in engineering codes for high temperature damage analysis may not be particulary appropriate for the advanced 9Cr steel family. For this reason two alternatives have been examined: (a) a strain based creep damage evaluation using the R5 ductility exhaustion approach and (b) a creep-fatigue continuum damage mechanics method. The potential advantages and disadvantages of both are discussed. In addition, results from low cycle fatigue and thermomechanical fatigue tests on crossweld specimens machined from welded joints in the Mod.9Cr1Mo alloy are evaluated. Even if the usual cyclic life reduction factor of 2 with respect to base material behaviour appears adequate to account for the mean trend of cross-weld results, the large variability observed risks making the use of such a factor non-conservative for accurate life prediction.