Karl Maile

Effect of damage on flexural modulus of C/C–SiC composites

Abstract The aim of the present work is to study the influence of damage in carbon–carbon silicon carbide composites (C/C–SiC) on their flexural resonant frequency as well as their flexural elastic modulus. Composite samples were examined by the grindosonic method and confocal laser scanning microscope before and after impact damage. Damage was introduced by the drop-weight (low-velocity) impact method using varying drop heights. The results show that the changes of the flexural resonant frequency after delamination/cracking of the matrix, compared to those of non-delaminated specimens, gave a good indication of the degree of damage, confirmed that the microcrack reduces the flexural resonant frequency and gave lower values of the modulus of elasticity.

(Cr-Al) bi-layer coatings obtained by ion assisted EB PVD on C/C-SiC composites and Ni-based alloys

Abstract Cr and (Cr–Al) bi-layers were deposited on carbon reinforced composites (C/C–SiC) and Ni-based alloy substrates by ion assisted EB PVD. The ion current in the substrate region was measured using a special ion detector. Surface morphology of the films was studied using optical, scanning electron, and atomic force microscopy. The variation of the deposited elements was determined by means of a line-scan electron probe on the cross-sections of the samples. Interdiffusion of the deposited elements at the interface was observed. It could have been caused by ‘ion mixing’ during the deposition process. X-ray diffraction (XRD) investigations show the formation of intermetallic compounds Cr3Si andCrAl. Comparative measurements proved that the microhardness of Cr layers on Ni-base alloys, obtained by ion assisted deposition was higher than that of Cr layers deposited without the acceleration of ions.

Behaviour of Ni-Based Alloys for Fossil-Fired Power Plant Components in the Long-Term Creep Regime

High efficiency steam power plants are planned to operate at temperatures higher than 700°C and at a pressure of up to 350 bar. Due to this increase of the steam parameters, Ni-based alloys are required for constructing these plants. Materials testing - based on appropriate manufacturing and design criteria - is necessary in order to have a reliable data base of the relevant design characteristics. Additionally, a better understanding of the specific material behaviour under service like loading conditions for the evaluation of possible damage mechanisms is essential. This paper describes research on the behaviour of thick-walled power plant components made of Alloy 617 mod. and Alloy 263. Results from basic qualification programs with standard specimens including welded joints show the applicability of the materials. Results from creep rupture tests of base material and welded joints and microstructural investigations to obtain information on precipitations and dislocations in the virgin and aged conditions are presented. Information on the influence of chemical composition on creep rupture was obtained by analysing creep rupture data sets.

Requirements for microstructural investigations of steels used in modern power plants

Abstract The thermo-mechanical behaviour of newly developed 9 – 12 wt.% Cr steels used in steam power plants depends strongly on the microstructure. In the present work, a combination of various microscopy techniques is used to study the microstructural features relevant for the creep resistance. These include the subgrain size, the number of free dislocations, the chemical composition and crystallographic structure of the occurring precipitates, and their density and size distribution. Accordingly, the techniques which were applied range from light microscopy to transmission electron microscopy in order to cover all length-scales from the millimeter regime to the nanometer level. These detailed investigations are important for alloy design and as input parameters for thermodynamical calculations and modelling of the microstructural evolution to develop steels for cleaner power stations with high thermal efficiency.

Microtomographic assessment of damage in P91 and E911 steels after long-term creep

Abstract Two flat hollow cylinders made of martensitic 9 wt.% Cr steels were creep deformed under in-service conditions typical of steam pipes at fossil-fuel fired power plants. Damage in the tubes was assessed through synchrotron X-ray microtomography by evaluating the shape, size and spatial-distribution of voids. The analysis of the size distribution of non-coalesced voids suggested that void growth is controlled by the plasticity constrained diffusional mechanism, a hypothesis verified by micromechanical simulations. A much higher void density was found in steel grade P91 compared to E911.

Effect of impact damage on flexural strength of the C/CSiC composites

Abstract The effects of impact damage on flexural strength of oxidised and unoxidised C/CSiC composites were investigated. Composite specimens were oxidised for 10 min at 1600°C. Each sample was impacted for pre-impact damage at a constant impact velocity of 20 m s−1 and with an impact energy of 90 J in a Split-Hopkinson Pressure Bar test instrument. The results indicate that the flexural strength of virgin C/CSiC composite is affected by the pre-crack damage and oxidising process. During this process, the C/CSiC composite loses 20% of its strength owing to an increased incidence of microcracks.

Finite element calculation of creep crack initiation on an IP-turbine rotor using the C*-parameter

Creep crack initiation results from a heat resistant turbine rotor steel of type 1%Cr-1%Mo-0.6%Ni-0.3%V are described by the parameter C*, which is calculated on the basis of an inelastic finite element analysis. The application of these results for the calculation of creep crack initiation of an assumed internal defect in the center of an IP rotor is discussed. The time to creep crack initiation estimated on the basis of the C*-concept is compared with a predicition by means of the analytical two-criteria- concept.

Characterisation of the generation of ions in electron beam evaporator for the control of metal deposition processes

By means of a specially designed ion detector, the dependence of ion current at the substrate on the electron emission current of the e-gun and bias-potential for the evaporation of Ti, Al, and Cr is investigated. It is illustrated by distinction of morphology of metal films, that the control of the parameters of bombarding ions can be a key for the control of the ion-assisted electron beam physical vapour deposition (EB-PVD) process.

Assessment of hot tears in cast steel components

Abstract Large scale specimens with real internal defects were taken from castings. The behaviour of the defects under service like loading conditions (creep, creep–fatigue and fatigue tests at 530°C) was studied, accompanied by non-destructive testing and post-test investigations.

Assessment of creep rupture life of weldments of martensitic steels

Abstract Martensitic steels are of major importance for modern high efficient power plants. Their long-term creep rupture characteristics are essential for the economic and reliable design of high temperature components. However, the weldments in such components have to be considered as a potential failure area under long-term creep loading as the heat affected zone of ferritic creep resistant materials (for example new martensitic steels) is exposed to significant microstructural changes due to the heat input during welding. Furthermore, the stress situation and multiaxiality of the stress state in the weld region changes permanently due to creep and relaxation. Evaluation therefore has to be based on experimental investigation that means uniaxial creep tests of crossweld creep specimens. The microstructrural characterisation of the heat affected zone and the comparison with the situation in the optimised base material has also to be taken into account. Heat affected zone simulation of specimens and the relevant experimental investigation deliver an important basis for the modelling of the stress – strain situation. All these aspects have to be combined in the inelastic computation in order to consider all relevant effects: the individual creep strength in the different regions of the heat affected zone as well as the influence of the subsequent state of stress multiaxiality. The paper highlights recent and on-going research activities in Germany in this domain. It also demonstrates the importance of inelastic calculation for the design of welded pipes.