Christof Sommitsch

Characterization of δ-phase in superalloy Allvac 718PlusTM

Abstract Nowadays, the trend goes to better and more efficient gas turbine engines with lower emissions, greater durability and lower cycle costs. To this end, new materials such as Allvac 718PlusTM, should enhance the high temperature performance. The appearance, morphology and control of the δ-phase are of special interest because of their critical influence on grain structure, grain size and mechanical properties. In this work, the evolution and the morphology of the δ-phase during heat treatment between 900 and 1000°C for annealing times of up to 8 h are investigated in order to determine the time – temperature – precipitation diagram and to understand the phase morphology. For this, different analysis methods are applied, such as light microscopy analysis with computer-aided quantitative metallography, dual beam focused ion beam and both scanning electron and transmission electron microscopy.

Precipitation evolution and creep strength modelling of 25Cr20NiNbN austenitic steel

25Cr-20Ni-Nb-N is a high strength and oxidation-resistant austenitic stainless steel intended for Ultra-Supercritical (USC) power plants. In this work, the precipitation evolution, and creep strength at 650 and 750°C for up to 100 000 h are predicted. Six precipitates are considered in the thermokinetic calculation by MatCalc: M23C6, η (Cr3Ni2SiN), σ, G, Z, Nb(C,N). For the creep strength prediction, three hardening mechanisms are taken into account: dislocation, precipitation, and solid solution hardening. Both matrix composition and precipitation evolution, calculated with MatCalc, are used for modelling the precipitation and solid solution hardening. It is found that the dislocation hardening, followed by precipitation hardening gives the largest contribution to the creep strength. The most important precipitates strengthening phases are found to be Z-Phase and Nb(C,N), which are nucleated at the dislocations. The model for the creep rate can represent how the creep exponent is raised with increasing applied stress and reduced temperature.

Fatigue Life Extension by Crack Repair Using Double Stop-Hole Technique

Drilling holes in the vicinity of the crack tip turns the crack into a notch and reduces the crack tip stress intensity factor. In this paper, a new idea is used in which instead of a single hole, two symmetric and interconnected holes are drilled at the crack tip. The main concept of double stop-hole method is to reduce the stress concentration at the edge of stop-holes in the cracked structural elements. The double stop drill hole method can be used to increase the fatigue life of the cracked components. The fatigue crack growth retardation is examined using an experimental investigation coupled with a stress analysis on the efficiency of proposed double stop-holes. The distance between the hole centers is considered as the main parameter affecting the efficiency of this method. The results show that the fatigue life extension caused by the double stop-hole method is significantly more than the conventional single stop-hole method.

Characterisation and quantification of cavities in 9Cr martensitic steel for power plants

Abstract This work focuses on the characterisation of cavities evolution in a P91 steel pipe in three conditions: as received, and after creep at 650°C and 60 MPa for 7000 and 9000 h. A field emission gun scanning electron microscope (FEG-SEM) equipped with focused ion beam (FIB) gun, a conventional scanning electron microscope (SEM) and a light optical microscope (LOM) have been employed for the investigation. This study reveals two types of cavities: the pre-existing cavities, which are rare in this type of heat resistant steels, with a mean diameter of 2·56 μm and the cavities produced during creep with diameters smaller than 0·6 μm. Lath boundaries and precipitates are found to be preferential sites for cavity nucleation. Furthermore, the number density and volume fraction of these small cavities are calculated from 2D measurements and compared to 3D results obtained by FIB serial sectioning.

Investigation of creep damage in advanced martensitic chromium steel weldments using synchrotron X-ray micro-tomography and EBSD

Abstract In recent years, a design concept for the stabilisation of the microstructure by addition of boron and nitrogen was developed. This so called martensitic boron–nitrogen strengthened steel (MARBN) combines boron strengthening by solid solution with precipitation strengthening by finely dispersed nitrides. Welded joints of MARBN steels showed no formation of a uniform fine grained region in the heat affected zone (HAZ) which is in general highly susceptible to Type IV cracking. In this work, the crossweld creep strength of a newly developed MARBN steel was analysed and the evolution of damage was investigated using synchrotron microtomography supported by electron microscopy. Three-dimensional (3D) reconstructions of the tested samples together with electron backscatter diffraction investigations revealed an intense void formation in a restricted area along small grains at prior austenite grain boundaries in the HAZ as the main reason for premature creep failures in the HAZ of welded joints.

A dislocation density model for the simulation of hot forming processes

Abstract An advanced process model for the calculation of the microstructural evolution of nickel base alloys during a hot forming process helps to optimise the formation and to achieve the desired microstructure. The simulation of the grain structure development during and after the forming process is based on a dislocation density model that is embedded in the FEM-program DEFORM™.

Optimised microstructure for increased creep rupture strength of MarBN steels

Abstract The efficiency of modern steam power plants relies heavily on the inlet fluid temperature and pressure. Thus it is necessary to improve the materials of cast, rolled and forged parts such as case, pipes and rotor. Two chemical compositions of 9%Cr creep resistant steels were investigated. Different heat treatments were applied to modify the prior austenite grain size (to diameters of 50, 100, 300 and 700 μm) and to modify the substructure. The microstructure changes were further investigated by LOM, SEM and EBSD. Then, these different microstructures were exposed to the uniaxial creep rupture test at 650°C and 150 MPa. Comparative results reveal the influence of the modified microstructure on creep rupture time, ductility and fracture.

Interaction of the Precipitation Kinetics of δ And γ’ Phases in Nickel-Base Superalloy ATI Allvac® 718PlusTM

In this paper, the precipitation behaviour of  (Ni3(Nb,Al)) and ’ (Ni3(Al,Ti,Nb)) phases in the nickel-base superalloy ATI Allvac® 718PlusTM, as well as their kinetic interactions are discussed. Important parameters such as volume fraction, mean radius and number density of precipitates are experimentally determined and numerically simulated as a function of the heat treatment parameters time and temperature. To match the experimentally observed kinetics, the predicted interfacial energy of the precipitates, as calculated for a sharp, planar phase boundary, is adjusted to take into account the interfacial curvature and entropic effects of a diffuse interface. Correction functions for the interfacial energies of  as well as ’ precipitates are presented. Using these modified interfacial energies, the calculated results show excellent agreement with the experimental measurements.

Innovative surface modification of Ti6Al4V alloy by electron beam technique for biomedical application

The low elastic modulus, high corrosion resistance and excellent biological response allow titanium alloys to be used for permanent orthopaedic devices. Furthermore, the design of specific multi scale surface topographies on titanium alloys can provide a fast osseointegration. This work highlights the use of electron beam as a promising technique to produce a designed surface topography and improve the tribological behaviour of Ti6Al4V alloy. The produced surface topography due to the transport of molten material is influenced by the deflection figure, the physical properties of the material and the energy input. The analysis of the surface roughness shows an increment of the area up to 26% and a canal shape in a range from 1.3μm up to 9μm depth and from 68.6μm up to 119.7μm width. The high solidification rate reached during the process affects the microstructure, provoking the formation of martensite and thus the improvement of hardness. In vitro studies with pre-osteoblastic MC3T3-E1 cells performed for several cultivation times show the cells with a polygonal shape and built connections through elongated filopodia. A notable increase of cell spreading area on surface structure with a finer canal shape is found after 48h cultivation time.

3D creep cavitation characteristics and residual life assessment in high temperature steels: a critical review

Abstract The need for a new paradigm to estimate remaining creep life of service exposed steels is critically assessed. New approaches to residual life assessment are proposed, in the light of a decade’s experience of the use of micro-tomography to characterise the three-dimensional (3D) nature of cavitation damage in structural materials. Imaging of conventional structural materials such as steels with high absorption to X-rays has been realised by synchrotron micro-tomography (SR-μCT), providing new insights into phenomena such as creep failure. The unique feature of SR-μCT studies is the direct imaging in 3D of cavities (hundreds of micrometres in size) present in the bulk, revealing the spatial characteristics and morphology of the creep voids. Quantitative analyses of the cavitation characteristics revealed by 3D datasets, when scaled with respect to time, stress and temperature, provide functional information suitable for developing constitutive equations for creep. The application of SR-μCT, a non-destructive technique providing high fidelity data, significantly reduces the ambiguity in developing functional relationships to predict creep failure. The explicit use of such constitutive equations to estimate the residual life of components in creep, and the consequent assessment of structural integrity, would prove invaluable. Micro-tomography studies related to creep in materials are reviewed, with special emphasis on a 10·86%Cr heat resistant steel, to demonstrate the type of data available for life assessment and design against creep failure. A brief discussion of current methods to estimate residual life in the light of recent 3D micro-tomography data follows. Finally, the possibility of new approaches, using micro-tomography data in conjunction with destructive 3D approaches such as serial sectioning, to formulate advanced residual life estimates, is briefly considered.