Volume 99, Issue 4

Displacive transformations involve the disciplined motion of atoms. As a result, there are clearly defined relationships between all aspects of the parent and product lattices. The theory for this is well established but has not been exploited in the calculation of transformation textures. This paper is a critical assessment of the methods for the estimation of crystallographic textures during the displacive transformation of austenite into martensite, bainite or Widmanstätten ferrite in steels. The discussion is limited to the case where austenite is not in a plastically deformed state prior to its transformation.

In steel weld metals and their heat affected zones, acicular ferrite or intragranular ferrite developed on selected oxide inclusions within prior austenite is a key to refining the microstructure and hence improving the toughness. In this study, to obtain a better understanding of the mechanism of the intragranular ferrite formation, different oxides were deposited onto C – Mn steel by a physical vapor deposition process, and ferrite formation was examined at the different oxide – steel interfaces after an HAZ-simulated thermal cycle. TiO, MgO, CaO and Al 2 O 3 were deposited on C – Mn steel containing 0.16% C and 1.4% Mn and then they were heated up to 1623 K and cooled down at a cooling rate of 20 K s −1 . Allotriomorphic ferrite was found to develop from TiO and MgO, while no allotriomorphic ferrite but Widmanstätten ferrite formed from CaO and Al 2 O 3 . Electron backscatter diffraction pattern analysis indicates that the allotriomorphic ferrite growing from the oxide has a specific orientation relationship either with the oxide or with prior-austenite. In the former case, the ferrite seems to be aligned so as to minimize the misfit against the oxide, implying that the likelihood of ferrite formation can be increased by the oxides having a good lattice match with ferrite. Manganese depletion was identified in the vicinity of TiO, but it is concluded that, although it encourages ferrite formation from the oxide, it is not a critical factor for the ferrite formation because of the fact that the ferrite formation was even found from TiO in manganese-free steel. First principles calculation indicates that the interfacial energy between oxide and steel is reduced when the magnesium in MgO is replaced by titanium, which suggests that the ferrite formation from TiO is also affected by the chemical effect of titanium.

In this work, MoSi 2 – ZrO 2 -composites with different volume fractions (15-, 25-, 35 vol.% ZrO 2 ) were examined. Zirconium oxide was successfully tetragonal stabilised (metastable state) through a high temperature isostatic pressure process, where the tetragonal – monoclinic phase transformation is inhibited by the pressure of the MoSi 2 -matrix. Stabilisers, e. g. Y 2 O 3 were not used. Increasing ZrO 2 content enhances strength and fracture toughness at room temperature. For temperatures above 900°C up to 1500°C no influence of the volume fraction of ZrO 2 was observed with respect to the compression tests. Wear of pre-oxidised samples (20 min at 1000°C) is higher for the 25 and 35 vol.% than non-oxidised material. Oxidation resistance is observed to be poor and decreases with increasing ZrO 2 content in the composite. Early crack initiation leads to poor creep behaviour and thermo-shock behaviour. This is probably due to the oxygen diffusing through the ZrO 2 -particles. Consequently no continuous protective SiO 2 -layer can be built as known from pure MoSi 2 -materials. In general the investigated material is not suitable for high temperature applications because of the poor oxidation behaviour.

Within the Collaborative Research Centre 561 “Thermally highly loaded, porous and cooled multilayer systems for combined cycle power plants” open porous and high-temperature stable Ni-based structures are being developed for the requirements of effusion cooling. A two-dimensional cooling strategy for the walls of combustion chambers, that allows the outflow of the cooling medium over the complete wall area of the combustion chamber, could be realised by an open porous metallic foam structure. The open porous metallic foam is produced by the “Slip Reaction Foam Sintering” process, a powder metallurgical process. For joining several foams to assemble structural elements laser beam welding and capacitor discharge welding have been used. Different joining strategies have been examined for both processes to find out the most suitable method for joining these foams. In this paper the process setups, settings of the different strategies and results of trials (seam geometry and strength tests) are discussed. The need for graded structures to combine the essential permeability and adequate weldability is also shown.

Fe – Co – Mo alloys show hardening behaviour comparable to that of precipitation hardened Al-alloys, which is characterised by a low hardness after solution annealing and a significant increase in the hardness during subsequent ageing at higher temperatures. In this study, the precipitation behaviour of Fe-25% Co-15% Mo (wt.%) has been studied applying non-isothermal ageing. High sensitivity differential scanning calorimetry and atom probe tomography have been used to characterise the precipitation sequence. Three overlapping exothermic reactions, which are related to precipitation reactions, are observed in the temperature range of 400 to 700°C. The investigations have shown that precipitation starts with diffusion of Mo, followed by the formation of the intermetallic μ-phase which reaches its equilibrium composition at a temperature of about 615°C.

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 718Plus TM , 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.

An experimental test melt of a boron alloyed 9Cr-3W-3Co-V,Nb steel for high temperature applications in the thermal power generation industry was produced by vacuum induction melting. This grade of steel typically displays a homogeneous tempered martensitic microstructure in the as-received, i. e. normalised and tempered, condition. However, after welding, this microstructure is significantly altered, resulting in a loss of its desired properties. The phase transformations during simulated thermal cycles typical of those experienced in the weld heat-affected zone were directly observed by in-situ X-ray diffraction experiments using synchrotron radiation. Heating rates of 10 K s −1 and 100 K s −1 up to a peak temperature of 1300°C are investigated here. The final microstructures observed after both simulated weld thermal cycles are primarily composed of martensite with approximately 4% retained delta ferrite and 4% retained austenite, by volume. With the temporal resolution of the in-situ X-ray diffraction technique, phase transformations from tempered martensite to austenite to delta ferrite during heating and to martensite during cooling were monitored. With this technique, the evolution of the final microstructure through both heating and cooling is monitored, providing additional context to the microstructural observations.

The effect of boron on creep deformation behavior and microstructure evolution during creep has been investigated for a tempered martensitic 9Cr-3W-3Co-VNb steel containing boron up to 140 ppm at 650°C (923 K) with the emphasis on long term behavior. Creep tests were carried out at 650°C for up to about 70 000 h. The base steel without boron exhibits degradation in creep rupture strength after times above about 1000 h. The addition of boron retards the onset of tertiary or acceleration creep, where the creep rate increases with time after reaching a minimum creep rate, at low stresses and long times. This effectively decreases the minimum creep rate and increases the time to rupture. The addition of boron reduces the coarsening rate of Ostwald ripening of M 23 C 6 carbides near prior austenite grain boundaries during creep, which retards the onset of acceleration creep. It is considered that boron atoms occupy vacancies in the vicinity of growing carbide interfaces near prior austenite grain boundaries, which makes it difficult to accommodate local volume change around growing carbides and hence causes the reduction in coarsening rate.

The stress versus strain curve of ASME Grade P92/T92 steels was analysed and the stress dependence of the creep deformation behavior was investigated. Good correspondence between 50% of 0.2% offset yield stress and a proportional limit was observed over a range of temperatures from 550 to 700°C. Stress dependence of the minimum creep rate was divided into two groups of high and low stress regimes with a boundary condition of 0% offset yield stress. Creep deformation in the low stress regime was considered to be controlled by dislocation climb. Large stress dependence of the minimum creep rate in the high stress regime was equivalent to the strain rate dependence of flow stress observed in the tensile test. Creep strength in the high stress regime is assumed to be influenced by plastic deformation, and should be excluded for a long-term creep strength evaluation.

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.

A novel steel grade has been developed recently exhibiting superior thermal stability. This is attributed to the combined precipitation of secondary hardening carbides and intermetallic phases. However, the precipitation behaviour of this steel is not completely understood yet. In this study, computer simulations of the precipitation kinetics during heat treatment of this steel are performed and compared to a complementary experimental characterisation of the precipitate microstructure, which was carried out previously. The simulations employ a novel model for nucleation, growth and coarsening of precipitates in multi-component, multi-phase systems. It is shown that the combination of experimental investigation and computer simulation provides a most comprehensive picture of the complex processes occurring in these materials during thermo-mechanical treatment, which cannot be obtained by the use of one single technique, experiment or simulation, only.

Modern advanced 9–12% Cr steels are complex alloys with excellent creep strength even at high temperatures up to 620°C. In this work, the experimentally observed creep rupture strength of the European COST alloy E2 is compared to the calculated maximum obstacle effect (Orowan stress) caused by the precipitates present in these steels for different heat treatment conditions. The applied model for precipitation strengthening is briefly outlined. It is shown that the differences in creep rupture strength caused by different heat treatments disappear after a long time under service conditions. This observation is discussed on the basis of the calculated evolution of the precipitate microstructure. The concept of boosting long-term creep rupture strength by maximizing the initial creep strength with optimum quality heat treatment parameters for precipitation strengthening is critically assessed.

In 9–12% Cr steels, nitrides (MX, M 2 X) and modified Z-phase ((Cr, V, Nb, Fe)N)) are of special interest because of their different contribution to the creep strength of the material. The changes in the chemical composition of complex nitrides and their crystallography were investigated using transmission electron microscopy. A method based on multiple linear least squares fit procedures was applied to extract the complete element content of the nitrides from the electron energy loss spectra. These results, augmented by X-ray spectroscopic data, were compared with the outcomes of the thermodynamic model implemented in the software package MatCalc. The elemental concentrations of precipitates under study i. e. the M 2 X, MX and modified Z-phase are in good agreement with simulations.

The transient displacement field caused by an arc weld was measured experimentally with an Aramis stereo-camera. The transient displacement field was also predicted using a computational weld mechanics program called VrWeld for ten different sets of model parameters. The objective was to identify the effect of model parameters on the deviation between experimental data and the data computed by computational weld mechanics. For each set of model parameters, the L2 norm of the difference between the experimental and computed displacement fields was computed for each of the 800 time steps. The structure being welded was a mm stringer fillet welded to a mm low alloy steel plate. The displacement field was measured on the ‘back’ surface of the plate. A robot made the metal-inert-gas weld. The parameters varied in the ten models included welding current, width of the power density distribution, convection coefficient and yield stress of austenite at high temperatures.

Heat transfer and visco-plastic flow during friction stir welding of Ti-6Al-4V alloy have been modeled in three dimensions by numerically solving the equations of conservation of mass, momentum and energy using temperature dependent thermo-physical properties and temperature and strain-rate dependent viscosity values. The computed results showed that five important model parameters, i. e., the spatially variable friction coefficient, the spatially variable slip between the tool and the workpiece, the extent of viscous dissipation, the mechanical efficiency and the spatially variable heat transfer rate from the bottom surface of the workpiece significantly affected both the temperature fields and the computed torque on the tool. An important problem in the modeling of friction stir welding is that the values of these five parameters cannot be specified from fundamental principles and, and as a result, computed results are not always accurate. Here we show that by combining the heat transfer and plastic flow model with a genetic algorithm based optimization scheme, the values of the five uncertain parameters can be determined from a limited volume of experimental data so that the model predictions of peak temperatures and cooling rates match well with the experimental results. The computed results show that for the welding conditions reported in this paper, close to sticking condition prevailed at the tool – workpiece interface for all the experiments. The extent of viscous dissipation converted to heat was fairly low indicating lack of intimate atomic mixing in the stir zone. Computed three dimensional pressure distributions and streamlines were consistent with defect-free reliable welds for all conditions of welding studied.