Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- Laser beam welding (6)
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- Solidification cracking (4)
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Eingeladener Vortrag
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Effect of cooling rate on microstructure and properties of microalloyed HSLA steel weld metals
(2015)
Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.
A new method for sampling wear particles directly from the lubricant reservoir has been developed and applied successfully for analyzing wear particles by high-resolution scanning electron microscopy in transmission mode having coupled energy-dispersive X-ray spectroscopy. The lubricated tribological testing was carried out with fully formulated as well as with non-formulated synthetic base oil. It was possible to analyze individual particles with dimensions as small as about 5–30 nm which are likely the 'primary' wear particles. A majority of the particles, however, are agglomerated and, thus, lead to the formation of larger agglomerates of up to a few micrometers. Chemical analysis led to the conclusion that most of the observed particles generated in formulated oil, especially the larger ones, are composed of the additives of the lubricant oil. In non-formulated base oil, the primary particles are of similar dimensions but contain only iron, chromium and oxygen, but most likely stem from the mating materials. This finding points to the fact that the main wear mechanism under lubricated conditions with fully formulated oil is more like a continuous shearing process rather than a catastrophic failure with the generation of larger primary particles. When the oil is non-formulated, however, several wear mechanisms act simultaneously and the wear rate is increased significantly. Generated larger primary particles are milled down to the nanoscale. When the oil is fully formulated, wear mainly takes places at the additive layer or tribofilm; thus, the steel surface is protected.
The influence of temperature and counterbody material on the tribological properties of a-C:H
coatings deposited on Cronidur 30 steel has been investigated in a lubricated ball on disk
contact situation with alpha-alumina and silicon nitride as counterbodies. The results show, that the wear volumes of the systems increase exponentially with increasing temperature, for alpha-alumina more than for silicon nitride. Two different wear mechanisms seem to have a
major influence: First, the abrasive action due to materials hardness and second, the tribo-oxidation when silicon nitride is counter material.
The integration of finite element method (FEM) into the least-squares adjustment presented in is further extended for a joint evaluation of an elastostatic model and displacement field measurement. For linear solids which obey the Hooke's law, the material parameters determination from measurements is being examined.
The kinetic energy of keV electrons backscattered from a rutile (TiO2) surface depends measurably on the mass of the scattering atom. This makes it possible to determine separately the angular distribution of electrons backscattered elastically from either Ti or O. Diffraction effects of these backscattered electrons inside the rutile crystal lead to the formation of Kikuchi patterns. The element-resolved Kikuchi patterns of Ti and O differ characteristically, but each can be described fairly well in terms of the dynamical theory of diffraction. Qualitatively, much of the differences can be understood by considering the relative arrangement of the Ti and O atoms with respect to planes defined by the crystal lattice.
Many industrial processes are run at non-atmospheric conditions (elevated temperatures and pressures, other oxidizers than air). To judge whether and if yes to what extent explosive gas(vapor)/air mixtures will occur or may be generated during malfunction it is necessary to know the safety characteristic data at the respective conditions. Safety characteristic data like Explosion limits, are depending on pressure, temperature and the oxidizer. Most of the determination methods are standardized for ambient conditions.
In order to obtain determination methods for non-atmospheric conditions, particularly for higher initial pressures, reliable ignition criteria were investigated. Ignition tests at the explosion Limits were carried out for mixtures of methane, propane, n-butane, n-hexane, hydrogen, ammonia and acetone in air at initial pressures up to 20 bar. The tests have been evaluated according to different ignition criteria: visual flame propagation, temperature and pressure rising. It could be shown that flame propagation and occasionally self-sustained combustion for several seconds occurred together with remarkable temperature rise, although the pressure rise was below 3%. The results showed that the combination of a pressure rise criterion of 2% and a temperature rise criterion of 100 K seems to be a suitable ignition criterion for the determination of explosion limits and limiting oxidizer concentration at higher initial pressures and elevated temperatures. The tests were carried out within the framework of a R&D project founded by the German Ministry of Economics and Technology.
The damage process of short glass fibre (30% weight) reinforced polyamide caused by mechanical loading was investigated from the beginning on micro cracking level to the incipient crack of mm-dimension. Based on high resolution computer tomography and the X-ray-refraction technique the inner surface due to micro-cracking at the short fibre ends and the fibre matrix debonding of the skin surface of the filament was determined quantitatively. With the knowledge of the fatigue crack propagation rate and fracture toughness of the material from former research projects, it was derived that the total inner surface due to micro cracks measured by X-ray refraction is much higher than the specimen could have withstand the load, supposed the surface is in a localized crack. Hence, the damage process could be described from micro to macro level. Accompanying fractographic investigations endorse the modelling based on the NDT-techniques.
The damage process of short glass fibre (30% weight) reinforced polyamide caused by mechanical loading was investigated from the beginning on micro cracking level to the incipient crack of mm-dimension. Based on high resolution computer tomography and the X-ray-refraction technique the inner surface due to micro-cracking at the short fibre ends and the fibre matrix debonding of the skin surface of the filament was determined quantitatively. With the knowledge of the fatigue crack propagation rate and fracture toughness of the material from former research projects, it was derived that the total inner surface due to micro cracks measured by X-ray refraction is much higher than the specimen could have withstand the load, supposed the surface is in a localized crack. Hence, the damage process could be described from micro to macro level. Accompanying fractographic investigations endorse the modelling based on the NDT-techniques.
Selective Electron Beam Melting (SEBM) is a promising powder bed Additive Manufacturing technique for near-net-shape manufacture of high-value titanium components. However without post-manufacture HIPing the fatigue life of SEBM parts is currently dominated by the presence of porosity. In this study, the size, volume fraction, and spatial distribution of the pores in model samples have been characterised in 3D, using X-ray Computed Tomography, and correlated to the process variables. The average volume fraction of the pores (b0.2%) was measured to be lower than that usually observed in competing processes, such as selective laser melting, but a strong relationship was found with the differentbeamstrategies used to contour ,and infill by hatching, a part section. The majority of pores were found to be small spherical gas pores, concentrated in the infill hatched region; this was attributed to the lower energy density and less focused beam used in the infill strategy allowing less opportunity for gas bubbles to escape the melt pool. Overall, increasing the energy density or focus of the beam was found to correlate strongly to a reduction in the level of gas porosity. Rarer irregular shaped pores were mostly located in the contour region and have been attributed to a lack of fusion between powder particles.
High resolution in situ monitoring of the initial cement hydration influenced by organic admixtures
(2015)
Numerous admixtures are used in the building practice to customize the properties of the cement paste during application. The influences of admixtures on the course of cement hydration and formation of hydrate phases have to be considered. Polycarboxylate ether (PCE) based polymeric superplasticizers (SPs) are known to retard the setting of the cement paste. The extent of the retardation differs depending on the molecular structure of the SP. Additionally, the presence of a stabilizing agent (SA) in the cement paste has a retarding side effect on the setting. The initial cement hydration processes and the detailed mechanisms of the retardation influenced by PCEs, as well as their interactions with particular SAs, are insufficiently understood. Up to now, only the results of phenomenological studies were taken into account to describe this retardation process. A detailed structure analysis monitoring the change of the phase composition during the hydration was never applied. Both SP and SA affect the adsorption of the sulphate ions on the clinker particles, causing changes in the formation of ettringite during the initial hydration, and are therefore a crucial part of the setting process itself. Here, the initial hydration of cement influenced by the interaction of SP and SA was monitored in situ by synchrotron X-ray diffraction. The high time resolution of the measurements allowed a continuous detection of the hydrates formed. The hydration was followed from the starting point of water addition and for couple of hours afterwards. The hydration of the levitated cement pellets containing starch as SA was initialized by adding aqueous solutions of different commercial SPs. Changes in the ettringite formation were detected in comparison to the reference hydration of pure cement.
Optimizing the performance of phase gratings for better visibility in Talbot- Lau interferometry
(2015)
We investigated the efficiency of phase gratings, i.e., the visibility upon variation of different parameters. Rotating around an axis parallel to the grid lines of the phase grating changes the grating’s shape and thereby the initial distribution of phase shifts. This yields high visibilities for shorter propagation distances than derived from box shapes. Tilting the grating in the scattering plane allows continuous tuning of the grating’s height that corresponds to an ideal phase shift for a particular photon energy. This opens the way for tuning the design energy suitable for the material under investigation. Our study included simulations for monochromatic sources with the sampling procedure.
Concrete of today has only little in common with the traditional concrete used a few decades ago. It has become a high performance material, which can be adjusted for high performance applications and according to ultimate user specifications. The reason for the rapid evolvement was the increasing awareness about how the rheology of concrete can be improved without negatively affecting the mechanical properties of concrete by chemical admixtures.
Combining CO2 Streams from Different Emitters – A Challenge For Transport And Storage Infrastructure
(2015)
The European Directive 2009/31/EC on the geological storage of CO2 envisages an open access of CO2 streams from different emitters to a nation- or even EUwide CO2 pipeline network if CO2 stream compositions meet “reasonable minimum composition thresholds”. As of today it is not known how such “composition thresholds” may be defined and which impurity levels may be viable in practical application.
To set up recommendations for criteria and respective threshold values for CO2 stream compositions, the project “CLUSTER” will investigate how a dynamic interplay – both in terms of mass fluxes and compositions – of CO2 streams from regionally clustered CO2 sources sharing a transport and storage infrastructure will impact corrosion, e.g., of pipelines and plant components, and geochemical alteration of cap rocks and reservoir rocks. In addition, the behaviour of such a highly dynamic CCS system will be considered for an overall optimization of system design including CO2 stream mixing schemes and facilities or interim CO2 storage.
Lightweight granules are mineral, spherical and porous particles with bulk density less than 2000 kg m . New types of lightweight granules are made from masonry -3 rubble as an alternative to the commonly used expanded clay and shale. They are produced in a multistage manufacturing process by thermal or hydrothermal treatment. Studies of the microstructure of the new lightweight granules are very important to optimise the engineering properties with regard to different applications from lightweight concrete to planting substrates and wastewater treatment. Here, the results of porosity and pore structure measurements are presented. Characteristic samples with different bulk densities of both thermally and hydrothermally hardened granules have been analysed by means of various methods.
The stress vs. strain curve of materials is affected the rate of imposed straining. Among the methods for dynamic testing the technique known as 'split Hopkinson pressure bar' (SHPB) has evolved into the most widely used one to exert high-speed straining. The theory behind it comprises simple equations to compute stress and strain. The reliability of the strain analysis can be assessed by digital image correlation (DIC). The present results indicate that the visually observed strain is smaller than predicted by theory.
Laser-GMA-hybrid welding of longitudinally welded large-diameter pipes of grades API- X80/ X120
(2015)
Potentials of the hybrid laser arc welding processes were investigated regarding reliable production of longitudinal welds of high strength pipe steels X80 and X120
Hybrid laser arc welding of the root pass:
Significant reduction in the weld cross section – savings of filler material
Lower heat input into the base material - improvement of mechanical properties of the weld joint
The arc type does not have any influence on the character of dilution in the laser part of the hybrid weld
14 mm deep root face was considered as optimum. No penetration of the filler material could be detected beyond this depth limit
Metal powder filler wires, micro alloyed with Ni and partly with Cr and Mo, guaranteed sufficient Charpy impact toughness at low temperature for the both investigated materials:
Hybrid laser arc welding is a very efficient process for the root pass in multi-pass welding and opens a large technical and economic potential for the manufacture of large pipes
Experimental study and numerical simulation of hot crack formation for novel laser weldability test
(2015)
Hot cracking susceptibility in laser beam welding was assessed for several stainless steels by means of the CTW test
The strain rate has a significant influence on the formation of the hot crack
It was possible to determine the critical strain and strain rate that led to shot crack formation
A material ranking of hot cracking resistance could be made.A three-dimensional FEM using the contact element technique to simulate hot cracking during LBW under external load conditions was developed.
Good agreement between the strain field measured using the DIC technique and the simulation results
Susceptibility to hot cracking can be predicted by using FEM with the correct mechanical and thermo-physical material properties