Search

This article presents the main possibilities for steel refining in Ladle Furnace (LF). These, are presented: steelmaking stages, steel refining through argon bottom stirring, online control of the bottom stirring, bottom stirring diagram during LF treatment of a heat, porous plug influence over the argon stirring, bottom stirring porous plug, analysis of porous plugs disposal on ladle bottom surface, bottom stirring simulation with ANSYS, bottom stirring simulation with Autodesk CFD.

Experimental studies of the interaction between amorphous hydrogenated carbon (a-C:H) film and short and ultrashort laser pulses in the near-infrared and visible spectral ranges (150 ns and 1064 nm, 10 ns and 1078 nm, 300 ps and 1078 nm, 220 ps and 539 nm, 100 fs and 800 nm.) are reported. The influence of the irradiation conditions (pulse duration, wavelength, laser intensity and the number of laser shots) on the structure and thickness of the laser-induced graphitized layer has been investigated. The effects of heat dissipation on the annealing duration and on the graphitized layer thickness are discussed for the case of laser processing with short pulses. It was found that the resulting morphology of the irradiated a-C:H film surface was determined by the concurrence of three processes: change of the mass density induced by structural transformations, multiple spallations of thin layers, and material evaporation. The laser-induced spallation is asserted to be the main factor limiting the laser microprocessing reproducibility for the examined a-C:H film; its effects were found to increase dramatically for longer (150 ns) laser pulses. The ablation (evaporation) rates of the a-C:H films and glassy carbon were revealed to be similar for femtosecond and picosecond pulses, but they essentially differed for nanosecond pulses. The ablation process demonstrated the same main features for both materials: (i) increase of the ablation rate with the pulse duration, and (ii) saturation of the ablation rate with fluence for picosecond and nanosecond pulses.

Laser material removal is an effective processing technique for hardmetals, which cannot be machined by chip-removal techniques. The basic physics of the laser–matter interactions and the influence of different laser parameters are discussed, with emphasis on sintered WC–Co specific features. The collateral affected zones and their occurrence mechanisms for laser machining with both nanosecond and femtosecond pulses are discussed. Experiments were carried out with pulsed laser systems operating in IR and UV, with ns and fs pulses and their results endorse the theoretical considerations. The use of direct or indirect laser processing (ns and fs pulses) in the surface engineering of coated/uncoated WC–Co parts is also presented. Subsequently, applications like laser microstructuring of tribological WC–Co surfaces and laser machining of integral chipbreakers are discussed.

Laser-induced multilevel spallation in diamond-like carbon films is observed and studied for a wide range of laser parameters (λ=539, 800, 1064, and 1078 nm; τ=100 fs, 300 ps, 10 ns, and 150 ns; E>0.1 J/cm2). Laser irradiation causes the detachment and removal of material layers, whose lateral size is comparable with the irradiated area and whose thickness of 10–200 nm is much less than the DLC film thickness. The influence of the laser parameters on the thickness, size, and number of removed layers is examined. A qualitative explanation of laser-induced spallation in DLC films is proposed, which is based on the fact that the absorbed laser pulse produces a rapidly expanding surface layer of modified structure and reduced density.

Diamond-like carbon (DLC) coatings were deposited on Si substrates using a hot filament diode discharge and they were irradiated with ultrashort laser pulses (800 nm, 150 fs, 2 ). The laser-treated films were examined using optical microscopy, Raman spectroscopy, SEM, AFM and white-light interferometery. Damage threshold of 0.16 J/cm 2 and ablation rates below 110 nm/pulse were determined. Changes in the structure of the laser-irradiated films were showed by means of Raman investigations. The laser-treated samples were etched and the depths of modified material layers were determined. Ablation experiments with longer laser pulses (1064 nm, 100 ns, 2 ) were also performed and the irradiated DLC films were afterwards analyzed using the same procedures. Dissimilarities in the structure changes induced by fs-and ns-laser irradiation were observed and comments are given.

Investigations on the evolution of the ablated depth over a large number of incident femtosecond laser pulses and the occurrence of structure and hardness changes in the immediate vicinity of the laser induced craters (for two sorts of steel and a hard metal substrate) are reported in this work. Experiments were performed in air with a Ti:sapphire laser (800 nm, 100 fs) at mean fluences of 2, 5 and 10 J/cm 2 . After the laser irradiation, cuts were made through the processed samples and pore cross-sections were obtained. Through their metallographical analysis it was possible to evidence crystalline structure changes in the immediate vicinity of the laser induced microholes; the extent of modified material zones increased for higher laser fluences. In such zones, a chemical composition quasi similar to that of the raw material was found and the performed nanohardness tests revealed hardness increases.

The friction and wear reduction in applications that allow only a minimal use of liquid lubricants is done with solid lubricant films or with protective coatings, such as diamond-like carbon (DLC). Further improvements are possible if the geometries of the contact surfaces are modified in a controlled way, as we have already demonstrated it for TiN and TiCN. In this work, the possibilities to generate patterned DLC coated low wear tribological surfaces by means of laser processing were investigated. In the first approach, a two step method was used: steel substrates were laser patterned and subsequently DLC films were deposited on them. The second considered approach was the laser processing of coated surfaces. DLC films were irradiated with laser pulses of different durations and energy densities (100 fs, 800 nm

The response of cemented tungsten carbide and of titanium carbonitride was investigated with respect to damage and ablation properties, under interaction with ultrashort laser pulses. These carbide materials present high microhardness and are of significant interest for tribological applications. The experiments were carried out in air with a commercial Ti:sapphire laser at energy densities on the target up to 6.5 J/cm2. The irradiated target surfaces were analyzed with optical, SEM and AFM techniques and the damage and ablation threshold values were determined using the measured spot diameters and the calculated incident energy density distributions.

Several ultrahard materials and coatings of definite interest for tribological applications were tested with respect to their response when irradiated with fs laser pulses. Results on cemented tungsten carbide and on titanium carbonitride are reported for the first time and compared with outcomes of investigations on diamond and titanium nitride. The experiments were carried out in air, in a regime of 5–8 J/cm2 fluences, using the beam of a commercial Ti:sapphire laser. The changes induced in the surface morphology were analysed with a Nomarski optical microscope, and with SEM and AFM techniques. From the experimental data and from the calculated incident energy density distributions, the damage and ablation threshold values were determined. As expected, the diamond showed the highest threshold, while the cemented tungsten carbide exhibited typical values for metallic surfaces. The ablation rates determined (under the above-mentioned experimental conditions) were in the range 0.1–0.2 μm per pulse for all the materials investigated.

Laser technique is employed to improve substantially the tribological behavior of microstructured stainless steel surfaces. The laser-generated patterns consisting of precisely ablated microcraters were produced using a flash lamp pumped, Q-switched Nd:YAG laser. The rims of resolidified melt around the craters are typical features of laser-produced microstructures and the laser parameters were established where their extent was notably reduced. The remaining rims could easily be removed by gentle polishing. The laser-structured substrates were tribologically tested by a sliding tribometer under standard conditions. The test results show clearly that the microstructures improve the lifetime of the samples.

The ability of femtosecond laser pulses to pattern coated tribological surfaces in order to improve their wear behavior was investigated. Experiments were performed with a Ti:sapphire laser (wavelength: 800 nm, energy density: 2 J/cm2, pulse duration: 100 fs) on TiN- and on TiCN-coated surfaces. Morphological analyses of the laser-treated surfaces were carried out and did not reveal any film delamination or other coating damage after laser processing. Tribological tests simulating rapidly increasing contact pressures under boundary friction were performed on both unpatterned and laser-patterned coated surfaces using a steel counter body. The patterned surfaces showed significantly better tribological performance with respect to stability and the value of the friction coefficient during testing. EDX analyses of the tested unpatterned samples revealed complete coating removal and material transfer from the counter body to the sample surface. In the case of the laser-patterned surfaces, only slight coating damage and an accumulation of debris from the steel counter body in the laser-induced pores were observed.

Mechanisms responsible for the limitation of the aspect ratio obtained by deep drilling of hard metals are investigated in the present work. Cemented carbide targets have been irradiated with laser pulses of 100 fs duration and 100 μJ maximum energy delivered by a Ti:sapphire laser system. The experiments are carried out in different gas environments (vacuum, air, helium up to atmospheric pressure) with incident laser fluences ranging from 1 to 20 Jcm-2. During deep drilling, the laser-induced ablation plume is characterized by means of in-situ plasma diagnostics. Fast imaging is used to observe the expansion behavior of the plasma plume whereas time- and space-resolved emission spectroscopy is employed to analyze the plasma composition. After irradiation, the laser-produced craters were examined by optical microscopy. A correlation between the ablation plume characteristics and the morphological changes of the mciro-holes is established. The results indicate that nanoclusters, that present a significant part of the ablated material, are responsbile for the alteration of the crater shape in the high laser fluence regime.

Laser ablation with fs laser pulses was performed in air on cobalt cemented tungsten carbide by means of a Ti : sapphire laser (800 nm, 100 fs). Small and moderate fluences (2, 5, 10 J/cm2) and up to 5×104 pulses per irradiated spot were used to drill holes with aspect ratios up to 10. Cross-section cuts from laser-irradiated samples were produced and they were analysed with optical microscopy and SEM. EDX analyses were carried out on selected zones. Quasi-cylindrical holes were found for 2 J/cm2, whereas for 5 and 10 J/cm2 irregular shapes (lobes, bottoms wider than hole entrances) were found to occur after a given number of incident pulses. Layers with modified structure were evidenced at pore walls. SEM revealed a denser structure, while EDX analyses showed uniform and almost similar contents of W, C, and Co in these layers. As a direct application, patterning of coated WC-Co was carried out with 2 J/cm2 and 100 pulses per pore. The resulted surfaces were tribologically tested and these tests revealed an improved friction and wear behaviour.

Polycrystalline SiGe is attracting more and more attention in micro and optoelectronics devices both at industrial and university level. Research on both devices and material growth techniques continues at a very rapid pace in the scientific world. Low cost production techniques, capable to produce such alloys with uniform and controlled grain size, becomes of particular attention. Excimer laser crystallization has proved to be a valuable how thermal budget technique for amorphous silicon crystallization. Its main advantages are the high process quality and reproducibility joint to the possibility of tailoring the grain sizes both in small selected regions and in large areas. This technique is here applied for producing poly-SiGe alloys from amorphous SiGe films deposited on glass.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Investigations on the occurrence of structure and hardness changes (for two sorts of steel and for a hard metal substrate) in the immediate vicinity of laser induced craters are presented in this work. Experiments with femtosecond pulses were performed in air with a Ti:sapphire laser (800 nm, 100 fs) at mean fluences of 2, 5 and 10 J/cm2. Series of microcraters were induced with 100 to 5,000 laser pulses per hole. Experiments with similar fluences, but 10 to 40 pules per hole, were performed on the same materials using a Nd:YAG delivering 100 ns pulese. After laser irradiation, cuts were made through the processed samples and the changes occurred in the crystalline structure of the target materials were evidenced by metallographical analysis of the resulting cross-sections. Hardness measurements were performed in points situated in the immediate vicinity of the laser-induced pores. Affected zones in the material surrounding laser induced pores were always found in the ns-regime, however with different properties for various laser parameters. In the fs-regime, zones of modified materials were also found and in such zones a significant hardness increasing was evidenced; the limit of the low fluences regime, where no structure changes occurred, was found to be slightly above 2 J/cm2.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The controlled laser patterning of solid surfaces improves their wear properties: laser generated microcraters of defined dimensions and morphology can act as lubricant reservoirs and as traps for wear particles. For generating such microstructures different techniques are possible; however laser ablation has the advantage of a great versatility, since it can be adapted to produce a wide range of structures crater arrangements. We have generated microstructures with ps- and ns- pulses on TiN coated steel, uncoated steel and on uncoated steel with was subsequently coated with TiCN. Laser patterning of metals with pulses in the 100ns range is very effective however the process is governed by local melting and vaporization. The melt ejection yields the formation of resolidified droplets and rims on the target surface. This undesired artifacts can be removed by gentle polishing. Furthermore, a recast layer of increased hardness and brittleness of about 1µm thickness which homogeneously covers the crater walls is observed. Tribological tests were performed using the ball-on-disk method. The lifetime of the structured samples, defined as the sliding distance after which the friction coefficient showed an abrupt increase, was found to be significantly enhanced for all structured surfaces. The enhancement ranged from a 30% lifetime increase when TiN coatings were directly structured to an increase of over 10 times for structured hard metal which was subsequently coated with TiCN. To evaluate the possibility of circumventing some of the drawbacks of laser ablation in the ns regime, mainly the melt rims on the surface and the recast layer film on the crater walls of the single microholes the quality of microstructures produced in various metals with pulses of femtosecond duration was studied. To that end the evolution of the ablated depth over a large number of incident femtosecond laser pulses and the occurrence of structure and hardness changes in the immediate vicinity of the laser induced craters was analyzed for two sorts of steel and for hard metal. The analysis evidenced changes in the crystalline structure of the target materials in the fluence regime above 2J/cm2 but only minor alterations up to this fluence. Hardness measurements were performed on the cross-section surfaces in points situated in the immediate vicinity of the laser induced pores. Affected zones in the material surrounding few pores induced in the fs- regime were found and in such zones a significant hardness increasing was evidenced.

Different laser beam techniques were applied to AISI M3 steel samples in order to produce well-defined surface microtextures consisting of pores, which can act as lubricant pockets (reservoir) as well as traps for wear debris. Both effects contribute to improve the tribological performance of textured functional surfaces. The tribological performance of lasertetxures was studied as function of the pore depth and diameter in unidirectional sliding tests under starved lubrication. The lasertetxured surfaces tested under those conditions were produced by a well-established ns laser technique using a Q switched Nd:YAG laser. The topographical microstructures of these laser-induced textures were characterized by optical and scanning electron microscopy as well as replica technique. A significant change in friction behavior as compared to untextured tool surfaces was observed when using microtextured surfaces.In another part of the study, the influence of new-developed fs laser technique of the tool steel was investigated. The fs laser texturing of the steel results in a change of the metallographic structure of the laser-affected zone, which is clearly detectable in cross-section samples. The structure of the material and chemical composition of the laser-affected, pore-near region was analyzed by transmission electron microscopy with combined EDX analysis. It could be shown that the laser-affected zone seems to consist of an amorphous or nanocrystalline material in opposite to the ‘macrocrystalline’ steel substrate. Nanoindentations revealed a two times higher hardness of the laser-affected zone than the steel bulk phase.

The controlled laser microstructuring of solid surfaces improves their wear properties: microholes induced on a friction surface can act like lubricant reservoirs and as traps for debris particles. In generating such microstructures, the laser has the advantage of its great versatility, since it can be used in various environments and it can be adapted to a wide range of desired structures. In this work metallic surfaces where precisely and reproducibly patterned on a micrometer scale by an industrial, Q-switch-operated Nd:YAG laser. The duration of the pulses was 100 ns FWHM. For laser ablation in this pulse length range local melting and vaporing govern the mechanisms of materials expulsion and the melt ejection occurs through the vapor pressure gradient, yielding the formation of resolidified droplets and rims on the target surface.

Summary form only given. Through controlled laser microstructuring of solid surfaces, the wear properties of solid surfaces can be improved: microholes induced on a friction surface can act like lubricant reservoirs and as traps for debris particles, ameliorating both friction and wear behaviours. In generating such microstructures, the laser has the advantage of its great versatility. Since it can be used in various environments and it can be adapted to a wide range of desired structures. In this work the laser beam coming from an industrial, flash-lamp pumped, Q-switched operated, Nd:YAG laser was focused on metallic surfaces, producing removal of the target material. The duration of pulses was 100 ns, FWHM for laser ablation within this temporal domain. Local melting and vaporising govern the mechanisms of material expulsion and the melt ejection occurs through the vapour pressure gradient.