The present investigation has been conducted in order to develop a rational approach, able to evaluate quite accurately the depth of a nitrided tool steel that should be removed previous to hard PVD ...coating deposition, and the dependence of such depth on the nitriding conditions, information that cannot be obtained easily by means of simple metallographic observations. The method thus proposed is based on the use of the hardness data corresponding to the hardness depth profile of the nitrided material, in conjunction with Fick's second law. The analytical procedure is described employing the experimental data obtained from a gas nitrided AISI H11 steel (X38CrMoV5.1), and has been validated by computing a rough estimate of the diffusivity parameters of nitrogen in the α-Fe matrix. In the particular case of the H11 steel, it has been determined that nitriding at a temperature of 580 °C for 8 h seems to have important advantages over the process conducted at 510 °C and 48 h, particularly in terms of the characteristics of the hardness profile obtained and the ability of the nitrided material to withstand indentation loads, a measure of its load-carrying capacity, as a feasible substrate for hard PVD coating deposition. The GDOS analysis that were conducted showed that within the first 8–25 μm of the sample depth, depending on the nitriding conditions, a layer of compounds with a high nitrogen content (7–19 wt.%) is formed, which is followed by a different layer with a lower nitrogen content, in the range of ∼
2 wt.%, presumably constituted by a solid solution of nitrogen dissolved in the octahedral interstitial position of the α-Fe, together with a large volume fraction of sub-microscopic coherent and/or semi-coherent nitride precipitates, which explains the high hardness, in the range of ∼
9–10 GPa observed. As expected, at a constant temperature, the extent of the compound layer was found to increase with the nitriding potential.
The present work has been conducted in order to develop a novel solution for the heat transfer problem during the continuous casting of steel billets and blooms, by using the element-free Galerkin ...method under a pseudo-transient moving cross-section slice approach. The transport laws, non-linear aspects and boundary conditions of the initial value problem have been specified. A detailed explanation concerning the characteristics inherent to the application of the element-free Galerkin method to this problem has also been provided. The feasibility and suitability of this novel approach has been verified by comparison with the numerical techniques proposed and the results reported by other researchers as well as an analytical solution of a simple 1-D alloy solidification problem. The results have revealed that this technique could be used successfully in the pseudo transient moving cross-section solution of the heat transfer problem involved in the continuous casting of blooms and square billets.
The present work has been conducted in order to develop a novel solution for the transient heat transfer problem during the startup phase of a direct-chill casting (DCC) process by using the ...element-free Galerkin method (EFGM). The material domain evolution is conditioned by a bottom block, which represents the material moving boundary. The governing equations have been solved on the basis of this global weak formulation under a Lagrangian description during the startup phase, and coupled with an Eulerian description once the block has reached the lower boundary of the computational domain along the casting direction. Finally, the EFG formulation has been adapted to an axisymmetric temperature transient problem over an aluminum alloy round billet. The feasibility and suitability of this novel approach has been verified by comparison with the numerical techniques proposed and the results reported by other researchers. The results have revealed that this technique could be used successfully in the solution of the DCC heat transfer problem during the startup phase.
Adhesion and hardness of Diamond-Like Carbon films are improved by nitriding of the steel substrate prior to PVD deposition. Since the mechanical properties of the nitrided steel layer are not ...homogeneous, i.e. a significant hardness decrease is observed in the upper nitrided layer close to the surface, an outer surface layer of ~
15
μm is removed prior to the film deposition. In the present work, a 316L stainless steel substrate is nitrided in a cyanide–cyanate solution at 570
°C during 3
h. The coated system involved the deposition of a hydrogenated, amorphous carbon (a-C:H) solid lubricant of ~
2
μm including a chromium carbide interlayer. The comparison between the hardness behavior of the DLC/steel and the DLC/nitrided steel systems reveals the existence of a very important hardness gap, which highlights the benefit of the nitriding treatment prior to coating deposition. In addition, the microhardness-depth profile is determined from a load–depth curve, by applying a simple hardness model. The predicted change in hardness is found to be in a very good agreement with the experimental profile, which allows the hardness determination both in the white layer and in the diffusion zone over ~
30
μm in total depth. However, only the composite hardness modeling allows the accurate determination of the intrinsic hardness of the film.
►Nitriding a 316L stainless steel substrate increases its load-bearing capability. ►Increasing the load-bearing capability provides adequate support for DLC film deposition. ►The nitrided steel layer close to the outer surface exhibits a decrease in hardness. ►Modeling Martens hardness data allows the determination of the hardness–depth profile. ► Frame compliance and rounded tip effect corrections of load–depth data are applied.
The present work has been conducted in order to determine systematically the influence of the spraying distance on the microstructure and mechanical properties of a Colmonoy 88 alloy deposited by ...means of HVOF thermal spray onto a SAE 1045 steel substrate. The spray distance varied between 380–470mm and the evaluation of the deposits characteristics and properties was carried out both on their surface and on cross section. Both hardness and elastic modulus of the coatings were determined according to the model of Oliver and Pharr. The yield strength of the coatings was also estimated following the methodology developed by Zeng and Chiu for the analysis of the loading and unloading curves obtained from nanoindentation experiments, as well as from classical static spherical indentation tests. The microstructural analysis indicated a significant increase in the unmelted particles volume fraction and the development of interlamellar microcracks as the spraying distance increases, leading to a decrease in the elastic modulus of the coatings. Both hardness and elastic modulus showed an anisotropic behavior and were found to be higher on the cross section of the coating than on the deposition plane. A satisfactory comparison between the predicted and experimental values of the coatings yield strength was observed for all the conditions investigated.
The present investigation has been carried out in order to study the erosion wear behaviour of WC-Co base thermal spray coatings. WC-12Co and WC-10Co-4Cr coatings were deposited by means of high ...velocity oxygen fuel (HVOF) thermal spraying. The erosion tests were conducted at impact angles of 30 and 90° using SiC particles of ∼50 μm in diameter as erodent, at a velocity of 83·4 m s
−1
. It has been found that the erosion rate for both coated systems was higher when the test was carried out at an angle of 90°. The through-thickness residual stresses of the coatings, as well as the microstructural characterisation, allowed an explanation of the results and the erosion mechanisms in each case. It has been found that, under the experimental conditions carried out in the present study, the WC-10Co-4Cr coating exhibited a higher erosive wear resistance as compared to the WC-12Co coating.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
The present study has been carried out in order to investigate the fretting wear performance of a Ni–Cr based alloy, containing B and C, deposited onto a SAE 1045 steel substrate by HVOF thermal ...spray. Tests were conducted on both the uncoated and coated substrate, under unlubricated dry conditions, at different applied normal loads, cycles and amplitudes in order to analyze the influence of these parameters on the wear behavior. It has been determined that the coated substrate exhibits a very good tribological performance in comparison to the uncoated substrate. The decrease of the wear volume of the coated substrate is of ~
95–97% of that determined for the uncoated substrate. The increase in the fretting wear resistance provided by the coating has been attributed to the presence of a large amount of dispersed Ni and Cr carbide and/or borides in the Ni–Co matrix. Examination of the fretted surfaces by SEM indicates that the uncoated substrate undergoes an abrasive wear mechanism. On the contrary, it has been observed that the wear mechanism of the coating–substrate system depends on the magnitude of the applied load. At loads of 30 N, a polishing wear mechanism has been determined, whereas between 40 and 50 N, the initiation and propagation of micro-cracks takes place through the coating. The numerical integration of the wear scar depth profile curves employed in the present work allows an estimation of the wear volume which is in good agreement with that determined experimentally by means of 3D profilometry. It has been determined that, at a constant wear amplitude the wear volume increases with the applied normal load and that at under constant load conditions, the wear volume decreases as the wear test amplitude also decreases, becoming insignificant when it is less than ~
100 µm.
•A constitutive description of a C–Mn steel under hot-working conditions has been developed.•Plane strain compression tests were conducted between 850°–1100°C, at strain rates of 0.3–24s−1.•The ...temperature and strain rate dependence of the flow stress is introduced in two different ways.•The formulation considers the interaction of dislocations with athermal and thermal barriers.•The flow stress is computed from the numerical integration of the work-hardening rate equation.
The computation of loads, torques and power consumption of hot rolling processes conducted at industrial scale requires a detailed analysis of the work-hardening rate of the material, as well as a precise description of the changes in flow stress with microstructure and deformation conditions. The present work describes two rational experimental methodologies that can be applied in order to accomplish this analysis, which encompasses the determination of the athermal stress, as well as the temperature and strain rate dependence of both the yield and saturation stresses of the material. The evolution of the flow stress in the course of plastic deformation is determined by means of the numerical integration of the phenomenological exponential-saturation work-hardening law advanced by Sah, J.P., Richardson, G., Sellars, C.M., 1969. Recrystallization during hot deformation of nickel. J. Aust. Inst. Met. 14, 292–297 expressed in differential form. In this way, it is possible to compute the current value of the flow stress in terms of its previous value and to update the changes in deformation temperature and strain rate that occur after each strain interval during the course of plastic deformation, as expected in industrial hot working processes. In the present work, these methodologies are applied to the analysis of the work-hardening transients of a number of stress–strain curves obtained for a C–Mn steel deformed under plane strain compression conditions, in the temperature range of 1123–1373K, at strain rates in the range of 0.4–24s−1. The results indicate that both the work-hardening rate and flow stress of the material can be satisfactorily described for most deformation conditions. It is shown that by employing the Sellars–Tegart–Garofalo model and the Zener–Hollomon parameter, in order to account for the temperature and strain rate dependencies of the stress parameters, the accuracy in the description of the experimental flow stress can be improved, but at the expense of an increase in the number of material parameters involved in the analysis. The limitations of employing a single internal state variable for the computation of the work-hardening rate and flow stress have also been discussed.
An investigation has been carried out in order to study the fatigue and corrosion–fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P (EN) deposit, in the as-plated ...condition, of approximately 38–40
μm in thickness and a high P content, of approximately 18
wt%. The results obtained, show that the EN coating can give rise to a significant improvement in the fatigue and corrosion–fatigue performance of the substrate, depending on the testing conditions. When the coated system is tested in air, it is observed that the increase in fatigue properties decreases as the alternating stress applied to the material increases. At stresses of the order of 0.4
σ
0.2% the increase in fatigue life is more than about 100%. However, as the stress increases to values in the range of 0.7
σ
0.2%, no improvement in the fatigue performance of the system is observed and the behavior is similar to that of the uncoated substrate. Under corrosion–fatigue conditions, the fatigue life is observed to increase between approximately 60% and 70%, depending on the stress applied. It is shown that fatigue cracks are associated with nodular-like defects present on the surface of the coated samples. The deleterious effect of such defects seems to be more pronounced as the alternating stress applied to the material increases. A crude estimate of the yield strength of the EN coating from tensile measurements indicates that such a parameter is in the range of 3.8 GPa, in agreement with the computation of the absolute hardness of the deposit, of about 4 GPa, by means of Meyer’s law. It is also shown that the EN deposit has a very good adhesion to the substrate even when the system is subjected to tensile stresses greater than the yield strength. Such characteristics as well as the higher mechanical properties of the EN coating in comparison with the aluminum alloy substrate and the preservation of its integrity during fatigue testing contribute to the better fatigue performance of the coated system.
The results of the mechanical and tribological characterization of a prototype a-C:Cr, Si sputtered coating are presented. The hardness and the elastic modulus of the coated system have been ...determined by means of nanoindentation taking into account the actual architecture of the bi-layer coating. Both mechanical properties were recorded continuously versus the indentation depth, h, up to approximately 2000nm, at a constant indentation rate and maximum applied loads of 700mN. The results were analyzed by means of the Oliver and Pharr method and modeled on the basis of novel approach proposed recently by some of the authors. Wear tests were conducted at 25°C, 400°C and 450°C against alumina, employing a contact pressure of 540MPa. Characterization of the worn surfaces by SEM and elemental X-ray mapping has also been carried out. A wear rate as low as 1.2×10−18m3/Nm was determined for the coating tested at 25°C, which is approximately one order of magnitude and three times less than those found from the tests performed at 400°C and 450°C, respectively. It has been determined that the a-C:Cr,Si coating exhibits a very good wear resistance even at temperatures up to 450°C, as consequence of the Si and Cr oxides formed due to the oxidation process. Also, it has found that at this temperature, a continuous oxide film is formed, which reduces the wear rate of the coated system in comparison to that determined at 400°C. However, the volume increase due to the oxidation process at 450°C and the elimination of CO2, Ar and H2O vapors, induces a severe surface cracking of the coating.
•High temperature sliding wear of a prototype a-C:CrSi coating has been investigated.•Mechanical properties of the coating have been determined by nanoindentation.•Elemental X-Ray mapping indicated the existence of Si and Cr oxides at 450°C.•The coated system showed satisfactory wear behavior at temperature as high as 450°C.
PDF