The progress of wear associated with the compound and diffusion layers of nitrided samples was studied by employing laboratory tests at low, medium and high contact pressures, simulating the ...conditions occurring during the hot extrusion of aluminium. It was found that with increasing of contact pressure also wear rates increase that indicates on predominately frictional removal of compound layer which was confirmed by scanning electron microscopy and back-scattered electron micrographs as well as energy-dispersive spectroscopy analysis of tested surfaces. Testing at medium contact pressures reveals some common features observed at testing at lower as well as at higher contact pressures. The essential difference between the testing at medium and low contact pressures is in the density of the obtained micro-craters and appearance of their extension in sliding direction at medium contact pressures. At higher contact pressure, removal of compound layer is already preferentially oriented in sliding direction in the first stage, while at medium contact pressure, this is observed only in later stages of degradation progress.
In the paper, five procedures for the assessment of the seismic performance of low-rise RC buildings at different levels of complexity are presented and discussed. They include simple procedures ...based on methods originally developed in Japan (levels 1 and 2), the N2 method with two variants of the mathematical model (levels 3 and 4), and non-linear dynamic analysis (level 5). The procedures have been applied to seismic assessments of three RC building structures. A small difference between the N2 and NDA results was observed, whereas the results of the procedures at the first two levels are much more conservative. On the other hand, the amount of input data and of computational work increases with the increasing complexity level. Research is still needed on the definitions of capacities, especially the shear capacity of the structural members, and on the capacity of the whole structure. The initial effective stiffness of the structure proved to be the most important quantity which determines the seismic demand.
The reasons for the formation of surface cracks during the hot rolling of tool steels are not well understood. However, we know that apart from the parameters of the thermo-mechanical processing, the ...chemical composition of the tool steel has a big influence on the formation of these surface cracks. The majority of examinations of the hot deformability (appearance of surface cracks) of various steel grades made so far were limited to studying the influence of a minor number of chemical elements on the formation of the surface cracks, where the databases were based on laboratory tests. This paper proposes a new approach to the study of hot workability by analysing crack formation during the hot rolling of AISI D2 tool steel. The database was formed on the results from the surface cracking of rolling stock in an industrial rolling process and the rolling stock’s chemical composition. The analysis of the spatial influence was performed with CAE neural networks, and included an analysis of the influence of carbon and carbide-forming elements, of manganese and sulphur, copper, tin, aluminium, etc. The results of the analyses revealed a new understanding of the influences, and thus also the possibility to reduce the amount of surface cracking if the chemical concentrations of the elements were to be closer to the exactly determined values, or closer to the more exactly determined ratios.
Formation of a sufficiently large database on tools for hot forging, which is necessary for successful prediction of wear at a given number of strokes, as well as for the prediction of the critical ...number of strokes when the acceptable tolerance of a forging is exceeded, is a relatively time-consuming process in the production practice. To overcome this problem, this article presents a starting point for quicker prediction of these quantities by means of conditional average estimator neural networks (CAE NN), namely by the so-called integral method and by the partial method. A comparison of the efficiency in prediction of these methods was carried out on the results of wear obtained in laboratory forging, which allowed a gradual and relatively quick tracing of wear contour progression on tools and thus the formation of a reliable database. The results presented show that in the case of a relatively small database, where, for instance, there are known data and wear parameters on at least three different tool steels, or, on differently heat treated steels, it is possible to effectively predict the wear of a fourth tool simply on the basis of the slightly perceivable wear profile of the tools. Here, the integral method gives better predictions. This conclusion is of great importance in practice: from intermediate control of gradual tool wear, we can predict its tool life.
A non-parametric model was proposed for modelling the influence of different technological and chemical parameters on the mechanical properties of the 6082 aluminium alloys during the hot extrusion ...process with a special consideration of human factor. It was shown that human factor (influence of process engineers) was important and that it could be efficiently modelled and taken into account by the proposed Conditional Average Estimator (CAE) method. Production might be improved (optimized) by a proper education and/or by eliminating critical process engineers. It was found that the highest values for elongation and yield stress did not coincide with the range of the most frequent combinations of input parameters.
In this study progress of the wear on the bearing surfaces of nitrided dies for aluminium (AA6063) hot extrusion is demonstrated and explained. Two groups of nitrided AISI H11 tool steel dies were ...analysed, with relatively short and long bearing surfaces. It was found that bearing surface length in relation to the microstructure of the nitrided exit edge should be considered especially in the case of relatively short bearing surface. Cracking and adhesive removal of die material were identified as the main wear mechanisms leading to the formation of craters, both deep and shallow furrows and removal of the exit edge of the bearing surface. On the exit edges with relatively short bearing surface, initiation of wear on the two characteristic spots (distances) from exit edge was observed while on the relatively long bearing surface, wear observed was mainly in the central area of the bearing surface. Formation of deep furrows originates from previously formed craters which then expand in the longitudinal direction. On the other hand formation of shallow furrows is based on the high-density of adhesively removed islets on the nitrided layer that leads to its removal in longitudinal direction as a consequence of its decreased shear strength. Oxygen and aluminium ingress was detected on the grain boundaries far beyond the crack tip, which accelerated grain boundary decohesion.
•Nitrided extrusion dies with relatively short and long bearing surfaces were analysed.•Relatively shorter bearing surface die exhibited two different pathways to failure.•One degradation pathway was found on die with longer bearing surface.•Formation of deep furrows originating from craters was observed.•High density of adhesively removed islets lowers shear strength of nitrided layer.
HOT FORMING OF AISI A2 TOOL STEEL Pirtovsek, T Vecko; Perus, I; Kugler, G ...
Metalurgija,
10/2008, Letnik:
47, Številka:
4
Journal Article
Recenzirano
Odprti dostop
For further increase of economy of production of AISI A2 tool steel a study of possibility of expanding the hot working range and better prediction of flow stress has been carried out. By employing ...hot compression tests it was proved, that initial microstructures have influence on the lower limit and chemical composition on upper limit of hot working range. A CAE Neural Networks was applied to predict the flow stresses for intermediate values of strain rates and temperatures. For optimization purposes the activation energies and constants of the hyperbolic sine function for two temperatures ranges (850-1000 deg C and 1000-1150 deg C) were calculated.
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