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Hot-work tool steels are intended for the fabrication of tools used for the shaping and forming of metals and other materials, such as die casting dies, drop hammer dies, tools for pressing, extrusion, rolling, deep drawing, and moulds used in glass industry.
In service, these tools are heated to temperatures above oC because the tool surface and the material being formed are in contact. Besides being subjected to high temperatures, these tools are also subjected to mechanical loads and there are tribological issues and chemical actions involved in the process.
Therefore, hot-work tool steels must have a wide range of specific properties. Hard coatings are generally used for tribological protection of cutting tools in order to improve their properties and thus to prolong their life and to increase productivity.
Advantages of the application of hard coatings that are well known in the field of cutting tools have been studied less extensively in the field of tools for shaping and forming. This fact can be accounted for by a complex geometry and great weight of the tool together with a lower hardness value of the tool base material, which have a negative effect alati the coating-substrate alahni, and thus on the surface layer properties.
In this doctoral thesis, new surface layers have been developed by using plasma nitriding and plasma assisted chemical vapour deposition PACVD with the aim of improving the properties of hot-work tool steels and of prolonging the tool life. The theoretical part of the thesis gives a detailed description of wear mechanisms in tools made of hot-work tool steels together with key requirements related to their properties.
In addition, an overview of modification and coating processes is given regarding their application to the hot-work tool steels.
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In the experimental part of the thesis, the celii achieved in the newly-developed surface layers are analysed in terms of layer characterisation and expected properties of hotwork tool steels, with special focus on resistance to thermal fatigue as the main cause of failure in hot-work tool steels. The developed surface layers on the X38CrMoV hot-work tool steel were tested and results were compared with the results of conventional heat treatment processes of hardening and tempering commonly used in practice.
Detailed characterisation of the surface layers was carried out by the analysis of their chemical composition using Glow Discharge Optical Emission Spectrometry — GDOES and Energy Dispersive Spectrometry — EDS and microstructure using a light microscope and a scanning electron microscopeby determining their crystal structure using X-Ray Diffraction — XRD and surface roughness, by testing the adhesion of coatings using the scratch test method and the Rockwell hardness test method – Rockwell C indenter and the thickness of layers alahni the ball indentation test and by testing the micro hardness of nitrided layers and PACVD coatings.
In order to quantify the effect of newly-developed surface layers on the properties and durability of hot-work tool steels, the following properties were tested: Resistance to thermal fatigue was tested using a device designed and manufactured by the doctoral candidate.
In addition, the coefficients of friction and sliding wear were determined using the ball on flat sliding test.
A die for the casting of aluminium alloy radiators was tested in real conditions in order to confirm the results of laboratory testing of the newly-developed surface layers and of their influence on the durability of the die. The analysis of results showed that the properties of the tool steel were significantly improved and the tool life prolonged by the newly developed surface layers, which was confirmed in real conditions.
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