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High Temperature Materials

Thermo-mechanical and oxidation behaviour of high temperature advanced metallic alloys

The main focus of this paper is on materials for radiant burners application. Two advanced metallic alloys, a Ni and Fe-based alloy are studied and compared to a reference ferritic stainless steel. Oxidation kinetics of such alloys at different temperatures are reported. Oxide formation mechanisms are discussed. Furthermore, thermo-mechanical resistance and eventual strengthening mechanism in temperature are studied. Finally, technical and brief tentative economical analysis of different alloys as potential candidates for the fabrication of radiant burners are given.

Influence of heat treatment and surface engineering on thermal fatigue behaviour of tool steel

Thermal fatigue (TF) is a common problem in many tool steel components. It is caused by thermal cycling in presence of internal constraints. The resulting thermomechanical stresses induce thermal cracking (heat checking). A laboratory test was developed to reproduce TF damage on a laboratory scale, under oxidizing conditions. Two different test configurations were used to induce unidirectional and bidirectional cracking. Nitriding impairs TF resistance of plain steel due to the easier propagation of cracks through the diffusion layer.

Initial Oxidation Behavior in Air of TiAl-2Nb and TiAl-8Nb Alloys Produced by Electron Beam Melting

Titanium aluminide alloys are good candidates for structural applications thanks to their low density and good balance of properties up to relatively high temperatures. However, their application is still limited by significant oxidation. Four γ-TiAl alloys with different content of aluminum and niobium were produced by electron beam melting: Ti-45Al-2Cr-2Nb, Ti-48Al-2Cr-2Nb, Ti-45Al-2Cr-8Nb, and Ti-46Al-2Cr-8Nb.

The effect of carbon, chromium and nickel on the hardness of borided layers

The variation in hardness of the phases (Fe, M)B and (Fe, M)2B (M ≡ Cr or Ni), which are the predominant components of the borided layer obtained on iron alloys, was defined and related to increase in chromium, nickel and carbon contents. It was found that chromium increases the hardness both of the borided layer as a whole and of the boride components, even though these values are systematically lower than those measured on pure borides.

Boronizing of Sintered Ferrous Materials

The chemical composition of sintered steels has been related to the physical and mechanical characteristics of the surface layers obtained by solid boronizing. Sintered samples of various composition and density were produced as bushings by mixing Hoganas powders and subsequent heating in industrial furnaces. Various conditions of temperature, time of treatment and chemical composition of the boriding agent were investigated. All the borided layers consisted of Fe//2B and FeB type borides.

Boriding of Steels. Some Notes on the Boron Diffusion Mechanism

The aim of this work was to ascertain if the alloying elements in steels influence the diffusion mechanism of the boron during boriding treatment and to relate the characteristics of the borided layer with the chemical composition of the matrix. Steels, Armco iron and some ferrous alloys containing Cr and/or Ni were borided for 7-24 hours at 1173-1223K using powders of composition B//4C equals 20 30%, KBF//4 equals 5%, SiC equals 65 75%. The borided samples were analysed by an X-ray diffractometer, electron microscope, EDS spectrometer, optical microscope and Vickers microdurometer.

Surface Hardening of Titanium Alloys: Chemical Characterisation of the Nitrided Layers

The investigations were made on alloys OT-4 (Ti-3Al-1. 5 Mn), IMI318 (Ti-6Al-4V) and IMI550 (Ti-4Al-4Mo-2Sn-0. 5Si) of the alpha-beta type. The pressure was fixed at 10 torr and the temperature was made to vary between 800 and 1,000 degree C. A gaseous mixture was chosen, composed of 60 nitrogen and 40 hydrogen vol% and times of 4, 8 and 16 hours were selected for treatment with an ionic discharge. Diffractometric analysis was used for identification of the type of phases present on the hardened surface. It was found that the alloying elements modify the morphology of the surface layer.

Texture of surface layers obtained by ion nitriding of titanium alloys

Titanium metal and some of its alloys of the type Ti-Al-Me (Me=transition metal) were submitted to plasma nitriding. The resulting compound layer was examined by optical microscopy and by XR diffraction in order to establish both its microstructure and texture degree under different nitriding conditions. To this aim a texture evaluation method was employed, which was formerly devised and applied on surface layers obtained by boriding of steels. In these nitrided layers the Ti2N nitride was clearly textured while the TiN did not show an appreciable degree of preferred orientation of crystals.