1987

To decrease the treatment time and to obtain better properties of the layers, the authors studied a cycle consisting in a pre-oxidation with sulfur dioxide, effected with the purpose of reducing the interconnected porosity, and a nitrocarburizing stage. Steel samples, containing little carbon (0. 01 wt pct) and copper (2 wt pct), with three different density ranges (6. 3 multiplied by (times) 10**3, 6. 7 multiplied by (times) 10**3 and 7. 2 multiplied by (times) 10**3 kg/m**3), were treated in a laboratory furnace with a small flow of SO//2 (1.

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.

Analysis of the process used to hydrate orthorombic anhydrite (CaSO4-II) obtained by the heat treatment of gypsum has shown that its reactivity is closely related to its specific surface. If, however, the BET surface is greater than 6-7 m2/g, reactivity is also dependent on the degree of microstructural disorder. These two parameters in turn are closely dependent on the thermal treatment for synthesis.

In order to clarify the influence of grinding on anhydrite reactivity to water, the authors have ground natural and synthetic products in different ways and investigated the evolution of microstructure and the rate of the conversion: CaSO4 II → CaSO4·2H2O, as a function of grinding. Different parameters have been used to characterize the anhydrite microstructure; nevertheless, only the specific surface BET and very short-range disorder, defined by the 'disorder coefficient' (Cd), can affect the product reactivity.

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.

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.

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.