PetroMat

The investment casting process, extensively used for the production of jewels, is made up of different phases that, for their intrinsic typology, are deeply influenced by many chemical, physical as well as metallurgical factors. Even though investment casting has been used for decades, it is absolutely not properly optimised, also in consequence of the difficulty in deeply understanding, and therefore controlling, all the different phases of the process. It is therefore evident that different, and unfortunately many, kind of defects may appear in precious art-crafts.

A basic and a modified AISI H11 steels were selected for the research. Impact toughness and microstructure in the quenched and tempered state were analysed. Cyclic immersions in molten aluminium with the aim of simulating the washout, typical of aluminium die casting tools, were applied. Optical and electronic microscopy analysis were used to evaluate damages provided by washout on the specimen surface. XRD measurements were applied to identify the corrosion and soldering products deposited on the steel surface.

In the course of a study whose aim was to obtain the optimum mechanical characteristics of chromium white iron, we decided to make a preliminary evaluation of how additions of boron and chromium affected the hardness and morphology of cementite. The samples were obtained from pure elements by melting in an arc furnace under argon atmosphere. The alloys were submitted to microdurometric, micrographic and diffractometric analysis. The composition of the individual crystals was defined with the use of an energy dispersive spectrometer.

Synthetic ferrous alloys containing chromium and/or nickel were prepared and borided at 1173 K with powders containing B4C, KBF4 and SiC for times varying from 20 to 60 h. The surface layers composed of borides of type (Fe, M)B and (Fe, M)2B were characterized by means of X-ray diffraction, microscopic observations, analysis with the microprobe and microhardness measurements. A quantitative study was carried out on the differentiated distribution of chromium and nickel in the phases constituting the borided layer.

In the present work the possibility to obtain large size castings of compacted graphite cast iron by inoculating ipereutectic pig iron melts with Fe-Si-Ca-Mg-Ti alloys with different calcium contents was first studied. Two alloys have been used: the first contained 4 divided by 5, 5 wt % Ca and the second less than 1 wt % of the element. The amount of inoculant alloy to be added to the melts was calculated so that the content of Mg in the casting agreed with the Sofroni equation. Casting temperatures were in the 1350-1400 degree C range.

The crystallographic characteristics and the equilibrium ratios for solid solutions containing 33 and 50 at% boron which belong to the Fe-Ni-B system are described. The study was carried out on samples obtained by diffusion at 1073 K, of mixtures of the elements in powder form. It was possible to confirm the existence of a complete solid solution between the compounds Fe2B and Ni2B.

A study has been made of the preferential distribution between the phases (M', M″)2B and (M M″)B present in the two-phase fields of the M'-M″-B systems for the transition metals with atomic numbers between 24 and 28. It has been ascertained that the metal with the smaller atomic number is systematically concentrated in the phase richer in boron.

Surface layers having elevated hardness were produced by ion nitriding of titanium and α-β alloys of Ti-6wt.%Al-4wt.%V, Ti-4wt.%Al-2wt.%Sn4wt.%Mo and Ti-4wt.%Al-2wt.%Mn. The treatment was conducted at temperatures between 1073 and 1273 K for times of 4-32 h, using gaseous mixtures of nitrogen and hydrogen containing 20-80 vol.% nitrogen. X-ray diffraction, optical microscopy and microhardness were used to characterize the hardened surfaces.