G. F. Bocchini

Bio: G. F. Bocchini is an academic researcher. The author has contributed to research in topics: Indentation hardness & Powder metallurgy. The author has an hindex of 5, co-authored 23 publications receiving 115 citations.

Microstructural and mechanical characterisation of some sinter hardening alloys and comparisons with heat treated PM steels

Abstract: Four grades of sinter hardening materials have been compared, using industrial equipment. Three powder types were completely prealloyed; the last one was a hybrid, combining prealloying and diffusion bonding. Different amounts of Cu have been added by mixing. The lubricated mixes, containing 0.6% graphite, have been compacted at different pressures, to form gears at green densities ≥7.0 g cm−3 ; the compacts have been sintered at 1120°C, under endogas from methane and fast cooled (at least 7 K s−1 within the range 850–400°C). The final step has been stress relieving, at 180°C, for 1 h. Material properties have been investigated, focusing on porosity, pore shape, hardness, microhardness, microstructure, local chemical composition and mechanical properties. For comparison, other gears, compacted in the same tool and at the same density level, but manufactured according to a more conventional cycle, i.e. starting from less alloyed powders and adding carbonitriding, quenching and stress relieving, ha...

Cooling rates of P/M steels

Abstract: The cooling curves of Jominy specimens made from a powder metallurgy steel (0.65 w/o C) based on a hybrid powder (4 w/o Ni, 1.4 w/o Mo, 2.0 w/o Cu) have been obtained using still air or high-speed water as the cooling fluid. It has been observed that cooling rate increases as the density (6.8 g/cm 3 and 7.0 g/cm 3 ) decreases. This result agrees with recent literature data. Analysis of variables affecting cooling rate shows that thermal diffusivity decreases as the density decreases, while the amount of heat transferred to the cooling fluid, depends on the heat-transfer coefficient and is a function of material density and effective extension of the exchange surface, considering porosity. The influence of porosity on the cooling rate inside a porous body is discussed, and made explicit by a simple formula involving only the porosity. It is demonstrated that the effect of density on cooling rate is reversed with the hypothesis of an infinite heat-transfer coefficient.

Sinter hardening of low-alloy steels: influence of part geometry and physical properties of the material

Abstract: Assuming that the cooling rate of parts depends on thermal conductivity of the material and on the surface/volume ratio, a two-ways study has been carried out. Firstly, the isothermal surfaces have been calculated by a numerical method on parallelepipeds of constant cross section and varying height and weight. Microstructures, depending on isothermal surfaces predicted by the numerical model, agree with microhardness and mechanical properties. A maximum weight of about 200 grams has been observed to be apt to a full sinter-hardening process and the surface/volume ratio seems to be the critical driving factor. Numerical analysis and experimental tests show that the thermal diffusivity of sintered steels increases as porosity increases.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

On the kinetics of plasma nitriding a martensitic stainless steel type AISI 420

Abstract: Plasma nitriding was employed to treat martensitic stainless steel type AISI 420. The ability to remove the passive film from the surface is an important advantage in this process in order to guarantee a homogeneous surface treatment. The resulting nitrided surface shows the presence of a compound layer and a diffusion zone. The interface between the diffusion zone and the substrate is flat, as a consequence of the high chromium content on the alloy. The precipitation of chromium nitrides is also responsible for the high levels of hardening obtained. A value up to 1350 HV 0.025 constituting a maximum hardness was obtained. Diffusion depth increases with an increase of the nitriding temperature as measured by optical microscopy and verified by the hardness profiles. Calculated diffusion coefficients of nitrogen were very low compared to the data for alpha iron, and allow calculation of the Arrhenius behaviour during the process.

Glow-discharge nitriding of sintered stainless steels

Abstract: The glow-discharge nitriding process is particularly suitable to harden the surface of sintered stainless steels components, owing to the high porosity levels of these materials. On wrought austenitic stainless steels this treatment produces a metastable phase, known as supersaturated austenite or S phase, which has shown high hardness values and good corrosion resistance. In the present paper the influence of glow-discharge nitriding process on the microstructural and mechanical properties of AISI 316L austenitic sintered stainless steel has been evaluated and it is compared with the results obtained with ion-nitrided martensitic (AISI 410) and ferritic (AISI 430L) sintered stainless steels. The ion-nitriding treatment, performed at 773 K for 4 and 8 h, produces modified surface layers. The microhardness profiles show high hardness values in the modified layers and a steep decrease to matrix values, thinner hardened layers and lower hardness values are observed on AISI 316L samples, in comparison with AISI 410 and AISI 430L samples. The S phase is detected on the modified layers of the ion-nitrided AISI 316L samples. The crystallographic characterisation has shown that a face centred tetragonal lattice as base for this phase fits well the diffraction spectra, in respect of the ‘traditional’ face centred cubic lattice usually adopted, since the lattice shows a strong distortion, in spite of this, the d -spacing values calculated with a f.c.t. base show a good agreement with literature data, when the used f.c.c. indexing is modified for the f.c.t. lattice.