David Bombac

Study of carbide evolution during thermo-mechanical processing of AISI D2 tool steel

The microstructure of a cold-worked tool steel (AISI D2) with various thermo-mechanical treatments was examined in the current study to identify the effects of these treatments on phases. X-ray diffraction was used to identify phases. Microstructural changes such as spheroidization and coarsening of carbides were studied. Thermodynamic calculations were used to verify the results of the differential thermal analysis. It was found that soaking temperature and time have a large influence on dissolution, precipitation, spheroidization, and coalescence of carbides present in the steel. This consequently influences the hot workability and final properties.

Response to Hot Deformation Conditions and Microstructure Development of Nimonic 80A Superalloy

Laboratory compression tests at different temperatures and strain rates have been performed on Nimonic 80A superalloy to define optimal hot forming characteristics. A mathematical expression connecting mean grain size and true stress is presented using a Hall–Petch-like equation. The evolution of microstructure at various sample positions in correlation with deformation temperatures, strain, and strain rates has also been investigated. Optimal hot-working conditions are determined using processing maps and obtained microstucture.

An integrated hot rolling and microstructure model for dual-phase steels

An integrated tool is presented for the estimation of the microstructure of steels that consist of a mixture of ferrite and martensite following hot deformation processing. The model is based on phase transformation theory and shows good agreement with experimental data.

Influence of diffusion asymmetry on kinetic pathways in binary Fe-Cu alloy: a kinetic Monte Carlo study

Kinetic Monte Carlo simulations with model asymmetry in binary Fe-Cu alloy leading to the same microstructure are presented. A method based on thermodynamic data for calculation of interatomic potentials dependent on model asymmetry is presented and evaluated. Results show that kinetic pathways are sensitive to model asymmetry and are compared to the classical growth and coarsening theories. Experimental diffusion data are used and compared to simulation results to determine a realistic combination for simulations.

Theoretical evaluation of the role of crystal defects on local equilibrium and effective diffusivity of hydrogen in iron

ABSTRACT Hydrogen diffusion and trapping in ferrite is evaluated by quantum mechanically informed kinetic Monte Carlo simulations in defective microstructures. We find that the lattice diffusivity is attenuated by two to four orders of magnitude due to the presence of dislocations. We also find that pipe diffusivity is vanishingly small along screw dislocations and demonstrate that dislocations do not provide fast diffusion pathways for hydrogen as is sometimes supposed. We make contact between our simulations and the predictions of Orianis theory of ‘effective diffusivity’, and find that local equilibrium is maintained between lattice and trap sites. We also find that the predicted effective diffusivity is in agreement with our simulated results in cases where the distribution of traps is spatially homogeneous; in the trapping of hydrogen by dislocations where this condition is not met, the Oriani effective diffusivity is in agreement with the simulations to within a factor of two. This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Thermal Fatigue Behavior of High Cr Roller Steel

In this work thermal fatigue resistance of 1.7C, 11.2Cr, 2.0Ni, 1.2Mo steel for hot working rolls was studied using our newly developed test rig with specially prepared test samples. Tests were carried out in temperature range between 500–700 \({^\circ }\)C whereas relevant characteristics related to cracks after 200, 500, 1000, and 2500 cycles were obtained. Average length of all cracks, their density, average length of five longest cracks, and relevant microstructural characteristics of tested specimens were determined. It was found that initiation of cracks is strongly related to the cracking and spalling of carbides at specimens surface layer and that cracks growth is related to the characteristics of carbides. For comparison also results for Indefinite Chilled Double Poured roll cast iron are given. Based on obtained results, possible improvements of thermal fatigue resistance of these two materials are discussed.

Amelioration of surface cracking during hot rolling of AISI D2 tool steel

ABSTRACT Reported is a relationship between a profile edge cracking during hot rolling of AISI D2 tool steel and material and processing parameters. Several months of observation of industrial hot rolling was done for neural network analysis and complemented with equilibrium thermodynamics calculations and laboratory hot deformation tests. Industrial results, in general, show that for the same chemical composition, hot rolling yield decreases with an increased profile aspect ratio. Cr content is significant for the soaking and strongly correlated with a hot workability at upper and lower limits of the hot working temperature range. Laboratory hot compression tests were employed to determine the optimal soaking temperature and to study hot workability to expand safe hot working temperature window.

INTERNAL FIXATION OF RADIAL HEAD FRACTURES WITH AN INTRAMEDULLARY NAIL

Comminuted fractures of the radial head still present significant technical and surgical challenges. In this article, we describe a novel fixation of comminuted radial head fractures with the help of an intramedullary nail. Experiments with solid, conventionally machined intramedullary nails showed some major drawbacks in the fixation of radial head fractures. Several design and manufacturing procedures were proposed. The general idea behind the new design was the concept of a nail which would eliminate the need for prefabricated bores. Experiments with a selective laser sintered thin-walled nail, designed with the help of CT images, fulfilled expectations. This thin-walled proximal radius nail thus offers a stable fixation of the radial head fracture fragments, with the ability to preserve the existing vascular supply to the radial head fragments, and therefore not just use the reconstructed radial head as a bioprosthesis.