Cast AerMet100 exhibits mechanical properties comparable to the wrought properties of competing ultrahigh-strength steels; however, the segregation behavior had not been quantified under casting conditions. A microsegregation profile of the as-cast ingot was simulated and homogenization treatments were predicted using Thermo-Calc and DICTRA software. Experimental composition analysis confirmed the simulated microsegregation profile, and the homogenization process occurred at a slightly faster rate than predicted. The convergence of theory and experiment demonstrates the feasibility of designing cast alloy compositions assisted by computer simulation to minimize segregation without casting a large experimental matrix of test ingots.
Precipitation of carbonitrides has been studied in as-cast slabs of one Nb and
one Nb and Ti containing HSLA steel. The precipitates have been quantified using LOM
and TEM. The measured size and number distributions was then compared to model
calculations of precipitate nucleation and growth using estimates of the cooling rates in
the austenitic range (1490oC to 800oC) during casting. Both average size and number
distributions could be modelled with good agreement using identical model parameters
(except for individual diffusion coefficients for the participating species).
The model is based on classic nucleation rate theory and a quasistationary
approximation for growth of spherical particles. Local equilibrium is assumed at the
phase boundary.
Carbide precipitation in martensitic low alloyed steels contributes to the mechanical properties through precipitation hardening. A high number density of carbides is desired to maximize the hardening effect, which is achieved through the precipitation of carbides on the dislocations in the martensitic structure. In this study, the nucleation, growth, and coarsening of vanadium and molybdenum carbides during aging at 600 °C for periods up to four weeks were investigated. The work covers characterization with atom probe tomography, which showed that the nucleation of V and Mo rich MC/M2C carbides takes place on dislocations. The growth of these carbides proceeds by the diffusion of elements to the dislocations, which has been modeled using Dictra software, confirming the rate of the reaction as well as the depletion of carbide formers in the matrix. For longer aging times, particle coarsening will decrease the number density of particles with a transition from dislocation-based carbides to separate rounded carbides.
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