Papers by Keyword: Microstructure Formation

Abstract: AlN particles can be synthesized by blowing Nitrogen gas into liquid metal. At high temperature Nitrogen molecules decompose to atoms and react with Al to create AlN. EDX diffraction shows that weight ratio of the particles fully coincides with the ratio of standard atomic mass between Al and N (about 2), confirming that the particles are AlN. The SEM images show that AlN particles size varies from several hundred nanometers to some micrometers, depending on the contact time between gas bubbles and liquid metal. A simple model shows that the growth rate of the AlN particles (0.148 nm/s) at the early stage is only a half of that at finishing stage (0.305 nm/s), this may be due to the difficulty of the nucleation at the early stage. The formation rate of AlN was calculated based on the X-ray diffraction and shows the same tendency, as the growth rate.

Abstract: The aim of this study is the characterization of hot-dip galvanized S355K2+N steel plates used as components for engineering civil structures. Two thin zinc coatings with a thickness of 145+/-14 μm and 329+/-23 μm, were developed at the surface of the plates. Several experimental techniques were performed to study the microstructure, the chemical composition at the surface of the galvanized plates. The residual stress field was also evaluated in the coatings and the top surface of the plates using the incremental hole drilling method, in the rolling and the transverse directions. The results show the presence of tensile stresses and compressive stresses respectively in the coating and the substrate.

Abstract: Microstructure of the powder metallurgy (PM) steels and especially mechanism of its formation differs significantly from the microstructure of the conventional steels even if composition will be exactly the same. The difference is not only connected to the presence of the pores, which are inalienable feature of the PM parts. Presence of the prior inter-particle boundaries, which can be contaminated by residual oxides, as well as microstructure heterogeneity are another characteristic features of the microstructure of PM steels. Microstructure heterogeneity is connected to the PM manufacturing process: powder mix, consisting of the base powder and additional alloying elements is compacted and then sintered. Fully prealloyed powder is not always possible to use in standard press & sintering route due to the solid solution strengthening of the ferrite resulting in bad powder compressibility. Hence, in order to provide good powder compressibility only pure iron or low-alloyed (typically <3 wt.%) powders are used. Required alloying elements and carbon (added as graphite) are further admixed in the powder form and are distributed during sintering by diffusion into iron particles at high temperatures. To assure satisfactory distribution of alloying elements, oxide layer, covering surface of the powder particles and hindering mass-transfer of the alloying elements, has to be removed first. This can be done by gaseous reducing agents as hydrogen and carbon monoxide. However, their cost and/or purity are of issue for modern alloyed PM steels. Admixed carbon, additionally to its function as alloying element, plays a role of effective reducing agent at higher temperatures. Paper summarizes the main features of microstructure formation during the whole sintering cycle (heating and isothermal sintering) and effect of alloying additives (different carbon sources, alloying elements) and processing parameters (sintering atmosphere composition, temperature profile) on the microstructure formation during conventional sintering process. Results indicate that for successful sintering of alloyed PM steels with homogeneous defect-free microstructure, hydrogen-rich atmospheres and high-temperature sintering are required.

Abstract: The influence of alloying elements on the chill formation in Compacted Graphite Iron (CGI) is investigated. Chill wedges cast in an industrial foundry were used to investigate the chill formation. A total number of 19 chemical compositions were studied, including three trials of varying nodularity treatment level; four trials of varying copper content; four trials of varying silicon content; four trials of varying tin content and four trials of varying carbide promoter content. Three wedges were cast for each alloy composition, of which one was used for measuring the temperature at three different heights in the wedge. Contrary to some previous reports, the results indicate that low-nodularity CGI is not more prone to chill formation (columnar white) than high-nodularity CGI. Trends regarding the effect of alloying elements on chill formation are shown to generally be in agreement with previous work on spheroidal graphite iron and lamellar graphite iron. Most of the samples also show carbide formation in centre line areas of the wedge (inverse chill), this occurrence is also discussed in the paper.

Abstract: A Cellular-Automaton Finite-Volume-Method (CAFVM) algorithm has been developed, coupling with macroscopic model for heat transfer calculation and microscopic models for nucleation and growth. The solution equations have been solved to determine the timedependent constitutional undercooling and interface retardation during solidification. The constitutional undercooling is then coupled into the CAFVM algorithm to investigate both the effects of thermal and constitutional undercooling on columnar growth and crystal selection in the columnar zone, and formation of equiaxed crystals in the bulk liquid. The model cannot only simulate microstructures of alloys but also investigates nucleation mechanisms and growth kinetics of alloys solidified with various solute concentrations and solidification morphologies.