B. Campillo

Effect of the Presence of SiCp on Dendritic Coherency of Al–Si-Based Alloys During Solidification

In order to explore the effect of the presence of SiC particles on dendritic coherency during solidification of Al–Si based metallic alloys, a factorial fractional two levels experimental design was implemented to allow an identification of the main effects of the particle content, silicon content, grain refinement, and cooling rate on the solid fraction at coherency. This solidification parameter was determined for Al-3wt%Si and Al-7%Si alloys, and Al-3%Si/SiCp and Al-7%Si/SiCp metal matrix composites. The cooling process during solidification was monitored by performing cooling curve measurements at two radial locations within samples poured into cylindrical molds at two cooling rates. The experimental cooling curves were numerically processed by the Fourier thermal analysis method to know the evolution of solid fraction as a function of time. The effect of grain size was included using samples with or without grain size refinement. The grain refinement was obtained by adding predetermined quantities of TiAlB master alloy. It was found that presence of SiC particles affects the coherency point of the metal matrix composites increasing the solid fraction at coherency. However this effect is relatively small when compared to the effect of grain refinement, cooling rate, and Si content on dendritic coherency of experimental probes.

On the local microstructural characteristics observed in sand cast Al–Si alloys

Abstract The aim of this work is to explore the phenomenology that describes the local solid formation and heat transfer occurring during sand cast alloys solidification in order to propose an explanation to the observed changes of local microstructural characteristic lengths in hypoeutectic and eutectic Al–Si based alloys. Microstructural observations are made in different radial positions of solidified rod castings. Also, solidification kinetics information is obtained using the Fourier thermal analysis method. A coupled heat transfer-solidification kinetics model is employed to predict the thermal history, the solidification kinetics and some microstructural parameters in order to compare the predictions with experimental results. The model and the experimental outcome suggest that there is a strong dependence of the local solidification kinetics on the local heat transfer. The analysis of this dependence is used to propose an explanation to the observed changes in microstructural characteristics at different locations within sand castings.

Effect of initial microstructure on austenite formation kinetics in high-strength experimental microalloyed steels

Austenite formation kinetics in two high-strength experimental microalloyed steels with different initial microstructures comprising bainite–martensite and ferrite–martensite/austenite microconstituents was studied during continuous heating by dilatometric analysis. Austenite formation occurred in two steps: (1) carbide dissolution and precipitation and (2) transformation of residual ferrite to austenite. Dilatometric analysis was used to determine the critical temperatures of austenite formation and continuous heating transformation diagrams for heating rates ranging from 0.03°C•s−1 to 0.67°C•s−1. The austenite volume fraction was fitted using the Johnson–Mehl–Avrami–Kolmogorov equation to determine the kinetic parameters k and n as functions of the heating rate. Both n and k parameters increased with increasing heating rate, which suggests an increase in the nucleation and growth rates of austenite. The activation energy of austenite formation was determined by the Kissinger method. Two activation energies were associated with each of the two austenite formation steps. In the first step, the austenite growth rate was controlled by carbon diffusion from carbide dissolution and precipitation; in the second step, it was controlled by the dissolution of residual ferrite to austenite.

Characterization of a solid solution of Cu in Al-Cu-SiCP metal matrix composites processed by spray atomization and co-deposition

The effect of silicon carbide particulate (SiCp) reinforcement on the formation of a solid solution of copper (Cu) in Al–Cu alloys during spray atomization and co-deposition is investigated. The extent of Cu solid solubility in sample compositions of Al–1.3, 5.9 and 18.3 wt% Cu and Al–1.5, 5.9 and 19.4 wt% Cu+6 vol% SiCp was characterized using X-ray diffraction scanning electron microscopy, (SEM) microanalysis and high resolution electron microscope techniques. The copper content retained in the α-Al solid solution in Al-alloys both with and without SiCp additions was determined by initially deriving the lattice parameter (a) values of the samples by X-ray diffraction and the copper content in the solid solution was determined using a plot of a versus copper content previously, reported in the literature. Results of SEM microanalyses performed on the above alloys in regions of α-Al solid solution showed a good agreement on the amount of Cu retained in solid solution with values determined by X-ray diffraction especially for alloys containing small amounts of Cu. High resolution electron microscopy images of the matrix and the matrix/SiCp interface were employed in order to determine values of the interplanar spacing (d) for the α-Al solid solution and to correlate these values using the plot of lattice parameter as a function of copper content retained in solid solution. The results were in good agreement with those determined by the scanning electron microscopy microanalyses.

Crack growth in microalloyed pipeline steels for sour gas transport

Different cracking modes in a sour gas environment were observed. These modes were mainly related to the microstructure obtained during the manufacturing process of two API X52 microalloyed steels. A banded ferrite/pearlite microstructure was found to be susceptible to hydrogen effects, whereas an acicular ferrite with a grain boundary bainite/bainite microstructure was found to be more susceptible to dissolution in crack-tip regions.

Precipitation of niobium nitrides in a high nitrogen HSLA steel

Analyse de la precipitation des nitrures de niobium cuboidaux ou des carbonitrures lors du moulage des billettes et lors de la fabrication de barres par laminage. Determination de la distribution de la taille des particules. Une fraction de Nb reste dans la solution solide des billettes car de petits precipites coherents sont observes dans les grains de ferrite dans les barres laminees. Ces precipites ont une structure hexagonale

Mechanical characterization of the welding of two experimental HSLA steels by microhardness and nanoindentation tests

The microhardness and nanohardness of the welding zone of two experimental HSLA steels were determined. The first steel has a microstructure of martensite and bainite, and the second one has a microstructure of quasipolygonal ferrite and acicular ferrite. In the bainitic - martensitic steel, softening of the heat affected zone was observed. This softening can be attributed to: the formation of polygonal ferrite in the recrystallization subzone, the formation of quasi-polygonal ferrite and the tempering of martensite in the intercritical subzone, and the tempering of martensite in the subcritical subzone. Besides the softening, with nanoindentation technique, hardening was observed at the position where the peak temperature reached the critical temperature Ac1, which can be attributed to a phenomenon of secondary hardening by precipitation of carbides of alloying elements. In the ferritic steel, a softening phenomenon did not appear since there was no martensite in its initial microstructure. Finally, it was noted that both polygonal ferrite and the bainite have similar behavior and nanohardness, this coincidence can be attributed to the effect of grain boundary.

Study of aging and embrittlement of microalloyed steel bars

The aging of hooks, anchors, and other bent reinforcing steel bars in concrete structures are considered in modern international standards. Rebend test procedures have been designed in order to predict the aging embrittlement susceptibility by submerging bent reinforcing bar specimens in boiling water. Subsequently the bars are rebent or straightened in order to determine the loss of ductility or embrittlement of the aged material. The present work considers the influence of carbon, sulfur, and niobium on the performance of reinforcing bars in rebend tests of 300 heats of microalloyed steel bars with a variety of compositions. The microstructural evidence and the statistical results clearly indicate the strong influence of carbon and sulfur on rebend failure, while niobium-rich precipitates contribute to the hardening of the ferrite grains during aging.

Dissolution and coarsening of large niobium carbonitrides in a microalloyed steel

Observations and measurements of the kinetics of coarsening and dissolution of large cuboidal niobium carbonitrides during solution treatments of a high nitrogen niobium microalloyed steel are reported. At temperatures between 1473 and 1573 K a competitive coarsening and dissolution process was established where the larger niobium carbonitrides grew at the expense of the smaller, or employing niobium and nitrogen which remained in solid solution. In this temperature range growth or dissolution rates and critical sizes could be determined from the analysis of the evolution of particle size distribution. At higher temperatures (1623–1723 K), only a dissolution process existed, where the dissolution rates as a function of particle size was found to increase with increasing temperature.

A study of the effect of nitrogen in two niobium microalloyed steels

Effet de la teneur en azote sur la microstructure et les caracteristiques mecaniques de deux aciers microallies au niobium