Héctor Javier Vergara-Hernández

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.

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.

Kinetic study of austenite formation during continuous heating of unalloyed ductile iron

The austenite formation kinetics in unalloyed cast ductile iron was studied on the basis of dilatometry measurements, and Avrami’s equation was used to estimate the material’s kinetic parameters. A continuous heating transformation diagram was constructed using heating rates in the range of 0.06 to 0.83°C·s−1. As the heating rate was augmented, the critical temperatures, Ac1 and Aα, as well as the intercritical range, which was evaluated as the difference between the critical temperatures, ΔT = Aα − Ac1, increased. At a low heating rate, the kinetics of austenite formation was slow as a consequence of the iron’s silicon content. The effect of heating rate on k and n, the kinetic parameters of Avrami’s equation, was also determined. Parameter n, which is associated with nucleation sites and growth geometry, decreased with an increase in heating rate. In addition, parameter k increased with the increase of heating rate, suggesting that the nucleation and growth rates are carbon- and silicon-diffusion controlled during austenite formation under continuous heating.

Effect of Heating Rate and Silicon Content on Kinetics of Austenite Formation during Continuous Heating

The first step in a heat treating cycle is the austenitizing of the as-received material. Despite its importance, this step has received relatively little attention. In this work, the kinetics of austenite formation during continuous heating tests of steel samples with low and high silicon content was determined as a function of heating rate. The microstructural evolution was characterized through dilatometric analysis of cylindrical samples (7 mm × 20 mm), continuously heated in a protective atmosphere at constant heating rates ranging from 2 to 40 °C/min. The critical temperatures and the transformation kinetics were determined from the derivative of the relative length change as a function of temperature. As the heating rate increases the critical temperatures and the transformation temperature range increase; the addition of silicon produces a more marked effect. The transformation kinetics data were correlated using an Avrami-type equation. The kinetic parameter n is nearly independent of heating rate while the parameter k is a strong function of the heating rate; in both cases, slightly larger values were obtained for the high-silicon steel.

Kinetic Study of the Austenite Decomposition During Continuous Cooling in a Welding Steel

The kinetics of austenite decomposition during the continuous cooling of a low-carbon welding steel was determined by dilatometric analysis. Based on the measurements, the critical transformation temperatures of austenite decomposition were determined for both ferrite and pearlite. The cooling conditions were established from the Stelmor® controlled cooling process for the manufacturing of wire rod. It was observed that the critical temperatures decrease when the cooling time was reduced as a result of an increased cooling rate in the range of 600–900 °C. Using the decomposition temperatures, a continuous cooling transformation CCT diagram was created and it may be observed that austenite decomposition occurs in two steps. The kinetic parameters for each step were determined and compared with the Johnson-Mehl-Avrami-Kolmogorov diffusive model. The final microstructure was analyzed using an optical microscope and evaluated by nanoindentation to determine the effect of the cooling rate on the nanohardness of each phase.

Investigation of the effect of inert inclusions on densification during solid-state sintering of metal matrix composites

Abstract Solid-state sintering is the most used process to produce composites. In this paper, the effect of inert inclusions on densification during sintering was evaluated for Cu-WC and Cu-W composites, which have several industrial applications. Dilatometry tests were performed to follow the densification of composites. The effects of the quantity, size, and interphase bonding on densification of the matrix were studied. Distribution of the inert particles inside of the matrix was observed by scanning electronic microscopy. The results show that densification is decreased as the volume fraction of inclusions increases. Two different behaviors are detected when two different sizes of inclusions are used. For <20% vol. of inclusions, smaller tungsten particles have a minor effect on the densification than those of tungsten carbide. On the contrary, higher volume fractions of smaller tungsten particles drastically decrease the densification. The microhardness of the copper matrix is improved up to 15% vol. of inclusions, being higher for tungsten carbide particles. It was found that 15% vol. of inclusions is the maximal quantity of inclusions that can be used, as higher quantities inhibit densification and reduce the mechanical properties of the composite.

Characterization of Constrained Sintering of Powders on Solid Substrate

Nowadays, sintering is a very useful technique to fabricate metal, ceramic and composites parts for different applications. This phenomenon has been extensively studied over 50 years and, most of the research related to it used a model based on two contacting particles. However, just a few jobs were focused on the powder sintering on a solid substrate. This work investigates the effect of two parameters; substrate shape and inclusion of the reinforced particles on the evolving microstructure during sintering of particles on a rigid substrate. Powders and solid bars of copper are used as a model material and particles such as tungsten carbide (WC) as reinforcing particles. Sintering was performed in an electrical furnace at 1050 °C under reducing atmosphere. The progress on sintering was evaluated by measuring the relative density close and far from the solid substrate by means of the image analysis from pictures taken by scanning electronic microscope (SEM). The effects on the constraint sintering were also identified. Heterogeneous densification and delamination of the film from the substrate were observed as densification increased which by the way is reduced by the inclusion of the reinforced particles.

Constrained sintering and wear properties of Cu-WC composite coatings

Abstract Coatings of metal matrix composites (Cu-WC) were fabricated by solid-state sintering. WC reinforcing particles in different quantities from 5% up to 30% (volume fraction) were mixed with Cu particles. After mixing, the powders were poured onto the surface of copper substrates. Sintering was carried out at 1000 °C under a reducing atmosphere in a vertical dilatometer. Sintering kinetics was affected by both rigid substrates and WC particles which retarded the radial and axial densification of powders. However, the coatings were strongly attached to the substrate, and WC particles were randomly distributed within the matrix. The addition of the reinforcing particles enhanced the microhardness and reduced the volume loss in wear tests to 1/17 compared to the unreinforced sample. The predominant wear mechanism was identified as abrasion at a load of 5 N. 20% WC (volume fraction) reinforcing particles led to the maximum values of properties for the composite coating.

Numerical simulation in 3D of a thermoelectrical system by ohmic heating

This paper presents the numerical simulation in 3D of a hollow cylindrical sample heated by Joule effect. The solution of the governing equations describing the phenomenon and the coupling of electrical terms coupling were performed, Also an user-defined function (UDF) was used with the computational program fluid dynamics ANSYS Fluent ©. The Thermo-physical properties of the material are in function of the temperature. The numerical results were validated experimentally.

Sintering kinetics of Ni2FeSb powder alloys produced by mechanical milling

Abstract A ternary Ni 2 FeSb shape memory alloy was fabricated by powder metallurgy route. Sintering kinetics was estimated from dilatometry tests; whereas the microstructure and morphology of the powder and consolidated bulk samples were evaluated by XRD and SEM, respectively. Microhardness tests were performed on the surface of sintered samples. The results indicated that milling time has an effect on the shape and particle size as well as the homogeneity of the crystalline structures of the powders. Samples with longer milling time presented higher relative densities, better distribution of the elements on the alloy as well as the L2 1 and martensite phases, which will give the shape memory effect. The estimated activation energy values ranged from 109 to 282 kJ/mol at temperatures between 750 and 1273 K, indicating that sintering is controlled mainly by volume diffusion. Microhardness was improved by increasing the milling time and the heating rate.