J.A. Juarez-Islas

Processing of ultra low carbon steels with mechanical properties adequate for automotive applications in the as-annealed condition

A series of ultra low carbon/Ti added steels were produced with the aim of evaluating the steelmaking route and processing conditions of slabs, in order to achieve mechanical properties on resulting annealed sheets adequate for automotive applications. Characterization of microstructure was carried out in the as-cast, deformed and annealed specimens by means of scanning and transmission electron microscope techniques. Slab specimens were hot rolled, coiled, cold rolled and annealed. From the resulting annealed sheets, evaluation of mechanical properties in terms of the percent of elongation and the normal anisotropy ratio was carried out on three experimental ultra low carbon/Ti added steels from heats of 230 t. From the resulting mechanical properties, it was noticed that the sheet steels achieved very high formability, especially for parts requiring good deep drawability, adequate for automotive applications.

On the processing of Fe-C-Mn-Nb steels to produce plates for pipelines with sour gas resistance

Several steel plates in the as-hot rolled plus cooled condition were studied, in order to evaluate the impact of the steelmaking route and the controlled thermomechanical processing plus the cooling media, on the resulting microstructure and mechanical properties of steel plates, which will be processed to produce large diameter steel pipes with sour gas resistance. The steelmaking route to produce the slabs involved the use of 100% sponge iron which was feed into an electric arc furnace, vacuum degassed, ladle treated and continuously casted. After soaking, a controlled thermomechanical processing schedule was applied to steel slabs followed by air or accelerated cooling of plates. Most of the resulting steel plates cooled in air showed a banded structure, which sometimes presented a central segregation region. The worst plates with a central segregation region showed intermetallics compounds in it. After modifications of the steelmaking route and to the controlled thermomechanical/cooling schedule, a steel plate with a ferritic microstructure plus 0.5 vol% of bainite was obtained. This microstructure together with the resulting mechanical properties, fulfilled the API grade 5L X-70 properties, required in modern steel pipes demanded by the oil industry.

EIS testing of new aluminium sacrificial anodes

Aluminium alloys In/Hg free, suitable as sacrificial anodes for cathodic protection, have been developed. Short-term electrochemical tests were performed on these new alloys to obtain their electrochemical efficiency and to reveal any tendencies to passivation. Samples were galvanostatically held at different current densities, which changed every 24 h according to DNV RP 401. Electrochemical impedance spectroscopy (EIS), polarization curves and electrode potential against time measurements were employed to study the main features of the processes taking place at the alloy–solution interface.

Nitride width and microhardness in H-12 ion nitrided steel

Abstract The ion nitriding technique was used on an AISI H-12 tool steel aiming to correlate measurements of the nitride width, microhardness profile and fractography measurements and nitriding time. The ion nitriding conditions were 500°C and 500 V, employing a 80% H 2 -20% N 2 gas mixture. The average width of the nitrides formed near the surface exposed to the plasma was linearly related to the nitriding time. The surface microhardness (about 15 μm into the surface) increased with time until a maximum is reached and a plateau was developed near the nitrided surface. However, when exposure time is above 27 ks the surface microhardness decreased. The hard case measurements employing fractography revealed a correlation with the microhardness measurements and nitriding time. The role of tungsten, which is contained in H-12 tool steels and absent in H-13, is discussed.

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.

Evaluation of an Al-Zn-Mg-Li Alloy/Potential Candidate as Al-Sacrificial Anode

This paper forms part of an overall effort to develop Al-sacrificial In/Hg free anodes; our research has been directed toward developing Al alloys appropriate for cathodic protection. The Al-Zn-Mg system has been particularly selected due to the presence of precipitates in the α-Al matrix, which are capable of breaking down passive films while presenting good electrochemical efficiencies. At the same time, the effect of Li additions on superficial activation of the anode by means of precipitation of AlLi-type compounds was examined. The microstructure was characterized in the as-cast and as-aged ingots, showing the presence of α-Al dendrites as well as eutectic of Al2Mg3Zn3 and precipitates of Mg7Zn3 in interdendritic regions. Electron microscopic observations performed on specimens with and without heat treatments showed in the α-Al matrix the presence of a uniform distribution of precipitates of (τ-Al2Zn3Mg3, Mg7Zn3, and δ-AlLi type. The electrochemical behavior of the alloy was investigated in a 3% NaCl solution simulating seawater at room temperature. After evaluation of the electrochemical efficiency, values up to 67% were obtained. The relationship between microstructure and electrochemical efficiency is discussed in this work and suggestions of future research are given in order to improve the electrochemical behavior of Al anodes in the field.

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.

Microstructural and mechanical evaluations of spray-deposited 7XXX Al-alloys after conventional consolidation

Abstract Alloys of the 7XXX type were prepared using the spray atomization-deposition method. These alloys were also hot extruded and subsequently heat treated at different temperatures. Microstructural characterization of the alloys was carried out by transmission electron microscopy and X-ray diffractometry. The mechanical properties were assessed through tensile tests using an Instron machine.

Analysis of chill-cast NiAl intermetallic compound with copper additions

This study carried out a characterization of chill-cast NiAl alloys with copper additions, which were added to NiAl, such that the resulting alloy composition occurred in the β-field of the ternary NiAlCu phase diagram. The alloys were vacuum induction melted and casted in copper chill molds to produce ingots 0.002 m thick, 0.020 m wide, and 0.050 m long. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) performed in chill-cast ingots identified mainly the presence of the β-(Ni,Cu)Al phase. As-cast ingots showed essentially no ductility at room temperature except for the Ni50Al40Cu10 alloy, which showed 1.79% of elongation at room temperature. Ingots with this alloy composition were then heat treated under a high-purity argon atmosphere at 550 °C (24 h) and cooled either in the furnace or in air, until room temperature was reached. β-(Ni,Cu)Al and γ′(Ni,Cu)3Al were present in specimens cooled in the furnace and β-(Ni,Cu)Al, γ′(Ni,Cu)3Al plus martensite-(Ni,Cu)Al were present in specimens cooled in air. Thermogravimetric analysis indicated that martensite transformation was the result of a solid-state reaction with Ms ∼ 470 and Mf ∼ 430 °C. Tensile tests performed on bulk heat-treated ingots showed room-temperature ductility between 3 and 6%, depending on the cooling media.

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.