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Properties of AISI M3:2 high speed steels produced by conventional casting and by powder metallurgy techniques
(2007-05-13) ARAÚJO FILHO, O. O.; AMBRÓZIO FILHO. F.
The aim of this study was to determine the properties of AISI M3:2 high speed steels produced by conventional metallurgy and by other powder metallurgy routes. PM Sinter 23 produced by hot isostatic pressing of gas atomized powders, M3:2 produced by axial cold compaction and vacuum sintering of green compacts obtained from water atomized powders and VWM3C manufactured by a conventional metallurgical route were investigated in this work. Specimens of Sinter 23 and VWM3C were taken in the longitudinal and transverse direction of hot forging. Transverse rupture strength (TRS) specimens of the five M3:2 high speed steels were manufactured as per the ASTM B 528-99 standard, heat treated by austenitizing and air quenched at 1140, 1160, 1180 and 1200 °C followed by triple tempering at 540, 560 and 580 °C. These specimens were fractured in three point bending tests with a bending rate of 2.5 mm/minute. TRS and hardness (Vickers and Rockwell C) of the high speed steels were determined. Scanning electron microscopy was used to study the microstructures of specimens that had undergone the different hardening treatments and the sizes of the primary carbides were measured at 1000 X on polished and etched specimens. The Quantikov image analyzing software was used to determine primary carbide distribution and at least six micrographs of the specimens in each condition were used. Correlation between TRS and size of the primary M6C and MC type carbides was found.
Hot extrusion and characterization of an Al-Fe-VorNb-Si powder alloy prepared by high energy milling
(2008-09-29) COELHO, R. E.; AMBRÓZIO FILHO, F.
This paper presents the microstructural and mechanical characterization of an alloy, Al90.8- Fe6.2-(VorNb)1.0-Si2.0 (at%), prepared by high energy milling elemental powders of Al, Fe, V and Si, followed by vacuum hot pressing and hot extrusion. The powders were milled for 10 hours at 800 and 1400 rpm and the extruded bars were characterized by X ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive analysis of selected areas. The following phases were found in the extruded bars: Al8Fe2Si, Al12(Fe,V)3Si in the alloy with V and Al8Fe2Si, Al12(Fe,Nb)3Si, and AlNb2 in the alloy with Nb. Tensile tests and Vickers hardness at room temperature were carried out on the extruded bars. The bars from powders milled at 1400 rpm showed the highest strength and hardness and are similar to those obtained by rapid solidification in the alloy Al-Fe-V-Si. The improved mechanical properties of these bars could be attributed to the dispersoids in the microstructure.
Pitting corrosion of UNS S41000 and UNS S42000 stainless steels
(2009-09-06) BORGES, A. O.; ALONSO-FALLEIROS, N.; Rodrigo Magnabosco
The aim of this work is to evaluate the pitting corrosion resistance, through potentiodynamic polarization tests, in two commercial martensitic stainless steels, named UNS S41000 and UNS S42000, by using four electrolytes of 0.10M Na2SO4, with different concentration of chloride: 0.01M NaCl; 0.05M NaCl; 0.10M NaCl; 0.60M NaCl (3.5%). The received samples showed a ferritic microstructure with spheroidized carbides (annealed), whereas it was observed a microstructure of tempered martensite after heat treatment. Considering the same condition - annealed or quenched and tempered - the hardness of steel UNS S42000 was always higher than steel UNS S41000. In potentiodynamic tests, the potential scan rate was defined as 1mV/s after exploratory tests which showed this parameter influence on the curve shape. From then on, it was possible to obtain polarization curves with scanning rate of 1mV/s, starting at open-circuit potential after a five minute immersion and 600 grit surface finish. The results showed that the higher chloride ion concentration, the lower the pitting potential. For a given electrolyte, pitting potential measured for annealed steel UNS S41000 is slightly higher than the same steel on quenched and tempered condition. The intense chromium carbide precipitation at the grain boundaries of former prior austenite, a phenomenon that can lead to sensitization, may be a reason for it. Through immersion tests, it was found out that pitting potential of UNS S41000, after heat treatment, is lower than corrosion potential in the electrolyte composed of (3.5% NaCl + 0.10M Na2SO4). Otherwise, steel UNS S42000 presented higher pitting potential after heat treatment. In this case, intergranular carbide precipitation was not observed and the quench and temper treatment, at first, generates a higher content of chromium in solid solution if compared with the annealing treatment, leading to a greater corrosion resistance. Pitting density and their sizes are chloride concentration dependent: the higher ion chloride concentration, the lower is pit density and the larger is pit width. According to the studied conditions, the most suitable electrolyte for pitting potential determination is composed of (0.10M NaCl + 0.10M Na2SO4). The shape of the obtained potentiodynamic polarization curves with this electrolyte allowed the identification and accurate determination of the pitting potential.
Influence of the composition and the tungsten particle size on the sintering of W-Cu composites
(2010-10-10) COSTA, F. A. DA; SILVA, A. G. P. DA; AMBROZIO FILHO, F.; GOMES, U. U.
Sintering is an alternative technique for producing W-Cu composites, but the system has a low sinterability. The increase of the sintered density for such a material is a need. The sintering mechanisms of W-Cu composites is investigated and the influence of the copper content and the tungsten particle size is analyzed by dilatometric measurements. The evolution of the structure during sintering is described. Copper plays the most important role for promoting densification. The sintering between the copper particles in contact is responsible for the densification in the solid state, but the main contribution is the rearrangement of the tungsten particles when copper melts. The higher the copper content the higher is the densification of the material. The use of fine tungsten and copper powders leads to higher densities and avoids the formation of large pores as copper melts.
Effect of grain size on machining strength in an austenitic stainless steel
(2010-07-26) COPPINI, N. L.; Júlio Cesar Dutra; BAPTISTA, E. A.; FRIACA, C. A.; FERREIRA, F. A. A.
This paper explores the effect of grain size on machining strength in an Fe-Cr-Ni alloy, namely AISI 316L. Ideal grain growth law, proposed by Burke,[1] was used to obtain the activation energy for this steel based on data published by Stanley and Perrota,[2] which was 185 kJ.mol-1. The initial grain size was measured - 12 0m - And by considering these values, both temperature and time values needed to achieve a final grain size ten times larger than the initial one could be calculated, which were 1200°C and 2 hours. Ternary phase diagram analysis showed that austenite was stable at this temperature. Following, samples of 200-mm length were annealed and quenched in water to prevent any formation of sigma (0) phase. Annealed and as-received bars were then used to compare their machining strength. Results showed that the machining strength is higher in the as received condition than the one after annealing (127 0m). It may be concluded that the bigger the grain size, the lower its machining strength. It is believed that this is caused by the pile-up of dislocations on grain boundaries, since this material exhibits large plastic deformation before fracture.
Experimental and numerical evaluation of crack growing curves (R-CURVES) of AISI 1020 steel using pin-loaded and clamped SE(T) specimens
(2010-07-26) MOREIRA, F. C.; OLIVEIRA, F. P. DE; Gustavo Donato; Rodrigo Magnabosco
SE(T) specimens are of great interest for the determination of fracture toughness and crack growth resistance of structural steels applied to pipelines and pressurized components, as a result of the similarity between stress fields in the near-tip region, which is not found in conventional C(T) and SE(B) specimens. This work applies the unloading compliance method supported by finite element models for the experimental evaluation of R-curves, using pin-loaded and clamped SE(T) specimens with equivalent crack depths. AISI 1020 steel samples were tested as an example for the experimental methodology. The obtained results allow critical evaluation of load vs. displacement and R-curves, and moreover allow the discussion of the experimental difficulties found in testing both specimen geometries.
Influence of the intercritical annealing time on the microstructure of a low carbon dual-phase steel
(2010-07-26) FORGAS JUNIOR, A.; Rodrigo Magnabosco
This work has the main purpose of analyzing the intercritical annealing time on the microstructure transformation kinetics of a dual-phase steel. Computational simulations confirmed the experimental observation, showing that annealing time at 755°C influences the formation of the austeno-ferri tic microstructure, and this can be attributed to the alloying elements partition and redistribution through ferrite-austenite interfaces. Kinetics calculations using a spherical model of diffusion, and a unidimensional model with bi-directional diffusion flux, resulted in equivalent results, which can partially describe the observed kinetics of transformation. Determination of basic metallographic parameters of the as-received material (martensite fraction and mean intercept), together to the knowledge of alloy elements distribution, are prerequisites to the success of the computational simulation.
UNS S31803 stainless steel phase transformations during 475°C and 650°C aging and its influence on the pitting potencial in a 0.6M NaCl solution
(2010-07-26) MELO, E. B. DE; Rodrigo Magnabosco
This work evaluates the phase transformations during aging at 475°C or 650°C of UNS S31803 duplex stainless steel and its influence on the pitting potential in a 0.6M NaCl solution. The material was solution treated at 1175°C and water quenched, and then aged at 475°C or 650°C for times up to 360h. Samples were a nalyzed by optical microscopy through quantitative metallography, and ferrite quantification was conducted with a ferritscope. Vickers microhardness measurements were also performed. Cyclic polarization tests were then conducted in a 0.6M NaCl solution, at a scan rate of 1 mV/s. It is found that aging at 475°C leads to a continuous increasing of material microhardness, probably related to the presence of alpha prime phase, and reduction in the pitting potentials were observed for samples aged for more than 12 h. Samples aged at 650°C show the presence of sigma and chi phases, which increased with aging time, and pitting potential is reduced for aging times greater than 4h. The pitting potential reduction in the aged samples is probably associated with the presence of ferrite and/or austenite impoverished in Cr and Mo, as a result of alpha prime formation at 475°C or sigma and chi formation at 650°C.
Influenzce of the sigma phase formation in pitting potential of UNS s31803 duplex stainless steel in 0,6M NACL AGED at 850 E 900°C
(2010-07-26) Daniella Caluscio dos Santos; Rodrigo Magnabosco
The main purpose of this work is to analyze the mechanisms and kinetics of sigma phase formation of UNS S31803 stainless steel after isothermal aging between 850 e 900°C, evaluating the influence of sigma phase formation on pitting potential in 0.6M NaCl solution. Solution treated samples of the UNS S31803 duplex stainless steel were isotermically aged at 850°C or 900°C up to 360 h. The microstructural characterization of the samples was made through quantitative metallography, ferritscope analysis and Vickers microhardness. The cyclic polarization tests used to the determination of the pitting potential were conducted in polished samples. The electrochemical behavior of aged UNS S31803 duplex stainless steel in 0.6M NaCl solution is strongly dependent on the microstructure, since both changes in mechanisms and kinetics of sigma phase formation influenced the pitting potential of the material. Increasing aging time lead to the decrease of pitting potential, due to the formation of chromium impoverished regions. Oscillations in pitting potential values were noted due to the chromium redistribution enabled by easy diffusion of this element in high temperatures.
Ferrite-to-austenite transformation during cold working of a duplex stainless steel
(2011-09-23) Rodrigo Magnabosco; TAVARES, D. B.; FORGAS, A.; DE MOURA NETO, C.
Ferrite-austenite duplex stainless steels (DSS) are replacing austenitic stainless steels in many applications due to its high mechanical and corrosion resistance. However, magnetic measurements of ferrite volume fraction using a ferritscope of cold worked samples indicate a reduction in the ferrite content with increasing mechanical work. Considering this fact, this work studies the influence of the cold working on the ferrite/austenite ratio. Samples of UNS S31803 DSS sheet, originally 3 mm thick, were solution-treated at 1175°C for 30 minutes and water quenched; after that, different levels of cold working were applied. Ferrite volume fraction was determined using two distinct methods: quantitative stereology with an optical microscope and magnetic measurements using a ferritscope; a detailed microstructural characterization, including X-ray diffractometry, was conducted to base the discussion of a possible ferrite-to-austenite diffusionless transformation.
Comparative study of ferrite quantification methods applied to duplex stainless steels
(2011-09-23) Rodrigo Magnabosco; SPOMBERG, S.
Duplex stainless steels (DSS) present approximately the same quantities of ferrite (magnetic phase) and austenite. These steels are replacing austenitic stainless steels in some applications due to its high mechanical and corrosion resistance. However, changes in the volumetric percentage of those phases can modify material properties, justifying the development of this study, which has as principal aim a comparison between three different ferrite quantification methods. Samples of two different DSS (UNS S31803 or UNS S32750), originally supplied as 20mm round bars, were solution-treated between 1000°C and 1250°C, in order to obtain different ferrite/austenite ratios. Ferrite quantification was performed in three sections: longitudinal, transverse and diagonally oriented to the rolling direction. The volume fraction of ferrite was determined using three distinct methods: quantitative stereology with an optical microscope, magnetic measurements using a ferritscope, and X-ray analysis by comparison of the relative intensities of peaks corresponding to the planes that generate maximum intensity of diffraction for single-phase samples. Results are discussed, analysing the reliability of the studied methods.
Carbon nanotubes experiment in microgravity
(2011-10-03) LA NEVE, A.; BELLODI, M.; MELO, M. A. A.; FERREIRA, M.; CASTRO, R. H. R.; PEREIRA, A. N.; F. S. Ortega
Carbon Nanotubes (CNTs) nanostructures are an interesting focus of research, due to their unique electrical and mechanical properties, including high specific surface area and electrical conductivity. These characteristics make them potential components for applications in microelectronics, field emitters and catalysts, among others. The purpose of this experiment is to obtain CNT homogeneous films on aluminum substrates both in microgravity and on earth, in order to study the real effect of gravity in the deposits organization. This experiment was carried out in microgravity, during a period of 4 minutes, aboard a VSB-30 Brazilian sounding rocket. To perform the experiment, it was developed an equipment, called CADEN, which consists of 3 sets of 4 reaction chambers, each one containing an aluminum substrate (in a total of 12), in addition to embedded electronics, responsible for the system control, data acquisition and monitoring of electric currents in each chamber. The chambers were loaded with a CNT solution in different concentrations, 0.1 mg/ml and 0.5 mg/ml; in each of them an electrolytic current flew according to the electrical potential applied to the aluminum plates, which acted as anode and cathode. In order to provide for a wide range of possibilities, which might be useful in further analysis, different DC voltages were applied to each pair of chambers, which were loaded with different CNT concentrations. On receiving the microgravity signal, on flight, the control system applies a different bias on each of the three sets. From this moment on, each pair of chambers will be active for 2 or 4 minutes time, accordingly to their programming. While the experiment is running, the electric currents that flow through all chambers, along with temperature and other variables, are periodically read and stored in the internal memory, and are also transmitted by telemetry. The launching and payload rescue were successful, and so was the equipment functioning. Electrodes electrical characterization was performed, focusing on the plates surface resistivity, and it was observed a significant increase in electrical conductivity. Additional analyses include plate surfaces electron microscopy, to verify both CNT deposition morphology and the nanotubes orientation on the surface. The results were compared with data obtained on earth under similar conditions, to identify the influence of microgravity and the role of the other variables in the experiment. The electrical resistance of plates with CNT deposited in microgravity was, in general, lower than that of plates with CNT deposited on Earth. Copyright ©2010 by the International Astonautical Federation. All rights reserved.
Fast sintering of SnO2 nanoparticles after chemical lixiviation
(2012-10-07) PEREIRA, G. J.; BASSO, P. M. R.
Tin oxide based powders without additives present low densification even at high sintering temperatures. Usually, metal cations are introduced as sintering aid, and the densification can be achieved when low temperatures as 1200°C are used, even within a few seconds. In this work, SnO2 based powders were prepared with different amounts of Mg2+ by a polymeric chemical process derived from Pechini's method. The powders were submmited to a lixiviation (washing) process. Both washed and unwashed samples were sintered by fast firing. All unwashed samples presented high initial densification rate, while washed samples showed low rate, but similar to samples containg few amounts of Mg2+. Since sintering is a high-temperature process strongly dependent on surface characteristics, a relationship between additive removal, and the sintering behavior can be established. All powders were characterized by specific surface area. The sintering was studied by means of final density. Surprisingly, samples achieved high densities within few seconds of sintering, and the traditional diffusion theory failed to explain this behavior. The washed sample shown less densification. A hypothesis based on the influence of the additives on the thermodynamic stability of pores was drawn.

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