Alejandro Toro

Correlations between microstructure and surface properties in a high nitrogen martensitic stainless steel

Nitrided and tempered AISI 410S stainless steel was tested under corrosion–erosion conditions and compared to conventional AISI 420 martensitic stainless steel. The corrosion–erosion resistance of the nitrided specimens was higher than that of the AISI 420 steel when tempered at 200 °C, but it decreased with tempering temperature in the range between 200 and 600 °C. The higher corrosion–erosion resistance of the high-nitrogen steel was credited to a more homogeneous distribution of chromium in martensite and a lower number of coarse second-phase particles, especially for tempering temperatures below 550 °C. The hexagonal -nitride was identified in specimens tempered at 200 °C, while finely distributed cubic CrN nitrides were observed in specimens tempered between 400 and 600 °C. Hexagonal Cr2N nitrides were observed at 550 and 600 °C. These coarse, high-chromium precipitates were responsible for the drop in corrosion resistance of the nitrided specimens.  2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The effect of testing temperature on corrosion-erosion resistance of martensitic stainless steels

Conventional AISI 420 and high-nitrogen martensitic stainless steels were tested under corrosion–erosion conditions in slurry composed by substitute ocean water and quartz particles. The tests were performed at 0, 25, and 70 ◦ C, with mean impact angles of 20 and 90 ◦ . Polarization tests in H2SO4 solution containing chloride ions were also carried out at the same temperatures. Both conventional and high-nitrogen specimens were tempered at 200 and 450 ◦ C before the tests. The high-nitrogen specimens were produced through gas nitriding of AISI 410S (13%Cr–0.03%C) and AISI 410 (13%Cr–0.15%C) stainless steels at 1100 ◦ C. These treatments allowed obtaining interstitial contents (nitrogen + carbon) at the surface of the specimens equivalent to the carbon content of conventional AISI 420 stainless steel. The best corrosion–erosion resistance was obtained in the nitrided AISI 410S samples tempered at 200 ◦ C and tested at 0 ◦ C under 20 ◦ -impact angle. Increasing testing temperature led to higher mass losses and wear rates due to the intensification of intergranular and pitting corrosion mechanisms, especially in the conventional AISI 420 stainless steels. In tests performed at 0 and 25 ◦ C, a reduction in the wear rate for longer testing times was observed, which was mainly associated to fragmentation and roughness changes of the abrasive particles. The mass losses under normal impact conditions were systematically higher than under oblique incidence, and some evidences of mass removal by brittle fracture were found after SEM examination of the worn surfaces.

Corrosion–erosion of nitrogen bearing martensitic stainless steels in seawater–quartz slurry

AISI 410S stainless steel was nitrided at 1473 K in N2 atmosphere, direct quenched and tempered at temperatures between 473 and 873 K. Martensitic cases with circa 0.52 wt.% N at the surface were obtained. Corrosion–erosion tests were carried out in slurries composed by quartz particles and tap or substitute ocean water. The concentration of solids, the impact angle and the pH of solution were fixed, while the temperature, surface changes and mass losses were monitored during the tests. Quenched and tempered AISI 410 and 420 stainless steels were used as comparison materials. The results showed that the erosion resistance and the corrosion–erosion resistance of the nitrided steel tempered at 473 K were higher than those of the AISI 410 and 420 steels tempered at the same temperature. This behavior was due to the higher hardness and better intergranular, pitting and generalized corrosion resistance of the nitrided alloy. The synergism between corrosion and wear was more important in the AISI 410 and 420 samples.

Genetic control of lysine metabolism in maize endosperm mutants

The capacity of three maize endosperm opaque mutants (o10, o11 and o13) to accumulate soluble lysine in the seed in relation to their wildtype counterpart, W22+, was investigated. The W22o13 and W22o11 mutants exhibited 278% and 186% increases in soluble lysine, respectively, while for W22o10, a 36% decrease was observed, compared with the wildtype. A quantitative and qualitative study of the N constituents of the endosperm has been conducted and data obtained for the total protein, non-protein N, soluble amino acids, albumins / globulins, zeins and glutelins present in the seed of the mutants. Following 2D-PAGE, a total of 38 different forms of zein polypeptides were detected and considerable differences were noted between the three mutant lines. The metabolism of lysine was also studied by analysis of the enzymes aspartate kinase, homoserine dehydrogenase, lysine 2-oxoglutarate reductase and saccharopine dehydrogenase, which exhibited major changes in activity, depending on the genotype, suggesting that the mutant genes may have distinct regulatory activities.

Genotoxicity Effects of Ceramic Coatings Applied on Metallic Substrates Using Single Cell Gel Electrophoresis Assay In Vitro

Sol-Gel coatings are a good choice for protection and bioactivation of metals used as dentistry and standard surgical implant materials. These films should both prevent degradation of the substrates by wear or corrosion, and bioactivate the material for inducing the formation of a hydroxyapatite (HA) rich layer onto the material surface, thereby permitting a natural bonding to living tissues. The aim of this work was to estimate the clastogenicity in vitro by Single Cell Gel lectrophoresis Assay (SCGE) or “comet” assay of coatings of TiN applied by magnetron sputtering and of hybrid layers obtained by Sol-Gel containing glass, glass-ceramic and HA particles on stainless steel AISI 304. Six test specimens were prepared: AISI 304 Stainless Steel coated with an hybrid silica single film (SF), applied by sol-gel process, AISI 304 SS coated with double film with bioactive glass (DFG), glass-ceramics (DFGC) and HA (DFHA) particles, AISI 304 SS coated with TiN multi films (MFTiN) applied by PVD and bare AISI 304 SS (304SS). Significantly lower DNA migration (p>0.005) was observed in the cells of the cultures corresponding to the samples coated with SF, DFG, DFGC, DFHA and MFTiN respect to the bare 304 SS. The comparison between negative control and the same coated samples did not reveal any statistically significant difference (p>0.005) in clastogenicity in vitro evaluated by SCGE.

Distribution of soluble amino acids in maize endosperm mutants

For human nutrition the main source of vegetable proteins are cereal and legume seeds. The content of total soluble amino acids in mature endosperm of wild-type, opaque and floury maize (Zea mays L.) mutants were determined by HPLC. The total absolute concentration of soluble amino acids among the mutants varied depending on the mutant. The o11 and o13 mutants exhibited the highest average content, whereas o10, fl3 and fl1 exhibited the lowest average content. In general, the mutants exhibited similar concentrations of total soluble amino acids when compared to the wild-type lines, with the clear exception of mutants o11 and fl1, with the o11 mutant exhibiting a higher concentration of total soluble amino acids when compared to its wild-type counterpart W22 and the fl1 mutant a lower concentration when compared to its wild-type counterpart Oh43. For methionine, the mutants o2 and o11 and wild-type Oh43 exhibited the highest concentrations of this amino acid. Significant differences were not observed between mutants for other amino acids such as lysine and threonine. The high lysine concentrations obtained originally for these mutants may be due to the amino acids incorporated into storage proteins, but not those present in the soluble form.

Development of low friction snake-inspired deterministic textured surfaces

The use of surface texturization to reduce friction in sliding interfaces has proved successful in some tribological applications. However, it is still difficult to achieve robust surface texturing with controlled designer-functionalities. This is because the current existing gap between enabling texturization technologies and surface design paradigms. Surface engineering, however, is advanced in natural surface constructs especially within legless reptiles. Many intriguing features facilitate the tribology of such animals so that it is feasible to discover the essence of their surface construction. In this work, we report on the tribological behavior of a novel class of surfaces of which the spatial dimensions of the textural patterns originate from micro-scale features present within the ventral scales of pre-selected snake species. Mask lithography was used to produce implement elliptical texturizing patterns on the surface of titanium alloy (Ti6Al4V) pins. To study the tribological behavior of the texturized pins, pin-on-disc tests were carried out with the pins sliding against ultra-high molecular weight polyethylene discs with no lubrication. For comparison, two non-texturized samples were also tested under the same conditions. The results show the feasibility of the texturization technique based on the coefficient of friction of the textured surfaces to be consistently lower than that of the non-texturized samples.

EVALUACIÓN DE LA RESISTENCIA AL DESGASTE ABRASIVO EN RECUBRIMIENTOS DUROS PARA APLICACIONES EN LA INDUSTRIA MINERA

The resistance to abrasive wear of two hardfacing alloys containing either high Cr or high W was studied. The alloys were deposited onto AISI A36 steel plates by a SMAW procedure. An alloy applied by oxyacetylene welding was used as comparison material. Wear resistance was measured in a dry sand – rubber wheel testing machine according to ASTM G65 standard, procedure A. The worn surfaces and the microstructure of the samples were analyzed in optical and scanning electron microscope, while the hardness of the deposits was measured in a Rockwell hardness tester. The best abrasion resistance was obtained in microstructures composed of primary (Fe,Cr)7C3 carbides and eutectic matrix, while higher mass losses were measured in Fe-Cr-C specimens containing primary austenite. The FeW-C hardfacing also showed good abrasion resistance due to the high hardness and deformability of the structure.

Experimental assessment of friction coefficient between polypropylene and human skin using instrumented sclerometer

Prosthetic devices are used to restore as much as possible not only the functionality, but also the self-esteem of patients who have been submitted to amputation surgeries. Typically, lower limb prostheses need a socket to act as a link with the human stump, so the contact stresses at the socket–stump interface are critical for the recovery process and the subsequent comfort perception of the patient as well. In this work, a broad experimentation to establish the coefficient of friction (COF) between socket material (polypropylene) and human skin was developed with the aid of an instrumented sclerometer, which was adapted to put in contact a polypropylene probe with human forearms. Seven factors were considered, but only sweat and hair skin were found to have a significant effect on COF, which varied from 0.22 to 0.45 in the tests. Lower values of COF were obtained when sweat was present at the interface, while the absence of both sweat and hair skin led to the highest value. The results are believed to be relevant for developing reliable finite element (FE) models for socket–stump interaction since the relation between normal and shear stresses at the interface of two interacting bodies is strongly determined by the COF.

Sintered High Nitrogen Stainless Steel Obtained using Pre-Mixed Powders

Nitrogen alloying in martensitic stainless steels has been considered as an alternative to improve mechanical properties and corrosion resistance. The use of conventional melting and solidification processes is not the most efficient way to produce high nitrogen steels (HNS) since nitrogen solubility in melt is quite low. Nowadays, HNS is being produced by Powder Metallurgy and, in this case, the nitrogen is alloyed in the solid state, especially considering reactions between high nitrogen sintering atmospheres and pre alloyed pressed bodies. This paper investigates another possibility regarding the introduction of nitrogen in stainless steel, i.e., the use of high nitrogen chromium powders (nitrided powders) mixed with ordinary atomized iron powder. This mixture was pressed and sintered at 1350 °C for 2 and 8 hours. As reference, two other set of nitrogen free samples were produced and characterized: sintered pre-alloyed 434L stainless steel (commercial powder); sintered pre mixed Fe + nitrogen free Cr. The discussion was based on shrinkage during sintering, microstructural aspects and corrosion resistance.