Caracterización microestructural y superficial de aceros laminados en caliente y en frío

Tesis (Maestro en Ciencias de la Ingeniería Mecánica con Especialidad en Materiales) U.A.N.L.UANLhttp://www.uanl.mx

Failure analysis of the fasten system of wheels used in mining pickup trucks

Regardless of their specific applications, all the vehicles used in mining operations are subjected to severe working conditions that reduce in a considerable amount, their active in-service life. In this work, the causes that promote failure of the fasten system and subsequent ejection of the wheels of passenger pickup trucks used in open-pit mines are analysed. By means of scanning electron microscopy, optical microscopy analyses and hardness tests, it was found that failure of the fasten system is characterised by a series of synergetic steps that include, the plastic deformation of the lug nuts caused by deficient tightening practices, fatigue and plastic deformation of the bolts. When combined, these phenomena leaded to the formation of cracks that propagated in the radial direction of these elements. The reasons that promote the development of this kind of failure are presented and discussed in this investigation. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Characterization of Corrosion Behavior of Painted Galvanized Steel under Accelerated Conditions

In the present study three systems of carbon steel (1008) are evaluated, which were provide of two corrosion control methods, barrier and cathodic protection (painted and galvanized respectively) commonly used in the construction industry. They were evaluated under accelerated conditions exposed in fog chamber, according to ASTM B-117, which specifies continues exposition of sodium chloride at 5% and 35 °C. The main aim of the research was to characterize the corrosion resistance and to determine the degradation mechanism under test’s conditions. The results after 1080 and 3500 hours of exposure are presented, with adhesion measure (ASTM D-3359) and scratch resistance measure (D-1654) for each exposure time, as well as the characterization of corrosion attack through the mapping analysis of Scanning Electron Microscope / Energy Dispersive X-ray. According to the results it is proposed that the corrosion of the systems under study begins at galvanized – metal base interface. Later advances due to formation of porous layer of zinc hydroxi-chloride, which it’s characteristic of environments with chloride ions, forming zinc’s corrosion products like zinc oxide and zinc hydroxide on the porous layer until iron starts to dissolve, producing iron oxide and iron oxy-hydroxide on the zinc’s corrosion products and porous layer

Oxidation and wear behaviour of a work roll grade high speed steel

The oxidation behaviour of high speed steel (1.55 % C, 7.70 % Cr, 4.90 %V, and 2.00% Mo) was studied isothermally at 550 and 615'C for three different environmental conditions, two of which consisted in a mixture of dry air and water vapour flowing at a rate of 0.45 or 1.45 cm'/min respectively, compared to dry air conditions. At 615'C and for the maximum water vapour content, the oxidation behaviour was initially logarithmic followed by linear stage. In contrast, for a water vapour flow rate of 0.45 CM3/Min, the oxidation was parabolic, which resulted in a greater mass gain of the samples after I hour oxidation, even though there was a lower water content. This was believed to be related to the partial pressure of oxygen and therefore to the amount of oxygen available for oxidation. When exposed to dry air, logarithmic kinetics were observed, with much lower mass gain compared with the other two environmental conditions. Reducing the test temperature to 550'C only reduced the mass gain, with the same oxidation kinetics for each condition, suggesting the same oxidation mechanisms at both temperatures. For the humid conditions, an iron-chromium spinel (Fe, Cr)304was formed along with magnetite Fe304 and hematite Fe203- In dry conditions the spinel and hematite were also present. In addition, a VO vanadium oxide layer was located at the top of the oxide layer, indicating oxidation of the MC vanadium rich carbides, promoted by the high partial pressure of oxygen of the environment. The steel was also exposed to cyclic temperature fluctuations, of two different frequencies, in an attempt to more closely simulate the conditions found during operation of rolls in industry. For the high frequency tests, it was difficult to establish a mathematical relationship for the oxide growth, with the kinetics being rather stochastic. The cyclic oxidation produced a very thin granular layer which appeared to be the spinel (Fe, Cr)304. Local surface regions exhibited high plastic deformation associated with cracks that facilitated the oxygen dissolution within the steel. In the low frequency cyclic tests, the oxidation kinetics were parabolic at both test temperatures. Quenching in water, resulted in the preferential spallation of the hematite by a mechanism known as "buckling", generated from compressive thermal stresses. Material was removed at a rate of -0.13mg/cycle and -0.07mg/cycle at 615 and 550'C respectively, leaving the surface only protected by a layer of iron-chromium spinel. The relationship between the wear of the high speed steel and oxidation was investigated at 600,500 and 4001C in a rolling-sliding disc on disc configuration using a 2.5 kg load for a sliding distance of III km for two different environmental conditions (water, both gaseous and liquid, and laboratory dry air). The specific wear rate of the high speed steel discs was greater for the dry tests compared to the wet ones. However, the specific wear rate of the dry tests was strongly temperature dependent, while for the wet tests, the wear rate was insensitive to temperature. The dry tests exhibited a combination of metallic and oxidational wear, while the wet tests were almost entirely oxidational, with a different oxide phase constitution to the dry tests. Surprisingly, the wet tests exhibited higher friction compared to the wet tests. The reasons for this and the difference in wear rates are discussed and compared to the static oxidation tests

Electrochemical Noise Measurements of Advanced High-Strength Steels in Different Solutions

Advanced high-strength steels (AHSS), are commonly used in the manufacture of car bodies, as well as in front and rear rails, and safety posts. These components can be exposed to corrosive environments for instance, in countries where de-icing salts are used. In this work, the corrosion behavior of four AHSS steels with dual-phase [ferrite-martensite (DP) and ferrite-bainite (FB)] steels were studied by means of electrochemical noise (EN) measurements according to the ASTM G199-09 standard in NaCl, CaCl2 and MgCl2 aqueous solutions at room temperature. The direct current (DC) trend data from EN were removed by a polynomial method of statistical and spectral analysis. According to the noise resistance (Rn) values obtained for the DP and FB dual-phase steels, both the martensite/bainite content and morphology of the phase constituents have an important effect on the corrosion behavior of these steels. The L.I. (localization index) (0.00054 to 0.15431), skewness (−6.18 to 7.35) and kurtosis (high values 37.15, 74.84 and 106.52) were calculated. In general, the results indicated that the main corrosion process is related to uniform corrosion. Corrosion behavior of AHSS steels exposed in NaCl solution could be related to the morphology of the phase constituents exposed in NaCl, CaCl2 and MgCl2 solutions