Journal of Metallurgical and Materials Engineering
Year:2010 | Volume:21 | Issue:2|Papers Count:7

Carbide layer produced by thermal reactive diffusion (TRD) technique onto metals and alloys has characteristic properties including wear، oxidation and corrosion resistance as well as relatively high surface hardness. In the present work، kinetics of formation of chromium carbide layer on carburized AISI 1020 steel at temperatures of 900, 1000 and 1100oC for 3 to 9 hours was studied. The presence of Cr7C3 phase formed on the surface of specimens was confirmed by X-ray diffraction analysis. Cross-sectional observations of optical and SEM micrographs showed that chromium carbide layer formed on the steel surface were continuous and relatively uniform. The surface micro hardness values for the coated steel samples were between 530 and 1321 HV (under 200 g load). The evaluation of kinetics of chromium carbide layer formed using salt bath method showed that there is a parabolic relationship between the layer thickness and the soaking time. In addition, the activation energy for chromium carbide formation on carburized AISI 1020 steel in borax salt bath was obtained to be 97.781 KJ/mol. Moreover, the contour diagrams were suggested for prediction of thickness of carbide layer and surface micro hardness. The possibility of using a series of mathematical equations for predicting carbide thickness and surface micro hardness was also investigated.

Effect of chromium content on the microstructure and inclusions formation in low alloy steel multipass welds was investigated. Manual metal arc welding was performed in flat position according to AWS A5.5-96 standard. The ferro-chromium content in the coating was varied such that the final weld metal consists of 0.05, 0.53 and 0.91 wt% chromium. Microstructure was studied by optical and scanning electron microscope in both columnar and reheated regions of the weld metal. The results showed increase in acicular ferrite formed at the expense of primary ferrite and ferrite with second phase with steady refinement of microstructure. Amount of microphases also increase as the chromium content of the weld metal increases. It is believed that chromium does not affect apparent characteristics (such as volume fraction and average size) and chemistry of inclusions.

Pitting corrosion of embedded mild steel in concrete is one of the main concerns of civil engineers. Introducing a reliable measuring method for prediction its corrosion has been intensely considered by corrosion researchers. The aim of the present study is to demonstrate the pitting potential measurement correlation between potentio dynamic polarization and zero resistance ammeter on two identical electrode techniques. Potentio dynamic polarization of mild steel in concrete pore solution (saturated Ca (OH2)) shows passivition prior to transpassivity due to water oxidation. Addition of 0.5 M NaCl to the above solution caused pitting corrosion at 327 mV. Meanwhile the presence of nitrite ions as an inhibitor by addition of 0.5 M NaNO2 to the concrete pore solution containing 0.5M NaCl caused an increase of 562 mV in pitting potential. Zero resistance ammeter (ZRA) technique was employed to measure the galvanic current density and potential of two coupled identical electrodes of investigated mild steel in the mentioned environments. The results of ZRA investigation revealed passivity behavior in concrete pore solution and concrete pore solution containing chloride and nitrite ions but pitting corrosion in concrete pore solution containing chloride ions after 300 second of immersion. Pitting corrosion occurrence was attributed to the establishment of pitting criteria by exceeding pitting corrosion from pitting potential. The value of pitting potential obtained by ZRA technique was very close to the one measured by potentiodynamic polarization technique.

A common process in auto-parts manufacturing consists of forging، normalizing، quenching and tempering. Normalizing treatment is usually employed to improve machinability of steel, and to homogenize and refine the microstructure. For simple parts, production cost would be reduced if normalizing stage were eliminated. In this paper, the effect of normalizing treatment on fatigue life of a quenched and tempered carbon steel has been studied. Samples were taken from forged، quenched and tempered parts at industrial conditions. Sixty four fatigue specimens were tested using a rotary bending fatigue machine (R=-1) at four different stress amplitudes between 500 and 600 MPa. Results showed minor effect of normalizing on microstructure, hardness and tensile properties. However، specimens normalized after forging indicated improved fatigue life at the same stress amplitude. Average fatigue lives at 508, 516, 534 and 583MPa were 261000, 194000, 119000 and 54000 cycles respectively for normalized quenched and tempered samples and 209000، 138000، 101000 and 46000 cycles for quenched and tempered samples. Due to variation of fatigue life results, Student's t-test was employed to prove any meaningful difference of fatigue life for specimens with or without normalizing treatment. Student's t-test is often used to determine if the averages of two samples are significantly different. The test approved the difference with 95% confidence. Fatigue life of normalized samples was estimated at least 18% higher than the life of samples without normalizing.

In this paper، how to use the abrasive wheel in the pin on disk test was evaluated and developed for tribological studies of steels. Parameters such as type of pin (steel), type of abrasive wheel، normal load on pin, linear velocity of disk and sliding distance were studied. The results showed that initial hardness can not merely determine the wear resistance of a metal whereas it may be related to the surface hardness resulting from work hardening during abrasive. In addition, in the pin on disk method, using the abrasive wheel is a good choice for evaluation of tribological behavior of high wear resistance metals such as Hadfield steels while, it is not appropriate and proper for mild steel.

Al-12%Si metal matrix composites reinforced with 6-25 vol% of Al2O3، TiC and B4C reinforcement particles, were fabricated by stir casting. The microstructure، density، porosity، micro-hardness، tensile strength of composites were evaluated. Microstructure and mechanical properties characterization showed uniform reinforcement distribution and increasing of hardness and tensile strength of composites samples with increasing of Al2O3 (25 vol%), TiC (18 vol%) and B4C (10 vol%). For Al-5% Al2O, various parameters of stir casting processing such as temperature of molten metal، rate and time of stirring، Type of agitator, were investigated. The results show that fabrication of composites in condition of 700- 750 rpm for agitator, 17 minutes of agitation for alumina and 6 min for carbides particlrs، using of zirconia-coated agitator with blades in angle of 30° in relation to rotation axis, adding of ceramic particles with speed of 0.375-1.2 gr/sec, temperature of casting of 700-750°C give the optimum reinforcement distribution، wetting and quantity of porosity.

In this paper aluminum nanopowder reactivity was investigated under air atmosphere. Aluminum nanopowder was produced through evaporation-condensation method. The thermal behavior was studied via non-isothermal oxidation of nanopowders with average particle size of 45 nm by differential thermal analysis and thermogravimetry analysis. Comparisons were also made to the micrometer sized particles. Several parameters have been used to characterize the reactivity of aluminum nanoparticles، including the onset and temperature of maximum oxidation rate, which were 490°C and 528°C, respectively. It was found that beside the superior surface area, the high reactivity of nanopowders may be attributed to the deviation of nanoparticles from spherical shape and broad particle size distribution.