مجله مهندسی متالورژی
پیاپی 60 (زمستان 1394)

Tantalum is a refractory metal which has often been considered for high temperature applications because of its high melting point (2996◦C). In the present study, the tantalum powder characteristics produced by calciothermic reduction of Ta2O5 under argon atmosphere at different amounts of flux was studied. A fixed value of Calcium was used for the reduction of Tantalum oxide which amounted to two times of the stoichiometric ratio while various amounts of CaCl2 flux of 1, 1.5, 2, 2.5 and 3 times of the stoichiometric amount was utilized. XRD analysis on the product showed that the obtained powder contains tantalum and calcium tantalate phases. ICP technique was used to determine the calcium content of the reduced samples. The minimum value of remained Calcium in the product was obtained when the CaCl2 flux amounted to 2.5 times of stoichiometry, which was 23614 ppm. The morphology of the products was studied by scanning electron microscopy. Based on the SEM results, the morphology of the tantalum powder was coral-like which the size of the particles decreases by increasing the amount of CaCl2 flux.

In this study, effects of over aging treatment and its temperature as well as Nb addition on the microstructure and tensile properties of cold-rolled dual-phase steel were studied. For this purpose, two different steel containing Nb and free Nb were considered. Steels were intercritical annealed at 770C for 5 minute and then were over aged in the salt bath with five different temperatures for 12 minute and finally were slow cooled in the air to the room temperature. Samples microstructures were evaluated by scanning electron microscopy. Uniaxial tension test was used for study of tensile properties. The obtained results implied that the over aging temperature affects microstructure and tensile properties of both steels significantly. This effect is also different in high and low temperature regime. In the low temperature regime; increasing of the over aging temperature, decreases yield and tensile strength significantly while elongation increases by a sharp rate. In high temperature regime; increasing temperature has not noticeably effect on the elongation. However, yield strength increased and tensile strength decreased. The consideration of dual-phase Nb steel showed that Nb addition increases strength significantly. Also, it was caused increasing in the strength stability in high temperature. Results were discussed through relation microstructure and tensile properties.

In this study, steel hollow spheres were manufactured using commercial iron powders by powder metallurgy technique. For manufacturing the hollow spheres, iron powders, copper powders (10 wt. %) and carbon powders (0.5 wt. %) were firstly mixed and then were added to the sodium silicate (10 wt. %) as binder to produce a homogenous suspension. Then, prepared suspension was coated on the polystyrene spheres. After drying the green spheres, two different heat treatment processes were carried out on the spheres including the melting and thermal de-binding and the sintering processes. The microscopic evaluations were performed by optical microscope and scanning electron microscope and mechanical properties of steel hollow spheres were investigated by the compression test. The results shown that sphere walls composed of four different regions including: the sintered iron powders region, the dispersed copper powders region, remained sodium silicate region and porosities region. In addition, the force-displacement curves of compressed spheres were revealed no significant differences in spheres with different sphere sizes. The fractures of spheres in compression forces were occurred based on the presence of the micro-cracks in the sphere walls and growth of them.

The aim of this research is in-situ production of WC-Co composite with different combination of 13% to 33% of cobalt from metal oxides WO3 and Co3O4 by use of carbothermic reduction method and then testing its physical and mechanical properties. In this study, cobalt and tungsten metal oxides powder with- different ratios with 17% carbon (30% more than stoichiometric value) after homogenization and mechanical activation in a planet ball mill with argon shielding gas for 20 hours, pass pressing, reduction heat treatment and sintering (temperature of 1500 oC for 2 hours and under shielding layer of alumina and carbon powder). Hardness (Vickers), toughness (Vickers indentation and crack length measurement) and density (Archimedes) of sintered sample were evaluated and relation of properties and microstructure was discussed using electron microscope observations. The results showed that in the milling stage no reduction reaction occurred even up to 20 hours and only powder activation took place. Between investigated composites, the composites with 23% cobalt showed highest density and strength while the composite with 33% and 13% cobalt showed highest toughness and hardness, respectively.

The purpose of the present paper is to investigate the mechanical and corrosion behaviors of an AISI 321 austenitic stainless steel plate produced by controlled hot-rolling and accelerated cooling (TMAC) route without and with subsequent annealing and stabilizing thermal treatments, based on ASTM A 240 and AMS 5510 standards. The subsequent experiments indicated the consistency of the properties of all the investigated specimens with standards requirements; however, the strength of the as-treated sample was much improved in comparison with the heat-treated samples. Furthermore, all three samples satisfied the standards requirements regarding the intergranular corrosion resistance, which was due to the precipitation of TiC carbides instead of Cr23C6 in grain boundaries. In addition, aging treatment at 600°C for 20 hours showed superior intergranular corrosion resistance. Finally, it was concluded that it is possible to benefit from the AISI 321 plate produced by TMAC route without subsequent heat treatment, in service conditions.