METALLURGICAL ENGINEERING
Year:2016 | Volume:19 | Issue:2 (62) |Papers Count:6

Ferrite-martensite dual-phase (DP) steels are a subset of advanced high strength steels which can be produced by applying inter-critical heat treatment on low-carbon steels. The strength and toughness of DP steels are greater than those in ferrite–pearlite steels with the same chemical composition. In this study, mechanical properties and forming limit diagram of ferrite–pearlite and DP steels with the same chemical composition were investigated and compared. For this purpose, inter-critical quenching heat treatment was applied on a low-carbon steel with ferrite–pearlite microstructure to produce ferrite – continuous martensite DP steel. Tensile and hardness tests were used to determine the mechanical properties, and Nakazima test was used to determine the formability of ferrite–pearlite and DP steels. Forming limit diagram of steels was also simulated using finite element method in macro scale, and compared with experimental results. The results of mechanical tests showed that the yield stress, tensile strength and hardness of produced DP steel were increased 65.91, and 87% respectively, in comparison to the same mechanical properties of ferrite–pearlite steel. Based on Experimental and simulation results of Nakazima test, the formability of DP steel is better than ferrite–pearlite steel. There was good agreement between simulation and experimental results.

Semi-solid metal forming offers many advantages in comparison with casting and forging. However, due to the high-melting temperature and related difficulties, there is relatively a few experimental data on the semi-solid processing of steels. Therefore, this study is subjected to the microstructure evolution during partial remelting of 304 stainless steel which is pre-deformed in solid state by a severe plastic deformation (SPD) technique. Thermodynamic calculations were carried out by Thermo-Calc software and the results were compared with the experimental observation. In this study, repetitive corrugation and straightening by rolling (RCSR) as a new SPD technique is utilized to induce a great strain in material prior to semi-solid process. Microstructural evolutions were studied in different temperatures and holding times during semi-sold process. The results indicate that finer globular semi-solid microstructure is achieved from 30 cycles RCSR processed specimen after 3 minutes holding at 1425.

In this study, the effect of Mechanical activation on Carbothermic reduction of Molybdenite in the presence of Magnesium oxide was studied. Mechanical activation of all powder mixtures (Molybdenite: Graphite: Magnesium Oxide=1: 2: 2) was performed in argon atmosphere for different time periods (0-50 hours), using a planetary ball mill. Phase changes in the powder mixtures were investigated by X-ray diffraction (XRD). The Results showed that no reaction occurred in the mill to produce new phases in the mixtures. Observations indicated that Carbothermic reduction of Molybdenite in the presence of Magnesium oxide was formed as easily as possible and goes on by formation of intermediate phases such as Molybdenum Oxide (MoO2) and Magnesium Molybdate (MgMoO4). The effects of Mechanical activation on the reduction of different samples were thoroughly investigated by simultaneous Thermal Analysis (STA). The samples were heated in argon atmosphere with the linear heating rates of 10, 15, 20oC/min. The plots of process conversion degrees showed that the reaction temperature in the reduction reaction starting temperature is strongly affected by the amount of mechanical activation. So that by doing 50 hours of milling, the temperature of the starting of reduction reaction was decreased by about 170oC. Kinetics reduction of powder mixtures was studied by model- fitting (Coats-Redfern) and model-free (Ozawa and Friedman) methods the dominating Model over the reaction was the chemical control at the intersection and the amount of activation energy and pre-exponential factor was determined to be about 251 kJ.mol-1 and 1´106 Sec-1.

The Inconel 713 Superalloy was used for turbocharged blades in the automobile industry. In this paper, changes in the microstructure and the hardness was studied as the temperature of the full and the partly solution heat treatments was changed in the range of 1000 to 1200oC in 1 hour. For studying the hardness of superalloy the Rockwell C hardness test was used and also the optical and the Scanning Electron Microscopy and the Electron Dispersive Spectroscopy and the X-ray Diffraction (XRD) were used to detect the microstructure and present phases in the superalloy. The results showed that the significant changes in the phases were observed when the temperature range was 100oC. The heat treatment caused to solute chromium carbides in the γ matrix. In addition, by increasing the heat treatment temperature, the g phase dissolved in (200) and (220) plane and this phase in the plane of (111) had the best stability in 1000oC. Phases of Ni5Al3, NiAl and Al0.42Ni0.58 were observed in the temperature of 1000, 1100 and 1200oC, respectively. As the temperature of the heat treatment increased the hardness increased 25%.

Titanium and its alloys are known as one of the most significant metallic materials used in the orthopedic and dental implants due to their excellent mechanical properties, corrosion resistance and biocompatibility. One of the main issues in dental implant is the fabrication of the biomaterials that have early and sufficiently strong bonding with the surrounding bone. In the present study, porous Ti6Al4V scaffolds were produced using magnesium as a space holder by powder metallurgy. The specimens were sintered in 950oC, below the β transition temperature, close to magnesium vaporization point. To evaluate the porosity and effect of magnesium on it, the micro structure was investigated by optical microscopy and SEM and then mechanical properties and electrochemical corrosion behavior of the specimens were studied. Biocompatibility was investigated by MTT test, and it was deduced that the cell proliferation and biocompatibility was increased with increasing the porosity. This investigation showed that the compressive strength and elastic modulus of the porous scaffold with 10% magnesium and 31% porosity are 155MPa and 9GPa, respectively and close to dental bone. Also, the corrosion results and cell proliferation showed the appropriate corrosion behavior and osseointegration of this scaffold. Due to the importance of strength in the dental implant and according to these results, the Ti6Al4V scaffold with 10%Mg could be an advanced alternative for clinical applications which two factors of strength and osseointegration are required.

Turbine blades operate on the different temperature and stress conditions. The various mechanisms contribute to the creep process and lead to turbine blade failure. The characterization of the creep mechanisms is critical to predict the creep behavior and design the useful life of turbine blade. This paper compares the creep behaviors of new and used blades (worked 3000 equal operating hours) based on IN738 superalloys by investigating the microstructure and using the creep models. Optical and scanning electron microscopes were applied to study the volume fraction, size of the primary and secondary precipitated phases (g’), change of the precipitated phases morphology (e.g. formed continuous carbide in grain boundaries or coarsening g’) and the formation of the creep pores. Furthermore, the dominated creep mechanisms are identified and compared the new blades with used blades at different stress rupture tests and real work conditions. The results show that at 760oC/586 MPa, the active creep mechanisms includes cutting and Orowan for both new and used blades. Whereas at 982oC/152 MPa, the dislocations climb, diffusion creep and grain boundary gliding are active for new blades and Orowan and the dislocations climb are operative for used blades.