Journal of Metallurgical and Materials Engineering
Year:2018 | Volume:29 | Issue:2 |Papers Count:15

Two groups of tool steel properties, namely high hardness and high toughness, are important for the performance of tool steels. Any increase in hardness occurs at the expense of reduced toughness. Deep cryogenic treatment may be used for increasing toughness, strength and hardness simultaneously. 12 sets of 1. 2542 tool steel specimens that 9 of which have been deep cryogenically treated for 24, 36, and 48h at-196 oC and have been tempered for 1, 2, and 3h at 200 oC and then double deep cryogenic treatment has been done. 3 sets were selected as standard specimens (control specimens). The best results, simultaneous improvement of 13. 8% in toughness, 8. 5% in hardness, 26. 4% in tensile strength and 23. 4% in yield strength were obtained for the specimen treated at-196 oC for 36 hours and then tempered at 200 oC for 1 hour (specimen 361).

In this study, the effect of retained austenite to martensite phase transformation has been studied in conjunction with wear behavior of white cast irons. For this purpose, the wear behavior of an alloyed white cast iron with a carbideaustenite microstructure has been studied in comparison to that of conventional white cast iron with a pearlite-carbide microstructure. The wear tests were conducted by using pin on disk test method with loads of 80, 100, 120 and 140N. The results show that retained austenite is stable and also work hardened during wear test conducted with loads less than 100N in alloyed white cast iron samples, while, the retained austenite transformed to martensite after using loads greater than 100N. The retained austenite to martensite phase transformation has been associated with significant improvement in wear resistance in the alloyed white cast iron.

The effect of heat treatment of nickel-aluminum bronze alloy on the interaction of the cathodic surface and its corrosion behavior in 3. 5% NaCl solution is studied. The alloy was studied under different industrially recommended heat treatments, including quenching, normalizing and aging heat treatments. The microstructure of the specimens was studied by optical microscopy. Polarization tests proved that with increasing immersion time, and the formation of the protective layer, helps extend the oxygen-diffusion controlled cathodic reactions and also increase corrosion resistance. Polarization curves showed that the anodic polarization behavior is almost the same for all samples, which suggests that activation is controlled by the heat treatment of the samples. The different behavior under the influence of cathodic polarization proves that the alloy also reflects the increase or decrease in the level of corrosion resistance.

A relatively new method, homogenization in liquid phase, was used to produce aluminum-matrix composite reinforced by graphite particles. The graphite content was chosen from zero to 4. 5wt. % in this study. Composite samples were produced using the mixed powder by two ways: spark plasma sintering and cold-press sintering under different pressure, temperature and time conditions. The specimens’ microstructure was investigated by optical microscopy and scanning electron microscopy. To evaluate the mechanical properties of the samples, compression tests and hardness measurements were performed. According to the results, homogenization in liquid phase method caused significant improvements in the distribution of graphite particles in the matrix. Improved mechanical properties were obtained at 3wt. % graphite. According to the results, the compressive strength of Al-2. 5wt. % Gr composite, obtained from this mixing process and cold-press sintering, was increased at least 113. 8% compared to the pure aluminum. Moreover, SPS process increased the strength of Al-3wt. % Gr composite to about 206. 2%, compared to pure aluminum.

Different amounts of nitrogen were added to nickel-free high manganese austenitic stainless steels with addition of a nitrided ferroalloy. The microstructure was evaluated by optical microscopy and the mechanical properties were determined by macro-hardness and tensile tests. The corrosion behavior was evaluated by electrochemical impedance spectroscopy and polarization. The results showed that the microstructure of high nitrogen and low nitrogen steels were austenite and ferrite and austenite, respectively. The hardness, yield strength and ultimate tensile strength of the steels increased with increasing nitrogen content. In addition, corrosion current densities decreased and corrosion potentials shifted to more positive values with increasing nitrogen content.

Nitinol shame memory alloy is very useful in manufacturing of medical implants and stents. Thermal oxidation and TiN coating are of common Nitinol surface modification processes and in return Fenton's oxidation using Fenton reagent is a new approach in modifying the surface of Nitinol for medical applications. To study the effects of this novel process, experimental tests including oxide surface characterization, investigation of corrosion behavior and biocompatibility of the created oxide surfaces in simulated body fluids were performed. Parameters such as temperature and duration of oxidation process were studied. Based on the results, the optimized conditions considering temperature and process duration were determined. Accordingly, Fenton's oxidation process at 65 oC for 24 hours can create an oxide surface without any cracks which produces desirable biocompatibility and corrosion resistance and helps to improve its in vivo performance.

In this study four layers of zirconia coating were applied on the surface of API5L steel samples by sol-gel method using zirconium isopropoxide precursor. In the first group of samples, after each layer of coating and for the second group after four-layer coating, heat treatment (calcination and sintering) were carried out at the desired temperatures (350-550  C). X-ray diffraction and differential thermal analysis were used to evaluate the physico-chemical reactions and phase changes in the obtained gels caused by heat treatment. Microstructural features, adhesion strength and corrosion resistance of the coated samples were determined by scanning electron microscope, microhardness, adhesion and electrochemical corrosion testing, respectively. The results showed that by increasing the sintering temperature from 350 to 550  C, the structure evolved from amorphous to crystalline state. For the first group of samples, sintering at 450  C and for second group, sintering at 550  C offered a more homogenous microstructure and improved corrosion resistance.

In this study, the pitting corrosion behavior of 403 martensitic stainless steel was investigated in NaBr and NaI solutions, by using potentiodynamic and potentiostatic techniques. The observations showed that halide ions have a bilinear effect on the pitting corrosion. The bromide ion due to its smaller ionic radius is more active in the initiation step of metastable pits, so that it increases the frequency of occurrence of metastable pits. On the other hand, the iodide ion has a greater effect on the propagation step, due to the low acidity of HI. Altogether, the bromide ion is more aggressive than iodide and has a lower pitting potential.

Zn– Ni alloy coatings were electrodeposited from acidic sulphate baths. In this study, the effect of plating parameters and bath composition on the cathodic current efficiency, as well as hardness, morphology and chemical composition of the coatings were investigated. The morphology and chemical composition of the coatings were investigated using scanning electron microscopy equipped with EDAX analyzer. Results showed that surface morphology and chemical composition of the films were strongly dependent on the electrodeposition parameters. Ni content and hardness of the coatings were increased and the cathodic current efficiency was decreased by enhancing the bath temperature and Zn (II)/Ni (II) ratios, or reducing the current density and bath pH. Electron microscopy images showed that cracks appeared on the coatings surface with increasing temperature due to the high residual stress. In addition, the structure of the coatings became finer with increasing the current density, resulting in the formation of voids and porosities. Studies on the bath pH revealed that only the coatings which were deposited in baths with pH=2 had a dense and uniform surface. Moreover, increasing the Zn (II)/Ni (II) ratios in the bath caused brittleness of the coatings.

Tissue engineering, as an interdisciplinary science, is culturing cell body or stem cells outside the body and depositing grown and differentiated cell complex on substrate biomaterials into a patient's body. PLGA and chitosan are commonly used for tissue engineering applications, respectively as synthetic and natural polymers. In this study, nano-biocomposite scaffold of tissue engineering was prepared by electrospinning/electrospraying method. Nanofibers with random and aligned arrangement were prepared. The average particle size and fiber diameter was measured using Image J analysis software.

In this research, effect of carbon fiber (Cf) and its content on sinter-ability, hardness and fracture toughness of ZrB2-SiC composites which were produced by spark plasma sintering, was studied. For this purpose, four composites with different Cf content (10, 20, 30 and 40 vol%) were sintered by spark plasms at 1800 ° C, 40 MPa pressure for 6 minutes by with a heating rate of ~50 ° C/min. In order to evaluate the microstructure, scanning electron microscopy, EDS and x-ray diffraction was applied. The hardness and fracture toughness were measured by macro-Vickers and SENB method, respectively. In this research, Cf addition does not have any significant effect on hardness. The results showed by increasing Cf, relative density decreases and open porosity percent increases. Fracture toughness of ZrB2-SiC composites was improved by increasing Cf content.

In the present study, the HA-TiO2 nanostructured composite coatings with 0, 10 and 20 wt. % TiO2 were fabricated by electrophoretic deposition on Ti-6Al-4V substrate. Micro-scratch tests in progressive load mode, were examined for evaluation of adhesion strength, tribology and fracture toughness of coatings. The critical contact pressures in micro-scratch test for crack initiation and delamination were increased by the addition of TiO2 content in the coating. Moreover, according to the linear elastic fracture mechanics, the maximum fracture toughness of coatings was obtained for HA-20 wt. % TiO2 sample. The scratch grooves were studied by scanning electron microscope.

The study purpose was to evaluate the effect of initial billet temperature, extrusion speed, extrusion ratio and angle on the tube extrusion process of AISI304, particularly the extrusion force, and achieve optimal parameters. Abaqus software was used for the simulation purpose. 12 samples were evaluated with different initial conditions. Two-dimensional axisymmetric model was used to simulate the process, due to the axial symmetry of the process. Mechanical-thermal coupled solution was used for process solution. Also, CAX4RT elements were used for achieving all the degrees of freedom. The initial temperature was found to have the strongest effect on the force.

In this work, Mn0. 5Zn0. 5Fe2O4 ferrite powders were prepared through glycine-nitrate combustion method at different molar ratios of glycine to nitrate. X-ray diffraction patterns showed that samples crystallized successfully in a spinel-type structure in all reactions. Magnetic properties were measured using a vibrating sample magnetometer. The saturation magnetization values of the as-synthesized samples were found to be in the range of 59. 5-63. 3 emu/g. Dielectric and electromagnetic properties were examined by a LCR meter. The sample synthesized at the lowest G/N ratio showed the lowest magnetic loss and the highest permeability. The complex impedance analysis illustrated only one semicircle indicating the predominant effect of grain boundary property of the material.

In this paper, the influences of metallurgical phenomena and texture in addition to the thermomechanical parameters on the warm flow stress of austenite in a gas line-pipe microalloyed steel have been investigated. Warm compression tests were carried out in the temperature range of 850 to 950° C under various amounts of strain, strain rates and holding times. The microstructure and texture of the samples were then investigated by scanning electron microscopy and electron back scatter diffraction technique. The results indicated that deformation temperature and strain via strain induce precipitation of microalloying elements during annealing impose the fraction of recrystallization in the material.