Abstract: Austempered ductile iron (ADI) has been widely used in various industries due to its excellent combination of high strength, ductility and good wear resistance. The tensile behavior of an unalloyed commercial ADI with a multiphase structure designed by a novel multi-step austempering treatment is investigated. The developed austempering process consists of austenitizing at 890°C for 20min, then initial rapid quenching to 180°C, and isothermal holding at 190, 220, 250°Cfor 120min, and finally air cooling to room temperature. The optimum mechanical properties with an ultimate tensile strength of 1350MPa, a yield strength of 1090MPa, as well as an elongation of 3.5% is achieved at 220°C. This is attributed to a synergistic strengthening effect of multiphase structure including a prior martensite with fine needle bainitic ferrite and film retained austenite.
Abstract: Dissimilar tungsten inert gas (TIG) welding of furnace tube alloys, T92, T91 and T22, using TG-S2CM, TG-S2CW and TG-S9Cb fillers is performed in the experiment. Post weld heat treatments (PWHTs) of specimens selected from wire cutting of the qualified weldments are carried out in order to evaluate carbon migration phenomena. The soft zone resulted from carbon migration can be evaluated by optical microscope and microhardness tests. Based on the long-term aging test results, obvious soft zone is found from the weldment with significant difference in alloying concentrations. There is no soft zone in T92/T91 dissimilar weldment using the TG-S9Cb filler after the aging test. In contrast, soft zones are observed from the interfaces, TG-S2CM/T92 and TG-S2CW/T92. The size of soft zone (~50 μm) using TG-S2CW filler is much less than that using TG-S2CM filler (~200 μm). The application of TG-S2CW filler shows potential in welding/repair welding of T92/T22 tubes in supercritical boilers.
Abstract: We evaluate the properties of friction welded steel tubes in terms of the relationship between microstructures and mechanical properties. For this work, steel tubes with a diameter of 60.5 mm, which has a thickness of 3.3 and 2.2 mm, was prepared by length of 170 and 70 mm, and friction welding was conducted at a rotation speed of 2,000 rpm and upset time of 5 s, respectively. Applying the friction welding led to the notable grain refinement so that average grain size was refined from 15.1 μm in base material to 4.5 μm in welded zone. These refined grains achieved the significantly enhanced microhardness (20% higher) and a slightly higher tensile strength relative to the base material, respectively. In particular, the tensile tested specimens showed the fracture aspect at the base material zone not at welded zone, which means the soundly welded state.
Abstract: In this study, the structures of Al-5Ti-1B master alloy and its influence on microstructures and mechanical properties of A356 alloy were investigated. The results show that Al-5Ti-1B master alloy consisted of the uniform distribution of lump-like TiB2 and network of TiAl3 on α-Al matrix. The addition of the Al-5Ti-1B master alloy can significantly reduce the grain size of A356 alloy. The mechanical properties of A356 alloy, i.e. ultimate tensile strength, yield strength and elongation were also improved. The use of Al-5Ti-1B master alloy as a grain refiner in the casting process of A356 alloy can effectively enhance the grain refinement and thus improve the mechanical performance of A356 alloy.
Abstract: In the present work, Al-Zn-Mg alloy having highest zinc content was deformed by one of the severe plastic deformation (SPD) technique, equal channel angular pressing (ECAP) and effect of ECAP on the microstructure evolution and the wear properties were studied. ECAP was performed in a split die and the channels of the die are intersecting at an angle of 120º. ECAP was attempted at least possible temperature and the alloy was successfully ECAPed at 423 K. Below this temperature samples were failed in the first pass itself. After ECAP, significant drop in the grain size was reported. Also, ECAP leads to significant raise in the microhardness of the alloy. Predominantly, after ECAP, upsurge in the wear resistance of the alloy was noticed. To figure out the response of ECAP on the wear properties of the alloy; worn surfaces of the wear test samples were analyzed in SEM.
Abstract: Compressive residual stress below the surface of material could increase fatigue life as it encounters the tensile loading applied on the material during operation. Shot peening process is a common surface treatment to introduce this stress. This study will investigate on how to introduce the same amount of residual stress by simulation using FEM as introduced in experimental shot peening process. Actual shot peening process was done using a particular sets of parameters and FEM with single shot is used to simplify the simulation procedure. Result shows that using a single shot simulation could also introduce the equivalent amount of residual stress as in the experimental multi-shots shot peening process. This value could be used in further study to study the relaxation of the stress after load is being applied.
Abstract: In the experiments of different process parameters (laser power, scanning speed and scanning distance),the low-time defects of forming part were studied by microscope,including air bubble, pore, micro-crack and macro-crack. The formation mechanism of bubble-defect was analyzed.Linear energy density (E=P/v) was introduced as synthetic parameter.According to analysis and test verification, the optimum technological parameters of 316L stainless steel were laser power 190-210KW, laser speed 800-1000mm/s and scanning interval 0.9-0.11mm,and the linear energy density was about 200J/m. There were no cracks, no bubbles, a small amount of porosity, and the product density reached 99.7%.
Abstract: High nitrogen stainless steel (HNSS) is becoming an increasingly important engineering material because of its excellent corrosion resistance and good mechanical properties. A hot rolled Cr19Mn19Mo2N0.7 HNSS, having a microstructure consisting of banded ferrite and dotted/dendritic nitrides dispersed in the austenite matrix, was solution treated at 1160oC for various times. It is shown that the nitride can be dissolved, and the shape of ferrite can be changed by the solution treatment. The microhardness of both austenite and ferrite decreases with increasing solution treatment time. This is attributed to an increased grain size and a homogeneous distribution of alloying elements. The results are helpful in controlling the thermomechanical processing of HNSSs in commercial practice.
Abstract: The corrosion behavior of galvanized steel in corrosive media of simulated concrete pore solution (SPS) with and without NaCl has been studied for applying it to porcelain insulator’s pins in high-voltage alternating current (HVAC) transmission line. It was found that zinc coating of galvanized steel was corroded increasingly for the first 3 days corrosion test in SPS mixed with NaCl solution. After that the passive layer was formed on the surface of galvanized coating and consequently, the corrosion rate was dramatically decreased nearly five times of initial corrosion rate after 21 days. In this experiment, the investigations have been performed by SEM, optical microscope and visual observation. The results show that this compact and well adherence passive layer can be effectively hindered the corrosion process.