Key Engineering Materials Vol. 658

Abstract: Sn-Zn alloy is one of the Pb-free systems that are promising because of its relatively low melting temperature and low cost. However, Zn exhibits poor corrosion and oxidation resistance that hinders its soldering applications. The objective of this research is to study the effect of Zn and Cu alloying contents on the Sn-Zn performance. Four compositions of Sn-Zn alloys were studied in this research including: Sn-7Zn, Sn-9Zn, Sn-9Zn-2Cu-Bi, and Sn-9Zn-4Cu-Bi. The microstructures were studied using Optical Microscope (OM) and Scanning Electron Microscope (SEM). The melting temperatures and corrosion resistance of the alloys were evaluated by Differential Scanning Calorimeter (DSC) and Potentiodynamic Polarization technique, respectively. The results showed that an increase in the Cu-to-Zn ratio led to better corrosion resistance. The selective corrosion of the Zn-rich phase could be visibly observed, with OM, on the post-corrosion samples. With the Cu alloying, the Cu and Zn formed an intermetallic compound resulting in a higher value of E_corr. However, the higher Cu content caused a significant increase in the liquidus temperature due to the Cu-Zn intermetallic compound, of which the melting temperature is higher than 400 °C, resulting in an incomplete melting at low temperature.

Abstract: Microstructural development and mechanical properties of as cast and heat treated 935 Ag-Cu and Ag-Cu-Be-Sn alloys without using solutionization were investigated. The conventional lost-wax process with vacuum assisted and induction heating has been performed for producing the as cast specimens. After pouring the melt into plaster mould, the casting has been prolonged for 15 minutes prior to quenching into water. Subsequently all specimens including the reference specimens, commercial specimens and Ag-Cu-Be-Sn additions specimens were aged at 250 and 350°C for a given period between 0 to 120 min according to normal aging procedure of the 935 AgCu alloy. In case of Be and Sn additions, the A1 alloy with aging at 350°C performed the combination of good mechanical properties and the surface finish with superior anti-tarnish and fire scale resistance and thus these are suitable for jewelry applications. Microstructures were characterized by using both scanning and transmission microscopes. The results show that the precipitates in the 935 AgCuBeSn alloy were detected and thus resulted in a better improvement in yield strength and modulus of resilience than those of the 935 AgCu alloy

Abstract: Advanced high strength steel (AHSS) was prepared using the conventional ‘press and sinter’ process.The compacts of ultralow carbon Fe-Cr-Mo powder with carbon additions (base metal powder admixed with 0.1,0.2 and 0.3 wt.% graphite) and without carbon addition (plain base powder) were sintered in a vacuum furnace at pressure of 1.28 x 10^-5MPa at 1280 °C for 45 min. After sintering, the sintered specimens were continuously cooled with different nitrogen gas pressures of 0, 2500 and 5000 mbars (or 0, 0.25, 0.5 MPa). Mechanical properties of the sintered alloys were strongly controlled by carbon contents and cooling rates after sintering. The sintered specimens, with 0.3 wt.% carbon and cooled by nitrogen of 5000 mbars, showing superior tensile strengths and good ductility, had microstructures dominated by carbide-free bainitic structures and some retained austenite. The sintered specimens with lower carbon contents and cooled under slower cooling rates, having lower tensile strengths but slightly higher ductility, had microstructures with lower bainite volume fractions and even without bainitic structures. The dominant phase in the sintered specimens with low strength but high ductility was ferrite.

Abstract: Effects of destabilisation heat treatment on microstructure, hardness and corrosion resistance of 18wt.%Cr and 25wt.%Cr irons have been investigated. The as-cast samples were heat-treated by destabilisation at 1000°C for 4 hour and then air cooling. The microstructure was investigated by light microscopy and scanning electron microscopy. The results show that the as-cast microstructure in 18wt.%Cr iron consists of pearlite, formed by decomposition of primary dendritic austenite, plus eutectic structure. In the 25wt.%Cr iron with lower hardness, the microstructure consists of primary dendritic austenite plus eutectic structure. The austenite had partly transformed to martensite, especially at areas adjacent to eutectic carbides. After destabilisation, the microstructure of both irons consists of eutectic and secondary carbides in a martensite matrix giving increased hardness. It was found that corrosion resistance of the irons was improved after destabilisation. The 25wt.%Cr showed superior corrosion resistance than the 18wt.%Cr iron due to greater residual Cr in the matrix to encourage passivity.

Abstract: Automotive parts made of ultra-high strength steels (UHSS) have been increasingly produced by hot stamping or press hardening of boron alloy steel. In case of novel hot formed components with tailored properties, different heating cycles needed to be applied for different zones, in which varying microstructure characteristics were generated. Mechanical properties of these parts were thus precisely controlled by the microstructure constituents. In this work, stress-strain behaviors of a boron alloy steel undergoing different heat treatment conditions with respect to that modified hot stamping procedure were predicted. Firstly, boron alloy steel sheet specimens were heated up to the austenitization temperature. Afterwards, they were abruptly cooled down to the bainitic temperature range, held for different holding times and finally cooled to room temperature. The microstructures obtained from each condition were characterized by optical microscope (OM) using color tint etching. The stress-strain responses of all generated microstructures were determined by tensile test. By the modeling, flow curves of the individual single phases were described taking into account a dislocation theory based model and their chemical composition. Subsequently, effective flow curves of the heat treated boron alloy steels were calculated by means of the isostrain and non-isostrain method and were finally compared with the experimentally determined curves.

Abstract: One of the surface modification processes for high-temperature oxidation resistance is slurry aluminizing process, forming protective layer of alumina (Al₂O₃). However, several important parameters such as annealing times and temperatures should be intensively considered. The objective of this study is to improve the process of slurry aluminide coating of ferritic stainless steels type AISI430 (16%Cr) combat to high-temperature oxidation. The specimens were cut, then ground, and finally sprayed with slurry mixture (Al powder + polyvinyl alcohol (PVA)). They were annealed in Ar at 1100°C for 15 minutes in order to eliminate PVA and form aluminide on their suface. The protective layer Al₂O₃ was finally formed in the temperature range of 900-1100 °C for 15-60 minutes. The cyclic oxidation tests were performed at 1000 °C for 24 hours. The surface morphology were then examined by XRD, SEM equipped EDS. The results showed that all oxidation kinetics of coated specimens were parabolic. The oxidation rate of uncoated specimens was apparently higher than that of coated specimens. Comparing with all coated specimens, the oxidation rate decreased with the increasing temperature and annealing time. In this study, the coating process at 1100°C for 60 minutes exhibited the lowest oxidation rate due to the most complete layer of Al₂O₃. The surface morphology showed the formation of continuous layer of Fe₂Al₅ and Al₂O₃, acting as barrier layer to oxide growth. Effect of temperature and time on oxidation resistance were discussed in this study.

Abstract: Due to high wear resistance, Cobalt-based alloys, such as Stellite12 (52Co30Cr8.5W), have presently been used as materials for hardfacing in several applications. Thermowell, a protecting part for thermocouple in petrochemical production, is also coated by Stellite12. Because of high deposition rate, the flux-core arc welding (FCAW) method was selected to be hardfacing process in the research. However, their welding parameters should be exactly controlled in order to obtain desired properties, depending on the microstructure of this material. The objective of this experiment is to study the influence of the FCAW parameters on the erosion resistance of AISI 304 (Fe-18Cr-8Ni-0.06C) welded by the cobalt base alloy filler, Stellite12 (Co-30Cr-8.5W-1.5C) as the hardfacing layer. The studied parameters were welding speed in the range of 2.1-8.5 mm s^-1, and wire feed speed in the range of 42.3-67.7 mm s^-1, leading to different heat inputs and cooling rates. The erosion resistance was investigated by using solid particle erosion test rig at ambient temperature. Surface characterization was then carried out by SEM equipped EDX and XRD. The results showed the relationship between erosion resistance and microstructure in welding and HAZ zone. The erosion resistance was depended on the formation of interdendritic phase in the welding zone. According to the surface examination, it was found that the formation of interdendritic, including size and shape played an important role on the erosion resistance. The heat input and cooling rate concerned with welding parameters was further discussed with the erosion behavior in this research.Keywords Stellite12, Cobalt base alloys, Flux-core arc welding, FCAW, Erosion-resistance

Abstract: Single Sensor Differential Thermal Analysis (SS-DTA) is a novel non-destructive testing technique for studying and detecting the phase transformations and structural changes in materials. It uses only one temperature sensor to measure the temperature in a particular point of interest in the material during actual and simulated thermal processing of the material. SS-DTA compares the temperature recorded in a tested specimen against a reference thermal profile which can be generated either by analytical formulae or piecewise linear approximation. The main advantage of piecewise linear approximation over the analytical formulae is that it does not need the knowledge of tested material and processing conditions to optimally estimate the parameters of reference thermal history. On the other hand, in order to apply the piecewise linear approximation technique we must specify the segment width which is normally fixed at a nominal value of 1.5 seconds. We have recently found that this nominal value might not be an optimal choice for the segment width as it does not guarantee to give the best detectability of phase transformation. Therefore, in this research work we proposed a technique to automatically select an appropriate value of the segment width. The performance of proposed method has been evaluated by investigating the phase transformations of welded stainless steel SUS 321and SUS 304. It was found that the appropriate segment width could be ranging from 1.25-1.75 seconds and by using this selection technique, we could detect the differential temperature more accurately than when using the nominal value.

Abstract: Photovoltaic (PV) has recently undergone impressive growth and substantial cost decreases. Basically wafer-based crystalline-Si PV technologies have the advantage of higher module efficiency as compared to thin-film PV, but thin-film PV has the advantage of lower production cost. The silicon-based solar PV needs light-induced charge separation at the p-n junction between two slices (wafers) of doped silicon in either single-crystal silicon (sc-Si) or polycrystalline (poly-Si). However until recently thin-film PV modules both amorphous silicon (a-Si) and non-silicon thin film technology have been advantageous developed. Metallic based modules such as cadmium telluride, CdTe and copper indium gallium diselenide, CIGS thin-film PV technologies have currently efficiencies of 16.1% and 15.7%, respectively. A high efficiency makes thin-film PV technologies more competitive with wafer-based crystalline-Si PV. This study investigates the electricity generation of both silicon based and non-silicon based solar PV modules. The implementation uses solar irradiation with average of higher than 18 MJ/m².day in high solar radiation provinces in Thailand. A High solar radiation is observed in mostly in central and the east regions of the country. The result shows that the commercial amorphous PV module is appropriate for large scale installation while wafer-based crystalline-Si PV can be installed both in cases of solar rooftop and solar PV farm. Thin-film PV modules both silicon based (a-Si) and non-silicon based is basically appropriate for small installation such as solar rooftop and building integrated PV (BIPV). But in the near future the metallic based PV modules will be competitive with crystalline-Si PV in terms of both efficiency and with its lower cost.

Abstract: The mechanical adhesion of thermal oxide scales formed on industrial hot-rolled steel strips produced through the electric-arc-furnace route was studied. The samples as-received steel were prepared with the specific shape fitting to the micro-tensile machine in the SEM chamber for observation of surface failure during the test. It was found that oxide transverse cracking and scale spallation are observed. This paper also presented a theoretical model for evaluate adhesion energy from strain and stress values at the first spallation. It was found that the oxide scales on the medium slab exhibited high mechanical adhesion energy. This might be due to the existence of oxide contained Si at steel-scale interfaces. It can promote adhesive interactions between steel-scale interfaces.