Key Engineering Materials Vol. 479

Abstract: Nickel-free high nitrogen stainless steel has become more important due to its impressive mechanical and corrosion properties. In contrast to high pressure processes, melting plasma in a normal atmosphere is an alternative way of obtaining high nitrogen content at low cost. However, melting in such an atmosphere will bring some surface impurities, like sulfur and oxygen, into the stainless steel through the refractories or the materials themselves. Therefore, this research aims the relationship between the sulfur and nitrogen content, and their influence on biocompatibility. Thermo-Calc is first used to design the composition of Fe-Cr based austenitic nitrogen stainless steel, and melted with different melting techniques, including plasma and an induction melting furnace. The results indicate that the nitrogen content varies with different sulfur content. It is also found that α-Fe (Ferrite) plates are observed near the grain boundaries when the sulfur content reaches a certain level. Besides, the Sulfur content has an obvious influence on the biocompatibility rather than nitrogen contents.

Abstract: The mechanism of brittleness due to slow cooling during quenching was experimentally investigated in 12% Cr martensitic heat resistant steel. The mechanical property tests and microstructural characterization by SEM、TEM and XRD were conducted. The results showed the impact toughness would decrease with the slowing of cooling rate during quenching, and the low cooling rate within the temperature range from 820 to 660 °C played a significant effect on the impact toughness . Different from the mechanism of temper brittleness, the main causes of embrittlement due to the slow cooling upon quenching were both the continuous precipitation of M23C6 along prior austenite grain boundaries during the process of slow cooling and that of M2C along prior residual austenite film during tempering, and this kind of quench brittleness was nonreversal.

Abstract: Inclusion in steel material plays a decisive role on the purity of steels that becomes more important in the energy critical age. This study was focused on the number and morphology of inclusions with different cooling rate in the continuous casting process. A low carbon steel with 3.36 wt% silicon content was used as test material, which was soaked at 1100°C, 1250°C and 1400°C for 2 hours. The analyzed results of microstructure and chemical compositions showed the inclusions were not dissolved into matrix but formed as compounds like oxide, sulfide, and nitride after reheating at 1100°C. However, the inclusion size and average number possessed increasable trend, compared to as-cast sample. Manganese sulfide began to be dissolved into matrix by reheating at 1250°C. Some evidences showed the dissolution of aluminum nitride under the reheating at 1400°C. The inclusion size varied from 8 μm to 3 μm, and average number decreased with increasing soaking temperature.

Abstract: The molybdenum alloy sheets composite strengthened by silicon and lanthanum oxide were prepared by powder metallurgy technology with Mo-La2O3(0.3wt%) and Si(0, 0.1, 0.3wt%) powders and thermo-mechanically processing. The influences of silicon content on the microstructure and mechanical properties of the final molybdenum alloy sheets were tested and analysized. The results show that the addition of lanthanum oxide and silicon can refine the alloys grain size. The introduction of lanthanum oxide particles can increase the yield strength. Although the molybdenum alloys with 0.3wt% silicon have solid solution strengthening effect, the alloys with 0.1 wt% silicon exhibits obvious solid solution softening effect at room temperature. The strengthening mechanisms are quantitatively assessed, which well explain the increase or decrease in yield strength with respect to grain size, lanthanum oxide particle and silicon solid solution.

Abstract: Magnesium materials are gaining an increasing interest especially in transportation industries. The goals are the reduction of fuel and the reduction of the green house gas CO2. To achieve these goals, the weight of vehicles must be reduced. Compared to Al based materials, Mg based materials offer the possibility of saving up to 30% in weight. Unfortunately the latter suffer from poor workability. The workability of AZ91B has been proven to be improved significantly by employing the rapid solidification process, Spray Forming. In this study, the AZ80 Mg alloy was synthesized via Spray Forming process. The microstructures were characterized and compared with conventionally cast materials. The workability of the spray-formed AZ80 Mg alloys was studies via a simple extrusion process. The effects of the extrusion speed and temperature on the workability were investigated.

Abstract: In this paper, mechanical performance test, slow strain rate test (SSRT) and optical microscopy analysis were employed in studying the effect of carbon migration on sulfide stress corrosion cracking (SSCC) behaviors of Cr5Mo/A302/0Cr18Ni9 dissimilar joints. The results showed that the resistance to SSCC of this joint improved first then decline after 98h aging treatments. Optical microscopy analysis showed fracture located in coarse grain heat-affected zone (HAZ) of as-welded joint, while in the joints aged for 56h and 98h, the fracture position transferred from coarse grain zone of HAZ to carbon rich belt in welds.

Abstract: The microstructures and mechanical properties of a dissimilar metal weld A508/52M/316L used in the primary water system of pressurized water reactor (PWR) nuclear power plants were investigated. The weld exhibits complicated microstructures, with significant change around the interfaces A508/52M and 52M/316L. The variations of main elements in 52M weld metal are greater than those in the A508 and 316L, with significant changes in the zones closed to the interfaces. The bulk 52M weld metal has higher and more uneven hardness than both of the base metals A508 and 316L. The HAZ of A508 exhibits the highest hardness value in the weld. The area around the A508/52M interface is the most weak part for stress corrosion cracking (SCC) resistance of the weld in simulated PWR primary water at 290°C. SCC was only found in the specimens tested at +200mV(SHE) but not in those tested at both -780mV and Ecorr (about -500mV).

Abstract: In the current study, the amorphization behavior of mechanically alloyed Ni57Zr20Ti22Ge1 powder was examined in details. The conventional X-ray diffraction results confirm that the fully amorphous powders formed after 5 hours of milling. The thermal stability of the Ni57Zr20Ti22Ge1 amorphous powders was investigated by differential scanning calorimeter (DSC). As the results demonstrated, the glass transition temperature (Tg) and the crystallization temperature (Tx) are 761 K and 839 K, respectively. The supercooled liquid region ΔT is 78 K. The appearance of wide supercooled liquid region may be mainly due to the Ge additions which cause the increasing differences in atomic size of mechanically alloyed Ni57Zr20Ti22Ge1 powders.

Abstract: Microstructure evolution of high energy milled Al-50wt%Si alloy during heat treatment at different temperature was studied. Scanning electron microscope (SEM) and X-ray diffraction (XRD) results show that the size of the alloy powders decreased with increasing milling time. The observable coarsening of Si particles was not seen below 730°C in the high energy milled alloy, whereas, for the alloy prepared by mixed Al and Si powders, the grain growth occurred at 660°C. The activation energy for the grain growth of Si particles in the high energy milled alloy was determined as about 244 kJ/mol by the differential scanning calorimetry (DSC) data analysis. The size of Si particles in the hot pressed Al-50wt%Si alloy prepared by high energy milled powders was 5-30 m at 700°C, which was significantly reduced compared to that of the original Si powders. Thermal diffusivity of the hot pressed Al-50wt%Si alloy was 55 mm2/s at room temperature which was obtained by laser method.