Key Engineering Materials Vols. 373-374

Abstract: The present paper reviews the scientific development of our understanding of S-Phase. It is now known that S-Phase formation is an example of para equilibrium phenomena. A necessary but not sufficient condition for S-Phase formation is the presence of an fcc structure at least in part with structure in the starting alloy. An essential requirement is for a nitride forming element to be present particularly Cr. After surface engineering with carbon, nitrogen or carbon and nitrogen to generate supersaturated solid solutions, the various tribological, corrosion, mechanical and microstructural studies are reviewed for the various alloy systems. The current industrial status of S-Phase technology on an international basis is examined and the potential for its acceptance in china is discussed.

Abstract: The novel low temperature plasma alloying technique that simultaneously introduces both nitrogen and carbon into the surface of austenitic stainless steel has been used in the past to create a hybrid N-C S-Phase. This S-Phase layer boasts of high hardness and wear resistance without any detriment to corrosion resistance. In this study, the afore mentioned hybrid N-C S-Phase was successfully implemented in the surface of two medical grade austenitic stainless steels: ASTM F138 and F1586. At an optimum process temperature of 430°C a very hard, 20μm precipitate-free S-Phase layer was created. Anodic Polarization tests in Ringer’s solution showed that the corrosion resistance of this layer was similar to that of the untreated alloys. Both dry-wear and corrosion-wear (Ringer’s) behaviour of the surface treated alloys showed an improvement of more than 350% and 40% respectively when compared to the untreated material.

Abstract: Thermal barrier coatings (TBCs) fabricated by electron-beam physical-vapor deposition (EB-PVD) were irradiated by high-intensity pulsed ion beam (HIPIB) at an ion current density of 100 A/cm2 with a shot number of 1-10. Microstructural features of the irradiated EB-PVD TBCs were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. All the HIPIB-irradiated EB-PVD TBC surfaces present smooth and densified features. The originated intercolumnar channels growing out to the top-coat surface and nanometer-scale gaps inside each single column were sealed after the remelting of TBC surface induced by HIPIB, resulting in formation of a continuous remelted layer about 1-2 μm in thickness. The dense remelted layer can work as a barrier against the heat-flow and corrosive gases, and gives the possibility of improving thermal conductivity and oxidation resistance of the HIPIB irradiated EB-PVD TBC.

Abstract: In order to improve wear resistance of carbon steel, laser cladding experiments were carried out using a 3kW continuous wave CO2 laser. The diameter of the laser beam was 3-5mm, the scanning velocity was 3-10mm/s, and the laser output power was 1.0-1.3kW. The experimental results showed that MoSi2/SiCP composites coating could be in-situ synthesized from mixture powders of molybdenum, silicon and SiC by laser cladding. A good metallurgical bond between the coating and the substrate could be achieved. The microstructures of the coating were mainly composed of MoSi2, SiC and FeSiMo phases. The average microhardness of the coating was about HV0.21300, about 6.0 times larger than that of steel substrate.

Abstract: The effect of thermal annealing temperature on the microstructure and phase composition of nitrided layer formed on AISI 304 stainless steel by plasma nitriding were investigated. The phase composition and structure of the nitrided have been analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis of nitrided samples at 420oC showed the presence of γN phase in the nitrided layer. The temperature at which the nitrogen expanded austenite started to decompose was about 450oC. Above this critical temperature the γN phase transformed into a mixture of CrN and α-phase. The surface hardness of nitrided layer also slightly changed with annealing temperature.

Abstract: The plasma nitriding (PN) process in the duplex surface treatment was controlled to create nitrided diffusion layers with depths of 0, 5, 15 and 80 m in the substrate of the DIN 1.2367 hot-work tool steel with the maximum microhardness values of 600, 700, 820 and 1000 HV, respectively. The scratch properties, i.e. the critical loads of cohesion (LC1), adhesion (LC2), breakthrough (LC3) and worn out (LC4), of the PACVD TiBN coating (boride, 5-7 at.%) on these substrates increased linearly with the maximum hardness of the PN diffusion layer. Instead of the composite hardness, the peak scratch hardness was used to describe the load-carrying capacity of the TiBN coating and PN substrate. Deep tensile cracks in the PN substrate with a hardness value of 1000 HV formed during the scratch test at a load as low as 90 N, indicating the low fracture toughness of the substrate. Therefore, an optimum balance between the scratch properties of the coating and the good fracture toughness of the nitrided substrate must be achieved through exercising the control of the PN and PACVD duplex process.

Abstract: Effects of stacking faults in a high nitrogen face-centered-cubic phase (γΝ) formed on plasma source ion nitrided 1Cr18Ni9Ti (18-8 type) austenitic stainless steel on peak shift and peak asymmetry of x-ray diffraction were investigated based on Warren’s theory and Wagner’s method, respectively. The peak shift from peak position of the γΝ phase is ascribed to the deformation faults density α, while the peak asymmetry of the γΝ phase is characterized by deviation of the center of gravity of a peak from the peak maximum (Δ C.G.) due to the twin faults density β. The calculated peak positions of x-ray diffraction patterns are consistent with that measured for plasma source ion nitrided 1Cr18Ni9Ti stainless steel.

Abstract: The tensile plastic strains and the residual tensile stresses caused by heat input during the laser cladding process are the main reasons for the cracking. In this paper, the laser cladding process on a type 1045 steel plate with Ni60 powder feeding was investigated and simulated by finite element method to analyze the temperature field and stress-strain field of the laser cladding process. In the temperature field model, the main considerations were given to the heat source data and the thermal boundary conditions. The interactions of laser, powders and base metal were mainly considered in the application of the heat source data. The relationship between the heat convection coefficient of work piece surface and the temperature variation was mainly considered in the application of thermal boundary conditions. In the stress-strain field model, the main consideration was given to the elastic-plastic characteristics of the materials, and the materials were assumed to be linear strain-strengthened. Moreover, the thermal stresses could be solved through the temperature field and were subsequently applied directly to the stress-strain field model as loads. Besides the temperature variations, the stress variations and the strain variations of some critical points (including the crest point of the cladded layer and intersection point of cladded layer and plate) were also obtained through the finite calculation. The temperature variations show that the heating curve is approximately a straight line while the cooling curve is like an arm of a hyperbola. The strain variations show that the thermal strain has a variation trend similar to the temperature variations. The elastic strain of each point is very low when compared to the plastic strain. The calculated results show that the tensile plastic strain of the crest point on the coating is the greatest in the cladding direction and the tensile stress in this direction of this point is great too. As a result, transverse crack can be easily initiated at the crest of the coating. While the tensile plastic strain at the intersection point of the base metal and coating is the greatest in the direction vertical to the plate thickness, the stress at this point (in the same direction) is compressive. Therefore, the intersection points tend to form a limited toe crack which can not grow.

Abstract: Magnesium alloy is an important engineering materials，but the wider application is restricted by poor corrosion and wear resistance. In the present study, an attempt was made to enhance corrosion resistance and microhardness of Mg alloy AZ91D by electron beam alloying. Flame spray method was used to prepare Al coating on the surface of AZ91D magnesium alloy, then remelted by high power electron beam. The microstructure and composition of the coating were analyzed in detail. Al-Mg diffusion was produced between the coating and the substrate to lead to the re-distribution of alloy elements in the melted layer. The coating was mainly composed of Al-Mg solid solution, Mg-Al intermetallic compounds and Mg-Al solid solution transition zone. Microhardness of the alloying layer was enhanced to 220 HV0.05 as compared to 70-80 HV0.05 of the substrate, due to the intermetallic phase formation, such as Mg2Al3 and Mg17Al12. These phases were good to improve anticorrosion property of AZ91D alloy．

Abstract: The micro-plasma arc welding technology（MPAW） was used to remanufacture the exhaust valve of W615 engine. The ST6 Co alloy powder was selected as overlaying material, the processing parameters of overlaying was optimized considering the geometry demand of deposited layer, and the current downslope method was adopted to weaken the arc crater. The microstructure characteristics and the micro-hardness of the weld were investigated, no defects were found in the deposited layer or at the interface, and the heat-affected zone among joint was not evident. The maximum hardness situated at the top layer being 657.8HV, with a slight decrease towards the bottom layer and fusion zone.