Papers by Keyword: Nb

Abstract: Four series of Mo₂FeB₂ based cermets with different Nb content between 0 and 6 wt.% had been successfully fabricated by liquid phase reaction sintering (LPRS). The densification behavior, microstructural characteristics and mechanical properties of Mo₂FeB₂ based cermets were investigated. The result shows that with the increase of Nb content, the grain size decreased, but more pores formed. Meanwhile, the hardness and transverse rupture strength both be increased first and then decreased. Mo₂FeB₂ based cermets doped with 4 wt.% Nb content have the highest hardness and TRS of HRA 90.6 and 1970 MPa.

Abstract: High-chromium ferritic stainless steels have been developed for applications such as exhaust systems that require good formability. To improve formability, continuous yielding is preferred. However, in high-chromium ferritic stainless steels an upper yield point is often present as a result of free interstitials and Cottrell atmospheres. The upper yield point can be removed by temper rolling but it would be better to avoid it via a suitable heat treatment. This paper describes how this can be done in the case of a ferritic stainless steel containing 0.011%C, 0.012%N, 18%Cr, 2,1%Mo, 0.33%Nb, 0.15Ti%. Despite the presence of Nb and Ti, which should bind the free carbon and nitrogen as carbides and nitrides, an upper yield point was still observed. Previously it has been suspected that this is due to an intermetallic Laves phase present in this steel depleting the Nb in the matrix so that some carbon remains free. A series of short-term annealing experiments showed that the upper yield point diminishes, when the annealing temperature increases above 550 °C, finally disappearing after a heat treatment at 750 °C. On the basis of Thermo-Calc calculations and EDS analyses, free interstitials in the matrix could be related to depletion of MX or insufficient time to reach the equilibrium state.

Abstract: Two kinds of C-Si-Mn-Cr-Mo tested steels were used to study the transformation rules and microstructure evolution during thermal mechanical experiment and rolling experiment. Effects of micro alloying element Nb on transformation and propertied were discussed. The result shows that Nb retards ferrite transformation, reduces ferrite volume fraction and refines the ferrite grain size and martensite colony size. After micro-alloying of 0.047wt%Nb, yield strength and tensile strength increase by 145MPa and 135MPa respectively under medium temperature coiling process. YS/TS ratio increases from 0.51 to 0.62, meanwhile, the decrease of elongation is not obvious. The grain refining strengthening and precipitation strengthening of Nb micro-alloying is effective to enhance the strength grade of hot rolled DP steel coiled at medium temperature.

Abstract: The increase of the strength of gray cast iron is mainly depended on alloying. However, with the improvement of strength, its processing performance will always decrease. So three different gray cast irons are studied in this experiment, including adding 0.1% Nb elements, adding 0.2% Nb elements and adding 0.3% Nb elements, to investigate the Nbs effect to the mechanical performance of gray cast iron, we adopt Dynamic Strain Amplifier to measure cutting force to evaluate processing performance, use Optical Microscope and Electron Microscopy observe each samples organization, explains the relationship between Nbs content and the mechanical and processing performance of gray cast iron from micro-level. Finally, we draw the conclusion: when the Nb comes to 0.3 percent, the appearance of E-type graphite and Nb carbide durum granular will greatly decrease its processing performance.

Abstract: The research about effects of Nb on martensite size and mechanics in B-bearing steel was completed. The results show that both steels have similar martensite due to high hardenability of B element which is responsible for high mechanical properties. However, there are differences in the microstructures between A steel (Nb-containing steel) and the B steel (Ti-containing steel). Microstructural observation of the quenched specimen shows that the formation of fine martensite is dominant in the A Steel than B steel due to Nb element. Martensite size in B-bearing steel was refined by Nb element for high fatigue property and toughness.

Abstract: Defects in unintentionally Nb-doped 6H-SiC grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). An EPR spectrum with a hyperfine (hf) structure consisting of ten equal-intensity lines was observed. The hf structure is identified to be due to the hf interaction between an electron spin S=1/2 and a nuclear spin of 93Nb. The hf interaction due to the interaction three nearest Si neighbors was also observed, suggesting the involvement of the C vacancy (VC) in the defect. Only one EPR spectrum was observed in 6H-SiC polytype. The obtained spin-Hamiltonian parameters are similar to that of the Nb-related EPR defect in 4H-SiC, suggesting that the EPR center in 6H-SiC is the NbSiVC complex in the neutral charge state, NbSiVC0. Photoexcitation EPR experiments suggest that the single negative charge state of the NbSiVC complex is located at ~1.3 eV below the conduction band minimum.

Abstract: (W,Ti)C-10%Co system cermets doped with Ta, Nb or V grain growth inhibitor were prepared in this work. The effect of Ta, Nb and V on the microstructure and mechanical properties of (W0.8783Ti0.0976VB0.0242)C-10%Co systems was investigated. The results show that the three elements play an important role on the WC grain growth and shape; the cermets appeared uniformly distributed as whiskers or thin platelets. V results in the hardest material, Ta causes the material to become harder compared with (W0.9Ti0.1)C-10%Co cermets. But the hardness of cermets with Nb addition decrease because of the increasing of porosity.

Abstract: Self-protection of flux-cored wire surfacing is a new way to improve mechanical parts surface properties, and can improve mechanical equipment in the harsh condition of wear-resisting, corrosion resistant, high temperature resistant and oxidation resistance abilities. This paper based on this technology, research on its reinforcement in the roof without bells chute board, first analyzes the board of chute working conditions, stress and aggrandizement requirements, and using ANSYS simulated the displacement field and the equivalent stress field during the impact process, to explain failure modes, Then from the view of experiment to research the effect of molybdenum and niobium self-protection of flux-core wire, the hard facing layer microstructure analysis show that hard facing layer can obviously increase the matrix micro hardness and impact toughness, The microstructure of Niobium surfacing layer is more delicate than molybdenum surfacing layer, impact toughness is higher and more suitable for strengthening chute lining board.

Abstract: In this study the precipitation of silicon in Type 441 steel (18%Cr-0.4%Nb-0.5%Si) was investigated and its influence on strength properties were determined. To simulate high-temperature service conditions, heat treatments with various ageing times up to 120 hours and temperatures up to 800 °C were performed. Following the aging treatments, micro-and macro-hardness and tensile properties were measured. Microstructure and precipitation were analyzed using scanning electron microscopy and energy-dispersive spectroscopy. Predictions for equilibrium pericipitation were calculated using the Factsage software. According to observations, coarse titanium nitrides (TiN) and niobium carbides (NbC, Fe₃Nb₃C) were present in all specimens including non-aged ones. These precipitates did not coarsen during ageing, which implies that their growth already occurred in the sheet production process. However, silicon started to precipitate in the course of prolonged annealing. Si contributed to the formation of a secondary phase resembling the Laves-phase (FeNbSi) on grain boundaries. Hardness and yield strength were found to decrease with prolonged ageing at high temperatures. Factors affecting the silicon precipitation are discussed.

Abstract: This paper considers for transformation behavior of Nb microalloyed steels, experiments had demonstrated the effect of Nb was obvious: lowered start temperature of ferrite transformation remarkably and forming more refined ferrite grain size. In this study, computer simulation of Nb-containing steels transformation during continuous cooling was carried out by adopting a classical nucleation and growth theory, the model was derived based on the theories and the experimental results, and the calculated start temperatures of ferrite transformation were compared with the results of experimental.