Jian Ting Guo

Stability of Microstructure and Mechanical Properties of GH984G Alloy during Long-Term Thermal Exposure

A low cost Ni-Fe-based wrought superalloy for 700 advance ultra-supercritical coal-fired power plants was developed. The stability of microstructure and mechanical properties of this alloy during long-term thermal exposure was investigated by SEM,TEM and tensile tests. The experimental results showed that the major precipitates in the alloy were spherical γ, MC and discrete M23C6 distributing along grain boundary after the long-term exposure at 700 and 750 and no harmful phases, such as σ phase and η phase, were found. However, after exposure at 800 up to 3000 h, small amount of lath-like η phase precipitated at grain boundary by consuming the surrounding γ. The η phase exhibited a fixed orientation relationship with the γ matrix. During thermal exposure γ coarsened with increasing the exposure time and exposure temperature. In addition, all major phases and their stability temperature ranges were calculated by JMatPro and these results were confirmed by the experimental results. The 700 tensile tests revealed that the alloy after exposure at 700 and 750 for 3000 h exhibited excellent ductility and strength. Therefore, the GH984G alloy possessed excellent stability of microstructure and mechanical properties between 700 and 750 up to 3000 h, and it is a promising material for 700 advance ultra-supercritical coal-fired power plants.

Degeneration of Primary MC Carbide in a Cast Ni-Base Superalloy

During long-term thermal exposure, the degeneration of primary MC carbide is a diffusion-controlled process. With the element exchange between the primary MC carbide and the γ matrix, three MC degeneration reactions, that is, γ + MC → γ + M23C6 (Reaction I), MC + γ →η+ M23C6 (Reaction II) and MC + γ →η+ M6C (Reaction III), sequentially operates. Of them Reaction III has never been reported in the previous references and is kinetically most difficulty. It is also shown that the grain boundary MC decomposes much more rapidly than the grain interior MC and that Reaction III is not as often seen at the grain boundary as in the grain interior.

Preparation of Nanocrystalline NiAl Compounds and Composites by Mechanical Alloying

During the past ten years, mechanical alloying/ball milling has been employed to successfully synthesize nanocrystalline NiAl intermetallic compounds (NiAl, NiAl (Cr,Co,Ti,Fe)) and composites (NiAl-TiC, NiAl-TiB2, NiAl-HfC, NiAl-HfB2) in our group, namely, by means of self-sustained reaction. During the mechanical alloying process, an abrupt temperature rise has been observed. The large heat release from the formation of NiAl, as well as HfC, HfB2, TiC, TiB2 compounds is suggested to be the driven force of the self-sustained reaction. It is interested to observe that nanocrystalline NiAl exhibits very well microstructure stability during high temperature annealing. The alloying additions can decrease reaction rate and prolong the reaction time. With the increase of alloying additions, supersaturated solid solution even amorphization of NiAl has been formed. The nanocrystalline NiAl compounds show a rise of the compressive strength and the ductility at room temperature compared with that of the cast coarse crystalline compound.

Effect of Hafnium on the Microstructure and Creep Property of a Hot Corrosion Resistant Superalloy

The effects of the selective addition of Hafnium (Hf) on the grain boundary, phase, carbides and creep properties of experimented nickel superalloy after standard heat treatment and long-term exposure were investigated. Predicted by the Bayesian neural network, the creep life is prolonged with Hf content of 0-0.6 mass%, which is more effective at low stresses. The decrease of creep life of Hf free alloy after long term exposure was pronounced. Comparative study showed that the mainly small, coherent, blocky and closely spaced MC(2) and M23C6 carbides precipitated on the grain boundaries in the 0.4wt% Hf contained alloy, and that relatively larger, incoherent MC(1) carbides precipitated on the grain boundaries in the Hf free alloy. During long term thermal exposure, fine discrete M23C6 carbides decomposed from primary carbide, inducing a layer along the grain boundary, and the coarsening of grain boundary in Hf free alloy is more pronounced. At high stresses, the Hf-free alloy exhibited a stronger tendency of rafting than the 0.4Hf alloy, while the tendency of appearance of rafting was very similar at low stresses. However, Hf can render the alloy prone to the formation of σ phase, according to D-electrons method. Thus, the Hf content needs to be controlled to a suitable level.

Effect of Hot Isostatic Pressing on the Microstructure of K417G Cast Turbine Discs

The present study focused on the effect of hot isostatic pressing (HIP) treatment on the microstructure of K417G superalloy. The experimental results showed that after the HIP treatment the size and volume fraction of the porosities significantly decreased. In addition, the dendritic structure and γ/γ eutectics in the as-cast specimens became obscure after the HIP treatment due to the improvement of segregation. The γ′ phases in the dendrite core were smaller than those in the interdendritic region, whether in the as-cast or HIP specimens. The slow cooling at the end of the HIP treatment leads to the irregular morphology of the γ′ phases.

Thermal Stability and Tensile Properties of GH984G Alloy for A-USC Power Plants

GH984G is a new Ni-Fe-based alloy which has been designed for use as superheater, reheater, and header materials for boilers in 700°C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the tensile properties and microstructure evolutions of GH984G were investigated during long-term thermal exposure at 700 and 750°C up to 15000h. The results show that the major precipitates were MC, M23C6 and γ ́, and no detrimental TCP or GCP phase appeared during long-term thermal exposure. The morphology of γ ́ was spherical and had no obvious change, but the size increases. After thermal exposure at 700°C for 15000h, the 700°C yield strength slightly increased, but the yield strength decreased after thermal exposure at 750°C for 15000h. The variation of strength is attributed to the coarsening of γ ́ precipitation. The deformation mechanism is the moving dislocations shear γ ́ precipitates and the stacking faults form in γ ́ precipitates at room temperature and 700°C. At 750°C, the deformation mechanism is characterized by the formation of dislocation loops and tangles.

The Influence of the Re-Melting Times on Microstructure and Mechanical Properties for Revert Alloy K452

The influence of the re-melting times on chemical composition, microstructure, and mechanical properties of revert alloy K452 were systemically investigated. It was shown that the key factor in the engineering application was the control of gas content and porosities level in revert alloy. By an advancing technology in combination with the superheat treatment, adding a small amount of alloying elements such as C, Al, Ti, and using ceramic foam filters during vacuum induction melting, the composition and mechanical properties of the revert alloy in addition of 50% scrap were similar to that in the virgin alloy, which has successfully been used in applications for nozzle vanes of some new gas turbines.

Effects of Nb/Ti Ratio on the Microstructures of Two Experimental Ni-Based Cast Superalloys

Effects of Nb/Ti ratio on the microstructures of two experimental Ni-based cast superalloys of the Nb/Ti ratio 0.46 and 0.03 were investigated. The Nb/Ti ratio had a significant effect on the dendritic microstructures, especially the γ precipitate, of the two alloys under the as-cast condition. The amount and composition of primary MC carbides were closely related to the Nb/Ti ratio for that the Nb atoms have a stronger tendency to form the MC carbides than the Ti atoms. During thermal exposure, the primary MC carbides gradually degraded through two reactions: firstly MC + γ M6C + γ and then MC + γ M23C6 + γ. The Nb/Ti ratio had little influence on the reaction formation but obvious influence on the decomposition degree of the primary MC carbides decomposition. In addition, the second M6C and M23C6 carbides on the boundaries (GBs) gradually formed a continuous chain during thermal exposure. The γ-free zones appeared along the GBs as the growth of the second carbides and γ precipitates on the GBs when thermal exposure time last up to 3000 h.

Effect of Nitrogen on Microstructure and Properties of a Cast Cobalt-Base Superalloy

The effect of the nitrogen on the microstructure and properties of a cast Co-based superalloy K640S was investigated with the different level ranged from 24 ppm to 85 ppm. The results indicate that almost all nitrogen exist in the form of nitride precipitation in the vacuum induction melting. It is found that high N level would extend the solidification window during the solidification process, and then increase the quantity of the carbide eutectic phases and led to the distribution in the network form of the eutectics in the interdendritic regions. High nitrogen content (more than 40 ppm) can reduced the mechanical properties of the testing alloy. So, it is suggested that the nitrogen in the recycled Co-based superalloys should be controlled at a suitable level in order to avoid the degradation of their properties.

Oxidation Behavior of a Disk Powder Metallurgy Superalloy

Oxidation behaviors of a spray-forming disk superalloy LSHR were investigated in the temperature range of 750-900°C. The composition and morphology of oxidation scales were investigated by X-ray diffraction (XRD), scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA). Oxidation kinetics was studied by the means of isothermal oxidation testing in air and weight gain measurement. The oxide scales were composed of Cr2O3, TiO2, Al2O3 and a small amount of NiCr2O4. The experiment results showed that oxidation kinetics and oxide layers followed a square power law as time extended from 750 to 900°C. With the oxidation temperature increasing, external scale thickness, and internal oxidation zone increased. The oxidation behavior was controlled by the diffusion of oxygen, chromium, titanium, and aluminum ions, as chromium, titanium, and aluminum ions diffused outward and oxygen diffused inward. Based on the standard HB5258-2000 spray-forming LSHR exhibited an excellent oxidation resistance in the whole test temperature range.