Search

Niobium is a strong carbonitride-forming element. The evolution of Nb-rich carbonitride in austenitic steels during welding has an important effect on the ductility of the heat-affected zone (HAZ). The new austenitic heat-resistant steel of 22Cr15Ni3.5CuNbN, a candidate material for ultra-super critical boiler superheater and reheater serviced at 620-650 ℃, contains 0.5% Nb, which will significantly affect the steel's weldability; therefore, it is necessary to study the microstructure and properties of the weld HAZ of the steel and provide a reference for the further applications of this new material. Because of the narrow weld HAZ of this material, the extended HAZ structure of 22Cr15Ni3.5CuNbN austenitic steel at different peak temperatures from 1150 ℃ to 1300 ℃ was obtained by Gleeble thermal physical simulation method in this study, aiming to simulate the thermal cycling process of the welding process, and impact performance tests were carried out. The results show that a certain amount of Nb-rich composite carbonitrides is present in the base metal of the experimental steel, which effectively pins the grain boundaries and entangl with a large number of dislocations. The Nb-rich composite carbonitride underwent a complex process of dissolution and re-precipitation during the simulated welding process. When the peak temperature was at 1150 ℃, only small particles of Nb-rich carbonitrides were dissolved, while when the peak temperature reached 1300 ℃, the Nb-rich composite carbonitride underwent dissolution and re-precipitation, showing a "mesh" structure, and its overall size increased. The evolution of Nb-rich composite carbonitrides led to changes in the impact energy of this steel. The impact toughness of the experimental steel subjected to welding thermal cycling condition was higher than that of the base metal. With the increase in the peak temperature, the impact toughness first increased and then decreased. The impact toughness of the steel reached the highest when the peak temperature was at 1150 ℃.

In the previous studies on the microstructure and orientation of titanium alloys, the microstructural and orientational evolution of typical titanium alloys during thermal compression have been studied in depth. However, studies of the correlation between hot compression and heat treatment processes on microstructural and orientational changes have been few. It is of great significance to further study this correlation during hot treatment. For this study, during hot compression deformation and subsequent hot treatment of TC17 titanium alloy on a thermal simulator using cylindrical specimens, the microstructure, grain size change, and orientation evolution of TC17 were studied using optical microscopy and backscattered electron diffraction analysis. Grain size, texture distribution, pole figure, and reverse polarity were analyzed. Law and the relation between structure and orientation results show that the primary α-content decreases dramatically and size decreases in tandem with deformation temperature. Most of the α phase grains are dispersed and located on the trigeminal grain boundaries of the high temperature β phase grains. After heat treatment, the α phase and β phase had a clear structure and distinct boundary. The primary α phase still exists and tends to be equiaxed, and the metastable β phase changes formed a lamellar β-transformed structure. The hot deformation reduces the density of the α phase texture. Additionally, with increasing temperature, the density value of the α phase texture also becomes small. The α phase is no longer strongly textured after thermal deformation, and the orientation of the α phase grains is considerably influenced by thermal deformation, which clearly improves the uniformity of orientation. The thermal deformation also reduces the texture polar density value of the β phase, but the effect is not obvious. However, there is still a density of orientation, and the uniformity of the orientation is relatively poor.

Nickel-base superalloy 617B is one of the most promising candidates for superheater and reheater pipes of advanced ultra-supercritical (AUSC) power plants.Hot extrusion is a key process during the manufacturing of superalloy 617B pipes.However, the high content of alloying elements in superalloy 617B makes microstructure control difficult during the hot extrusion process.Furthermore, to date, no systematical theoretical investigation has been conducted in the hot extrusion process control of superalloy 617B.Hence, in this work, the hot extrusion process of superalloy 617B tube was studied by finite element simulation using DEFORM-2D finite element software.The microstructure evolution during hot extrusion was considered by combining the microstructure evolution model of superalloy 617B and finite element simulation software.The microstructure evolution model was programmed using FORTRAN language and was developed using the finite element simulation software.The hot extrusion characteristics of superalloy 617B were systematically analyzed by the simulation.As a result, the evolution of temperature, grain size, and loading could be predicted quantitatively.At the same time, to optimize the hot extrusion parameters, microstructure-based hot extrusion control principles, including temperature principle, loading principle, precise microstructure control principle, were proposed considering practical hot extrusion process.Moreover, the control mechanism and application process of these principles were elaborated in detail in this paper.The hot extrusion parameters of superalloy 617B tube were optimized based on the proposed microstructure-based hot extrusion control principles.Under the guidance of the microstructure-based hot extrusion control principles, superalloy 617B tube with uniform axial dimension and good surface quality was extruded successfully in the factory.The practical extrusion result agrees well with the simulated one.Therefore, the establishment and validation of the simulation method and microstructure-based hot extrusion control principles can provide theoretical guidance for the hot extrusion process optimization of nickel-base superalloy tube in practical applications.

The solidification behavior and microstructures evolution of cast Ni-based superalloy K424 under different cooling rates were investigated by isothermal solidification quenching experiments and differential scanning calorimetry (DSC). The microstructures and segregation characteristics of K424 were analyzed at different isothermal temperatures and cooling rates using optical microscopy, scanning electron microscopy, and energy-dispersive spectrometry (EDS). The isothermal solidification, microstructure, and distribution characteristics of elements at the final solidification stage were also investigated, and the liquidus, solidus, and the formation temperatures of the main phases were evaluated. Furthermore, the influence of cooling rate on the morphology and size of MC carbides, (γ +γ') eutectic, and precipitated γ' phase were discussed. The results show that the solidification of the K424 alloy follows the sequence:(1) L → L + γ at 1345℃:the solidification begins with the formation of primary γ, and the liquidus temperature is 1345℃; (2) L → L + γ + MC at 1308℃:as the solidification continues, Ti and Nb are enriched in the liquid interdendrite, which results in the formation of MC carbides at 1308℃; (3) L → γ + (γ + γ') at 1260℃:the precipitation of the (γ + γ') eutectic occurs at about 1260℃ because of non-equilibrium solidification. Finally, the solidification ends with a solidus temperature of 1237℃. Furthermore, it is found that the precipitation of (γ + γ') eutectic at the interdendtitic regions is closely related to the cooling rate and the severe segregation behavior of Al and Ti into the residual liquid at the final solidification stage. With the increase of cooling rate, the quantity and size of MC carbides and eutectic first increase and then decrease. Moreover, with increasing cooling rate, γ' shapes transform from irregular petal-like structure to near cuboidal and spherical patterns, and the size scale of γ' precipitates decreases from 2 μm to 60 nm.

The effects of cyclic heat treatment and hot deformation on the spheroidization and orientation of the TC17 titanium alloy were investigated using a light microscope (LM) and electron backscattered diffraction (EBSD). The experimental results indicate that in the two-phase temperature range of a simple, cycle heat treatment, the spheroidization of the lamellar microstructure of the TC17 titanium alloy is finite. After cyclic heat treatment and compression deformation, the Widmannstatten structure disappears, the spheroidized α lamellae is more obvious, but the uniformity of its orientation is not greatly improved. In addition, the recrystallization velocity, and the strength and toughness of the two phases in deformation lead to a difference in their orientation, and the recrystallization velocity of the α phase is faster than that of the β phase. In the deformation process, the anisotropy of the α phase is preferentially reduced. On the other hand, during the hot deformation process, the degree of deformation of the α phase is lower than that of the β phase, because the α phase is harder than the β phase. The strain is primarily concentrated on the softer β phase adjacent to α phase, which produces greater uniformity in α phase than in the β phase.

Hot tearing tends to occur when K424 superalloy is used for a turbocharger turbine wheel. To solve this problem,the alloy characteristics of K424 and K418 superalloys and the factors impacting the susceptibility to hot tearing were studied by microstructure observations and casting simulations. The research proposed the main factors which affected the susceptibility to hot tearing and also the method to reduce hot tearing in the turbine wheel. It is found that the susceptibility to hot tearing of the turbine wheel is influenced by both the alloy characteristics and casting characteristics. K424 superalloy shows a higher susceptibility to hot tearing when compared with K418. Because of high amount of aluminum and titanium in K424 superalloy,the size and the amount of γ + γ' eutectics increase,resulting in more prior forming and propagating areas of hot tearing. On the other hand,due to a smaller thickness and a larger curvature change in the blade margin of the turbine wheel,the uneven distribution of the thermal stress is induced in the end of the solidification process,which leads to hot tearing in this paper. In order to decrease the susceptibility to hot tearing of the turbine wheel,the contents of aluminum and titanium should be controlled,and the reasonable technological parameters of casting should be set to ensure the final effect.

To study the microstructural stability of GH4169C alloy with high content of phosphorus and niobium, the microstruc-ture and hardness of GH4169C alloy and GH4169 alloy after long time aging were comparatively analyzed in this paper. These two su-peralloys were subjected to aging treatment at 600, 650, 704 and 720℃ for different times from 30 h to 10000 h. The microstructural evolution of the alloys was characterized by using a field emission scanning electron microscope and a Brinell hardness tester. The re-sults indicate that, at the service temperature of 650℃, GH4169 alloy and GH4169C alloy show excellent structure stability. Howev-er, above the service temperature, GH4169 alloy and GH4169C alloy have poor structure stability. In a short period of time, the two alloys lose stability. In contrast, the microstructure stability of GH4169C alloy is higher than that of GH4169 alloy with aging at 704℃, while GH4169C alloy shows a lower stability compared with GH4169 alloy at 720℃. It is thought that the stability of γ'phase in GH4169C alloy increases with increasing Nb and P contents. GH4169C alloy shows a better microstructure stability than GH4169 alloy at 704℃. However, the content of δphase will also increase with increasing Nb content, leading to the decrease in microstructure sta-bility of GH4169C alloy. At ultrahigh temperature (720℃), the microstructure stability of GH4169C alloy is inferior to GH4169 al-loy. This means that, compared with GH4169 alloy, the modified GH4169C alloy can be used in a higher temperature, whereas the temperature increase is limited, the microstructure stability of GH4169C alloy will decrease at ultrahigh temperature.

The relationship of creep rupture life and microstructural evolution under different high temperature oxidation conditions for Cr35Ni45 Nb alloy was studied by using SEM and quantitative electronic probe analysis. It was found that the microstructure gradually varied both in the edge and inside of the tube with increasing treatment time. The oxidation sequence showed that discontinuous oxidation films firstly developed at different places from the surface of the tube specimen,then they became continuous,and the carbide depleted zone developed as well during the high temperature oxidation process. As time went by,the oxidation films ruptured with temperature fluctuation,the characteristic of a higher oxygen partial pressure during the aging process resulted in the untimely recovery of the broken oxide films,and internal oxidation occurred at grain boundaries. Furthermore,creep rupture tests under different pretreating conditions of oxidation in air and argon gas atmosphere showed that the newly-built thin and continuous oxidation films improved the creep properties,while with increasing aging time,carbide depleted zone formation,oxidation film rupture and internal oxidation could decrease the creep properties of this alloy.

The fatigue crack growth rates of powder metallurgy (P/M) superalloy FGH97 with different grain sizes, γ' phase sizes and Hf contents were measured at a 650℃ high temperature condition and compared with those of FGH95 and FGH96 alloys. The fatigue fracture behavior characteristic of FGH97 alloy in each stage was analyzed by the quantitative analysis method. It is found that FGH97 alloy with coarser grains has a lower fatigue crack growth rate. Reasonable match of secondary and tertiary γ' phases and Hf addition can get a higher fatigue life. Compared with FGH95 and FGH96 alloys, FGH97 alloy has the highest fatigue cack intiation resistance and exihibits the lowest fatigue crack propagation rate at high temperature.

A standard heat treatment(1105℃/4 h/(oil cooling) + 650℃/24 h/(air cooling) + 760℃/16 h/(air cooling)) and a subsequent long term exposure at temperatures of 720℃ for 0-5000 h and 800℃ for 0-1000 h were proposed to investigate the microstructural evolution of γ' precipitates in GH720 Li alloy. The size,morphology and quantity of the primary γ' phase have no significant variation at 720℃. However,at the same temperature,obvious grain coarsening and the grain size inhomogeneity of secondary γ'particles appear with the aging time up to 200 h and 500 h,respectively. The descending slope of hardness decreases first and when the aging time exceeds 500 h it flattens. At 800℃,the primary γ' phase coarsens after 500 h of aging,while both obvious grain coarsening and the grain size inhomogeneity of secondary γ' particles happen after 100 h of long term exposure. The descending slope of hardness decreases first,and unlike the results at 720℃,it is still downward when the aging time exceeds 500 h. The coarsening behavior is controlled by lattice diffusion. There shows a linear relationship between the average radius of the γ' phase and the cube root of aging time,which accords with the L-S-W(Lifshiz-Slyozov-Wanger) theory.

The microstructure of as-cast GH625 alloy was investigated on the basis of experimental methods such as Thermo-Calc software,scanning electron microscopy and energy dispersive spectroscopy. The effect of casting temperature on the microstructure was also analyzed by comparing different samples. The microstructure of as-cast GH625 alloy is a dendritic structure and M6C carbides accompanied with δ phase precipitate in the interdendritic area. Both primary dendrite arm spacing(λ1) and second dendrite arm spacing(λ2) increase with the increase of casting temperature; however,the microsegregation is affected by both diffusion distance and diffusion time. The comprehensive interaction of these two factors leads to the greatest microsegregation degree of Nb at 1420℃,which contributes to δ phase precipitation in the interdendritic area. As a result,diffusion distance and diffusion time should be comprehensively considered in making the casting process,so as to choose a proper casting temperature(or cooling rate).

The hot deformation behavior of 5Cr9Si3 valve steel was investigated by hot working simulation test at different hot working parameters. When the deformation temperature increases from 850 to 900℃ or from 1000 to 1100℃, the peak stress decreases; but when the deformation temperature increases in the range of 900 to 1000℃, the peak stress rises. Further microstructural and phase transformation analyses show that a transformation from α-ferrite to austenite occurring within the deformation temperature range from 900℃ to 1000℃ causes austenite phase transformation strengthening, inducing the increasing of hot deformation resistance. Also, with the increasing of deformation temperature, the dissolution of carbides leads to solution strengthening for the matrix. Thus, phase transformation strengthening and solution strengthening at elevated temperatures are the main reason for the abnormal variation of flow stress for 5Cr9Si3 valve steel.

The mierostructure and texture evolution of Ti-6Al-4V alloy under different forging processes on a free forging hydraulic press were investigated by scanning electron microscopy, electron backscattered diffraction and X-ray diffraction. It is found that the microstructures of Ti-6Al-4V alloy under different forging modes, unidirectional forging and cross upsetting and stretching, are nearly the same. The distributions of ct phase and β phase in the difficult deformation zone, small deformation zone and large deformation zone are very similar, while the strain is distinctive in different forging regions. Further observations indicate that, in the edge region with small strain, the slip system is mainly the {0001} basal plane and forms the basal texture, {1120} and {1010} prismatic textures occur in the internal high strain region, while { 1122} and {1011} pyramidal textures appear in the place where the stress concentrates. The two forging processes can both improve the uniformity of deforming textures; in addition, cross upsetting and stretching is better than unidirectional forging.

Microstructure and texture evolution in Ti-6Al-4V alloy under different hammer forging processes were investigated by electron backscattered diffraction and scanning electron microscopy. The results showed that the inicrostructure became more and more uniform along with the frequency of forging, and the directionality turned more and more invisible; after heat treatment, the distribu-tion uniformity of α and β phases and the micro-texture were both increased to somewhat extent because of the release of deformation energy and the tilting of α and β phases, especially highlighted in the forging modes of uniaxial 3 times and cross upsetting and stretching 3 times. As for the texture distribution of this alloy, the close-packed hexagonal structured α phase was more sensitive to hot working modes than the body centered α phase. For raw materials and one-time axial upsetting and stretching, the texture of α phase mainly consisted of the {0001 } basal texture; with the increase of frequency and deformation energy, the {1010} and {1120} prismatic slip and the {1011} and {1212} pyramidal slip were strengthened, accordingly inducing the more uniform of orientation because of the texture components and the dispersion of texture distribution.

Abstract: Hot corrosion behavior of FGH96 powder metallurgy (P/M) superalloy in 25 % NaCl + 75 % Na2SO4 molten salts at 650, 700, and 750 °C was investigated in this paper. The methods of mass loss measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were used here. The experimental results show that hot corrosion kinetics follows a square power law at 650 °C, while a linear one at 700 and 750 °C. The corrosion layer is detected to be composed of Cr2O3, NiO, and Ni2S3 at each temperature. The cross-sectional morphologies and corresponding elemental maps indicate that the corrosion layer is a stratified structure of oxide and sulfide. The results and analyses confirm that the hot corrosion mechanism of FGH96 P/M superalloy is a cooperate process of oxidation and sulfidation. Furthermore, the relatively higher concentrations of Cr, Co, and Ti provide better corrosion resistance to the attack of S and Cl−. [ABSTRACT FROM AUTHOR]