Band 37, Heft 9-10 -

In this article, the electron beam welding and post welding heat treatment (PWHT) coupling process of nickel alloy GH80A is carried out using ABAQUS software. The thermo-mechanical coupling (TMC) model is considered in the simulation. The simulation results obtained from finite element method (FEM) including thermal history, heat source sharp, residual stress and stress relaxation. The simulation objectives are analysis of thermal field and heat source sharp which is compared with experimental results. These results also can determine the heat affected zone and residual stress during welding and stress relaxation which is produced by the post welding heat treatment process.

In the present study, two commercially available Ni-base superalloys, namely Rene 80 which contains Ti and IN 792 containing Ti and Ta, were investigated during isothermal oxidation at 1,050°C in Ar–20% O 2 gas up to 50 h. It was found that IN 792 showed a slightly lower oxidation rate as compared to Rene 80. Also differences in oxide scale microstructures and thicknesses were found. Both alloys formed qualitatively similar oxide scales consisting of outer TiO 2 on top of a Cr 2 O 3 scale and internally oxidized Al 2 O 3 . The IN 792 formed a thinner oxide scale in general as well as a thinner outer TiO 2 scale. Also the depth of the internally oxidized Al 2 O 3 precipitates is smaller in case of IN 792. These findings were correlated with the formation of TiTaO 4 , which in turn hampered outer Ti and inward oxygen flux.

The effects of Ce addition on the inclusions and the pitting corrosion behavior of ferritic stainless steel were investigated. The results showed that Ce 2 O 3 ·SiO 2 and Ce 2 O 2 S inclusions were mainly found in 434 ferritic stainless steel containing 0.011% or 0.023 wt% Ce, which is the most appropriate addition of Ce that could contribute to produce the dispersive and fine rare earth inclusions. According to the thermodynamic analysis, Ce inclusions could be produced by rare earth elements combining with oxygen, sulfur, or preexisting SiO 2 . While the kinetic analysis and mechanism for formation of inclusions suggested that the area ratio of the pure SiO 2 inclusion decreased with Ce increasing, and the difference of [Ce] concentration between the original inclusion interface and the SiO 2 solid unreacted nuclear interface is the main reason to cause the different process of inclusion modification. Meanwhile, when the content of Ce was 0.011% or 0.023%, the resistance to pitting corrosion increased with the interface areas between the inclusions and the steel matrix decreasing. However, the excessive Ce addition resulted in the increase of size and quantity of inclusions, which would reduce the resistance to pitting corrosion.

A thermodynamic model for calculating the mass action concentrations Ni{N_i} of structural units in Fe–Al binary melts based on the atom–molecule coexistence theory, i. e ., AMCT–Ni{N_i} model, has been developed and verified to be valid by comparing with reported activities aR,i{a_{{\rm{R, }}i}} of both Al and Fe relative to pure liquid as standard state in Fe–Al binary melts over a temperature range from 1823 to 1973 K (1550  to 1700 °C). Furthermore, Raoultian activity coefficients γi0\gamma _i^0 of both Al and Fe in the Fe-rich corner or Al-rich corner of Fe–Al binary melts as well as the standard molar Gibbs-free energy changes ΔsolGm,i(l)→[i][%i]=1.0Θ,%{\Delta _{{\rm{sol}}}}G_{{\rm{m, }}i{\rm{(l)}} \to {{{\rm{[}}i{\rm{]}}}_{{\rm{ [ \% }}i]{\rm{ = 1}}{\rm{. 0}}}}}^{\Theta {\rm{, \% }}} of dissolved Al or Fe for forming [% Al] or [% Fe] as 1.0 in Fe–Al binary melts have also been determined by the developed AMCT–Ni{N_i} model and verified to be accurate. The reported activities aR,i{a_{{\rm{R, }}i}} of both Al and Fe from the literature can be well reproduced by calculated mass action concentrations Ni{N_i} of free Al and free Fe in Fe–Al binary melts. A small effect of changing temperature from 1823 to 1973 K (1550  to 1700 °C) on reaction abilities of both Al and Fe from the available literature is also confirmed by calculated mass action concentrations Ni{N_i} of free Al and free Fe in Fe–Al binary melts. The obtained activity coefficients γi{\gamma _i} of both Al and Fe in Fe–Al binary melts can be described by a quadratic polynomial function and a cubic polynomial function, respectively. Furthermore, accurate expressions of Raoultian activity coefficients γi0\gamma _i^0 of both Al and Fe in Fe-rich corner or Al-rich corner of Fe–Al binary melts are also obtained as lnγAl0=−9,646.5/T+2.196\ln \gamma _{{\rm{Al}}}^0 = - {\rm{9, 646}}{\rm{. 5}}/T{\rm{ + 2}}{\rm{. 196}} and lnγFe0=−6,799.1/T−0.01367\ln \gamma _{{\rm{Fe}}}^0 = - {\rm{6, 799}}{\rm{. 1}}/T - {\rm{0}}{\rm{. 01367}}, respectively. In addition, expressions of the first-order activity interaction coefficients εii\varepsilon _i^i or eiie_i^i or hiih_i^i of both Al and Fe coupled with three activity coefficients γi\gamma _i^{} or f%,if_{ \% {\rm{, }}i}^{} or fH,if_{{\rm{H, }}i}^{} relative to three standard states are also obtained from the developed AMCT–Ni{N_i} model for Fe–Al binary melts.

Up to now, there are few reports on the yield behavior of Ni-based superalloy during plastic deformation. However, an accurate yield stress model is significant for simulating the plastic forming process by cellular automaton or finite element methods. Therefore, the yield behavior of a solution-treated Ni-based superalloy is studied by hot compression tests. In order to evaluate yield stresses from the measured flow stress curves, the yield process is analyzed in terms of dislocation theory. Then, yield stresses at different deformation temperatures and strain rates are clearly determined. The experimental results show that the yield stresses are highly sensitive to deformation temperature and strain rate. The determined yield stress almost linearly increases with the increase of the logarithm of strain rate or the reciprocal of deformation temperature. A yield stress model is developed to correlate the yield behavior of the studied solution-treated Ni-based superalloy with deformation temperature, strain rate, and strengthening effect of alloying elements. The developed model can well describe the yield behavior of the studied solution-treated Ni-based superalloy.

In order to study the stress relaxation behaviour of SA302Gr.C low-alloy steel, the stress relaxation experiments were performed at the temperatures of 400, 500 and 620 °C with the constant strain of 0.5 %. Based on the experimental stress relaxation curves, the relationship between creep strain rate and stress were derived and the creep strain rate–stress curves at different temperatures were obtained. Then, the creep constitutive equations at different conditions were developed and the constants in the equations were determined. In order to verify the developed creep constitutive equations, the finite-element (FE) model of stress relaxation of SA302Gr.C low-alloy steel was established with the FE software MSC.Marc. The comparison results show that the simulated stress relaxation curves agree with the experimental ones, which illustrates that the developed creep constitutive equations are appropriate for the tested steel.

In the brazing process, some brazing defects like semicircular or straight type are generated due to incomplete filling. In this paper, the creep damage and creep crack initiation (CCI) time of Hastelloy C276-BNi2 brazed joint with defects are investigated by a ductility exhaustion damage model. The effects of defect dimension and filler metal thickness are also discussed. The results show that the different defects have different creep damage distributions and CCI times. The maximum creep damage is located at the defect frontier due to the larger stress concentration. With the increase of semicircular defect radius and straight defect length, the CCI time decreases. The creep fracture is inclined to generate in semicircular defect for the smaller defect area ratio, while it is easy to generate in straight defect for the bigger defect area ratio. As the filler metal thickness increases, the CCI time increases. For the thicker filler metal, the creep crack is easy to initiate in semicircular defect.

A thorough understanding of hot deformation behavior plays a vital role in determining process parameters of hot working processes. Firstly, uniaxial tensile tests have been performed in the temperature ranges of 150 °C–600 °C and strain rate ranges of 0.0001–0.01s −1 for analyzing the deformation behavior of ASS 304 and ASS 316. The phenomenological-based constitutive models namely modified Fields–Backofen ( m-FB ) and Khan–Huang–Liang ( KHL ) have been developed. The prediction capability of these models has been verified with experimental data using various statistical measures. Analysis of statistical measures revealed KHL model has good agreement with experimental flow stress data. Through the flow stresses behavior, the processing maps are established and analyzed according to the dynamic materials model (DMM). In the processing map, the variation of the efficiency of the power dissipation is plotted as a function of temperature and strain rate. The processing maps results have been validated with experimental data.

Mill scale (MS) has a potential to use as an iron source because of its high iron content. MS mainly consists of a mixture of iron oxides, metallic iron and other base metal oxides. MS is formed on the surfaces of steel ingots during continuous casting as a waste material. In this study, the use of MS as an iron source for the production of carbon-free iron containing alloys (unalloyed iron, Fe-Ni, Fe-Cr-Ni, Fe-Cr-Ni-Mo) via a metallothermic reduction process was investigated. Thermodynamic calculations and the experimental studies were performed on the basis of 100 g of MS. The effects of different stoichiometric amounts of MS and aluminum (Al) powders (as reductant) were investigated for the production of unalloyed Fe. While, different amount of metal oxide ratios and their effects on metal recoveries, compositions and microstructure of final alloys were studied during Fe-based alloys production. The highest iron recovery during unalloyed Fe production was obtained as 95.14 % by using 100 g of MS and 100 % stoichiometric Al (28.6 g) containing mixture. In Fe-based alloys production series, the highest metal recovery values were reached up to 95.0 % for Fe, 95.1 % for Ni, 68.3 % for Cr and 77.2 % for Mo, respectively.

In order to improve the performance of the steel for brake discs, including strength, toughness and thermal fatigue resistance, Nb and V are added to the steel. The effect of Nb and V on austenite grain growth behavior of the Cr-Mo-V steel for brake discs was studied by analyzing the phase equilibria as well as the microstructures. The precipitation behaviors of precipitates were also investigated based on transmission electron microscopy and energy-dispersive spectroscopy. The results showed that V-rich M 8 C 7 and NbC particles existed at the experimental temperatures. The abnormal grain growth in Nb-free steels was affected by the partial dissolution and coarsening of M 8 C 7 particles. With increasing V content, the grains were refined, but the mixed grain phenomenon became more serious. There were many small NbC particles in Nb-bearing steel, so the grains were effectively refined. Furthermore, a prediction model, which calculates the austenite grain size considering precipitate pinning effect, was established, and the model was well consistent with the actual situation in the Nb-bearing steel up to 1,100 °C. The grains of the Nb-free steels were sensitive to the increments of temperature and time due to the rapid dissolution or coarsening of M 8 C 7 particles.

The recovery of nickel from the oxidic nickeliferous laterite ores is receiving increasing attention due to the difficulty of recovering this metal from the sulphide ore deposits. One possible solution is to selectively reduce the nickel oxide in the ore, which could then be upgraded by, for example, magnetic concentration. In this article, a thermodynamic study was performed on the reduction of a limonitic laterite ore by methane. Methane was selected as the reducing agent as it has a lower environmental impact than carbon due to the reduced carbon dioxide emissions. The effects of temperatures and methane additions on the nickel recovery and nickel grade were investigated. High recoveries of over 95 % were predicted, but the grades were limited to about 2.5 % due to the formation of magnetite. The thermodynamic simulations for reduction by methane were in agreement with the experimental results in the literature for other reducing agents, reflecting the fact that the nickel oxide in the limonitic ore is relatively unstable. Thus, high recoveries could be achieved irrespective of the reducing agent involved.

The electrochemical behaviour of Si(IV) ions on Fe electrode in the NaCl-NaF-KCl-SiO 2 molten salt system (0.177 mol·L −1 SiO 2 ) at 1,103 K was studied by using cyclic voltammetry, square wave voltammetry, chronoamperometry, and chronopotentiometry．Experiments conducted using the first two methods indicated that the reduction of Si(IV) occurred in two steps: Si(IV) → Si(II) → Si(0). Furthermore, the electrochemical reaction is a quasi-reversible process, which is controlled by both the ion diffusion and electron transport rates. The electrochemical crystallization of Si was found to be a transient, the three-dimensional nucleation process. The cyclic voltammetry curves indicate that the diffusion coefficient of Si(IV) is 1.16×10 –5 cm 2 ·s −1 . The phases formed on the surface of the deposit were analysed by scanning electron microscopy and X-ray diffractometer. The results show that Fe and Si have formed intermetallic compounds Fe 3 Si, FeSi, and Fe 5 Si 3 .

The hot compressive deformation behavior of pure aluminum has been investigated at different conditions of strain rate range of 0.005–10 s −1 and temperature range of 523–823 K. Based on dynamic materials model Malas criterion, processing maps were constructed to evaluate the efficiency of energy dissipation and flow instability regions. The optimized condition was around strain rate of 0.005–0.01 s −1 and temperature of 773–823 K with peak energy dissipation efficiency of 42 %. Meanwhile, the rule of strain hardening exponent, strain rate sensitivity exponent and temperature sensitivity exponent have been studied, then 3D processing maps considering the strain variable was developed, and the energy dissipation maps and the dynamic instability maps were changed with the strain increasing. Results showed that region at strain rate of 10   s −1 and the region at temperature of 823 K, strain rate of 0.005   s −1 showed high-energy dissipation rate. The 3D dynamic instability maps showed instability region of different parameters, the proper process parameters was ought to be kept away from this region. The conditions of the cracking microstructure occurred proving that the region was determined by the 3D instability maps.

This paper describes the performance under cyclic oxidation of (Mo,Cr) 3 Si compound with different Cr additions for possible coating application. Cyclic oxidation was carried out at 1,000 °C at different intervals during 250 h. Oxidized surface samples were analysed by scanning electron microscope where epitaxial oxide scales were observed mainly in samples with higher Cr content which may provide protection against surface oxidation. X-ray diffraction studies have shown the Cr 2 O 3 and SiO 2 formation as the main oxide scale; after analyses, it was found that these oxides are responsible for the best oxidation protection, with 36 at.% Cr being the optimal chromium concentration. At lower chromium concentrations, pest reaction occurred in the oxidized samples at times less than 25 h as a result of the formation of the unstable molybdenum oxide.

The solidification structure of ferritic stainless steel can be refined by controlling the contents of Ti, O, and N in the liquid steel through the thermodynamic analysis and high-temperature experiment. It is found by the scanning electron microscopy technology, in which the composite core of Ti nitride-enwrapping Ti oxide can be formed in the solidification front, which promotes the nucleation of δ iron and refines the solidification structure. Meanwhile, the structure analysis of the composite core by the transmission electron microscope technology proves that the Ti oxide that exists in the centre of the composite core is Ti 2 O 3 and the Ti nitride that exists in the outer layer of the composite core is TiN.

In this paper, empirical dynamic recrystallization (DRX) models for the semicontinuous AZ31 magnesium alloy were established based on the stress–strain curves and microstructure observations generated from hot compression tests. The DRX evolution during the seamless tube extrusion of the alloy was studied by numerical and experimental methods. The extruding temperature and the ram speed were two important parameters during the extrusion. With the increase of the two parameters, the volume fraction of DRX and the recrystallized grain size were observably increased. The predicted DRX fraction and grain size were in an excellent agreement with the experimental results.

The study on the behaviour of decarburization, temperature increase, change of phosphorus and oxygen content of liquid steel during reblow process was presented in this paper. On the basis of traditional three-stage decarburization theory and thermal equilibrium theory, models for the variation of carbon content and temperature were established. Moreover, with application of data mining method, models for change of phosphorus and oxygen content were built based on large amount of production data and thermodynamic theory. By considering the variation of composition and temperature of molten steel, the reblow model was obtained and the software for reblow operation was developed with the combination of production technology. The test results indicated that the reblow rate (reblow more than once) dropped from 3.99 % to 0, and the average tap to tap time was reduced from 37.43 min to 36.35 min. It could provide an effective solution for medium-high carbon steel melting.

The effect of B 2 O 3 on slag-metal reaction between CaO-Al 2 O 3 -based mold flux and high aluminum steel was investigated. The results showed that the addition of 5 % B 2 O 3 , the slag-metal reaction hardly occurred. When the content of B 2 O 3 was increased, the reaction rate increased rapidly. This indicated if only considering the slag-metal reaction, it’s better for high aluminum steel casting if the addition content of B2O3 is less than 5%. The chemical reaction was greatly influenced by the reaction time. With higher content of aluminum ([Al]=0.1 %), the early stage of reaction was greatly affected by the reaction time, and furtherly, the influence was decreased. When the aluminum content was increased, the effect of [Al] on the slag-metal reaction was comparatively small in the initial 10 min, and the content of [B] was increased slightly. But when the reaction time increased to 1 hour, the slag-metal reaction acutely occurred, and the content of [B] increased rapidly.

In this paper, a hybrid strategy incorporating finite element computation, artificial neural network (ANN) method and Random Forest (RF) algorithm is proposed for computation of the thermal residual stresses (TRS) in SiC/SiC composites with multi-layered interphases. The finite element method is adopted to calculate the TRS of the unidirectional SiC/SiC composites. The ANN with the structure of Random Forest (RFANN) is employed to approximate the non-linear relationship between the multi-layered interphases thicknesses and the TRS of the SiC/SiC composites. The well-trained RFANN is finally used to compute the TRS of a unidirectional SiC/SiC composite with six layers of interphases.

High temperature corrosion of the water wall tube in a 50 MW thermal power plant was investigated which caused several boiler accidents. X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS) were used to observe the cross-sectional morphology of the tube and analyze the oxide scales. Results show that the boiler water and the coal quality did not meet the requirements. High temperature corrosion of water wall tubes was attributed to the using of coal which had a higher ash content and lower received lower heating value. Higher dissolved oxygen and incrustation in the boiler water caused serious corrosion at the inner surface of water wall tube, which led to the possibility of decarburization and degradation of the steel. Suitable coal blending and stability of the thermal load were the effective means to prevent the high temperature corrosion of the tube.

Particle size dependence of CO 2 absorption rate of powdered Ba 2 Fe 2 O 5 with three kinds of particle size was studied by XRD, SEM, particle size distribution measurement and thermogravimetry. From the particle size distribution measurement, average particle sizes of the powdered Ba 2 Fe 2 O 5 samples were estimated about 52.5 μm, 77.5 μm and 100.0 μm, respectively. Results show that the reaction of CO 2 with Ba 2 Fe 2 O 5 was controlled by the diffusion step in the product layer, and the kinetics process could be described by the RPP model (Real Physical Picture). Moreover, Ba 2 Fe 2 O 5 exhibits a good recycling performance though the adsorption capacity is reduced slightly during the cycle processes, and the pathway of the CO 2 adsorption–desorption on Ba 2 Fe 2 O 5 has also been obtained, which suggested that Ba 2 Fe 2 O 5 is a promising CO 2 absorbent material.

This paper investigated the effects of MnS inclusions on inducing pitting in steel. Three different cooling methods were used to control the morphology, size, and distribution of MnS inclusions. AC (alternating current) impedance spectroscopy experiments and potentiodynamic polarization experiments were carried out for different sizes of MnS inclusions. The corrosion immersion experiment was conducted to study the pitting corrosion process in different morphologies of MnS inclusions. The experiment results showed that pitting corrosion occurred around MnS inclusions and pitting was induced along the junction of MnS inclusions and their surrounding steel matrix. Also, the sensitivity of inducing-pitting of MnS inclusions was changed with the change of the average size of MnS in steel and it had a critical size of MnS. The deeper the MnS inclusions were buried in steel matrix, the more difficult it was for it to dissolve and fall off. Herein, it would cause greater damage.