Zhang Maicang

INVESTIGATIONS ON DISSOLUTION MECHANISM OF LAVES PHASE IN GH4169 ALLOY INGOT BASED ON CLASSICAL DYNAMICAL MODEL

It is an urgent thing how to control the quality of large size GH4169 ingots nowadays. The high Nb element content in this alloy can increase the tendency of freckle defect formation. Though almost all the investigators consider that the segregation of Nb-riched Laves phase is the key factor of the freckle defect, how to avoid this phenomenon is still a hard-to-solve problem in engineering practice. In this work, a new prototype system based on classical dynamical model related to basic metallurgy theory was established to simulate the dynamical dissolving process of precipites evolution in nickel base superalloys. In this prototype system, the parameters related to the thermodynamic equilibrium state can be got from thermodynamic software of Thermo-Calc, the solute element diffusion coefficient at any temperature and time iterative can be got from dynamic software of Dictra. By using this prototype system, the dissolution process of Laves phase during homogenization process with different initial particle sizes for GH4169 alloy was simulated, and then series remelting experiments with different cooling rates and different Laves phase distributions were carried out, and the calculated results were in good agreement with the experimental results. This newly developed prototype system may give great help to homogenization process design in engineering use.

INTEGRATED SIMULATION OF THE FORGING PROCESS FOR GH4738 ALLOY TURBINE DISK AND ITS APPLICATION

In order to control the grain size of forged turbine disk of wrought superalloy like GH4738 more effectively, constitutive equations and grain structure evolution models of GH4738 alloy are used in Deform 3DTM for achieving integrated simulation of whole forging process of GH4738 alloy turbine disk(from preheating billet for upsetting to die forging). By using of integrated simulation, the variation of temperature, average grain size,etc., during the whole forging process has been explored, making it possible to control these parameters quantitatively. Comparing with traditional simple stage simulation, results of integrated simulation are more consistent with corresponding experimental results of forged turbine disk(300 mm in diameter). Therefore, the reliability of the integrated simulation is verified. Finally, with the application of integrated simulation, GH4738 alloy turbine disk with a diameter of 1450 mm has been successfully forged by 8×104t forging press. This work provides a more practical simulation method for helping the process design of forging large turbine disk.

Evolution of Grain Boundary Carbide and Cr-depletion Region in Alloy 690 during Isothermal Treatment

The precipitation behavior of grain boundary carbides and the evolution of Cr-depletion region in Ni-based superalloy 690 after solid solution treatment (1 020 ℃/15 min) followed by heat treatment at different temperatures (650 ℃,715 ℃,800 ℃) for various times (1～40 h) are studied by means of transmission electron microscopy.Based on the analysis of microstructure,an optimal heat treatment is given.It is concluded from the present investigation that carbides precipitate on both sides of the grain boundary at 800 ℃.In addition,the distribution of Cr near grain boundary is most smooth at the treatment of 715 ℃/10～20 h.As a whole,the optimal heat treatment process is 751 ℃/10～20 h.

HIGH TEMPERATURE VACUUM CARBURIZATION BEHAVIORS AND PHASE EVOLUTION MECHANISMS OF Cr35Ni45Nb ALLOY UNDER SERVICE CONDITION

Carburization in Ni-Cr-Fe-based alloys is an important phenomenon, especially in ethylene cracking tubes which serve at high temperatures under highly carburizing environment. In this work, the Cr35Ni45 Nb tube subjected to service condition for 6 a was carburized by low-pressure vacuum carburizing(LPVC) at 1080 ℃.The carburization behaviors and corresponding mechanisms of phase evolution in the inner wall were comprehensively analyzed through SEM, XRD and EPMA. The results showed that oxidation behaviors of the tube at high temperature were consisted of external oxidation of Cr and internal oxidation of Si, resulting in formation of com-posite oxide scales. Depletion of Cr in the subsurface caused by surface Cr2O3 leaded to carbide dissolution and formation of carbide free zone and carburized zone. The critical concentration of Cr for carbide dissolution is about19.0%(mass fraction). By comparing carburization behaviors of specimens whose oxide scales were retained or removed, the carburization resistance of the composite oxide scales in carburizing environment was systematically investigated. The results showed that the composite oxide scales formed previously acted as an effective barrier to carbon infiltration. However, the outermost Cr2O3 scale tended to be carbonized to form carbide scale to spall from the surface in the strongly reducing environment with low oxygen partial pressure, while the Si O2 kept stable all along due to its excellent thermodynamic stability. However, a certain amount of carbon was still capable to penetrate the alloy interior through gaps of the Si O2 scale due to its discontinuity. Therefore, continuity, density and high-temperature stability of the oxide scales were crucial for the alloy to achieve excellent anti-carburizing performance. Once the oxide layers were removed or carbonized adequately, inconceivable internal carburization occured widely. Large amounts of secondary carbides precipitated again in the previous carbide free zone due to high carbon activity. Widespread precipitations of graphite called metal dusting in the range of about 0.5 mm in depth occurred after long exposure of specimens to the carburizing environment. The carbon activity gradually decreased with increasing distance from the surface. The primary carbides within the deeper carburized region were transformed from M23C6 to M7C3in situ, which were accompanied by precipitation of vermicular g phase in the primary carbides, phase transition from h to Nb C and decomposition of intragranular secondary carbides. Severe coalescing and coarsening of carbides and metal dusting caused the serious degradation of microstructure, formation of macrocracks and final thinning of the Cr35Ni45 Nb tube wall.

ELEMENTS DIFFUSION LAW OF DD407/FGH95 DIFFUSION BONDING UNDER HOT ISOSTATIC PRESSING I.Building Diffusion Bonding Model

Turbine platform and blade are two main parts of aero engines and gas turbines. Due to different requirements in practice, platforms are always fabricated by single crystal superalloys, which have high temperature strength and resistance to hot corrosion and oxidation. The platforms employed at relatively lower temperatures can be made of powder superalloys. Therefore there is a great demand for bonding single crystal superalloys to powder superalloys. Because of high content offorming elements, traditional fusion welding methods employed in bonding the two materials are high susceptibility to cracking. Hot isostatic pressure (HIP) bonding is a preferable technique now to join nickel base superalloys. However, using experimental methods to explore appropriate HIP bonding parameters is time consuming and costly. This work puts forward a calculated method to simulate diffusion process and phase distribution of diffusion couples obtained by HIP diffusion bonding. In this work, the numerical model of HIP diffusion bonding was built, and distribution of elements and phases of DD407/FGH95 diffusion couples under different HIP temperature and bonding time were calculated

ELEMENTS DIFFUSION LAW OF DD407/FGH95 DIFFUSION BONDING UNDER HOT ISOSTATIC PRESSURE II.Model Verification and Experimental Analysis

Manufacturing dual superalloy turbine rotor is becoming an important direction of development in the field of aviation and aerospace.In order to improve working temperature and service life of turbine rotor,blade materials have been evolved to single crystal superalloys,turbine disc materials have also been developed to powder superalloys.Therefore the connection of these two materials has become key problem in improving turbine performance.Nowadays hot isostatic pressure(HIP) bonding is a preferable bonding technique due to no fusion and macroscopic deformation in bonding area.In HIP bonding process,elements diffusion is the key factor in forming microstructure of interface,and determining interfacial properties.At present,articles about HIP diffusion bonding mainly focused on interfacial microstructure and mechanical test of bonding couples,while few articles explored the relationship between elements diffusion and microstructure forming.In this work,diffusion couples of DD407 and FGH95 alloys were conducted under 1120,1170,1210℃ and 120 MPa HIP for 3 h,the microstructure and elements distribution of interface were studied to verify simulated resultsin the last article,and explore interface formation mechanism.Then the relationship between simulated phase distribution and actual microstructure of interface was built,which can be used to forecast interface morphology.And also the recrystallization in bonding zone was studied.Combining simulated and experimental results,this work put forwards principles for HIP bonding process controlling.