Microstructure Control And Mechanical Properties Optimization Of GH4706 Wrought Superalloy

Alloy GH4706, a nickel-iron superalloy developed from IN718, has superior characteristics to IN718 in segregation tendency, hot workability and economics, and is suitable for large forgings. As a critical hot component of the F class heavy industrial gas turbines, GH4706 alloy forging is the largest turbine disc made by the wrought superalloy in the world, with a diameter over 2000mm and a weight over 5t, of which manufacture technology represents the top level within the industry. Within the capabilities of existing melting and forging capacity, the domestic procedure is each ultimate challenge of the equipment limits. Undoubtedly, it is a truly'extreme manufacture'. Therefore, the hot deformation behavior and microstructural evolution law of GH4706 alloy is studied, to provide data and experimental support for the accurate prediction of forging load and microstructure control. Furthermore, the effects of the content of P and B elements, and heat treatment parameters on the microstructural and mechanical properties are investigated, in order to solve the deterioration of mechanical properties caused by the size extension of the supersize GH4706 alloy turbine disc forging.It is found that the true stress-true strain curves the temperature range of 900?1150? and strain rate range of 0.001?1s-1 indicate that flow softening occurs in these conditions. The constitutive equation can be described by the hyperbolic sine-type equation with the deformation activation energy of 435.36 kJ/mol and the stress exponent of 4.13. Based on the constitutive equation and processing map, the forging load prediction and hot process optimization of the supersize GH4706 alloy turbine disc can be achieved by using the finite element technology.The hot processing map can reflect the effects of deformation temperature and strain rate on the deformation mechanism and microstructure of GH4706 alloy, which can be described quantitatively by lnZ parameter. The results show that, at a higher strain rate and a temperature lower than 950?, the power dissipation efficiency ? is less than 0.23 in the processing map, expressed in terms of local flow instability; at a lower strain rate and a temperature range of 940??970? and 1040??1140?, there are two peak zones with the ? value of 0.32 and 0.35 that correspond to a phenomenon of two dynamic recrystallization(DRX) mechanisms, respectively. When deformed at a lnZ value lower than 36, the abnormal grain structure of GH4706 alloy will form easily, for the DRX mechanism transformed into a nucleation mechanism of subgrain rotation. Moreover, the abnormal grain structure is harmful to the rupture properties of GH4706 alloy. Therefore, the hot work process of GH4706 alloy with a lower deformation temperature and a higher strain rate is favorable.The influence of deformation temperature and strain on the microstructure of GH4706 superalloy is studied through double-cone sample compression combined with finite element numerical simulation. The results show that the DRX mechanism of GH4706 superalloy is a discontinuous process associated with a strain induced grain boundary bulging phenomenon leading to the formation of nuclei. It is found that the critical temperature (TDRX) is 975?, while the critical strain (?DRX) of DRX depends on both the solution of ? phase and deformation generating heat. When the deformation temperature is slightly lower than the TDRX, ? phase will be partially retained in the alloy, which then hinders the migration of sub-grain or grain boundaries. Therefore, finer grain of GH4706 superalloy can be obtained by deforming with a larger strain at a temperature below TDRX.The study finds that GH4706 with an appropriate limit of P and B elements, the rupture life can be improved by 200% without any loss of tensile and impact properties, which will provide a novel method for the optimization of mechanical properties for the supersize GH4706 alloy turbine disc.The effects of heat treatment process, solution treatment parameters and stabilization treatment time on the microstructure and mechanical properties of GH4706 alloy are studied. The results show that, the main precipitations of as-heat treated GH4706 alloy, which is used for the supersize turbine disc, are ?' phase and ?'-?" co-precipitation and ? phase. The effects of the as mentioned precipitates on the tensile, impact and rupture properties at room or high temperature of GH4706 alloy are discussed. Further, it is suggested that the solution process of GH4706 alloy used for the supersize disc forging should be at 980? with a slower cooling rate, and the favorable stabilization time is 1?3h.