Study On Hot Deformation Behaviors And Hot Tube Extrusion Mechanism Of GH690Alloy

GH690is a nickel-based wrought superalloy of high chromium content (around30wt%). Not only does it possess the good intergranular corrosion resistance of other nickel-based alloys, like Inconel-600, for example, but also appears to be less vulnerable to stress corrosion cracking. Therefore, it is suitable for manufacturing the heat exchanger tubes of steam generator in the nuclear power plant industries. At present, the heat exchanger tubes of steam generator in the Chinese nuclear power plant industries are mainly depending on import. Along with the imminence requirement for the localization of alloy GH690nuclear heat exchanger tube in recent years, it is necessary to deeply develop the basic research of the hot deformation behavior of alloy GH690. The deformation behaviors at high temperatures for alloy GH690have been investigated in this thesis by isothermal compression testing. The interdependences of strain, strain rate, deformation temperature and flow stress for the alloy at the temperatures and the strain rates of interest have been studied with the aids of OM. EBSD, TEM and regression analysis. Microstructure evolution and dynamic recrystallization of the alloy have also been discussed. The temperature field, strain field and stress field of the extrusion process of GH690alloy tube have been investigated by numerical simulation and extrusion experiment, and extrusion limit diagrams of the alloy GH690tubes have been constructed. The results are as follows:(1) At the investigated condition with a temperature of950℃to1250℃and a constant strain rate of0.001s-1to Is-1, the flow stress and deformed microstructure were greatly affected by strain rate and deformation temperature. The flow stress increased with the increase of strain rate and decreased with the increase of deformation temperature; it indicated that GH690alloy was a kind of positive strain rate sensitive material. The flow stress of alloy GH690during high temperature deformation could be expressed by a Zener-Hollomon parameter in the hyperbolic sine function.(2) At the investigated condition with a nominal strain of0.7and a constant strain rate ranging from0.001s-1to Is-1, the temperature needed for fully dynamic recrystallization increases with the increase of strain rate. There is almost no change in the size of the DRX grains with the increasing strain when the microstructure has totally turned to be dynamic recrystallization grains.(3) A discontinuous dynamic recrystallization (DDRX) with nucleation mechanism of bulging of the original grain boundaries is the operating nucleation mechamism of DRX of GH690alloy. A continuous dynamic recrystallization (CDRX) with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurrs simultaneously with the DDRX.(4) Mixed grain structure are easy to appear in the internal wall of the GH690alloy extruded tube because of the large temperature drop of billet. To avoid the appearance of mixed grain structure, it is needed to increase extrusion speed, thereby reducing extrusion time and the temperature drop of billet. In addition, it is found that cracks appear easily on outer surface of GH690alloy extruded tube because of tensile stress. The friction between billet and extrusion die is the primary cause of producing tensile stress. In order to lessen or avoid the cracks, it need to develop a better high-temperature lubricant to decrease friction coefficient.(5) Extrusion limit diagrams of alloy GH690were developed. The extrudability of the GH690alloy tubes depends on the extrusion variables, such as extrusion ratio, extrusion temperature and extrusion speed. The increase of extrusion ratio decreases the available scale of extrusion temperature and extrusion speed. In order to improve production efficiency and avoid appearing mixed grain structure, it should be selected extrusion condition at high temperature and high extrusion speed which near to incipient meltingline.