【作者】 彭海健；

【导师】 李德富；

【作者基本信息】 北京有色金属研究总院 ， 材料科学与工程， 2014， 博士

【摘要】 GH690合金是一种铬含量30%左右的镍基变形高温合金,在不同温度的水溶液中均具有优良的耐应力腐蚀和抗晶间腐蚀的性能,非常适用于制造核工业中蒸汽发生器的传热管。目前国内核工业中蒸汽发生器使用的传热管仍然需要进口,随着近年来需求量的不断增加,迫切需要深入开展国产化研究。本文通过高温等温压缩试验研究了GH690合金的高温塑性变形行为。通过OM、EBSD、TEM和回归分析等方法,研究了GH690合金高温塑性变形过程中的应变、应变速率、变形温度和流变应力的相互关系、组织演变规律及GH690合金动态再结晶规律等；通过GH690合金管材热挤压变形的热—力耦合模拟和管材热挤压工艺试验,分析了GH690合金管材热挤压过程中的温度场、应变场和应力场,研究了GH690合金管材热挤压变形特性,并获得了GH690合金管材热挤压成形极限图。结果表明：(1)GH690合金在950-1200℃,0.001-10s-1条件下热变形时属于正应变速率敏感材料。采用Zener-Hollomon参数的双曲正弦函数可以较好地描述GH690合金高温变形时的流变行为。(2)在应变ε=0.7,ε≤10s-1时,GH690合金获得完全动态再结晶组织的温度随应变速率的增大而升高；在已获得完全动态再结晶组织后,随着变形程度的继续增加,动态再结晶晶粒尺寸变化不大。(3)GH690合金在950-1200℃,0.001-10s-1条件下进行热变形时发生了动态再结晶,其动态再结晶机制主要为应变诱导晶界弓弯形核的非连续动态再结晶(DDRX)机制；同时,GH690合金在晶内形变带处也发生了动态再结晶。(4)在GH690合金管材热挤压过程中,在管材的内表面附近容易出现混晶组织。其主要原因是由于挤压时坯料的温降太大。为避免出现混晶组织,需提高挤压速度,从而减少挤压时间,减小挤压管坯的温降。另外在挤压管材外表面会容易因拉应力而产生裂纹。坯料和工模具的摩擦是产生拉应力的主因,为减轻或解决挤压管材的表面裂纹,需要开发出更好的高温润滑剂,降低摩擦系数。(5)建立了GH690合金管材挤压成形极限图。GH690合金管材的热挤压成形性能由挤压比、挤压温度和挤压速度决定,取决于GH690合金在具体挤压条件下的流变应力水平。增大挤压比将大大减小GH690合金管材的许可挤压成形条件范围。为提高生产效率和避免在管材内壁出现混晶组织,挤压宜选择在靠近极限温度曲线附近的较高温度和较高挤压速度的变形条件区域内进行。更多还原

【Abstract】 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.更多还原