GH625合金管材热挤压成形工艺及组织演变的研究

Hot Extrusion Moulding Process and Microstructure Evolution of GH625 Superalloy Tubes

利用卧式挤压机对GH625合金进行了管材热挤压试验，研究了挤压温度和挤压比对GH625合金管材挤压过程中的力能参数及挤压后管材不同部位的显微组织的影响。结果表明，随着挤压温度的降低和挤压比的升高，最大挤压力逐渐升高。管坯在固定挤压速度40mm·s-1，预热温度为1150～1200℃和挤压比为3．46～4．10的条件下，可成功挤压出3种规格的GH625合金管材；挤压后的管材由于在挤压过程中发生了动态再结晶组织明显细化，管坯横向组织为等轴的动态再结晶晶粒和原始晶粒组成，纵向组织则由等轴的动态再结晶晶粒及被拉长的原始晶粒组成，呈条带状组织；挤压后管材的外壁、中心、内壁与管材的头部、中部与尾部在热挤压变形过程中，由于变形不均匀发生了不同程度的再结晶，因而存在不同程度的混晶组织。为消除混晶组织，结合设备能力与GH625合金的变形特征，可通过提高坯料挤压的变形温度和挤压比来控制变形的均匀性，并通过切头，去尾和对管材内壁进行少量机加工的方法，可获得具有完全动态再结晶组织的挤压管材。

GH625 superalloy tubes were hot extruded using a horizontal extruder. The influence of extrusion temperature and extrusion ratio on the extrusion force and microstructures of different positions of extruded tubes were investigated. It was suggested that the maximum extrusion force during extrusion was increased with the decreasing extrusion temperature and the increasing extrusion ratio. It was also found that three sizes of GH625 superalloy tubes could be successfully fabricated with a stable extrusion speed of 40 mm·s-1, extrusion ratio from 3.46 to 4.10 and preheating temperature from 1150 to 1200 ℃. The microstructure of extruded tube was obviously fined due to the occurrence of dynamic recrystallization （DRX） during extrusion. The horizontal microstructure of the tube was composed of equiaxed DRX grains and original grains, while the longitudinal microstructure was composed of equiaxed DRX grains and elongated original grains. Different degrees of mixed grain structure were observed in outer wall, center and inner wall of the tubes due to the different degrees of DRX caused by inhomogeneous deformation, which was similar to that in the head, middle and tail of the ex- truded tubes. Considering the equipment capacity and deformation characteristics, the mixed grain structure could be eliminated by in- creasing the extrusion temperature and the extrusion ratio, then extruded tubes containing fully DRX grains could be obtained through cutting the head, tail and machining a small amount of the inner wall.