Fracture causes of forging hinge beam of cubic press

ObjectiveThe cubic press is the main equipment for synthetic diamond production in our country, and the hinged beam is its core component. The hinged beam with a forging structure can significantly improve the working pressure of the structure and is a structural type with great development potential. However, inadequate control in various processes such as steel making, forging, and heat treatment can also lead to fracture failure of the hinged beam in the forging structure. This paper systematically analyzes typical fracture specimens of the hinged beam to reveal the fracture mechanism, improve the existing manufacturing process, and ultimately enhance the overall performance of the structure. MethodAccording to the typical hinge beam fracture specimen, the crack direction and crack initiation position of the cylinder bottom are determined by analyzing the fracture appearance of the hinge beam. To analyze the causes of crack formation, morphological inspections are first conducted on typical fracture specimens of the hinged beam, including macroscopic inspections and microscopic morphology analysis of the fracture area. To determine the plane strain fracture toughness of the material, fatigue cracks are pre-cracked in fracture toughness specimens, and the microstructure of the fracture region is found to exhibit quasi-cleavage fracture morphology. Furthermore, low-magnification microstructure inspections are performed on the fractured samples, including grain size and inclusion statistics, metallographic structure analysis, inspection of the morphology below the fracture surface, and chemical composition and gas content analysis. It is found that there are numerous small cracks near the surface of the fracture, and in some local areas, cracks can be seen traversing the pearlite-ferrite structure, with some cracks initiating from non-metallic inclusions within the material. Finally, mechanical property tests are carried out on the workpiece samples, including tensile tests, hardness tests, fracture toughness tests, and simulated heat treatment experiments. Results(1) The macroscopic inspection results show that the crack initiates from a circular area with a diameter of approximately 20 mm located at half the wall thickness. Within the circular area, a mixed fracture morphology of intergranular and quasi-cleavage is observed. While a few regions exhibit intergranular fracture morphology, the majority display quasi-cleavage fracture morphology. The fracture surface in this area is similar to the quasi-cleavage fracture surface in the pre-cracked fatigue region of the fracture toughness specimen, indicating that the circular area is formed by the propagation of internal cracks under fatigue stress. The results of the mechanical property tests indicate that the material at half the wall thickness has higher tensile strength, lower yield strength, and lower elongation after fracture. (2) The results of the fracture toughness experiments indicate that the material at half the wall thickness has poor resistance to crack propagation, and rapid fracture occurs when the critical size is reached. Therefore, the failure process of the hinged beam is as follows: pre-existing microcracks inside the hinged beam act as fatigue sources and propagate under the cyclic stress of pressurization, holding, and unloading during several months of use. When the crack reaches the critical size, rapid fracture occurs, exhibiting macroscopic characteristics of fast fracture and quasi-cleavage fracture.ConclusionThe failure process of the hinged beam is as follows: During the quenching process, quenching cracks form inside the material. During use, cracks undergo fatigue propagation under the stress of pressurization and holding. When the crack exceeds the critical size, instantaneous brittle fracture occurs at the bottom of the cylinder under the combined effect of operating pressure and internal stress. The unevenness of the metallographic structure near the fatigue source affects the tensile and impact properties at that location, resulting in significant differences from the standard requirements. The unevenness of the microstructure is caused by the generation of significant structural stress during the heat treatment process. The insufficient forging of the workpiece and the microsegregation between dendrites affect the solid-state phase transformation process, leading to microstructural inhomogeneity after heat treatment. The metallographic structure near the surface of the cylinder bottom contains ferrite and bainite, which differs from the results of laboratory-simulated heat treatment, indicating an improper heat treatment process. The fracture toughness results indicate that the cylinder material has low resistance to crack propagation. The chemical composition of the material meets the standard requirements.