Analysis Of Hammer Forging Die Fatigue Fracture And Research Of The Remanufacturing Process Based On Damage Mechanics

Hammer forging die is an important branch of hot forging die,which can produce complex structural component by using small tonnage hammer equipment.Apart from the thermal and mechanical loadings endured during the forging process,the hammer forging die is also suffered from high-frequency impact loading.Therefore,failures such as fatigue fracture,wear,plastic deformation would be more likely to happen in the hammer forging die compared to the common hot forging die.Aiming at the severe fatigue fracture of hammer forging die,a forging die of ultra-supercritical steam turbine blade is studied in this dissertation.Based on continuum damage mechanics(CDM)and fracture mechanics,the damage evolution,crack propagation and failure process of forging die material were studied deeply with the aid of finite element method(FEM).In addition,the fatigue failure was analyzed systematically by means of mechanical properties and fracture characteristics.The gradient surface welding process was proposed in this study based on different failure modes of hammer forging die,i.e.,according to different performance requirements of different regions of forging die,corresponding characteristics of welding materials and their combination were designed for the die re-manufacturing,thus to ensure the entire forging die with excellent overall performance.By adopting such surface welding method on the re-manufacturing of steam turbine blade hammer forging die,the cost was reduced by 1/2,the production cycle was shortened by 2/3,and the service life was increased by 100%,compared to that of original ones.Following are the main contributions of this study:(1)Analysis of service state of hammer forging die:The stress state,strain,temperature and wear state of the hammer forging die of a steam turbine blade were predicted and analyzed by using the FEM of metal plastic forming.The fatigue failure analysis and the re-manufacturing process of the hammer forging die were carried out.Meanwhile,some improvements have been made in terms of the die design and forging processes to reduce the stress of the hammer forging die.(2)Establishment of cyclic plastic damage constitutive model and its FE implementationA cyclic plastic damage constitutive model was established based on the CDM to predict the material deterioration and internal macroscopic cracks in the forging process of hammer forging die.Such model was further programmed using the UMAT subroutine in ABAQUS for FE simulations.The damage evolution and macroscopic crack propagation of the hammer forging die under cyclic load conditions were predicted and analyzed,and certain conclusions in terms of damage evolution were obtained.(3)Fatigue fracture analysis of hammer forging dieAiming at solving the problem of fatigue fracture of hammer forging die in the process of service,This paper investigates the whole process of fatigue fracture of hammer forging die under cyclic loading via finite element simulation in the framework of continuum mechanics,from damage mechanics to fracture mechanics,combined with mechanical properties,microstructure,fracture analysis and other analysis methods to achieve a more systematic analysis of such problems.The fatigue life of the die,the mechanical properties of the die,the microstructure,and the fracture condition were obtained,and the failure process of the fatigue fracture was analyzed.(4)Re-manufacturing process of hammer forging dieThis paper introduces the process of re-manufacturing hot forging die by bimetallic gradient overlaying,and points out the different failure modes,such as fracture,wear and plastic deformation,and the causes of these failures in the application of this method in recent years.Based on the different failure modes,a new method called Gradient Overlay Re-manufacturing is proposed.(5)Re-manufacturing application of a steam turbine blade hammer forging dieBased on the failure analysis of the hammer forging die of a turbine blade,the relevant surfacing welding material was designed,and the basic experiment,such as microstructure and mechanical property,was designed and combined.The re-manufacturing of the hammer forging die is completed by the gradient welding re-manufacturing process based on the different failure modes.Via production verification,the re-manufacturing of the hammer forging die is compared with the original system of the new hammer forging die,reducing costs by 1/2,shortening the cycle by 2/3,and incresing life expectancy by 100%.