| 17-4PH stainless steels have been widely used in applications such as nuclear,chemical,aircraft,and naval industries due to their excellent mechanical properties,excellent corrosion resistance and weldability.It is mainly used as turbine blade steel at home.Quality blade product is one of the prerequisites for the development of high power steam turbine and the improvement of the economy of the unit.It has been an important task for researchers and engineers to improve the stability of blade quality and improve the reliability of forging process to meet the need of upgrading and upgrading of steam turbine technology.In the manufacturing process of the blade forgings,it will be heated to a certain temperature and then formed.The quality of the forgings is closely related to the grain size,microstructure and thermal deformation ability of the material after heating.In the production,the microstructure should be controlled while ensuring the required shape and size.Microstructure is an important factor affecting the mechanical properties of materials.On the one hand,the grain size should be controlled when the forgings are large.Coarse grain or heterogeneous structure seriously affects the mechanical properties of the products.On the other hand,for 17-4PH steel,?-ferrite is formed at certain heating temperature,which is a major cause of microcrack under external force.Avoiding the precipitation behavior of ?-ferrite is very critical.To require the shape and size of the blade,the macro deformation behavior of material should be mastered.It's necessary to establish the material model for numerical simulation of process development in production.The main research was carried out as follows:Through heating experiments,the grain growth law was studied.The effect of heating temperature and holding time on the grain size of 17-4PH steel was analyzed.A mathematical model was established to describe grain growth.The precipitation of ?-ferrite was investigated under different heating conditions and the temperature inducing the formation of ?-ferrite was detected.According to the observed results,the forging temperature of 17-4PH steel was determined.The hot upsetting experiments of specimens at different temperatures were carried out and the hot deformability of the specimens with ?-ferrite and without ?-ferrite was compared and analyzed.The metallographic structure of the specimen with ?-ferrite was observed and the influence of ?-ferrite on the thermal deformation ability was analyzed.Based on the experimental data of thermal simulation experiments,the flow stress curves of 17-4PH steel was obtained and their variation was analyzed.A flow stress constitutive equations of the work hardeningdynamic recovery period and dynamic recrystallization period was established.It lays the foundation for numerical simulation. |
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