Experimental And Numerical Study On Dynamic Recrystallization Of316LN Nuclear Power Steel

Heavy forgings are easy to produce various kinds of defects due to their big volumeand large size. Unreasonable production process will lead to the failure of heavy forging’smanufacturing. This will cause huge economic loss, or even affect other heavyequipment’s development in the situation of heavy forgings’ high cost and longmanufacturing cycle. From a micro perspective, product’s mechanical properties failure isprecisely caused by the bad internal microstructure. Therefore, obtaining fine and uniformgrains is a key to improve the mechanical properties of heavy forgings. In the hotdeformation, dynamic recrystallization (DRX) is an important mechanism ofmicrostructural evolution and it can enhance the strength and ductility by generating fineand uniform grains. In addition, the softening mechanism of DRX can remarkably reducethe hot deforming resistance of material. And before hot forging, the workpiece is neededto be heated to high temperature and held for a long time for stable single-phase austenitemicrostructure, which can be a good microstructure input for the practical forging process.Therefore, it is of great importance to study on the grain growth behavior during heatingprocess and the DRX behavior during hot working.Regarding to the nuclear power material316LN, the phenomenological models fordescribing grain growth behavior during heating process and DRX behavior at elevatedtemperature were developed, respectively. In order to simulate the complex DRX process,a CA model was developed, which can continuously display the structure evolution andeffectively reflect the physical mechanism. Thus, the prediction and control ofmicrostructure evolution of316LN steel during hot deformation was realized. The detailsof the research are as follows:Through heating experiments on the grain growth behavior of316LN, the influenceof heating temperature and holding time on grain size was studied. The initial forgingtemperature range was determined and the mathematical model for describing graingrowth behavior was developed, which can provide basis to acquire good initialmicrostructure and optimize the process. By thermal simulation experiments, flow stress curves of316LN steel was obtainedand their variation was analyzed. A constitutive model of flow stress and microstructureevolution models (including DRX kinetics model and DRX grain size model) wereestablished and validated by comparison of the calculated results and measured ones. Thedata used for CA simulation were extracted from the flow curves.Based on the fundamental metallurgical principles, a cellular automaton (CA) modelfor simulating DRX process, by tracking the variation of dislocation density, wasdeveloped. A parameter calculating module based on experimental data was programmedand incorporated into CA, which made it possible for DRX-CA simulation withthermomechanical parameters as input.The CA model was used to simulating the flow stress curves and microstructureevolution under different deformation conditions. The results showed that the simulatedresults were in good accordance with the actual data. By changing the strain rate,temperature, strain and initial grain size, the influence of thermomechanical parametersand initial microstructure on DRX behavior are investigated. The research showed that themetallurgical principle-based CA model can precisely predict the microstructuralevolution and flow stress during DRX and thus can provide theoretical basis for theprediction and control of mechanical properties of product.