| As one of the key mechanical components in the nuclear island, the primary coolant pipe is known as the artery of the pressurized water reactor (PWR). AP1000 primary coolant pipe is a large special-shaped pipe mainly consisting of two parts, i.e., body part and two nozzles exhibiting 45° each other. The pipe, made of 316LN austenitic stainless steel, is manufactured by integral forging technology. There are two factors restricting the quality improvement of the pipe. Firstly, lateral surface cracks of the large forging. Secondly, coarse grains and mixed grains, especially those on the nozzle areas. Therefore, during the forging process of the pipe, both the grain sizes and boundary dimensions on different parts of the workpiece should satisfy the design requirements. As well, cracks should be completely avoided. These factors make the forging process of the pipe a superior difficulty. In this dissertation, the hot-working properties, rheological properties at high temperature and microstructure evolution laws of the 316LN austenitic stainless steel were studied, and the material database was systematically established. Also, by using the method combining physical modelling and finite element numerical simulation, the lateral surface crack, coarse grains and mixed grains were researched. And the relationships among forging parameters, surface cracks and microstructure evolution were determined. In addition, scientific and reasonable intergral forging technology of the AP1000 primary coolant pipe was formulated, realizing the localization of the pipe. The main research contents and conclusions are as follows:(1) The high temperature rheological mathematical models, recrystallization and grain growth models of 316LN austenitic stainless steel were established, and the mechanism of recrystallization was studied. By using the secondary development function built-in Deform-3D software, the 316LN material database that are compatible with Deform-3D was systematically established. The experiment results indicated that the 316LN material database was accurate and reliable.(2) The processing maps of 316LN austenitic stainless steel were studied. The research results indicated that this steel has a superior hot-working property in the temperature ranges 975?1175? and strain ranges 0.03?0.1 s-1.Meanwhile, an approach was put forward to determine whether the cracks will form or not on the lateral surface on the 316LN forging during upsetting. According to this approach and the processing map, the processing parameters ranges of the 316LN forging were suggested as:temperature ranges 975?1175?, strain ranges 0.03?0.1 s-1,reduction ranges 0?55%.(3) A new forging technology was proposed to control the grain size of the nozzle parts of AP1000 primary coolant pipe. The effects of various forging speeds and friction coefficients on the grain size and distribution of the microstructure during forging were researched. The results showed that the grain sizes in the wall of nozzles and upper parts of the pipe increased slightly with increasing forging speed, and the uniformities were desirable. The grain sizes in the lower parts were double, and the uniformity became worse with increasing forging speed. The friction coefficient had a significant effect on the grain size. The forging speed was suggested to be less than 20 mm/s, and effective lubricants should be used to decrease the friction coefficient.(4) An integral forging process for AP1000 primary coolant pipe was drawn up, and the grain size evolutions during forging were studied. A modified forging process was put forward aimed at the fact that the numerical simulated results did not conform with the actual working condition, namely, add the heat preservation process. However, the grains became coarsing at boss part resulted from heat preservation. Consequently, another modified forging process was put forward to get the optimized one, namely, local forge boss part after integral forging process. The results showed that the final grain sizes were 50?123 ?m and<50 ?m in boss part and others of the workpiece, respectively.(5) The integral forging forming process of AP1000 primary coolant pipe was simulated by Deform-3D. Then, the distribution and evolution laws of the temperature, effective stress, and microstructure were investigated, and the forging defect generation probabilities were analyzed. The results indicated that the initial forging temperature should be strictly controlled within the interval of 850?1150?, otherwise ferrite or ? phase would be precipitated to deteriorate the plasticity of the steel. During forging, in the zones where the workpiece contacted the dies, the temperature drop was high, but the effective strain increased minimally, dynamic recrystall ization hardly occurred and the initial coarse grain structure was retained. While in other zones, the temperature increased slightly, the effective strain increased steadily, complete dynamic recrystallization occurred around higher temperature, the grains were refined, and some voids at the core of the workpiece were welded together. Cracks were more likely to appear on the workpiece surface when forged to a large deformation. The average grain size in the middle body part was smaller than 30 ?m with an extremely uniform distribution, but the grain size uniformities of both end parts were not as good as that in the former. The grain sizes ranged from 30 um to 60 ?m. Boss parts had relatively homogeneous microstructure with the average grain sizes from 30 ?m to 44 ?m.(6) Additionally, an experiment at industrial scale was performed for verification. The research results showed that the mechanical properties of the forged pipe were qualified. No cracks, voids, ferrite or ? phase occurred during forging in the forged workpiece. A good agreement in grain size was achieved between the simulation and experiment results, and all grains in the workpiece were controlled within the range under 180 ?m, which met the designer's demands. The experiment indicated that this integral forging forming process was scientific, reasonable and feasible. |
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