Microstructure Evolution And Fracture Behavior Of 316LN Austenitic Stainless Steel For Nuclear Power Plant During Hot Deformation

Nitrogen alloyed ultralow carbon 316LN austenitic stainless steel, is a good candidate for the primary piping of the third generation API 000 nuclear power plant (NPP), due to its good formability, excellent mechanical performance and intergranular corrosion resistance. Differing from the casting primary piping of the second generation NPP, the AP1000 primary piping is formed by integral forging, including two nozzles. The biggest challenge of forging process is to prevent mixed grain microstructure and obtain uniform and fine microstructure. In compaction stage, such as ingot upsetting and stretching, the cracks especially the surface cracks should be prevented effectively. Westinghouse put forward strict requirements on the grain size of AP1000 primary piping, and they demand that the overall grain size is finer than ASME grade 2. Since 316LN steel is a single phase austenitic stainless steel, its microstructure cannot be improved by heat treatment, so the microstructure after hot forging plays a decisive role on the final mechanical properties of the primary piping. Therefore, it is very important to master the microstructure evolution and fracture behavior of 316LN austenitic stainless steel during multi-pass hot forging process of AP1000 primary piping.In this study, in order to understand the microstructure evolution during the forging of the AP1000 primary piping, a large number of Gleeble testing and heat treatment experiments were carried out. Dynamic recrystallization (DRX) behavior, static recrystallization (SRX) behavior and meta-dynamic recrystallization (MDRX) behavior, austenite grain growth and the fracture behavior of 316LN steel during multi-pass hot deformation process were studied, and the recrystallization models, grain growth model and ductile fracture model were established. The experimental results will be helpful for microstructure control and crack prevention during forging process. The work will also provide reference data and numerical simulation model for the optimization of manufacturing process.DRX, SRX and MDRX behavior of 316LN steel during multi-pass hot deformation were studied by one-pass and two-pass hot compression deformation. It was found that dynamic recovery (DRV) is difficult to occur in 316LN stainless steel, and the main softening mechanism is DRX during hot deforamtion, the ratio of DRX critical strain and peak strain is only 0.38. DRX is prior nucleating on grain boundaries, triple junctions and twin boundaries, and also near the deformation bands. The main nucleation mechanism of 316LN steel is strain-induced boundary migration. If the applied true strain is less than DRX critical strain, SRX occurs within forging pass interval. If the applied true strain exceeds critical value of MDRX. MDRX occurs within forging pass interval.The problem of simulation results and experimental values for recrystallization microstructure evolution show larger gap was effectively solved, which is caused by that the DEFORM-3D finite element simulation software using equivalent strain and equivalent strain rate as variables in the calculation process. The relationship between the recrystallization kinetics, grain size and true strain, true strain rate was established by classical methods. These models were introduced into the DEFORM-3D finite element software, and simulated the recrystallization behavior of 316LN steel. The simulation results showed larger gap between simulation results and experimental values. So the recrystallization models should be modified by using equivalent strain and equivalent strain rate as variables in stead of true strain and true strain rate. By finite element method, the true strain and true strain rate could be converted to the corresponding equivalent strain and equivalent strain rate. The recrystallization models were reconstructed using equivalent strain and equivalent strain rate. DEFORM-3D simulation results using the reconstructed recrystallization models agree very well with the experimental values.A number of heat treatment experiments were carried out, and the austenite grain growth behavior of 316LN steel was studied under 900～1200℃ for 0.25～10 hours. At temperature lower than 1000℃, austenite grains grow slowly. When the soaking temperature is higher than 1050℃, austenite grains grow very fast. When the time is shorter than 0.5 hours, austenite grains grew up fast. When the holding time is longer than 0.5 hours, austenite grains grew up slowly, due to the driving force for grain growth decreasing. The grain size after long time soaking at a high temperature is nearly not affected by the initial grain size. The relationship between the austenite grain size and the temperature, time and initial grain size was established, and the austenite grain growth constant n is 2.47.At the temperature of 900～1200℃ and strain rate of 0.01～1 s-1,it was found that deformation around the neck is more intense after necking. At 1200℃ and strain rate of 1 s-1, recrystallization nucleation rate and recrystallization grain growth rate near the fracture are higher, and under the joint action of nucleation and growth, the recrystallization grains near fracture are the finest and the most uniform, the percentage of area reduction is the highest, and plasticity is the best. Under 900～1100℃ and strain rate of 0.1 s-1,there was a peak percentage of area reduction. There is a linear relationship between critical fracture factor based on the normalized Cockcroft&Latham criterion and the logarithm of Hollomon-Zener parameter of 316LN steel.