| Ultra-high strength stainless steel is an advanced material with outstanding strength, toughness and corrosion resistance. It has been widely used as structural components in applications such as aviation, aerospace and marine areas. However, hot plasticity of the alloy is poor due to its high contents of alloying elements and complicated precipitation, which considerably limits its further development. Therefore, it can make a significant theoretical sense and has a value of practical application by conducting systematize research on strengthening and toughing mechanism and hot deformation behavior of the steel.In this work, the microstructure of USS122 was studied by employing the methods of physicochemical phase analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that there are a large amount of M23C6 phase, M^C phase and x phase distributed on the original grain boundaries. There are lath marten site with high-density location and minor retained austenite for the steel after 1050℃ solid solution treatment for 1 hr, and precipitated phase is almost completely dissolved. The precipitated phase is not completely dissolved when saluting at 1000℃, which deteriorate toughness of the steel. Being solution treated at 1100℃, the yield strength decrease significantly as a result of the structure coarsening rapidly.The mechanical properties of the steel are Rm>1900MPa, AKU2=50J, after 1050℃×1hOQ,-73℃×8hAC,540℃×4hAC(peak aging treatment) treatment. Large amount of Laves phase were found in the martensite. The precipitates are very fine and dispersedly distributed in the martensite matrix (10-15nm), which improved the strength of the steel. Before aging peak, precipitation hardening isn’t strong enough due to the insufficient precipitates. After aging peak, precipitate was over-aging and coarsens.The critical strain, as a function of the deformation parameters, was measured and the DRX grain size model of the steel was constructed. A method are developed to determine the activation energy and material parameter in the hyperbolic sine constitutive equation based on the datas of flow stress at a temperature range of 850-1150 ℃ and strain rate range of 10-2～101s-1. The activation energy determined is 452.024kJ/mol for the steel in peak stress.The strain rate sensitivity, the efficiency of dissipation and the instability parameter have been studied in the temperature range of 850-1150 ℃ and strain rate range of 0.01～10s-1 by employing hot compression test, which were based on the dynamic material model, and the hot deformation map along with the best deformation area was established and confirmed. |
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