The precipitation and coarsening behaviors of Cu-rich particles and the effect on the hardness of S30432 tube after aging and creep at elevated temperatures

Abstract

The precipitation and coarsening behaviors of Cu-rich particles and the effect on the hardness of S30432 tube after aging and creep for up to 25848 h at the elevated temperatures were systematically investigated by the means of transmission electron microscopy (TEM), three-dimensional atom probe (3DAP) and Vickers hardness measurement. Results show that Cu-rich particles form rapidly during short-term aging and creep at high temperature, and they are always coherent with the γ-matrix in S30432 steel. Cu-rich particles coarsen slowly, and they are still very fine even after 25848 h of creep. The coarsening of Cu-rich particles in S30432 steel follows the diffusion-controlled Lifshitz-Slyozov-Wagner (LSW) theory, and the coarsening activation energy was estimated to be 238.87 kJ/mol. Increasing temperature effectively increases the coarsening rate of Cu-rich particles by promoting the diffusion of Cu atoms. The coherent relationship between Cu-rich particles and γ-matrix leads to low interfacial energy of Cu-rich particle/γ-matrix interface. Based on the experimental results and our former investigations, the interfacial segregation of Mn and Al retards the coarsening of Cu-rich particles by not only decreasing the interfacial energy but also acting as a barrier for the diffusion of Cu atoms. With increasing the time, the retarding effect increases due to the enhanced interfacial segregation of Mn and Al. Cu-rich particles tend to coarsen more quickly during creep than aging in S30432 steel. The precipitation and coarsening behaviors of Cu-rich particles play a vital role in the hardness variation of S30432 steel, appropriate addition of Cu, Mn and Al may be useful for promoting the precipitation and controlling the coarsening of Cu-rich particles to further optimize the strength at the elevated temperature.