The Microstructure Evolution Of Cu-Rich Precipitate In Low-Alloy Ferritic Steel

The samples for this study were taken from low-alloy ferritic steel having higher Cu content. The samples have three kinds of chemical composition. They were heat treatment of 0.5h at 880℃ and water quenching, the samples were tempered at 660℃ for 10 h followed by air-cooling to room temperature. Samples were then isothermally aged at370℃ and 400℃ for different times up to 6000 h. The precipitation of Cu-rich clusters and crystal structural evolution of Cu-rich precipitates in low-alloy ferritic steel were investigated by means of TEM, HRTEM, STEM, EDS and APT. The main conclusions are described as follows:(1) Low-alloy ferritic steel steel were isothermally aged at 370℃ for 3000 h. when size of rich Cu atoms cluster was at around 2nm, not partial of Ni and Mn atoms. However,when size of rich Cu atoms cluster grew to 5nm, Ni and Mn atoms began appearing in partial poly.Generally, partial poly occur in rich of Cu atoms within clusters.(2) Low-alloy ferritic steel simulate aging 4000 h at 400℃, a large number of Cu-rich clusters form, which is mainly formed at grain boundaries, indicating that the grain boundaries favor the formation of Cu-rich clusters, and C, Mo, Mn, Ni, P atoms at the grain boundaries have varying degrees of segregation; Low-alloy ferritic steel simulate aging 4000 h at 370℃,There was a large number of Cu-rich clusters precipitateat at the interface of the Mo2 C and ferrite phase, At this time, diffusion rate of Cu atoms along the phase interface became faster, This indicated that the Cu-rich clusters are formed in the phase interface priority. Therefore, precipitation of Mo2 C will promote the precipitation of Cu-rich clusters.(3) Low-alloy ferritic steel steel was at 370℃ aging 3000 h, the first Cu-rich phase coherent with the matrix, the phase transition occurs coexist structure of monoclinic 9R and orthogonal 9R, and then converted to 3R structure, and ultimately into fcc lattice structure of the loss of coherent volume relationship; RPV steel simulate aging 3000 h at370℃, we found that each two close-packed layer of rich Cu atoms were separated by a close-packed layer of rich Fe atoms. we propose to structural phase transition model of 9R to 3R. When the shear force parallel to the 9R/3R interface, 9R/3R interface will gradually move upward in parallel shear force, thereby increasing the width of 3R stacking area,Shockley partial dislocations or stacking cut so close-packed plane order from ABC/BCA/CAB...change to ABC/ABC/ABC..., 9R structure ultimately make structural changes to 3R.