Influence Of Grain Boundary Engineering On Intergranular Corrosion Resistance Of Alloy 690

Steam Generator (SG) is one of the most important equipments of Pressurized Water Reactor (PWR) nuclear power plant. SG heat exchange tube is the heat exchange interface between primary and secondary loop as well as pressure boundary of primary coolant, whose integrality has great effects on operation safety of nuclear plant. Therefore, the materials for SG heat exchange tubes must meet stringent technical requirements, among which resistance to the corrosion in primary and secondary coolant corrosion is of the greatest concern.With prolonged service lifetime and improved Performance being demanded by the nuclear energy industry, the need to improve the resistance to intergranular stress corrosion (IGSCC) and intergranular attack (IGA) in Alloy 690 should also be considered.Statistics from recent years show that: IGSCC and IGA, which are related to grain boundaries, are the main reasons for SG tube damage and leakage.In 1984, T Watanabe promoted the concept of"Grain Boundary Design and Control", which was developed into the"Grain Boundary Engineering"in the 1990s. This theory proposes that the ratio of special boundaries and its distribution can be improved and optimized through suitable rolling deformation and heat treatment. Therefore we can reach the aim of improving the mechanical and corrosion property of materials.The effects of thermomechanical treatments on GBCD in alloy 690 were studied by using optical microscope, scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and orientation imaging microscopy (OIM). The effects of rolling deformation on the grain size of alloy 690 were also investigated. The intergranular corrosion resistance of alloy 690, which has different special boundaries, was studied by using ASTM G28-A and electrochemistry methods.It is possible to reach the following general conclusions that are given below:1) The ratio of special boundaries in alloy 690 can be improved by using Grain Boundary Engineering method.2) In alloy 690, small strain(5%) and Subsequently high temperature(1120℃) annealing for short time (5min) can produce high proportion of low∑CSL grain boundaries (more than 67%),which mainly are∑3~n (according to Brandon's criterion)3) As the rolling deformation increases, the ratio of special boundaries of alloy 690 decreases; and the grain size increases; the performance on resistance to intergranular corrosion was evaluated using ASTM G28-A method, and results show that weight loss of specimens with 5% deformation reaches the minimum.4) Performance on resistance to intergranular corrosion was also evaluated by using electrochemistry method, and results show that Ra of specimens with 5% rolling deformation reaches the minimum.