Effect of pre-precipitation thermomechanical treatment on precipitation behavior of CLAM steel

Abstract

China Low Activation Martensitic steel (CLAM) is a cladding material used in thermonuclear fusion reactors. Its high temperature performance is closely related to precipitation strengthening. The MX phase with strong thermal stability is the key to improving the high temperature performance of CLAM steel, and the M23C6 phase which is easy to coarsen under long-term high temperature service conditions is one of the important factors causing creep failure. In order to increase the content of MX phase and reduce the content of M23C6 phase, this paper first pre-precipitates near the precipitation temperature of M23C6 phase (which is also close to the maximum precipitation temperature of MX phase), which limits the precipitation of M23C6 phase and promotes the maximum precipitation of MX phase. Then the subsequent thermomechanical treatment (TMT) is carried out. The results show that the content of M23C6 phase in the samples after pre-precipitation TMT is greatly reduced, and the size is slightly reduced compared with the samples with only thermal deformation. The size of the MX phase is reduced by 10.6%, the volume density is increased by 88%, the volume fraction is increased by 34.1%, and the dislocation density is slightly increased. At the same time, it was found that a large number of MX phases precipitated at the lath boundary and pinned the boundary, which delayed the coarsening of the lath during tempering. The changes of these microstructures improve the precipitation strengthening, dislocation strengthening and boundary strengthening, so the room temperature tensile properties and high temperature tensile properties are improved, but the work hardening makes the total elongation and room temperature impact properties decrease.