Microstructure and mechanical properties of HAZ of RPVS clad with duplex stainless steel

Abstract

The microstructure and mechanical properties of HAZ of reactor pressure vessel steel (RPVS) clad with duplex stainless steel were investigated for understanding the effect of heating during the cladding process on the RPVS. A hardened area (up to ΔHV=50 in comparison to base metal), which can be referred to as clad-HAZ, was present in RPVS just beneath the cladding up to a depth of about 6 mm from the cladding boundary. It was also shown that there was a sharp hardness peak on the cladding boundary in the hardness profile across the boundary. The sharp hardness peak did not always exist along the boundary suggesting that the presence of the peak was affected by the location suffering from different thermal history during the cladding process. A nanoindentation hardness test revealed that the hardness peak width was about 30 μm. Line analyses of FE-EPMA revealed that the cladding boundary was enriched with carbon, and the width of the high carbon concentration region almost coincided with the peak width of nanoindentation hardness. Moreover, TEM/EDS analysis of the surrounding area of the hardness peak revealed the presence of both martensite formation, which was attributed to the increased segregation of carbon in the boundary of the cladding/base metal and the phase separation of Fe/Cr in which the Cr content was increased by Cr diffusion from the stainless steel cladding. The DBTT of clad-HAZ of RPVS measured with 1/3 size specimens was about 22 K higher than that of the base metal of RPVS, while USE was not influenced by the cladding process.