Stress Corrosion Cracking Growth Rate of Cold Rolled Thermally Treated Alloy 690 and Solution Annealed Alloy 690 and their Mechanisms in Simulated PWR Primary Water

摘要

The Stress Corrosion Cracking Growth Rate (SCCGR) measurement tests and high-resolutional scanning electron microscopy, etc., were conducted on the many level of cold rolled (CR) Thermally Treated (TT) or solution annealed (SA) Alloy 690 and mill annealed (MA) Alloy 600 using commercial heats and laboratory heats, to clarify the mechanism on the large SCCGR for heavily CR TT Alloy 690. The number of micro-cavities near grain boundary (GB) carbides is strongly affected by the CR ratio. They are not detected in ≤ 10% CR commercial TT Alloy 690, in CR SA Alloy 690 and in ≤ 20% CR MA Alloy 600 (no or small number of GB carbides). No stress dependency for their formation was observed. The formation of micro-cavities in CR TT Alloy 690 is unrelated to creep damage. However, the number of micro-cavities near GB carbides in heavily CR TT Alloy 690 increased with increasing of time or temperature heated in air or exposure at 360°C in simulated PWR primary water and with increasing of Hydrogen concentration in simulated PWR primary water. The SCCGRs of CR TT Alloy 690 at 360°C in simulated PWR primary water show the good correlation with the number of micro-cavities near GB carbides in CR TT Alloy 690. Micro-cavities ner GB carbides in the TT Alloy 690 are deduced to be generated by shear strain due to CR. A high density of lattice defects is observed near GB carbides after CR and they agglomerate near GB carbides during heating at temperatures where lattice defects are sufficiently mobile. New micro-cavities or coarse cavities can be generated by heating and stabilized by Hydrogen absorbed from simulated PWR primary water. Nevertheless, in case of ≤ 10% CR (the likely upper limit of effective CR in practical components), the generation of micro-cavities near GBs is negligible.