Resistance of 10GN2MFA-A Low Alloy Steel to Stress Corrosion Cracking in High Temperature Water

In the period from 1986 to 1995 cracks due to stress corrosion cracking (SCC) have been revealed after 7000 to 60 000 hours of operation in the ligaments between tube holes on the cold collectors in 25 steam generators at 9 units operated in the former Soviet Union (WWER SC 076,1993; Matocha, Wozniak, 1996, IAEA-EBP-WWER-07, 1997). The cause analysis of collector cracking, carried out in the former Soviet Union, showed that the steel used, in all cases where cracking was experienced, has been produced by the open hearth furnace process (IAEA-EBP-WWER-07, 1997). Open heart furnace melted collector metal had local impurity concentrations, including manganese sulphide, which have deleterious effect on SCC resistance of low alloy steels in high temperature water environment. Since the beginning of 1989 a number of modifications have been introduced in the former Soviet Union in the steam generators or in their operational conditions process (IAEA-EBPWWER-07, 1997). One of them was the modification of the steelmaking process. Only steel produced by electroslag remelting has been used for the manufacturing of collector bodies. The electroslag remelting process (ESR) allows to enhance considerably micro-cleanliness of the steel and consequently the resistance of the 10GN2MFA steel to SCC in high temperature water environment. In the period from 1991 to 1994, the eight steam generators were manufactured in VITKOVICE, J.S.C. for WWER 1000 Temelin NPP. The collector bodies of Temelin NPP