Effect of Cooling Rate for Quenching on IGSCC Suseptibility of CrMoV Steel in Deaerated Water at 383K
Hitomi Itoh
Abstract:CrMoV steel has been widely used in H-LP turbine rotors. Due to their configuration, in many cases wet zones are produced in the downstream regions of such rotors. Accordingly, the possibility that stress corrosion cracking may be initiated in the rim attachments and other sections subjected to high stress during long-term use have become a concern. The specifications of CrMoV steel make it difficult to change the tempering conditions to decrease material strength in order to increase resistance to stress corrosion cracking. Accordingly, the authors studied the effect of cooling rate for quenching in particular, on susceptibility to stress corrosion cracking. The results were as follows:(1) With low cooling rates for quenching, furnace cooling, susceptibility to the initiation of stress corrosion cracking becomes high. (2) Crevices have no impact on the stress corrosion cracking of CrMoV steel in deaerated pure water. (3) There were corrosion pits at the origins of almost all of the stress corrosion cracks. (4) Since metallic inclusions became the origins of corrosion pits, improving the cleanliness of the steel through ladle furnace refining would increase the resistance to stress corrosion cracking. (5) Fracture mechanics can be applied to the initiation of stress corrosion cracking. In these tests, the threshold value of stress intensity factor exhibited at the K1SCCP was 6.1 Mpa E m 0.5. It seems that the lowering of the cooling rate for quenching reduces this threshold value of stress intensity factor increase for the initiation of stress corrosion cracking. (6) Stress corrosion cracks of CrMoV steel in deaerated water have intergranular feature, and the grain boundary was a factor in crack propagation. When cooling rate for quenching is slow, bulky Fe3C is precipitated at the grain boundary, and this may be a factor that increases susceptibility to stress corrosion cracking. Key Words:IGSCC,H-LP turbine rotor, CrMoV steel, stress corrosion cracking, corrosion pit, non-metallic inclusion, quenching, deaerated water, fracture mechanics