Strength Dependence of Intergranular Stress Corrosion Cracking Susceptibility on 3.5% NiCrMoV Steel at 403K

Abstract

Rotors and discs used in low-pressure turbines must be made of a material possessing high strength and toughness and be capable of manufacturing extremely large components. For these reasons, 3.5% NiCrMoV steel is widely used. It has been previously reported that intergranular stress corrosion cracking in this material is initiated in wet deaerated steam when the temperature reaches approximately 400K. As no rotor and disc material with high strength and toughness that can serve as a substitute for 3.5 NiCrMoV steel has been found, efforts to resolve this problem have been conducted to reduce material strength and lower the susceptibility to stress corrosion cracking.
The purpose of this study was to elucidate a relationship between material strength and intergranular stress corrosion cracking susceptibility. Slow strain rate tests (SSRT) were performed in a neutral atmospheric environment at a test temperature of 403K. The results were as follows:
(1) The SSRT found that, as the 0.2% offset strength decreased, the area of the intergranular fracture was reduced. For the test specimen with an 0.2% offset strength of 777 MPa, only a tiny intergranular fracture was observed; at 0.2% offset strengths of 697 MPa and below, cracking disappeared completely. (2) From the precipitation status of the two carbides M₃C and M₇C₃ precipitated at the grain boundaries, it was found that the disappearance of the M₃C carbide coincided with the disappearance of the intergranular fracture. From this observation, it is thought that differences in composition of these carbides have an effect on the susceptibility of 3.5% NiCrMoV steel to stress corrosion cracking. (3) It was confirmed that the M₃C carbide disappears when the 0.2% offset strength is approximately 700 MPa or below, and that these materials achieve the target value for increased resistance to intergranular stress corrosion cracking in neutral environments.