Mechanical and metallurgical properties of retired 3.5NiCrMoV low pressure steam turbine discs

Abstract

A study has been made of chemical composition, metallurgical structure and mechanical properties of many low-pressure turbine discs that had been retired from service, due to in-service cracking. Determination of the chemical composition involved both bulk chemical analyses and trace element analyses. The metallurgical examination involved determination of material microstructure, grain size, and grain boundary composition (Auger analysis). The mechanical properties measured included tensile strength, notch toughness, and fracture toughness. It was found that variations in bulk chemical composition do not affect the susceptibility to stress corrosion cracking and no abnormal composition that had contributed to in-service cracking was identified. The turbine discs that had cracked in service had widely varying tensile strengths, indicating that stress corrosion susceptibility of these particular discs were independent of their yield strength in the range 734 to 1131 MPa (106 to 164 ksi). The Charpy V-notch impact tests and the fracture toughness tests performed showed that the 3.5NiCrMoV steels generally had good toughness. Temper embrittlement of these steels led to reduced toughness and was related to increased phosphorus segregation to the prior austenite grain boundaries. The steels examined had similar tempered bainite microstructures, but varying grain sizes. There was no significant microstructural difference among the materials, and there was no evidence of inherent microstructural defects or abnormalities that may have contributed to the in-service cracking.