Determination of the toughness of in-service steam turbine disks using small punch testing

摘要

Knowledge of the material toughness is crucial in assessing the integrity of heavy section steel components. Conventional tests to determine the toughness involve extraction of large blocks of material and therefore are not practical on in-service components. On the other hand, conservative assumptions regarding toughness without regard to actual data can lead to expensive and premature replacement of the components. Previous EPRI studies have demonstrated the use of a relatively nondestructive technique termed the "small punch test" to estimate the fracture appearance transition temperature (FATT) and fracture toughness (K{sub}(IC)) of high-temperature turbine rotor steels and nuclear reactor pressure vessel steels. This paper summarizes the results of research into the feasibility of extending the small punch test to characterize the toughness of the 3 to 3.5% NiCrMoV (3-3.5NiCrMoV) low alloy steel used for fossil and nuclear power plant low-pressure (LP) steam turbine disks. Results of the present study show that the small punch transition temperature, T{sub}(sp) is linearly correlated with FATT, so that measurement of T{sub}(sp) permits estimation of the standard Charpy FATT through empirical use of the correlation. The statistical confidence prediction uncertainty bands for the correlation were found to be narrow enough to make the small punch- based FATT estimation practical for this alloy. Additionally, independent T{sub}(sp) measurements made by PowerGen, UK, on some of the same test materials were in excellent agreement with measurements made here, indicating that the small punch T{sub}(sp) measurement can be reproducible across laboratories. Limited testing for fracture initiation toughness showed, as has been demonstrated for other materials, that the small punch test-based initiation fracture toughness (K{sub}(IC)) determination was within ±25% of the ASTM standard measurement of K{sub}(IC), suggesting that the test method can be used for direct determination of fracture initiation toughness.