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

Knowledge of the material toughness is crucial in assessing the integrity o f heavy section steel components. Conventional tests to determine the tough ness involve extraction of large blocks of material and therefore are not p ractical on in-service components. On the other hand, conservative assumpti ons regarding toughness without regard to actual data can lead to expensive and premature replacement of the components. Previous EPRI studies have de monstrated the use of a relatively nondestructive technique termed the "sma ll punch test" to estimate the fracture appearance transition temperature ( FATT) and fracture toughness (K-Ic) of high-temperature turbine rotor steel s and nuclear reactor pressure vessel steels. This paper summarizes the res ults 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 allo y steel used for fossil and nuclear power plant low-pressure (LP) steam tur bine disks. Results of the present study show that the small punch transiti on temperature, T-sp, is linearly correlated with FATT, so that measurement of T-sp permits estimation of the standard Charpy FATT through empirical u se of the correlation. The statistical confidence prediction uncertainty ba nds for the correlation were found to be narrow enough to make the small pu nch- based FATT estimation practical for this alloy. Additionally, independ ent T-sp measurements made by PowerGen, UK, on some of the same test materi als were in excellent agreement with measurements made here, indicating tha t the small punch T-sp measurement can be reproducible across laboratories. Limited testing for fracture initiation toughness showed, as has been demo nstrated for other materials, that the small punch test-based initiation fr acture toughness (K-Ic) determination was within +/- 25% of the ASTM standa rd measurement of K-Ic, suggesting that the test method can be used for dir ect determination of fracture initiation toughness.