An Investigation into Creep Cavity Development in 316H Stainless Steel

Creep-induced cavitation is an important failure mechanism in steel components operating at high temperature. Robust techniques are required to observe and quantify creep cavitation. In this paper, the use of two complementary analysis techniques: small-angle neutron scattering (SANS), and quantitative metallography, using scanning electron microscopy (SEM), is reported. The development of creep cavities that is accumulated under uniaxial load has been studied as a function of creep strain and life fraction, by carrying out interrupted tests on two sets of creep test specimens that are prepared from a Type-316H austenitic stainless steel reactor component. In order to examine the effects of pre-strain on creep damage formation, one set of specimens was subjected to a plastic pre-strain of 8%, and the other set had no pre-strain. Each set of specimens was subjected to different loading and temperature conditions, representative of those of current and future power plant operation. Cavities of up to 300 nm in size are quantified by using SANS, and their size distribution, as a function of determined creep strain. Cavitation increases significantly as creep strain increases throughout creep life. These results are confirmed by quantitative metallography analysis

Correlation Between Thermal Desorption Spectrum Features and Creep Damage

The analysis of the spectrum features of thermal desorption spectroscopy (TDS) using the desorption-rate profile against temperature is widely applied to investigate the hydrogen kinetics including diffusion and trapping in metallic materials, which is related to hydrogen embrittlement. Recently the TDS spectrum features such as the peak magnitude and the peak area have been used for qualitative assessment of creep damage, although there is still a lack of theoretical understanding on the correlation between TDS spectrum features and creep damage. In this paper, creep voids inducing creep damage are considered as the only kind of hydrogen traps in steels. The relationships between the TDS spectrum features and creep damage of ferritic steels are investigated through parameter analysis of the modified McNabb-Foster model together with the Oriani assumption, which can describe hydrogen evolution during thermal desorption. It is found that the peak area of TDS spectrum is independent of the trap binding energy, and it is proportional to the trap density, demonstrating that it could be a good indicator for creep damage. The creep damage can be characterized as a power-law function of the peak area of TDS spectrum, indicating TDS as a promising semi-destructive characterization method for creep damage of metallic steels

Stress driven creep deformation and cavitation damage in pure copper

The stress dependence of creep deformation and cavitation damage in pure copper at 250 °C under uniaxial loading is studied using a flat hourglass test specimen under uniaxial tensile load. In-situ digital image correlation (DIC) is used to monitor time dependent surface creep deformation, ex-situ small angle neutron scattering (SANS) applied to measure volumetric cavitation damage, and scanning electron microscopy used for surface characterisation. A self-consistent discolation model is successfully applied to explain the full field multi-stress creep deformation behaviour measured by DIC. Through approximating a range of shaped cavities with a model distribution of spherical voids, a minimum stable cavity nucleation diameter range of 600 to 1200 Å, depending on the applied stress level, is clearly observed in the SANS results. This finding supports the validity of the classical surface energy/work balance expression defining the minimum stable cavity size. All cavities observed in interrupted life samples were facetted in nature. The SANS data imply continuous cavity nucleation and growth throughout creep life, with a nucleation rate at stresses less than 100 MPa linearly related to the creep rate. This is in accordance with the double ledge grain boundary sliding nucleation model of Sandström and Wu [1]