Anisotropic intergranular damage development and fracture in a 14Cr ferritic ODS steel under high-temperature tension and creep

Abstract

This work addresses damage mechanisms at high temperature of a hot extruded 14Cr oxide dispersion strengthened steel rod, in particular, anisotropy induced by its strong morphological and crystallographic texture. Under uniaxial tension, intergranular damage and unstable fracture increased with test temperature; they appeared much easier along the rod axis than perpendicular to it. Creep fracture of smooth specimens involved crack initiation from the side surfaces and intergranular crack propagation, accompanied by intergranular damage.In order to get rid of fracture surface oxidation by the air environment, a methodology promoting crack nucleation from inside the specimen was built. From tensile and creep tests on notched specimens the fracture initiation location and fracture stability were first mapped as a function of test temperature, maximal stress triaxiality and loading direction. Then, for the first time to the authors' knowledge, interrupted tests on notched tensile and creep specimens, followed by low-temperature fracture allowed revealing fracture surfaces formed at high temperature, but unconnected to the external environment. Intergranular fracture and grain boundary grooving were evidenced there, together with strong chemical reactivity at the surface of internal cavities during damage development at high temperature.

Creep and damage anisotropies of 9%Cr and 14%Cr ODS steel cladding

“…Two types of mechanical anisotropies are observed on the studied ODS steel tubes, in addition to the microstructural anisotropic features of the 14%Cr ODS steel tube. First, a strong effect of the loading direction is seen in Figure 5 for the stress to strain rate relationship on the 14%Cr ODS steel tube, consistent with bulk ferritic ODS steels with similar chemical composition [37,38,[40][41][42][43][44]. The 9%Cr ODS steel tube mechanical behaviour seems unaffected by the loading direction.…”

“…Fracture surface observation on the creep ODS steel specimens, both tiles and rings, showed the presence of oxidized intergranular fracture zones and ductile zones. This has been observed on other 14%Cr bulk ferritic ODS steel [37], with the following proposed two-step crack propagation mechanism: a slow stable intergranular crack propagation (suggested by the strong oxidation of such zones) followed by unstable ductile fracture as soon as a critical stress intensity factor is reached at the tip of the crack. Present observations concur with such fracture phenomena.…”

“…For the ring specimen, only cracks perpendicular to the loading direction can be seen. This is typical of ferritic ODS steels with elongated grains [3,6,[37][38][39], for which the grain boundaries, where titanium enriched unwanted particles tend to align as well, facilitate crack nucleation and propagation. Fracture surface observation on the creep ODS steel specimens, both tiles and rings, showed the presence of oxidized intergranular fracture zones and ductile zones.…”

Strengthening mechanisms of reduced activation ferritic/martensitic steels: A review

“…Fig. 7 shows a comparison of the high-temperature creep performance and radiation resistance between ODS and RAFM steels in recent years [100][101][102][103]. The creep life of the 9Cr ODS steel is ~1000 times longer than that of the F82H and Eurofer 97 steels under 650°C/130 MPa.…”

“…By contrast, the creep lives of the Eurofer 97 and F82H steels are merely 17, 50, and 150 h under identical creep conditions. The creep life of the cold deformed 9Cr2WVTa steel is 30 times higher than that of the F82H and Eurofer 97 steels under 550°C/270 Rupture time / h MA957 ODS [100] 12Cr ODS [100] 9Cr ODS [100] 14Cr ODS [100] 12Cr ODS [100] MA957 ODS [100] 14Cr ODS [100] 14Cr ODS [101] 14Cr ODS [102] Eurofer97 [100] P92 [100] F82H [100] HT MPa. The comparison of creep lives among ultrafine subgrained 9Cr2WVTa, typical Eurofer 97, and F82H steels is shown in Fig.…”