Stability of fatigue cracks at 350°C in air and in liquid metal in T91 martensitic steel (original) (raw)

Behaviour of short and long cracks in air and in liquid metal in T91 steel

MATEC Web of Conferences, 2018

The low cycle fatigue behaviour of a 9Cr1MoNbV martensitic steel has been investigated at 350°C in air and in lead-bismuth eutectic (LBE). Total strain controlled tests were performed from ∆ε t = 0.40% to 1.2%. The material exhibited a pronounced cyclic softening in both environments. LBE reduced the fatigue resistance. LBE accelerated the formation of the long crack by promoting the growth of the first short cracks. A clear change in propagation mode was observed. In air, ductile fatigue striations were observed while in LBE a brittle fracture decorated by voluminous and largely spaced striations were visible. From EBSD analysis, it was concluded that in LBE, the long crack advanced quickly by repeated and discontinuous cleavage.

Fatigue Behaviour of a 9Cr1MoNbV Martensitic Steel in a Liquid Metal*

Materials Testing, 2009

In this paper, the low cycle fatigue (LCF) behaviour at 300 8C in air and in liquid lead bismuth eutectic (LBE) of the T91 martensitic steel is studied. From LCF tests in the range D3 t 0.4-2.4%, it is shown that the cyclic stress response consists of a cyclic softening. Monotonic and cyclic stress-strain curves do not change with the environment, whereas the fatigue life is reduced after LCF in the high strain range in LBE compared to air. Metallographic and fractographic observations suggest that the reduction in the fatigue resistance is due to a liquid metal enhancement of the crack propagation rate.

Temperature dependence of liquid metal embrittlement susceptibility of a modified 9Cr–1Mo steel under low cycle fatigue in lead–bismuth eutectic at 160–450 °C

Journal of Nuclear Materials, 2015

Liquid metal embrittlement 9Cre1Mo steel Leadebismuth eutectic (LBE) Low cycle fatigue Fatigue endurance trough a b s t r a c t Low cycle fatigue properties of a 9Cre1Mo ferritic-martensitic steel (T91) have been tested in a low oxygen concentration (LOC) leadebismuth eutectic (LBE) environment and in vacuum at 160e450 C. The results show a clear fatigue endurance "trough" in LOC LBE, while no such a strong temperature dependence of the fatigue endurance is observed when the steel is tested in vacuum. The fractographic observations by means of scanning electron microscopy (SEM) show that ductile microdimples are prevalent on the fracture surfaces of the specimens tested in vacuum, whereas the fracture surfaces produced in LOC LBE at all the temperatures are characterized by quasi-cleavage. Interestingly, using electron backscatter diffraction (EBSD), martensitic laths close to the fatigue crack walls or to the fracture surfaces of the specimens tested in vacuum are found to have transformed into very fine equiaxed subgrains. Nevertheless, such microstructural modifications do not happen to the specimens tested in LOC LBE at 160e450 C. These interesting microstructural distinctions indicate that liquid metal embrittlement (LME) is able to occur throughout the fatigue crack propagation phase in the full range of the temperatures investigated, i.e. LME is not very sensitive to temperature during the fatigue crack propagation.

Temperature dependence of liquid metal embrittlement susceptibility of a modified 9Cr-1Mo steel under low cycle fatigue in lead-bismuth eutectic at 160 to 450°C

Journal of Nuclear Materials, 2014

Liquid metal embrittlement 9Cre1Mo steel Leadebismuth eutectic (LBE) Low cycle fatigue Fatigue endurance trough a b s t r a c t Low cycle fatigue properties of a 9Cre1Mo ferritic-martensitic steel (T91) have been tested in a low oxygen concentration (LOC) leadebismuth eutectic (LBE) environment and in vacuum at 160e450 C. The results show a clear fatigue endurance "trough" in LOC LBE, while no such a strong temperature dependence of the fatigue endurance is observed when the steel is tested in vacuum. The fractographic observations by means of scanning electron microscopy (SEM) show that ductile microdimples are prevalent on the fracture surfaces of the specimens tested in vacuum, whereas the fracture surfaces produced in LOC LBE at all the temperatures are characterized by quasi-cleavage. Interestingly, using electron backscatter diffraction (EBSD), martensitic laths close to the fatigue crack walls or to the fracture surfaces of the specimens tested in vacuum are found to have transformed into very fine equiaxed subgrains. Nevertheless, such microstructural modifications do not happen to the specimens tested in LOC LBE at 160e450 C. These interesting microstructural distinctions indicate that liquid metal embrittlement (LME) is able to occur throughout the fatigue crack propagation phase in the full range of the temperatures investigated, i.e. LME is not very sensitive to temperature during the fatigue crack propagation.

Influence of testing and tempering temperatures on fatigue behaviour, life and crack initiation mechanisms in a 5%Cr martensitic steel

Procedia Engineering, 2010

The effects of temperature as well as the initial hardness in the range 42 to 50HRC on the fatigue behaviour and life of a 5%Cr tool steel are investigated. Total strain controlled isothermal fatigue experiments between 200°C and 600°C are carried out under 1Hz frequency. The significant softening induced by tempering and/or cyclic loading is related to a strong reduction of the dislocation density clearly observed by transmission electron microscopy and x-ray diffraction. The coalescence of secondary carbides is also involved in the yield strength decrease above 550°C and during cyclic loading. Increasing the initial hardness reduces the cyclic softening rate and increases the fatigue life. Moreover, several isothermal fatigue experiments with regular interruptions at different fractions of life are performed in order to investigate the crack initiation mechanisms. Three main crack initiation sites were observed depending on testing temperatures: non-metallic inclusions, prior austenitic grain boundary and lath boundary.

Low-cyclic fatigue behavior of modified 9Cr–1Mo steel at elevated temperature

Materials Science and Engineering: A, 2014

The low-cycle fatigue behavior of indigenously developed modified 9Cr-1Mo steel has been evaluated using a constant strain rate (1 Â 10 À 3 s À 1) at ambient temperature (25 1C) and at elevated temperatures (500-600 1C) over the strain amplitudes varying between 70.7% and 71.2%. Cyclic stress response showed a gradual softening regime that ended in a stress plateau until complete failure of the specimens. The estimated fatigue life decreased with the increase in test temperature. The effect of temperature on fatigue life was more pronounced at lower strain amplitudes. The cyclic deformation behavior at different temperatures has been analyzed from hysteresis loop and also in view of the changes taking place in dislocation structure and dislocation-precipitation interaction. Evaluation of low-cycle fatigue properties of modified 9Cr-1Mo steel over a range of test temperature can help in designing components for in-core applications in fast breeder reactors and in super heaters for nuclear power plants.

Fatigue crack propagation in steel prone toward deformation twinning

Materials Science, 1982

In the last decade many investigations have been made of the influence of structure factors, temperature, medium, and mechanical test conditions on the kinetics of fatigue cracks. There is special interest in studying the rules of formation of the threshold stress intensity factors AKth corresponding to fatigue crack propagation with a rate not exceeding i0 -I~ m/cycle and, in particular, determining more accurately the role of the structure of the materials in this process.