Abstract
The influence of hydrogen on the high cycle fatigue (HCF) behaviour of Inconel 718 has been studied at room temperature in asymmetric push–pull mode using an ultrasonic HCF test rig. Fatigue tests have been carried out in gaseous hydrogen (GH2) and in Ar at a pressure of 30MPa. Oscillating stresses with amplitudes (σa) up to 450MPa and mean stresses (σm) up to 600MPa have been applied. For a given σa and σm, the lifetime in Ar is generally longer than in GH2, which is explained by a hydrogen-induced embrittlement of the material. For a constant σa of 218MPa, the lifetime in Ar and in GH2 is very similar for high σm, but the difference in lifetime increases as the mean stress decreases. An approach is presented to describe the number of cycles to failure Nf as a function of σa and σm.Microstructural analysis has been performed on the specimens tested at σa=218MPa and two values of σm (300MPa and 600MPa). SEM analyses of the fracture surfaces of these samples indicate embrittlement of the material when tested in hydrogen atmosphere.
Highlights
Gas phase embrittlement of materials caused by the presence of damaging species such as oxygen, hydrogen, sulphur or chlorine has been an important issue for a number of industries over the years
The microstructure and fracture surface of the samples were observed with a scanning electron microscope (SEM) (Zeiss Supra 50) equipped with energy dispersive X-ray (EDX) analysis
In order to establish a generalised formulation of Eq (2), the Goodman equation has been modified: rNf has been replaced by the stress amplitude rf at which the specimen fails after Nf cycles at a given mean stress level rm – 0 and rUTS is replaced by a parameter C0
Summary
Gas phase embrittlement of materials caused by the presence of damaging species such as oxygen, hydrogen, sulphur or chlorine has been an important issue for a number of industries over the years. Hydrogen tends to be attracted to regions of triaxial tensile stress where the crystalline lattice is dilated and it is drawn to regions ahead of cracks under stress [7,8] For this reason, the diffusion of hydrogen is facilitated by the formation of cracks during fatigue tests. Increasing lifetime of many engineering components has led to a growing need for materials data and in particular for fatigue data at high number of cycles. To satisfy this demand, fatigue testing by ultrasonic excitation has been pioneered in the second half of the 20th century [9] and has since been established as a means for testing to 109 cycles and beyond. In this paper the dependence of the fatigue life of Inconel 718 on the environment and stress conditions is studied with the aim of characterising the impact of a high pressure (30 MPa) gaseous hydrogen (GH2) atmosphere on the HCF behaviour of Inconcel 718 in the range of 106—107 cycles
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