Crack growth in alloy 709 at high temperatures under fatigue, creep and dwell-fatigue loads

Yan, Jin ORCID: 0000-0002-6693-3394 (2021). Crack growth in alloy 709 at high temperatures under fatigue, creep and dwell-fatigue loads. University of Birmingham. Ph.D.

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Abstract

Alloy 709 is a new generation austenitic stainless steel under consideration for structural materials of Sodium-cooled Fast Reactors (SFR) at the highest operating temperature up to 550°C. Compared with conventional austenitic stainless steels, this advanced austenitic stainless steel has superior creep resistance. Safe operation of nuclear power plants requires comprehensive understanding of material performance under prolonged stressing in complex operating cycles and environments. One essential element is the capability to predict remaining service life and determine safe inspection intervals of components containing flaws. This relies on understanding of crack growth resistance. Due to the temperature and loading change at start-up and shutdown, understanding crack growth resistance under dwell-fatigue conditions is also an important aspect and in addition to that under pure creep (static) loading. As a relatively new alloy, such databases and detailed mechanistic understanding have not been established to date. This study aims to characterise crack growth resistance in Alloy 709 under creep and dwell-fatigue loading conditions at three temperatures, 550, 650 and 750°C. Fatigue crack growth behaviour was also studied. Furthermore, a small number of tests were carried out on a conventional austenitic stainless steel, 316H, to compare with results obtained on Alloy 709.

To aid the research on crack growth resistance, the microstructure of Alloy 709 has been characterised in both as-received and overaged conditions, using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A series of ageing conditions were selected: 500, 1000 and 2000 hours, at 550, 650 and 750°C respectively. The alloy is shown to be strengthened by fine Nb(CN) precipitates and nano-sized Z phases which form on dislocations. The coarsening of those precipitates during long term thermal exposure and formation of different new phases are studied.

In addition to crack growth testing, tensile and creep testing were also investigated. An increase in yield stress was observed after ageing at 650°C for 2000 hours. Excellent strain hardening behaviour and creep strength are measured, especially at 550°C, which is nearly service temperature of the Sodium-cooled Faster Reactors. This temperature also is close to a transition of fracture mode and/or fracture strain from a relatively brittle (intergranular) to completely ductile (microvoid coalescence) in creep tests.

Crack growth tests were conducted under fatigue (0.25 Hz), dwell-fatigue (1 hour hold time at maximum load) and creep (static) loading using compact tension specimens. Both a linear elastic fracture mechanics (LEFM) parameter, \(\Delta\)K, and an elastic-plastic time dependent fracture mechanics parameter (the C*-integral) was adopted to characterise crack growth behaviour. Tests were performed at 550, 650 and 750°C in air. A small number of tests were also conducted in vacuum to clarify the effects of air environment on transgranular and intergranular crack growth. A maximum load of 8 kN was applied for most tests to allow comparisons of crack growth resistance to be made for very similar testpiece geometries. Under fatigue loading (0.25 Hz) only small changes in fatigue crack growth resistance have been confirmed over the temperatures investigated, with largest differences seen at lower growth rates within the Paris regime. In contrast, crack growth resistance varies significantly for intergranular crack growth under creep and dwell-fatigue loading. As shown in plane-sided creep tests, excellent creep crack growth resistance is observed at 550°C. It is at this temperature alone that interaction of fatigue and creep (where the fracture is a mixed mode of transgranular and intergranular crack growth) is confirmed with dwell-fatigue loading and this increases crack growth rates compared to those obtained under (static) creep loading. At a temperature of 650°C, it is found that overageing does not degrade creep crack growth resistance. Also, a significant improvement in creep crack growth behaviour is confirmed in Alloy 709, compared with 316H. When using the C*-integral to characterise creep crack growth resistance which involves signification deformation ahead of the crack tip, all crack growth data collected from various temperatures and different alloys have fallen into a narrow scatter band for all tests suggesting that it is a parameter that can characterise crack growth rates uniquely for a given testpiece geometry.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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