Start date and planned duration: January 2019, 24 months
- Investigate, using X-ray microscopy (XRM), the pit-to-crack transition stage on pre-strained 316L with various sizes of single corrosion pits under fatigue loads.
- Generation of fatigue life curves of 316L under combinations of with and/or without corrosion pits, pre-straining and environment conditions.
- Correlate changes in various parameters obtained from CT measurements with the extent of damage as seen via traditional microscopy (SEM and EBSD).
- Use of CT results such as the crack initiation point, crack shape and progression to feed into the lifetime modelling.
The project aims to develop safe working envelopes for Corrosion Resistant Alloys (CRA) with different strain histories and surface conditions (pre-pitted specimens). Using an environmental cell and XRM, the likelihood, onset and location of corrosion pit-to-crack behaviour will be investigated as a function of material condition (level of pre-strain, smooth/pre-pitted) and fatigue loads, in air and saline environments (3.5% NaCl, aerated and deaerated). The main benefits of the approach are as follows:
- Corrosion pits can be considered as ‘blunt notches’ and the influence of the latter on corrosion fatigue performance is important for design engineers and operators.
- The combined effect of pre-strain and pitting on fatigue performance in corrosive environment is currently unknown, yet in practice such scenarios are common in the offshore marine environment.
- Stainless steel grade 316L was selected due to its widespread application in tubing and pipework within the chemical, pharmaceutical, nuclear and oil and gas industry.
Stainless steel grades such as 316L exhibit complex behaviour under cyclic loading, which can also influence the corrosion behaviour, as plastically deformed material is more soluble. Under certain circumstances, 'dead' corrosion pits may reactivate and continue to grow. The use of in-situ XRM will be crucial to this study. The introduction of a corrosion pit might increase, decrease or have no effect on the pit-to-crack initiation time and/or crack location (eg near the pit surface/ at the pit wall/ at the bottom of pit). To obtain this information, the 3D interior structure of the test specimen has to be visualised and X-ray CT is ideal for the purpose. Further examination of the test specimens will use standard metallographic techniques including Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD).
The project will investigate which parameters have the biggest effects, using a simple test matrix. The information gained can be used to develop fatigue cracking initiation and lifetime models, and environmental knock-down factors.
Benefits to Industry
A successful outcome will allow industry to extend the use of this important class of materials safely into a wider range of applications and environments.
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