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Phased array ultrasonic inspection of stress corrosion cracking in austenitic cladding


SCC damage from less than 0.25mm to greater than 3mm in through-wall size.
Figure 1: The range of SCC damage from less than 0.25mm to greater than 3mm in through-wall size.

Stress corrosion cracking (SCC) in austenitic stainless steel cladding is a topic of considerable concern to several major industries. TWI is presently involved in a range of Research and Development projects to improve the detection and characterisation of this damage mechanism in order to improve the structural integrity assessment of components post-fabrication and while in-service. Several methods of cladding exist in industry suited to different kinds of application. TWI has gained and continues to develop capability in a range of the most widely used cladding types, including metallurgical bonding, weld overlay deposition and mechanical lining. Each of these cladding types offers different challenges for inspection which require considerable design effort and validation.

TWI has recently completed a project where a component in a processing plant containing both metallurgical and overlay cladding across a welded joint was required to be inspected while in-service. The requirement was raised following the identification of SCC in the cladding, adjacent to the weld taking on both a longitudinal and transverse profile. TWI was provided with a representative specimen containing the damage by the Industrial Member. A project was raised to perform a failure investigation to identify the reasons for the degradation, through an analysis of the materials and the process conditions. Simultaneously a project was raised to design and develop an ultrasonic technique for the inspection of the cladding system in-service from the outside surface. Phased array ultrasonic techniques were designed using reference specimens and sensitivity was set on the actual damage found in-service. In order to be sensitive to very early onset of damage and to accurately characterise the nature of the damage high frequency probes were selected. Additionally, the inspection configuration to detect the cracking which was found to be in all possible orientations required the generation of a unique set of tools to successfully detect the full range of damage. The techniques were then demonstrated to the client in TWI with a witnessed series of inspections and sectioning in TWI's facilities. Following this successful demonstration the client requested TWI to undertake the inspections on site to complete the assessment of the components in-service, preventing the need to remove them destructively and towards a long term monitoring programme.

Ultrasonic image of transversely oriented SCC
Figure 2: Ultrasonic image of transversely oriented SCC allowing for characterisation of morphology and extraction of critical information such as through-wall size for Engineering Critical Assessment.

TWI is presently engaged in a Core Research Programme project to develop ultrasonic techniques for detection and characterisation of SCC and small flaws in austenitic weld overlay cladding in 316 stainless steel and nickel based Alloy 625. Past experience has shown that effective development of such techniques depend on access to real flaws, hence Industrial Members of TWI who are able to provide specimens in the above mentioned overlay cladding systems also containing SCC (confirmed through surface techniques such as dye penetrant inspection and which can be sectioned as part of the development) are being sought to engage with this project. The inspection techniques developed will be available to TWI Membership to adapt and apply post-fabrication and in-service.

Contact the NDT department for further information.