Towards a Viable Field Deployable Ultrasonic Technique for Detection of Type IV Creep Damage in CSEF Steels at an Early Stage

Abstract

It is now apparent that welds in many of the creep strength enhanced ferritic (CSEF) steel grades are susceptible to Type IV creep damage. Furthermore, due to the complex nature of incubation and growth of localized creep damage in such alloys, state-of-the-practice non-invasive techniques such as hardness, replication and strain measurement alone are insufficient for reliable assessment. Consequently, there is concern in the industry regarding the integrity of existing and proposed installations that utilize CSEF steels such as ASME Grade 91 and Grade 92. To address this concern, in addition to pressing demands for increased efficiency and from environmental regulation, extensive research is underway on various fronts including fracture assessment, online health monitoring and life extension technologies. These rely heavily on the effectiveness of non-destructive testing (NDT) techniques. Therefore, volumetric non-invasive techniques that enable detection and characterization of damage are sought to facilitate effective assessment of welded components operating at high temperature and pressure. Several NDT methods were reviewed in order to understand the current state-of-the-art in terms of their sensitivity to early stage Type IV damage and their readiness for field implementation. Most of the advanced methods proposed for assessment of creep damage are based on the inversion of certain parameters to correlate to the extent of damage. This limits their selectivity, ability to characterize and determine the severity of localized damage. Using recent developments in electronics and signal processing instrumentation, ultrasonic testing was identified as having the potential to be developed as a reliable approach for detection of Type IV creep damage at an early stage. This paper presents the outcome of an industry-focused research effort with the goal of developing and validating an ultrasonic technique for reliable detection of Type IV creep damage at an early stage. In this framework, supported by the Core Research Programme at TWI, an ultrasonic technique was developed and tested on a number of creep-exposed specimens. Ultrasonic data was processed and correlated with controlled metallographic investigations to determine the detection, positioning and characterization performance of Type IV creep damage within the heat affected zone of welds.