A comprehensive approach towards detection of sub-surface microscopic damage in low alloy ferritic steels using ultrasonic technique

Abstract

Service induced micro-damage poses an indiscriminate threat to the integrity of piping and pressure vessels used in various energy sectors. Type IV creep and high-temperature hydrogen attack (HTHA) are two typical examples of such high-temperature damage mechanisms known to accumulate in the material's volume prior to the formation of macro cracks. This accumulation of microscopic damage effectively reduces the stress bearing cross-section of the material and may result in the sudden failure of the component. Therefore, the industry is increasingly interested in non-destructively detecting such degradation at an early stage and subsequently monitoring its progression during service. The ultrasonic testing was identified as a potential, viable, field-deployable method for providing information on the presence of early-stage localised damage in the material’s volume. The availability of specimens containing representative damage was considered crucial for developing and validating the ultrasonic technique. This research is explicitly concerned with the generation of samples containing Type IV creep and the development of an ultrasonic technique for the earliest detection of Type IV creep while in the secondary stage of progression. Cross weld tensile specimens machined from electron beam (EB) and arc welded ASTM Grade 91 steel pipe section were subjected to uniaxial creep test. Creep test temperature and stress were optimised within the industrially relevant range to induce localised creep damage under accelerated conditions. Specimens representing a spectrum of damage from early secondary-stage to late tertiary-stage creep were successfully generated for ultrasonic technique capability assessment and further development. It was found that under the uniaxial creep test, the localised creep initiates and develops into a crack underneath the surface. In most creep-tested cross-weld samples; the Type IV creep progresses faster in the heat-affected zone (HAZ), corresponding to a larger bevel angle. In general, despite a wider HAZ, the electron beam welded specimen's creep life was longer compared to the arc welded sample. Several ultrasonic techniques utilising advances in electronics and signal processing capabilities were examined to bring down the detection threshold. The experimental investigation performed on industrially relevant specimen containing verifiable representative damage reveals the inadequacy of state-of-the-art full matrix capture (FMC) / total focusing method (TFM) technique for Type IV creep detection at an early stage. However, at the stage when damage can produce weak reflections, FMC/TFM may offer good spatial resolution and improved SNR due to synthetic focusing in the region of interest. Furthermore, TFM could be advantageous for characterising the advanced stage damage when it starts to orient. A focused ultrasonic testing technique utilising custom-designed single crystal probe applied with optimised parameters enabled improved detection of Type IV creep at the mid secondary stage of progression. This work establishes the significance of focusing ultrasound energy during the forward propagation into the region of interest to detect microscopic damage in ferritic-martensitic steels. The performance comparison with the FMC/TFM technique available on portable commercial equipment, supports the fundamental, that detection of microscopic damage, scattering ultrasound, is primarily governed by the intensity of ultrasound energy transmitted into the region of interest. Once the region is insonified appropriately, any limitation on detecting such scatterers largely relates to the material and receiver characteristics.