Microstructural influences on the damage evolution and kinetics of high temperature hydrogen attack in a C-0.5 Mo welded joint

In the refining industry, steels can be susceptible to a damage mechanism known as high temperature hydrogen attack (HTHA). The detection of damage in such structures through non destructive testing (NDT) requires insight into the damage development in various parts of welded joints. Once damage is detected, the tracking of the damage progression and estimation of remaining life is problematic. The performance of C-0.5 Mo steels in such environments is also variable, and the current tendency is to regard the HTHA resistance of C-0.5 Mo as similar to that of carbon steels. The current investigation explores the possibility of using high-temperature capsule strain-gauges for the in-situ evaluation of swelling – which generally accompanies HTHA in susceptible steels – and, thereby, evaluate the development and kinetics of HTHA damage for a C-0.5 Mo steel welded joint. Specimens were prepared representing the base metal (BM), the HAZ region, both fine-grained and coarse-grained, and the weld-metal (WM). The specimens were exposed to hydrogen at 200 bar at a temperature of 500 °C in an autoclave, with certain specimens fitted with high-temperature strain gauges; i.e. instrumented test labelled as IWM, IHAZ and IBM. Metallographic evaluation of the specimens – regarding the three locations mentioned – were performed; with exposure times reflecting both incipient and advanced damage. In the case of the base metal and the fine grained HAZ, and after pronounced damage, macrocracks were observed. The orientation and location of these cracks were aligned with the rolling direction, and the cracks were evident in the through-thickness plane of the plate. It was found that the reason for this observation was due to preferential damage accumulation within banded regions, where high concentrations of carbides were observed. A sigmoidal equation was developed that described the swelling kinetics of the HAZ. The expression of swelling-induced strain rate (SISR) versus time showed a low initial SISR until incipient damage is formed, followed by accelerated attack. The peak SISR was observed midway through the damage evolution, where after the SISRs decreased as the presense of carbides – that feed the methane reaction – diminishes. It is proposed that the attachment of the encapsulated strain gauges to structures or components where HTHA damage is suspected, will assist in the tracking of damage and the estimation of remaining life; if used in conjunction with the developed sigmoidal equation.