Measurement and modeling of temperature-dependent hydrogen embrittlement of chromium-molybdenum steel to enable fitness-for-service life prediction

Thick-wall vessels in petrochemical applications, fabricated from 2.25Cr-1Mo steel, operate in pressurized H2 at elevated temperature for more than 20 years. There is a concern regarding the interactive effects of temper-embrittlement and hydrogen-embrittlement on fitness-for-service during startup/shutdown near ambient temperatures. The database of degraded material properties is inadequate to enable accurate assessment. Specifically, H loss from small fracture mechanics specimens was substantial during either long-term or elevated temperature experiments. In addition, the influence of temperature on H-embrittlement of Cr-Mo steel is not fundamentally understood. The objectives of this research are to (1) design a novel laboratory method to retain H in small fracture mechanics specimens, (2) characterize the temperature dependent internal hydrogen embrittlement (IHE) of Cr-Mo weld metal using the developed method, and (3) model H distribution near a stressed crack tip in a H-trap laden bainitic microstructure to fundamentally understand the temperature dependent IHE.; The new slotted CT specimen approach, with 3.0 wppm total H produced on the slot surface from acidified thiosulfate charging, quantitatively characterized the temperature dependent threshold stress intensity (KIH and K TH) and kinetics (da/dtRISE and da/dtHOLD) of IHE in Cr-Mo weld metal during both rising and slowly falling K loading. IHE was produced successfully and damage was more severe during rising K loading due to the role of crack tip plasticity in H cracking of low to moderate strength steel. The critical temperature at which embrittlement ceased is in the range 45°C < Tc ≤ 60°C for the weld metal and H content studied. This method provides a useful new tool to generate fracture mechanics based fitness-for-service data. A three-dimensional finite element diffusion model, that accounts for the effect of crack tip plasticity and trapping on H transport, established K, dK/dt and temperature dependencies of H distributed about the stressed crack tip in the slotted and standard CT specimens. The slot approach provides higher H levels for long times and/or elevated temperatures, and solves the problem of H loss during testing.; The diffusion model was used to understand temperature dependent ME Stress field interaction energy (EH) vs. temperature at the blunted crack tip for Cr-Mo steel is lower than the estimated binding energies (EB) for the various surrounding reversible trap sites; indicating with probability calculations that H is unlikely to repartition from these traps to the stress field. Hydrogen transport to the fracture process zone (FPZ) from the surrounding bulk is by diffusion, enhanced by a plasticity-related mechanism. Interfaces and boundaries within the FPZ in the dilated region at the crack tip are the sites that form the interconnected H-fracture path. Trapped H concentration in these fracture sites critically governs the temperature dependent IHE, with negligible effect of temperature (≤100°C) on the crack tip stress field. The measured KIH for subcritical H cracking under rising K decreases systematically with increasing H trapped in the FPZ, as established by diffusion modeling for a variety of H cracking and temperature conditions. Diffusion model predictions of the critical trapped H concentration indicate that the Tc at which IHE is eliminated from Cr-Mo weld metal should be ≥110°C for a thick-wall hydroprocessing vessel with total-peak H of ≈4.0 wppm.