Influence of hydrogen and carbides on high temperature cracking in cast Inconel

Abstract: Hydrogen embrittlement is a well-known source of cracking in metallic mechanical elements. This work studies the influence of hydrogen and carbides on the crack formation in a castnickel-alloy-component in a selective catalytic reduction system. It was found in literature that the alloy is prone to hydrogen embrittlement, and could therefore be a key factor in the crack formation. Cracks were found in the component, which is subjected to urea vapor (which contains a lot of hydrogen) at high temperatures. The propagation path of the crack seems to follow the carbide network in the material. From these observations two objectives for this project was formulated. The first objective of the work is to determine if hydrogen embrittlementis a plausible theory as a cause of the cracking. The second objective of this work is to investigate whether local stress levels in the vicinity of carbides is further raised by hydrogen. A modified boundary formulation of a crack tip is coupled with hydrogen diffusion, in order to realize the first objective. The coupled mechanical and diffusion problem is solved with a finite element model. The finite element model approximates the concentration of hydrogen that is diffused into the body during the working time, by the process of hydrogen diffusion for various parameters: hydrogen concentration in the crack, carbide trap density, carbide trap energy and more. A literature study is carried out and relevant intervals of such parameters is determined. It is found from the FE model that the concentration of hydrogen, 0.5mm ahead of the crack tip, can be approximately 1000 appm for smaller carbide trap energies (weak traps). For largercarbide trap energies (strong traps), the hydrogen concentration 0.5mm ahead of the crack tipcan be as high as 10000 appm and above. As a range of feasible hydrogen concentrations has been established, the second objective can be considered. The second objective is determined by considering dislocations and hydrogen in the vicinity of a carbide with a discrete dislocation dynamics (DDD) model. A conservative hydrogen concentration of 1000 appm, as well as hydrogen free setting is considered in the DDD simulation. The presence of hydrogen is shown to elevate the local dislocation density byapproximately 20%, which in turn elevates the local stress levels. It is highly plausible that stress levels may be further elevated by hydrogen as hydrogen concentrations from the first objective may be much higher than 1000 appm.