The Role of Silicon Content on Environmental Degradations of T91 Steels

T91 grade steels showed a gradual enhancement in tensile ductility at ambient temperature due to an increase in Si content from 0.5 to 2.0 weight percent (wt.%). However, the ultimate tensile strength was reduced only above 1.5 wt.% Si. The corrosion potential became more active in an acidic solution with increasing temperature. The cracking susceptibility in a similar environment under a slow-strain-rate (SSR) condition was enhanced at higher temperatures showing reduced ductility, time to failure, and true failure stress. Cathodic potentials applied to the test specimens in SSR testing caused an enhanced cracking tendency at 30 and 60 °C, suggesting hydrogen embrittlement as a possible mechanism of failure. Cracking of precracked and wedge-loaded double-cantilever-beam specimens was enhanced at higher initial stress intensity factors. In general, steels with 2.0 wt.% Si showed inferior corrosion resistance. A combination of cleavage and intergranular brittle failure was seen in the tested specimens depending on the type of testing.