Exploring the effect of W on the W-containing Laves precipitates and internal oxidation mechanism of cobalt-containing austenitic stainless steel

This study systematically investigates the effect of tungsten (W) on the cyclic oxidation behavior of cobalt-containing austenitic stainless steel (Fe-25Cr-20Ni) at 1000 °C. The addition of W led to the precipitation of a tetragonal σ-FeCr phase in the austenitic matrix, with W being enriched in these precipitates. The specimen with 5 wt% of W (W-5) consistently exhibiting a higher weight gain than that of W-free specimen (W-0). The oxidation products for both specimens primarily consisted of MnCr₂O₄, Fe₃O₄, and Cr₂O₃, without any W-containing oxides detected. The oxidation scale of W-5 stainless steel peeled off severely, since the co-existence of precipitates of Laves-(Fe,Cr)₂W and σ-FeCr phases and obviously internal oxidation; meanwhile the precipitation of Laves-(Fe,Cr)₂W phases reduced the adhesive strength between the oxidation scale and the matrix. The precipitates provided additional diffusion paths for Cr, facilitating its diffusion toward to the surface and the formation of a dense Cr₂O₃ layer; and this also accelerated the diffusion of O ions into the matrix, resulting in significant internal oxidation and a deterioration of oxidation resistance.