Continuously cooled microstructure and creep rupture strength of low C-2.25Cr-1.6W-V-Nb steel

The effect of the cooling rate after austenitizing on both the microstructure and the creep rupture strength of low C-2.25Cr-1.6W-V-Nb steel were investigated. As the cooling rate became lower, the as-normalized microstructure changed from bainitic ferrite (alpha (B)) to granular bainitic ferrite (alpha (B)) with polygonal ferrite (a,). The hardness of materials after austenitizing followed by cooling at rates of 0.09 to 6.94Ks(-1) and tempering was found to be about 190HV. Hardness was low at cooling rates of 0.03Ks(-1) or under because of the increased volume of ferrite. The extrapolated 10(5)h creep rupture strengths at 798, 823, and 848K with cooling at rates of 0.09 to 6.94Ks(-1) were nearly the same, while the trend at 873K differed from others, with a peak observed at 0.09 Ks(-1). However the creep rupture strength of 0.03Ka(-1) in the lower stress region was close to the creep rupture strength with cooling at rates of 0.09 to 6.94Ka(-1). Three types of creep behavior tested at 873K and 175MPa were observed, since the microstructure changed due to the cooling rate when normalizing. It was found that, for cooling rates of approximately 0.09 Ks(-1) and greater, sufficient creep rupture strength could be secured. This rate can be attained through air-cooling for normal boiler application thickness.