Effect of Cerium on Solidification Structure and Eutectic Precipitates in Electroslag Remelted Cr-W-Co Martensitic Heat-Resistant Steel

The effect of cerium on secondary dendrite arm spacing, Laves phase, eutectic carbides, and the microsegregation of alloying elements in electroslag remelted Cr-W-Co martensitic heat-resistant steels was studied. The number density and size of Ce-containing inclusions in the as-cast ingots with different cerium contents were analyzed. The secondary dendrite arm spacing of remelted ingots decreases from 55 to 34 mu m with increasing the cerium content from 0 to 0.03 mass pct. The solidification microstructure of the as-cast ingots is refined by rare earth metal cerium in two ways: (I) the heterogeneous nucleation of primary ferrite (delta-Fe) provided by Ce2O2S and Ce2O3 inclusions, (II) the growth of dendrites inhibited by the enrichment of alloying elements at the solidification front. The area fraction of Laves phase and eutectic M23C6 carbides in the as-cast ingots decreases from 0.954 to 0.831 and 0.282 to 0.229 pct, respectively, as the cerium content is increased from 0 to 0.03 mass pct. The decrease in the area fractions are originated from the compression of the growing spaces of Laves phase and eutectic (Cr,Fe,W)23C6 carbides by both the microstructure refinement and the decrease in the contents of alloying elements Cr and W in the interdendritic regions.