Effect of Holmium on Inclusions and Machinability of the As-Cast High Sulfur Steel
摘要
The influence of holmium (Ho) additions on the formation of inclusions and the machinability of sulfur-bearing steel was investigated. Scanning electron microscopy (SEM) was employed to characterize the inclusions in the as-cast steel with varying T.Ho content. The machinability of steels was analyzed by the lathe. The results indicated that the inclusion transformation proceeded in a particular order as the T.Ho content increased: MnS → MnS + Ho2O2S → MnS + Ho2O2S–Ho2S3 → MnS + Ho2O2S–Ho2S3 + Ho2S3 and the MnS inclusions located at the grain boundaries progressively changed into spherical Ho-bearing inclusions with uniform distribution. As the T.Ho content rose from 43 to 1228 ppm, the number density and area fraction of Ho-bearing inclusions in steels continued to increase from 4.0 #/mm2 and 46.6 × 10−6 to 40.8 #/mm2 and 1844.9 × 10−6, respectively. The fractions of Ho-bearing inclusions exhibiting an aspect ratio below 3 were measured as 20.57, 27.44, 53.97, and 78.92 pct at T.Ho content of 43, 183, 864, and 1228 ppm, respectively. As the cutting speed was 110, 355 or 560 r min−1, feed rate was 0.1 mm r−1, and cutting depth was 1 mm, the surface roughness of steels decreased with the increase of T.Ho content. With the feed rate (0.1 mm r−1) and cutting depth (1 mm) held constant, the surface roughness of the as-cast steel with the same T.Ho content showed an increasing trend as the cutting speed increased from 110 to 560 r min−1. An increase in T.Ho content reduced both the precipitation temperature and the number of MnS inclusions formed during solidification.