| Summary: | The paper presented the findings obtained by industrial research and experimental development on the use of rare earth metals (REMs) in the production of heavy steel ingots and their impact on the internal quality of the 42CrMo4 grade steel forging. REMs alloying was carried out after vacuuming the steel. A relatively large melting loss of cerium (about 50%) and its further decrease in casting due to reoxidation were observed. Refinement of structure and better mechanical properties of forged bar containing about 0.02 wt.% of Ce compared to that of the standard production were not achieved. The wind power shaft with content of about 0.06 wt.% of Ce showed high amount of REM inclusions, which were locally chained, and in some cases, initiated cracks. Four stoichiometrically different types of REM inclusions were detected in forgings, namely (La-Ce)<sub>2</sub>O<sub>2</sub>S + (La-Ce)O<sub>2</sub> + SiO<sub>2</sub> (minority); oxygen, phosphorus, arsenic, and antimony bound to lanthanum and cerium probably bonded with iron oxides La + Ce, MgO, Al<sub>2</sub>O<sub>3</sub> a SiO<sub>2</sub>; (La-Ce)<sub>2</sub>O<sub>2</sub>S, FeO, SiO<sub>2</sub>, and CaO or CaS.
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