Understanding the role of cerium during VIM refining of nickel-chromium and nickel-iron alloys
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INTRODUCTION
I T has long been recognized that the presence of trace amounts of dissolved gases can adversely affect the mechanical properties of nickel-containing alloys and superalloys.~ The introduction of Vacuum Induction Melting (VIM) in conjunction with mixing by carbon monoxide boil has permitted the vacuum refining of many of these dissolved gases. 2's There are, however, several sources even within the VIM process where oxygen can be introduced into the melts, especially from the dissociation of the crucible refractory materials and from leaks within the vacuum chambers. Eventually some balance is reached at the vicinity of 10 to 20 ppm and further vacuum refining occurs only with the addition of process oriented alloy elements. Cremisio 4 pointed out that, by using calcium and magnesium, oxygen and sulfur in nickel-base alloys can be decreased further. However, the elements which might have the strongest effect on the further reduction of oxygen and sulfur during VIM of nickel-containing alloys are the lanthanides or rare earth elements (REM), such as lanthanum, cerium, and mischmetal, which are known to be strong oxide and sulfide formers. 5-9 These and other studies indicate that the REM-caused refining can result in mechanical properties improvements such as hot workability, 8'9 ductility, 1~ and creep ductility. 7'~r The amount of REM addition must, however, be controlled since excessive amounts can cause severe hot shortness, s'9 A recent study 9 has shown that the recovery of hot forged 80 Ni-20 Cr billets is markedly dependent on the residual content of REM additions. An analysis of the performance of many forgings has shown the presence of a residual REM content "window" for improved hot workability, as shown in Figure 1. The optimum recovery of forgings is obtained when the REM residue is between 0.025 and 0.05 wt pct. For the alloy studied, all ingots with residual REM content below 0.01 wt pct or more than 0.07 wt pct suffered severe hot cracking. Furthermore, when 0.04 wt pct Ca and 0.04 wt pct Mg are added along with mischmetal, optimum residual content of REM is shifted down to between 0.015 and 0.025 wt pct (Figure 1). D. LI and J. K. TIEN are Visiting Scholar and Professor, respectively, at the Henry Krumb School of Mines, Columbia University, New York, NY 10027. F. COSANDEY, formerly at Columbia University, is now Professor at Rutgers University, Departmentof Mechanics and Materials Science, Piscataway, NJ 08854. G.E. MAURER and R. FOOTE are Manager and Research Assistant, respectively,at the Special Metals Corporation, New Hartford, NY 13413. Manuscript submittedOctober 14, 1981. METALLURGICAL TRANSACTIONS B
Although the beneficial effects of REM on properties is now fairly well documented, little is known of the details of the refining process. Since the melt O and S content depends on a variety of parameters such as the type of rare earth, the amount of REM addition, the time at which they are introduced, and the type of furnaces and melt procedure, a research program was conducted i
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