THEORETICAL AND PRACTICAL ASPECTS OF VACUUM INDUCTION MELTING OF HIGH-STRENGTH STEELS

摘要

Optimum vacuum induction melting, refining, casting and solidification procedures are employed to develop methods of improving general quality and, in particular, obtaining mechanical properties heretofore unattainable in conventional high-strength 4340 steel castings. This is accomplished by theoretically considering the vacuum furnace as a research instrument wherein considerable control can be exercised over crucible melt reactions, deoxidization, desulfurization, degasification, dissociation, volatilization and simultaneous deoxidation, crucible-melt and distillation reactions. These theoretical considerations are then applied to the actual conduct of vacuum heats. Residual gas contents of oxygen, hydrogen and nitrogen of 0.0003, 0.00001 and 0.0005 weight percent, respectively and lower, are attained in vacuum refined castings:this is a reduction in these gases of 83.7:1, 18.0:1 and 5.8:1 from the charge material. Reduction is size and frequency of nonmetallic inclusions is achieved in vacuum melting over conventional air melting procedures. Optimum casting and solidification techniques developed in these investigations are to pour under dynamic vacuum conditions to gain a stream de gassing effect. Immediately upon filling the mold with molten steel, an inert atmosphere is introduced, the pressure of which is well above that associated with the equilibrium gas concentration in the metal at the time of pouring.