Advances in ladle metallurgy enhance casting quality.

As steel foundries continue their push for improved casting quality, ladle metallurgy techniques are playing an increasingly important role. Recent enhancements in utilizing these refining processes are making them increasingly attractive to the steel foundry.

In describing the development of plasma refining for steel foundries (90-26) J.M. Svoboda, Process Metallurgy international, Inc, reported that "Recent research has clearly demonstrated that ladle metallurgy practices, such as desulfurization and calcium treatment, are important contributors to the production of 'clean steel.' The small size of foundry ladles and consequent heat losses have prevented the full application of these processes."

Several new developments are being used to overcome heat loss difficulties. Svoboda cited plasma Technology as one of these. Refining techniques "can be enhanced by applying supplemental heating to the ladle to allow the use of more fluid and reactive slags, and to increase processing time. The use of transferred and nontransferred plasma torches is applicable to these processes, as is the use of DC) arc furnace technology. The use of argon as the plasma gas also provides a protective cover for the melt, helping miminize gas pickup and reoxidation."

Plasma is an ionized gas made up of molecules, atoms, ions (in their ground or in various excited states), electrons; and protons. Overall, plasma is electrically neutral. For steel applications plasma is provided by either a plasma torch or through the electrodes in a DC arc furnace.

Another approach to overcoming heat loss during ladle refining was described by N.P. Cignetti and R.U. Swaney, Inductosteel (90-130). According to the authors, because of advancements in the electronics field-particularly the high-current diodes and SCRs in the past decade-indliction melting equipment manufacturers have been able to produce extremely efficient power supplies in excess of 97% electric efficiency.

This has allowed higher than ever before field densities to be applied in coreless induction furnaces. This technology, they say, led to the development of the Powered Window Ladle Refiner (PWLR). Reportedly, the power supply to the PWLR can be run virtually from zero power to fully rated power in order to control stirring and heating.

The PWLR uses a metallic ladle rather than ceramic material because of the potential for metal spill. Because of the unique construction of the ladle skin, more than 60% of the developed electromagnetic field passes through the ladle skin and couples with the molten metal contained in the ladle.

The advantages offered by the ladle metallurgy device, according to the authors, are to remove the metallurgical work from the primary melter, and to overcome the heat loss problems that come with unheated ladles "because the liquid metal only sees the open atmosphere at tap and later at pour, metal has been held for six hours with no ill effects to the ladle or metal."

The gating and risering of steel castings was another area that received special attention this year. T.S. Piwonka and N.H. El-Kaddah, Univ of Alabama/Tuscaloosa, presented work they have done on gating steel based on tundish flows (90-109).

According to the researchers, "Much of the difficulty in understanding gating system design for steel castings arises from the necessity to balance competing requirements in the runner system. In particular, the requirement that the runner serve to remove dross and inclusions from the metal stream and prevent reoxidation, while still providing rapid metal delivery at maximum yield, is particularly troublesome. However, recent research into the flow of molten metals in tundish systems suggests that a number of simple variations in runner design may substantially improve metal quality."

The use of tundishes rather than ladles to deliver metal to the mold should be regarded as a method to ensure that only clean metal is introduced to the gating system. The gating system itself should be regarded as the last chance to clean the metal before it enters the mold. Use of the principles developed in the steel industry for tundish design appears to offer a methodology for the rational design of gating systems which perform all of the functions required of them."

The proper feeding distance in steel castings makes a significant contribution in producing a sound part as well as minimizing shrinkage and deserves another look, according to N. Wukovich, Foseco, inc (90-38). Steel feeding distance rules were developed in the 1940s and 50s and have not changed during the past 40 years. "Yet, today some do not use the information, don't use it correctly or don't appreciate what a valuable tool they are," says Wukovich.

Feeding distance is defined as "the distance from a riser, or the last portion of a casting to solidify, that is sound or without significant shrinkage. Generally, this soundness is measured by radiographic inspection.

These levels of shrinkage are compared to the casting based on section thickness with the amount and intensity or shrinkage shown on the standard radiographs of the inspection specification." Secondary shrinkage in steel risers is an example of feeding distance being violated.

Wukovich concluded that understanding and applying feeding distance rules can help immeasurably in producing sound steel castings. "Data, information and materials are available for the foundryman to apply to produce sound castings. The materials include chills, padding materials and molding sands. The proper application can reduce casting rejects or costly repairs. Product liability and customer pressure will increase, making understanding this subject more demanding with time."