Vacuum process produces thin-wall iron castings.

By limiting the oxygen in the melt, this process combines with standard iron casting to improve graphite formation, elongation and metal flowability for thinner walls.

As the applications for castings continue to grow, foundries are going to be asked to push the limit in terms of the complexity and mechanical properties their cast components can achieve. Ever-increasing weight reductions by end users, such as the automobile industry, in conjunction with the need to push engines and other components hotter and faster, has resulted in the metalcasting industry researching and developing methods to produce thin-wall, structurally sound iron and aluminum castings that maintain high mechanical properties.

One development for iron casting, which still is undergoing testing but currently is in production at one Japanese foundry, is Superior Viscosity Casting (SVC), a thin-wall iron casting process engineered by the Japanese firms Shinko Metals, Inc. and Nikkyo Sangyo Corp. According to the developers, the SVC process achieves 1-2-mm wall thickness (with 3 mm the norm) in gray and ductile iron castings without chill or shrinkage problems and without excessive risers or chills. By improving the flow of the molten metal and controlling the amount of active oxygen ([O.sub.2]) in the melt, the process reportedly reduces the shrinkage cavity in thin-walled iron castings and can improve casting yield by 15-20%. In addition, for gray iron, the process promotes ferrite and graphite growth to improve casting machinability and, for ductile iron, the process reduces the inoculant-material requirements while increasing elongation by up to 26%.

These results are possible because the process is based on the same iron to [O.sub.2] relationship that is used to remove impurities in steelmaking. The high control of active [O.sub.2] achieved in the SVC process is what allows the extremely thin walls to be achieved, instead of the 4-mm limit previously known for gray and ductile iron castings.

SVC Process

The inventors of SVC began their research to meet the weight and cost reduction demands of many of their industrial product OEMs. The result was a process that is adapted to standard iron casting by adding one extra step to traditional melting, transfer, inoculation and pouring procedures.

The process begins once the 2732-2786F (1500-1530C) molten iron has been transferred from the melting furnace to a specially designed pouring ladle [ILLUSTRATION FOR FIGURE 1 OMITTED]. This transfer temperature ensures that the iron is above the temperature of primary crystallization. The control of eutectic graphite crystal generation due to over-cooling and active [O.sub.2] in the molten metal is critical to the processes success.

Once the transfer to the pouring ladle is complete, the ladle is placed in a vacuum chamber, which undergoes a depressurization to force a degassing (of the [O.sub.2] and other impurities) and cleaning of the molten iron. The length and amount of degassing are controlled automatically, but for a 200 kg ladle the treatment time is 90 sec and for a 400 kg ladle the treatment time is 120 sec. This degassing also improves the fluidity of the metal and closes the gap between the liquidus and solidus temperature stages of the molten metal to promote uniform solidification of the casting. Proper execution of the process is confirmed by comparing the amount of active [O.sub.2] in the final molten metal with that in the original molten metal via the system's control unit. Once deoxidization has been completed, the ladle is removed from the vacuum chamber and the molten iron is inoculated and proceeds to the pouring line.

The iron can be poured at 2282-2426F (1250-1330C).

The degassing under vacuum results in a molten metal with less than 0.1 ppm active [O.sub.2] and from [10.sup.-20]-[10.sup.-22] atomic weight of [O.sub.2] partial pressure within the degassing chamber. As a result, little to no [O.sub.2] film is formed on the molten metal at 2192F (1200C). As long as the molten metal proceeds through inoculation and then to pouring within normal time specifications, little to no reoxidization will occur.

Benefits

The four main benefits reported for the SVC process are:

* increased fluidity of the molten iron - the result is gray and ductile iron castings with 1-2 mm wall thickness and up to 20% improved casting yields (which reduces the riser and chill requirements);

* increased ferrite in gray iron castings - this reduces cutting resistance by up to 20% because of the promotion of graphite growth;

* increased elongation in ductile iron - the up to 26% increase results in increased casting toughness without affecting tensile strength. Table 1 shows a comparison in elongation for a ductile iron manifold. In addition, the ferrite effect suppresses cementite formation and inhibits chills;

* decreased shrinkage cavities in castings with thick/thin wall thickness - SVC raises the eutectic temperature by 18F (10C) and stretches the eutectic reaction time to 1 min, which results in simultaneous solidification and less of a shrinkage cavity in the thick walls.

Production

The developers of the process have targeted four primary markets for the technology: automotive (exhaust manifolds, cylinder heads, and transmission and crank cases); plumbing (pipes, valves and bushings); electric motors (motor frames and covers, fans and impellers); and art casting. One Japanese foundry - Yasukawa Electric Corp., Nakama, Japan - currently is licensing the process and is using it in production as well as prototyping.

Table 1. Comparison of Elongation of a Traditional Cast vs. SVC
Treated ASTM A536-84 (grade 65-45-12) Ductile Iron Manifold

Mechanical properties      Traditional cast      SVC treated

Tensile strength            45.2 kg/sq min      44.1 kg/sq mm
Elongation                        16%                 26%

Yasukawa has installed a SVC processor with a 400 kg ladle for the production of electric motor cases, robot parts and sensing arms for machine tools. With the ductile iron motor case, the foundry has achieved 1-mm wall thickness, which improved yield by 10% and machinability by 15%. In addition, the defects such as pinholes and shrinkage were reduced. The foundry is investigating other casting applications and the possibility of adding a 800-kg ladle for SVC processing.