Permanent mold tools: cast or forged? The metallurgical and economical characteristics of castings can provide many benefits in the processing of permanent mold tools when compared to billets.

The material to make steel permanent mold casting tools can be processed in one of two ways--through casting or forging. Forgings begin as individual cast ingots or are continuously cast as slabs. This hot working process changes the billet shape, as far as possible, to that of the final product. Castings, on the other hand, are inherently closer to the finished shape of the end-product and can achieve those results with less machining.

In addition to shape changing, the hot working of forging must be done to such a degree that it will consolidate porosity due to solidification and break up segregation streaks due to chemical elements or inclusions. If excessive needle-like inclusions are left, they can initiate cracks by serving as stress raisers.

By its very nature, metalcasting produces a permanent mold tool much closer to the final shape than a forged billet and enhances quality by minimizing the segregation of chemical elements due to the smaller section size during solidification. Also, unlike with wrought material used as the foundation for billets, the inclusions in castings tend to be well-rounded and not elongated.

This article details the metallurgical and economical characteristics of castings vs. billets for permanent mold tools and shows cases in which castings have provided substantial savings over billets for permanent mold tools.

Metallurgical Characteristics

From a metallurgical view. steel castings are isotropic, possessing equivalent properties in all directions. Forged steel billets, depending on the hot working procedure, usually are anisotropic, displaying higher properties parallel to the major flow lines (forging movement direction) and lesser values in radial or transverse directions.

Examples of anisotropic properties are that of an 8-in. thick, 120-in. long, medium carbon steel forging. It was heat treated to a minimum of 60,000 psi ultimate tensile strength and tested by removing both longitudinal and transverse test specimens. Transverse tensile tests showed the ultimate tensile strength and the yield strength decreased by 3%. More significantly, there were decreases in the ductility properties of elongation and reduction of area. Percent elongation in the transverse test was 27% compared to 31% in the longitudinal tests. Reduction of area was 40.3% compared to 53.3% in the longitudinal tests. Charpy impact tests used to measure toughness have shown that transverse test results are less than half of that for longitudinal tests in wrought steels. In addition, hydrogen-induced cracks (sometimes known as flakes or fisheyes), which can lead to failures, are less likely to occur in castings than in forgings unless the latter has been degassed.

Welding of cast steel molds for repairs while in service will present fewer problems than that of forged steel. Work conducted at the Univ. of Tennessee, Knoxville, Tenn., and reported by the Steel Founders Society of America (SFSA), Crystal Lake, Ill., compared the welding qualities of five grades of cast and lorged AISI 8630 and manganese silicon steels. In all tests, fewer underbead or cold crocks occurred in the cast steel versus the forged steel. Also, it was noted that lower preheat temperatures could be used in the casting than in the forging without underbead cracking.

The most definitive thermal fatigue laboratory tests simulating permanent mold life cycles have been conducted at Case Western Reserve Univ., Cleveland. Using the lengths of cracks that developed from thermal fatigue, it was demonstrated that the basic method of manufacturing permanent steel molds (cast or forged) has little effect on the life cycle (Fig. 1). Cast steel was found to be equivalent to double-refined forged steel of the same composition and hardness with only a slight perception of difference across the test cycle range. The difference is that double-refined steel is a costly process because it requires the metal to be melted twice. The research also emphasizes the importance of heat treating in obtaining the desired martensitic microstructure and hardness.

[FIGURE 1 OMITTED]

Castings vs. Billets

The shape makes the difference. Because castings are inherently closer to the finished shape of the end-product than those made by any other steel making process, the beneficial result is considerably less machining Anecdotal evidence indicates that when geometry is severe (maximum metal removal required) and all costs compared or factored in (material, labor, assets employed, etc.), the casting process yields the highest value and lowest total cost tool.

The 4140 billet and near-net-shape casting of a permanent mold for a suspension component used in the transportation industry are compared in Table 1. The component has deep halves and irregular parting. Preparing the material to the same degree of rough completion favors the near-net-shape casting (Fig. 2) because it results in a 58% cost reduction.

[FIGURE 2 OMITTED]

In another case. Winona Pattern and Mold Co., Winona, Minn., chose AISI H-13 steel castings over billets for two pieces of a three-piece permanent mold. Castings were more cost-efficient and produced a near-net-shape mold block. The use of the castings was driven by cost, geometry. leadtime and machine capacity.

Table 2 breaks down the costs incurred when using one conjoined pattern to produce the two H-13 castings. This comparison includes both material and processes to cavity finish machine stage, but it does not take into account a total spindle time comparison up to this point. The casting (Fig. 3) in this case had lower overall spindle time and added some additional value.

[FIGURE 3 OMITTED]

Value With Castings

Based on the data, it can be concluded thaT the use of H-13 cast steel, properly heat treated, will give service life equivalent to forgings and provide more overall strength, toughness and better repair weldability along with cost efficiency. Designers, purchasers and users are looking for serviceability and value. Cost reduction is a common request that could more easily be achieved if the material specifications allowed the use of castings. The reality is that the perceived advantage of wrought or forged tools is not well-founded, The real value comes with castings.

For More Information

"Castings or Forgings? A Realistic Evaluation," M. Blair and R. W. Monroe, Engineered Casting Solutions, Spring 2000, p. 26.

"Mold Materials for Aluminum Alloys Cast in Permanent Molds," J.F. Wallace, D. Schwam and Y. Wang, Proceedings from the AFS 4th International Conference on Permanent Mold Casting of Aluminum, Nov. 1997

Thomas Kablach has more than 40 years of experience in working with steel forging and steel casting. Currently, Kablach serves as president of Tech Mart Inc., Crown Point, Ind., a technical consulting company specializing in heavy steel components.