- The mold steel's chemical composition along with the
manufacturing techniques used during its production, will determine its
ability to perform well in a given service environment. State-of-the-art
technologies such as specialized, remelting techniques, thermal diffusion
treatments and high forging ratios all play a significant role in
influencing the characteristics of mold steels.
The Limitations of "Off-the-Shelf" Mold Steels
Many grades of tool steel that are used for building molding components also are used for other industrial applications. For example, AISI S7 and H13 are commonly used for plastic injection molds; however, S7 also is used for metalforming operations and H13 for forging. The properties that are important for forging or stamping are quite different from the properties that are important to a moldmaker or plastics molder. Therefore, one must take precautions to ensure that they are using a mold quality steel. To do that one must consider the microcleanliness level of the mold steel, the degree of micro and macrosegregation and the restrictions on the number and size of large, primary carbides.
Standard Mold Steel Production
From the standpoint of the steel manufacturer, there are basically two means for improving existing steels used for molding applications:
(1) Adjusting the chemical composition. Adding specific alloying elements and balancing their levels can significantly influence the characteristics of the material.
(2) The actual steelmaking process. With the use of specialized melting techniques, mold steels can be produced which possess a very high microcleanliness level and homogeneous microstructure. These are two extremely important properties with regard to the steel's polishability and etching/texturing characteristics.
The Mold Steelmaking Process
In order to distinguish the different quality levels that are available for mold steels it is important to first have a fundamental understanding of the mold steelmaking process.
Mold steels are manufactured by melting starting material in an electric arc furnace (EAF). The starting material is comprised of carefully selected, low alloy scrap steel with the lowest possible level of impurities. Typically, the EAF units can melt as much as 50 tons of starting material per heat. Once the initial melting is complete, the molten steel is transferred to a ladle or refining vessel, where the composition of the steel is adjusted to provide the final chemistry. Additional steps such as slag treatments and degassing procedures also are involved to remove undesirable elements.
Following the refining stage, the molten steel is poured into large molds where it solidifies into a simple form called an ingot. These ingots will take several hours to completely solidify. This relatively long period of time will lead to significant amounts of chemical segregation, resulting in a variation in composition throughout the ingot's cross-section.