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..
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