Polishing, the final step in the mold building process, is highly dependent on the experience and technique of the polisher. However, if the material is not a “mold quality steel,” it may be virtually impossible to bring it up to the required finish.

For example, AISI materials specifications for S7, H13 and 420 stainless steel have relatively high limits with regard to elements such as sulfur. This leads to the formation of undesirable particles, referred to as nonmetallic inclusions. These foreign particles are formed during the conventional mold steelmaking process and their origins may be traced back to the tramp elements found in the original starting material. During primary melting these elements will react with dissolved gases in the melt to form these impurities.

Since these impurities are essentially insoluble, they will precipitate out of the molten steel during the solidification process and form as hard particles. These precipitates are comprised of oxides, sulfides, silicates or other constituents. Such particles are inherently harder than the surrounding steel matrix. If the steel contains a large percentage of nonmetallic inclusions, problems may arise during the polishing operation.
Polishing consists of using an abrasive medium (stone, diamond paste, etc.) to remove a thin surface layer of steel and create a series of finer-and-finer “scratches” on the mold’s surface.

Therefore, during the polishing operation the softer steel matrix that surrounds an inclusion will be removed at a greater rate. Under such conditions there is a tendency to lift the inclusion particle out of the steel matrix and leave behind a void. Several problems are associated with polishing a mold steel that contains large, non-metallic inclusions.
The number of inclusions found in the steel is directly proportional to the level of tramp elements such as sulfur. Therefore, the higher the sulfur level, the greater the amount of sulfide stringers will be in the mold steel.

The presence of large or numerous nonmetallic inclusions often leads to problems during the polishing operation. At this stage a large amount of time and money has already been invested into the machining and heat treatment of the mold. Typically, very little time is available for the polishing operation and any problems at this point will jeopardize the production schedule. Therefore, in order to ensure that the mold steel possesses good polishability characteristics, it is necessary to consider its microcleanliness level.