Aren't all steels made the same?
The most crucial quality-related difference between steel types is its chemical composition, however, even steels with identical chemical composition can vary in quality. For example, carbide distribution and inclusion content often determine the overall quality of the steel. It's because of this that secondary refinements and thorough batch testing are required to ensure the highest quality of steel possible.
How is Steel Quality Defined?
Understanding essential steel attributes and processes can help tablet producers determine tool quality and make the best tool supplier decision:
- Cleanliness - Cleanliness refers to the degree to which a steel is free of impurities or nonmetallic inclusions. The presence of nonmetallic inclusions causes an increase in internal stress that can lead to tooling failure. Monitoring and controlling the inclusion content is imperative to tooling quality.
- Homogeneity of Microstructure - Homogeneity (or the consistency) refers to an even distribution of a steel's alloying elements, carbides, impurities and the chemicals that comprise the steel matrix. While eliminating impurities in steel completely is impractical, clusters of impurities are detrimental to steel quality and can result in premature tooling failure due to the inconsistency of its structure.
- Grain Size - Grains are individual crystalline lattice structures with unique orientations within the steel matrix. Grains form during the solidification process, and grain size is dependent on the rate of solidification. The smaller the grain size, the higher the strength of the steel.
- Carbide Size and Distribution - Carbide size and distribution are also important because steels with small, well-dispersed carbides perform better than steels with coarse carbides. Tooling suppliers intentionally design carbides to be part of the metal matrix by means of alloying to enhance the steel's performance. Carbides that are too large or that form clusters can be detrimental to the tooling's life. Therefore, monitoring and controlling carbide distribution is essential to steel quality.
There are secondary refining methods that can be used to yield inherently higher quality steel by controlling the inclusion content. ESR (ElectroSlag Remelting) ensures the removal of impurities by placing a raw steel into a chamber with reactive molten slag that has high-current electricity passing through it, melting the steel. As the raw steel melts, it passes through the reactive slag, which removes impurities. As the slag passes through the chamber, its walls chill and re-solidifies the liquid steel. The process continues until the entire batch of steel has passed through the slag and re-solidified on the opposite side, producing a very clean, homogeneous steel with a refined cast structure.
Tablet producers should ask their tooling supplier if the supplier performs due diligence by having an independent lab randomly test steel chemistry and microstructure and analyse steel samples to ensure they meet quality specifications. Several standards exist worldwide to ensure consistent and defined measurements of a steel's cleanliness and quality, including the American Society for Testing and Materials (ASTM), German Institute for Standardisation (DIN), Japanese Industrial Standards (JIS), and International Organisation for Standardisation (ISO). A tooling supplier should quarantine all incoming material until an outside laboratory verifies that it meets all the specifications and standards of the appropriate standards organization. Image analysis can further quantify the steel's cleanliness, either comparatively with representative standards or with software designed to sort and classify inclusions.
Heat treating further enhances tool steel quality
While not a determinant in the quality of stock steel, heat treating is a process that the tooling supplier must carry out properly to ensure maximum tooling performance. Improper heat-treating of a quality steel will result in shortened tooling life and ultimately tooling failure. Each steel type requires unique heat-treatment parameters to achieve the best tooling performance for a given application. Understanding how the characteristics of steel can change during heat-treatment procedures is vital to tooling longevity.
Using only the highest quality steels in the manufacture of tablet compression tooling is vital for long tool life and for producing consistent, high-quality tablets. By understanding the processes and components of stock steel, you can make informed decisions when it comes to your tooling selection.