August 2023 Volume 5

EQUIPMENT & TECHNOLOGY

vary in the time they are in contact with a heated billet. A hammer, for example, has relatively little workpiece contact time with a high rate of deformation during impact. Ductility and impact resistance are most important in this situation. In the case of presses, higher workpiece contact times and slower deformation rates require additional alloying elements to enhance specific desired properties.” Thermal treatments are used to achieve the demanding properties of tool steel, such as hardness, toughness, ductility, and wear resistance. However, heat treating is not a ‘one size fits all’ process. Depending on the steel type, heat treaters have developed specialized processes that result in the right combination of properties for specific die applications. This is where the use of time/temperature/ transformation curves come into play. These define the relationships between treatments at temperature, the times held at temperature, and the steel phase changes that occur during the process. A raw die block must be heat treated to prepare it for service. The initial heat treating and quenching cycle will harden the steel, but also make it brittle. Quenching the steel is critical to the end result, The extension of tool life in the machining process has long been a desired goal. As such, tool coating technology became a reality in the 1970s with the advent of titanium nitride (TiN), but it wasn’t until the 1980s that ultra-thin film TiN was commercially released. These events took a while, since as a business matter cutting tool manufacturers resisted the technology that would extend the life of their products and reduce sales. Nonetheless, with the advent of new and more versatile machine tools and the means of programming them, in conjunction with the development of increasingly complex alloys and machinable materials, coatings on the cutting edges of tools took hold. Also, there are now numerous types of coatings available, such as titanium carbon nitride, titanium aluminum nitride, chromium nitride (used for machining nonferrous materials), and others. Cutting tool coatings minimize the effects of abrasion and maximize the tool’s thermal resistance, leading to longer tool life and faster cutting speeds. Tool coating technology has advanced to the point that coatings of various types can be applied in layers, effectively customizing the tool to suit the material it is cutting. Layered coatings help extend tool life even more since microcracks propagating in one layer can be stopped at the interface to the next. The resultant higher metal removal rates improve the efficiency of sinking dies and tool production, ultimately benefiting the customers of tool and die shops, such as forges. Initially, coated tools were manufactured by chemical vapor deposition (CVD), which is a vacuum deposition method in which metal tools at high temperature are exposed to gases in the coating chamber that react with the tool surface and deposit a coating. Gases introduced into the coating chamber then react and deposit a layer of coating on the tool surface. This same process is used in the semiconductor industry to produce thin films. More recently, Cutting Tool Coatings Improve Tool Life and Cutting Speed

and the rate of quench matters. A rapid quench results in a harder and stronger workpiece, which then requires tempering cycles to reduce brittleness and increase toughness. McInerney sums it up as follows: “Regardless of the tooling being created several parameters must be considered. First, tool steel requires a series of tempering cycles (draws). Two are often used, but I recommend three draws for better results. Second, tooling used in presses should never be below 44 Rc hardness. Finally, hammer tooling should be in the range of 36-45 Rc for optimal die life results. In their as-cast or as-forged conditions, all steels need to be thermally treated to be useful as they are made into finished goods. In recent decades, cryogenic research has discovered that a ‘thermal’ treatment in the range of -300°F can be used to enhance the performance of tool steels in service. The goal for heating and quenching tool steels is to convert the steel microstructure to a 100% martensitic (the hardest of the steel phases) microstructure, but because this is a virtual impossibility on a commercial scale tool steels allow for a small percentage of retained austenite in the physical vapor deposition (PVD), which uses ionization techniques to deposit an ionized coating onto an oppositely charged workpiece, has become common. PVD coatings can also be used with plasma nitriding to increase the coating’s durability in use. Both CVD and PVD have their preferred applications. PVD is performed at lower temperatures and is preferred for most conventional cutting and milling tool applications. However, CVD has excelled in the application of diamond coatings to cutting surfaces.

FIA MAGAZINE | AUGUST 2023 19