August 2023 Volume 5

EQUIPMENT & TECHNOLOGY

material. Exposure to cryogenic temperatures helps transform some retained austenite into martensite, thereby improving the tool material’s durability. Cryogenic treatment also enhances the formation of carbides in the steel and relieves residual stresses from the manufacture of the material itself, as it was once solidified from a molten state. Cryogenic die treatments tend to be proprietary in nature by the companies that offer them. Forging dies are subject to among the harshest of service conditions as any industrial component. The act of applying these dies in a press or hammer subjects them to thermal fatigue, mechanical fatigue, abrasive wear, plastic deformation, high rates of workpiece deformation, and erosion. All these stresses occur and wear upon the die at its surface. As such, increasing service life and improving die performance is a constant goal among those who formulate tool steels, those who heat treat them, and those who sink the die cavities. Dies are coated to improve their service life (more production cycles). Additionally, a coated die surface not only enhances its mechanical properties, but also improves the lubricity between the workpiece and the die surface. Carburization is one way to improve the mechanical properties of a die surface. If the steel die is elevated to a high temperature and exposed to a carboniferous source, the carbon reacts with elements on the steel surface to form carbides in the metal matrix near the die surface which, in turn, enhance hardness and resistance to wear. Similarly, nitriding refers to a family of processes in which nitrogen is diffused into the surface of a die to create a case-hardened surface. This is a diffusion process in which nitrogen, usually in the form of ammonia, is exposed to the heated die surface. The depth of the hardened surface depends on how much time the die surface is exposed to the ammonia. Nitriding requires the presence of alloying elements such as chromium or molybdenum to form nitrides, which elevate the hardness and wear resistance of the die. “Simple gas nitriding relies on the surface of the steel alloy as a catalyst for the nitrogenous gas to form nitrides. The die for this process must be absolutely clean, as even oil from a fingerprint will prohibit nitriding in that spot,” says McInerney. “Ion nitriding, or plasma nitriding, requires no catalyst, instead using bombardment of the die’s surface with nitrogen ions at an elevated temperature. Ferritic nitrocarburizing (FNC) includes a range of case-hardening processes that diffuse nitrogen and carbon into ferrous metals during a salt bath. This was developed as a gas nitriding alternative to produce more uniform case hardening due to the surface contact with the liquid salt.” Die treatments and coating combinations have been used in combination to good effect. A common “hybrid” layer might consist of a nitride layer, upon which a PVD coating would be applied. Such a hybrid treatment could impart enhanced properties to a die surface that the separate treatments alone would not achieve.

Cryogenic treatments can improve tool steel durability. Courtesy of Gemini Group.

Forge bolster that will eventually hold quick-change tooling cassettes. Quick change tooling saves considerable time for tooling changeovers in forge presses of all sizes. Courtesy of Gemini Group.

Graph illustrating thermal treatment of tool steel. Courtesy of Gemini Group.

FIA MAGAZINE | AUGUST 2023 20