Many stamping organizations are shifting away from the standard practice of Chroming their dies for improved life and surface quality. As opposed to surface bonding associated with the Chroming process, Ion Nitriding relies on diffusion of nitrogen in the surface of the steel, forming a metallurgical bond with greater strength and durability. This surface treatment still allows Tool and Die makers the ability to work the punch, cavity and/or binder surfaces to improve a surface condition.
Ion Nitriding consists of a holding vessel, Cathode, anode, heat, alloying elements, and a controlled atmosphere. The die component will be placed on a plate and loaded into the holding vessel. The internal nitrogen enriched atmosphere is heated slowly to around 950⁰F, where any remaining contaminants are burned off from the casting and some of the residual stresses from the machining processes are relaxed.
In this low-pressure process (Figure 1), a voltage is applied between the component and the furnace wall, resulting in a glow discharge with a high ionization level generated around the parts. On the surface area that is directly charged by the ions, nitrogen-rich nitrides are formed and decompose, bombarding active nitrogen into the surface. The nitrogen forms compounds with alloying elements in the casting (Figure 2), such as CrN; these yield extreme hardness (>58 HRC) and great wear and fatigue resistance . The case depth of this metallurgical layer ranges in depth from 0.0006 inches (15 microns) to as much as 0.0035 inch, depending upon the application requirements and intended life between maintenance treatments.
The die is held at this temperature for a specified length of time to achieve the intended case depth of the hardened layer and is then slowly cooled to room temperature. Slow cooling ensures uniformity of the hardness and minimizes the risk of stress cracks on the surface. After the die is brought back to room temperature, the surface will have a slight layer of scale from this process. The scale is removed, and the die is ready for polishing.There are a few things that should be considered before Ion Nitriding the die. First, negative conditions such as a bad radius prior to Ion Nitriding will be preserved by this process– in this example, the bad radius will remain after Ion Nitriding. Second, the surface shine will not be the same as chroming, but when polished correctly, the reduction of friction will last longer because the hardness depth is greater compared to a traditional chrome layer.