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Ion nitriding - a ready-to-wear solution for dies and molds.

Ion nitriding can be the answer to the rigors of high-tonnage sheet-metal press working and the inevitable wear this causes for even case-hardened and conventionally nitrided stamping dies. It can boost wear resistance two to ten times, and significantly reduce costly repairs and press downtime. Corrosion resistance and fatigue strength are also improved, and the process can be applied to all types of die steel.

According to Sun Steel Treating, South Lyon, MI, ion nitriding offers a low-temperature, environmentally nonhazardous alternative to liquid and gaseous methods of surface treatment for stamping dies and plastic injection molds. They have eight ion-nitriding furnaces and a depth of expertise in applying this technology. They use computer-driven processing and control techniques to carefully monitor and maintain workpiece temperature, vessel pressure, power levels, gas composition, and cycle times. It's all critical to repeatability of layer composition, case depth, and hardness.

A wide material spectrum

Ion processing is not a radically new technology. Well established in Europe and Japan for many years, it was advanced in the US through the use of vacuum technology, computer controls, and gas-mixing techniques. The range of materials that have benefited from ion nitriding range from low-alloy materials in the auto industry; to medium-grade materials used in plastics, textiles, and autos again; to high-alloy, heat-resistant and heat-treatable grades used in aerospace, aircraft, military, and machine-tool applications.

Understanding the process

What is ion nitriding? It's a thermal/chemical process that is repeatable, with limited variability, for a wide range of materials and applications. From a process standpoint, it is less expensive to operate than other wear-surfacing alternatives, and very reliable, once you've mastered the processing technology and adapted the equipment to your intended range of applications.

There are four key parameters to control:

1. Voltage and current. These are critical for maintaining workpiece temperature to achieve proper case and core hardness.

2. Pressure. This controls the density of the glow seam and, therefore, the case uniformity of differently configured parts.

3. Gas composition. This controls the type of layer or surface composition you achieve.

4. Time. Generally, the longer the cycle, the deeper the case hardening. Shorter times yield shallower cases. However, case depths vary greatly with alloy composition.

Knowing what parameters you must control is only the first step. Any furnace manufacturer can provide you with these controls. Knowing why each must be controlled, and the effects of their interrelationship, is the expertise you must develop to get the desired result. Understanding the physics of glow discharge is mandatory to get the case depth, hardness, and surface composition you require economically, efficiently, and most importantly, repeatably.

Although you may have some understanding of how this process works from a physics standpoint or metallurgical view, here's a simplified explanation:

1. A workpiece is placed inside a classic vacuum furnace on a plate that is insulated from the balance of the furnace.

2. The vessel is pumped down to evacuate the atmosphere.

3. Inert hydrogen gas is injected and the part is preheated using either internal AC radiant heating, DC plasma heating, or both. Initially, not much happens-a little surface cleaning or sparking, but nothing significant until you start ionizing the gas.

4. When the furnace reaches its ionizing temperature, the hydrogen atom throws off a negatively charged particle, leaving a positively charged ion. This is accomplished because of the presence of the high DC voltage (200 to 1000 V) between workpiece and furnace and results in an ionic bombardment of the negatively charged workpiece.

5. This bombardment, coupled with AC generated heat, causes the workpiece to heat up further.

6. When the workpiece reaches 600 F, nitrogen gas is added, in a predetermined ratio for the desired surface layer and underlying composition. It also ionizes and bombards the workpiece.

7. As the workpiece continues to heat up and approach the desired final setpoint temperature, voltage and current to the furnace are manipulated to achieve that target temperature without overshooting.

At the start of ionization, a violet-colored glow is seen in the furnace. As temperature rises, the mixed gas pressure is increased to assure that all holes, slots, and depressions in the part are equally uniform in glow-seam thickness. This assures a uniform case depth, regardless of part configuration. Then, by holding temperature, pressure, and gas mixture for the prescribed time period, the desired case depth and hardness are achieved. Part characteristics

The main goal of ion nitriding is achieving increased wear resistance. A compound layer is developed on the surface, consisting primarily of iron nitrides, Fe[sub.4]N called gamma prime or Fe[sub.2-3]N called epsilon.

The structure of this compound zone is mainly determined by the carbon content of the plasma. A carbon-free atmosphere will generate only gamma prime, which is ductile and has excellent wear characteristics. A carbon-rich atmosphere will result in an epsilon layer, excellent for resistance to stress, shock loading, impact, and wear, but less ductile than a gamma-prime layer.

Controlling this compound layer's thickness is easily achieved by reducing nitriding temperatures and nitrogen levels in the plasma. The thinner the layer, the greater the ductility-a near complete suppression of nitrogen will result in no compound layer at all.

The underlying diffusion zone is a transition zone with declining hardness, from the high hardness at the surface down to the hardness of the core material. The hardness and case depth of the diffusion zone is determined by steel composition, prior heat treatment, temperature, and time. A deeper diffusion zone adds fatigue resistance by diffusing nitrogen into the crystal lattice of the iron present. These fine nitride precipitants form in the diffusion zone when alloys such as chromium, molybdenum, and vanadium are present, and lead to an increase in hardness. Process characteristics

Areas can be masked to allow batching by both chemical and mechanical means. Even light chemical paint-type masks retard case, but don't eliminate it. For batch runs, part location and proximity become very important. Arcing between parts can prevent a run from ever getting started, and high-voltage arcing can substantially damage arts.

Although not abnormally high, maintenance costs for the process are significant, and calibration, cleaning, and inspection systems are time consuming. These are vital for the ion unit to perform properly. The process is very susceptible to dirt and contamination. Unclean parts are the primary problem. Running large volumes of work makes the task of totally cleaning each part almost impossible. As a result, the furnace and its atmosphere get contaminated, and this shortens the life of furnace components and can interrupt cycles and cause downtime.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Jul 1, 1990
Words:1093
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