When case hardening the surface of forging dies, aluminum extrusion dies or tool inserts, options in the forging industry have traditionally included one of three processes: carburizing, salt-bath nitrocarburizing and gas nitriding. Each of these has its advantages and disadvantages.
However, those seeking more precise control of the diffusion-layer formation, depth of case hardening and preservation of component dimensions are increasingly turning to advanced pulse-plasma nitriding. This process enhances not only surface hardness, wear resistance, heat resistance and fatigue strength of forging dies, but it also reduces heat checking and surface adhesion.
Although pulse plasma has been utilized for decades, superior controls for the DC pulsing signal, along with improved chamber design and construction, allow for more precise temperature control and uniform distribution of the heat zone throughout the hot-wall chamber.
The result is extremely consistent and uniform batch-to-batch nitriding with less gas consumption. The benefits are more precise control of the diffusion layers and its broader appeal to heat treat steel and even materials like aluminum.
For steel and steel alloys, case hardening can be achieved by carburizing, nitriding, cyaniding or carbonitriding. Although carburizing and nitrocarburizing of steel is a traditional approach, the part must be raised above the A1 temperature (727ºC) on the iron-carbon diagram, usually in the temperature range of 900-930ºC. Since the solubility of carbon is higher in the austenitic state than the ferritic state, a fully austenitic state is required for carburizing.
Along with the high temperatures and time-at-temperature associated with carburizing, parts can be distorted. Therefore, a post-carburizing heat treatment is required, at a minimum, to reduce internal part stresses. Depending on the part and its geometric tolerances, limited machining may also be required.
An alternative to carburizing is nitriding, which is a lower-temperature, time-dependent, thermochemical process used to diffuse nitrogen into the surface of metal.
Gas nitriding (500ºC) and gas nitrocarburizing (540-580ºC) are universally accepted procedures and, compared with pulse-plasma nitriding, typically require a high concentration of ammonia (NH₃) and a high amount of carrier-gas flow (normal pressure process). The elemental nitrogen gas constituent diffuses into iron and forms hard nitrides. Because of the reduced temperature compared to carburizing, no quenching is necessary, and the chance for distortion and cracking are therefore lower. The disadvantages of gas nitriding are that it requires the use of flammable gases like ammonia; it requires high gas consumption; and it is not able to nitride treat rust- and acid-resistant steels.
With recent advancements in pulse-plasma nitriding, however, a new level of precision and control is possible, which results in uniform and consistent case hardening. Together with the advantages of using only environmentally friendly gases – in contrast to the use of ammonia in gas nitriding – plasma-based nitriding has become a focal point for additional innovations and a requirement for those that seek a more environmentally safe solution.
In pulse-plasma nitriding, parts are loaded into a heated vacuum chamber after evacuating the chamber to a working pressure of 50-400 Pa on a supporting fixture and covered by a bell chamber. The chamber is evacuated to below 10 Pascals prior to heating, and a pulsating DC voltage of several hundred volts is applied between the charge (cathode) and the chamber wall (anode). The process gas in the chamber is then ionized and becomes electrically conducting. For this type of process, nitrogen and hydrogen gas mixtures and gases with carbon additions (like methane) are often utilized.
Depending on treatment time and temperature, nitrogen atoms diffuse into the outer zone of components and form a diffusion zone. This can be atomic nitrogen dissolved in the iron lattice, as well as in the form of nitrate deposition.
Adding further precision, innovators in pulse plasma have discovered methods to optimize the process through better control of the pulses. In the patented PulsPlasma process we developed, for example, a precisely regulated gas mixture of nitrogen, hydrogen and carbon-based methane is used. A pulsating DC voltage signal of several hundred volts is delivered in less than 10 microseconds per pulse to ionize the gas. This serves to maximize the time between pulses for superior temperature control throughout the chamber.
“If you have a temperature variance of ±10ºC within a batch, you will get completely different treatment results,” said Dietmar Voigtländer, sales manager at PlaTeG, a product group within PVA Industry Vacuum Systems (Wettenberg, Germany), the manufacturer of PulsPlasma nitriding systems. “However, by controlling the pulse current by means of exact pulse on-and-off time management, the overall temperature can be precisely managed with a uniform distribution, from top to bottom, throughout the hot-wall chamber.”
A unique feature with this approach is that the system can be switched on to a stable glow discharge at room temperature. Most systems cannot do this because the generators do not supply stable plasma. To compensate, those systems must first be heated to 300-350ºC before plasma can be applied, which adds time to the process. With PulsPlasma, that time can instead be used to prepare the surface by giving it a fine cleaning.
Even the materials of construction used to manufacture the nitriding furnace itself have been optimized. In all systems, PlaTeG uses insulative materials developed in the aerospace industry to create a furnace wall as thin as 40 mm, compared to the industry standard of 150 mm. With less wall mass, the furnace requires less energy and time to heat while still protecting workers that may accidentally touch the outside of the chamber. With better overall control, PulsPlasma nitriding furnaces offer multiple heating and cooling zones, each controlled by its own thermocouple.
“This will create a very uniform temperature distribution within ±5ºC from the bottom to the top of the furnace,” Voigtländer said.
Uniformity of temperature within a chamber pays a dividend beyond the consistency of nitriding results. With an even temperature throughout the chamber, the entire space is available for loading components, which effectively increases the chamber’s capacity.
For forging operations, pulse plasma offers significantly more precision in nitriding through the control of the mixture of gases, the controllability of glow-discharge intervals, the design of the pulsed signal and the use of a highly insulated hot-wall nitride furnace. Together with innovations in the design of the furnaces to streamline batch management in nitriding operations, forgers can benefit from dies with superior surface hardness, wear resistance, heat resistance and fatigue strength.
Author Thomas Palamides is Senior Product and Sales Manager at PVA TePla America, a global supplier of custom plasma equipment used for surface modification of a variety of components and materials. He can be reached at firstname.lastname@example.org or at 951-741-7365. For more information on pulse-plasma nitriding, visit PVA Industry Vacuum Systems at www.pvatepla-ivs.com.
All images provided by the author.
The International Journal of Forging Business & Technology