Advantages with hotwall and pulsed plasma
Development with in the plasma nitriding technology has given birth to more efficient systems, consisting of hot wall chambers and pulsed dc power generators. The hot wall chambers, containing external heating elements, allow heating the target parts without the use of plasma which thereby only is needed for the actual nitriding. This allows the process to operate at lower voltages, thus reducing the arcing risk. Additionally, hot wall chambers provide better control of the temperature throughout the process.
This because it is equipped with cooling fans which cool the exterior walls of the chamber in order to avoid overheating of the system.
The use of pulsed dc power allows for better control of the plasma and, in combination with the overall reduced voltage, avoids the problem of arcing. In these type of systems, temperature control is such that parts with different geometry can be treated at the same time with satisfying results, something that is practically impossible with cold wall, continuous dc power systems.
General introduction to nitriding
Nitriding is a case hardening thermochemical process in which nitrogen is diffused into the surface of a material, generally (but not exclusively) different kinds of steel, forming nitrides with the different alloying elements of the substrate in layers that are harder than the core material. This results in enhanced mechanical and chemical properties at the surface, such as:
High surface hardness.
High fatigue strength.
Improved corrosion resistance (for non stainless steels).
Reduced friction coefficient and thereby increased adhesive wear resistance.
Nitriding is generally carried out at temperatures between 390-600 °C (734-1110 °F),at which no phase transformation occurs in the steel.
For this reason, hardness of the material can be maintained at the core and dimensional changes due to the processing temperature are small, allowing previously hardened material to be subjected to the advantages of nitriding.
There are three types of nitriding processes: gas, salt bath, and plasma. These processes differ in the media through which the nitrogen is diffused into the substrate to form nitrides. In gas nitriding, dissociated ammonia is used as source of nitrogen, whereas salt bath nitriding consist in immersing the substrate into a bath of molten nitrogen containing dangerous salts, such as cyanide.
Advantages with plasma nitriding
Plasma nitriding is the most recent and modern form of nitriding. The process uses a plasma, ionized gas atoms, generated by an applied electrical field to form nitrides at the surface of the parts. This is an environmentally friendly process, using clean gases (mainly pure hydrogen, nitrogen, and argon), which requires only low amounts of energy.
Energy savings are achieved due to the lower processing temperatures of this type of nitriding with respect to gas- and salt bath nitriding, and a more efficient nitrogen deposition rate compared to gas nitriding allowing shorter processing times.
Another advantage with plasma nitriding is that it allows treatment of a broader range of materials, for example stainless steels and recently, some developments have also been made in aluminum which cannot be nitrided by gas or salt bath methods.
The earliest plasma nitriding process uses the plasma as only heating source to heat the work piece prior to treatment and during the actual nitriding. A wall containing water cooling encloses the chamber in which nitriding takes place.
The temperature control is heavily related to the understanding of the process, the expertise of the operator and the way the parts are geometrically located inside the furnace. Furthermore, a continuous dc power generator is used to form the plasma. Over time some problems were detected with this type of current, arcing being the most pronounced.
Arcing occurs when current density drastically increases and leads to short circuits, producing local overheating of the material which results in metallurgical and mechanical defects, reducing its quality.