Plasma nitriding is a plasma-assisted thermochemical hardening process used to improve wear resistance, surface hardness of metal, and fatigue strength by creating a hard layer containing compressive stresses.
Benefits of using the plasma nitriding method
The advantages of gas nitriding can be surpassed by plasma nitriding. Especially in the case of high-alloy metals - plasma nitriding provides high surface hardness, thus improving resistance to wear, abrasion, and scuffing. The increase in strength is mainly due to the generation of compressive surface stresses. Nitriding of stainless steel is a good choice when the part needs to have both nitrided and softened areas. The ability to create a diffusionless composite layer is often used in plasma nitriding before applying PVD or CVD coatings. Non-standard layers and hardness profiles can be obtained during appropriate heat treatment.
Plasma Nitriding - Application
Typical applications make this technique for metal surfaces appear more often when it comes to gear wheels, crankshafts, camshafts, cam lifts, valve parts, extruder screws, pressure casting tools, forging dies, cold forming tools, injectors and plastic molding tools, long shafts, axles, clutches and engine parts. Plasma nitriding is generally better than equivalent gas processes that require masking. Plasma nitriding is suitable for all ferrous materials, but also for sintered steels, cast iron and high-alloy tool steels with high porosity, even with a chromium content above 12%. Stainless steels and nickel-based alloys can be plasma nitrided and retain most of their corrosion resistance at low temperatures. A special application is plasma nitriding of titanium and aluminum alloys. In the case of large loads of large parts of machines, such as shafts and spindles, it is very beneficial to nitride with special chrome and aluminum steels, as plasma nitriding gives a surface hardness exceeding 1000 HV.
Details of the plasma nitriding process
Plasma nitriding is a modern thermochemical process that takes place in a mixture of gases releasing nitrogen, hydrogen, and (optionally) carbon. During this low-pressure process, tension is applied between the batch and the furnace walls. A high-level ionization discharge (plasma) is generated around the element. Nitrides rich in nitrogen are formed on surfaces where ions are directly charged, which decompose, releasing reactive nitrogen to the surface. Thanks to this mechanism, shielding can be easily obtained by covering the appropriate area with a metal blanket. Nitriding allows for surface modification according to desired properties. By adjusting the gas mixture, it is possible to obtain layers and hardness distributions tailored to the customer's needs: from surfaces without low-nitrogen composite layers up to 20 microns thick to composite layers of carbon gases with a high nitrogen content (plasma nitriding). The wide range of temperatures applied means that many applications go beyond the capabilities of gas or salt bath processes. One of the biggest advantages of plasma heat treatment compared to heat treatment in a controlled atmosphere furnace is its lesser impact on the environment. For example, ammonia is commonly used for nitriding in controlled atmosphere furnaces. However, in plasma nitriding technique, steel can be nitrided with nitrogen and hydrogen. In addition, plasma nitriding only heats the treated object and does not require heating the entire interior of the furnace, which is required in the case of a controlled atmosphere furnace, and the decomposition of heated nitrogen molecules further supports the nitriding process.
Advantages of Plasma Nitriding
1. Plasma nitriding is significantly faster than other conventional nitriding techniques.
2. Proper control of temperature, atmosphere composition, and discharge parameters can result in an excellent microstructure and better control of the surface composition, structure, and properties of the final product.
3. Plasma nitriding is environmentally friendly.
4. Unlike conventional nitriding methods, this process can occur at temperatures up to 350°C. Low-temperature nitriding allows achieving high surface hardness while maintaining high strength of the core of steel hardened at low temperature. Additionally, processing at such low temperatures minimizes distortions.
There are several disadvantages of the plasma nitriding process:
- the cleanliness of component surfaces is crucial to prevent the formation of unstable arcs during heating cycles,
- parts require repair to avoid overheating,
- due to the power/area dependency, components of different iron alloys of similar size cannot be plasma nitrided in one batch,
- high initial cost of plasma.
Plasma nitriding: process
Plasma nitriding (also known as pulsed plasma nitriding and cold nitriding or plasma hardening) is a thermochemical heat treatment process used to improve the reliability and resistance to wear of mechanically stressed metal parts. The surface treatment in a particularly gentle manner improves fatigue strength and material protection against corrosion. Under the influence of heat, plasma nitriding causes a chemical transformation of the surface layer through the diffusion of nitrogen, which forms nitrides with the material being processed. This results in an increase in surface hardness and a significant improvement in wear resistance. Compared to the conventional hardening process, the object being processed is at a much lower temperature, ensuring high dimensional accuracy in this heat treatment. As a result, the costly final machining of carburized parts is no longer necessary or can be reduced to a minimum, making plasma nitriding even more cost-effective in the process chain. Raw materials can often be produced to the final size in a soft state and can be made after heat treatment with plasma with little or no final processing. In addition, steels subjected to heat treatment at a very low tempering temperature can be processed without loss of core strength. In principle, various processes can be used for nitriding. In addition to plasma nitriding, bath nitriding and gas nitriding are well known. Among hardening processes, plasma nitriding stands out due to its repeatability, environmental friendliness, and energy efficiency.
Physical principle of plasma nitriding
Plasma nitriding is a vacuum-assisted process. The object being processed forms the cathode, and the furnace wall forms the anode. After emptying the charge tank, an electric field is applied between the batch and the furnace wall. The supplied process gas is cracked and ionized in the electric field. It forms a conductive gas - plasma. When current flows to the cathode, the nitrogen ions contained in it are accelerated and hit the surface of the processed object with high energy. The effects of the above process are as follows:
– the surface of atoms is thoroughly cleaned,
– passivation layers (e.g. on stainless steel or stainless steel and titanium) are dissolved,
– surface activation occurs,
– the batch is heated for nitriding,
– nitrogen diffuses onto the surface of the processed object, which causes the absorption of the produced nitrogen to occur differently.
After reaching the processing temperature, the holding time begins. This depends on the type of material and the required depth of nitriding hardness. In the case of plasma nitriding, this time usually ranges from 12 to 50 hours. Compared to gas nitriding, plasma nitriding requires only about half of the holding time. After the appropriate processing time, the pressure is equalized by filling with gas. The charge is then cooled in a controlled manner, and the finished part can be removed at a low temperature.
Summary
Nitriding processes are intended for steel surfaces where there can be no major pressures and physical changes. This process ensures proper temperature control, microstructure and better surface composition control - even for special steel varieties. It does not require additional heat treatment and reduces machine wear. Let's also remember that the nitriding process is also associated with the use of ammonia.