Nitriding process

1. Introduction to Nitriding

Nitriding is a chemical heat treatment process in which nitrogen atoms diffuse into the surface layer of a workpiece in a specific medium at a certain temperature. Common types include liquid nitriding, gas nitriding, and ion nitriding. Traditional gas nitriding involves placing the workpiece in a sealed container, passing flowing ammonia gas through it, and heating it. After holding at a relatively high temperature for a long time, the ammonia gas thermally decomposes to produce active nitrogen atoms, which are continuously adsorbed onto the workpiece surface and diffuse into the surface layer, thereby altering its chemical composition and structure and yielding excellent surface properties. If carbon is simultaneously diffused into the workpiece during the nitriding process to promote nitrogen diffusion, it is called nitrocarburizing. Gas nitriding and ion nitriding are commonly used.

2. Nitriding Principle and Application
Nitriding, penetrating steel, forms iron nitrides with varying nitrogen contents from the surface inwards, and also combines with alloying elements in the steel to form various alloy nitrides, especially aluminum nitride and chromium nitride. These nitrides possess high hardness, thermal stability, and high dispersion, thus enabling nitrided steel parts to achieve high surface hardness, wear resistance, fatigue strength, anti-galling properties, resistance to atmospheric and superheated steam corrosion, resistance to tempering softening, and reduced notch sensitivity. Compared to carburizing, nitriding involves a lower temperature, resulting in less distortion. However, due to lower core hardness and a shallower nitrided layer, it generally only meets the requirements for wear resistance and fatigue resistance under light to medium loads, or for machine parts with certain heat resistance and corrosion resistance requirements, as well as various cutting tools, cold-working and hot-working dies, etc.

3. Common Nitriding Methods

The most common methods are gas nitriding, ion nitriding, and nitrocarburizing.

3.1 Gas Nitriding

The main purpose is generally to improve the metal’s wear resistance, which requires a high surface hardness. It is suitable for nitriding steels such as 38CrMoAl. The surface hardness of the workpiece after nitriding can reach HV850~1200. Nitriding offers low temperature and minimal workpiece distortion, making it suitable for parts requiring high precision and wear resistance, such as boring bars, boring machine spindles, grinding machine spindles, and cylinder liners. However, due to the thin nitrided layer, it is not suitable for wear-resistant parts subjected to heavy loads.

Gas nitriding can be performed using either a general nitriding method (i.e., isothermal nitriding) or a multi-stage (two-stage, three-stage) nitriding method. The former maintains a constant ammonia temperature and ammonia decomposition rate throughout the nitriding process. The temperature is generally between 480 and 520°C, the ammonia decomposition rate is 15-30%, and the holding time is nearly 80 hours. This process is suitable for parts with shallow nitrided layers, strict distortion requirements, and high hardness requirements, but it takes too long. Multi-stage ammonia nitriding involves using different temperatures, ammonia decomposition rates, and times at different stages of the nitriding process. The total nitriding time can be shortened to nearly 50 hours, yielding a deeper nitrided layer, but this requires higher nitriding temperatures and greater distortion.

Normal gas nitrided workpieces have a silver-gray surface. Sometimes, due to oxidation, they may also appear blue or yellow, but this generally does not affect their use.

Common gas nitriding processes include isothermal, two-stage, and three-stage nitriding.

(1) Isothermal nitriding: Also known as one-stage nitriding. It is a nitriding process that involves long-term heat treatment at a constant temperature of 510~530℃. The nitriding process curve is shown in Figure 1. The first stage, heat preservation for 15~20h, is the nitrogen absorption stage. A relatively low ammonia decomposition rate (18%~25%) is used in this stage. A nitrogen concentration difference is formed between the surface of the part and the core due to the large number of nitrogen atoms after cleaning. The second stage is the diffusion stage. In this stage, the ammonia decomposition rate is increased to 30%~40% to reduce the number of active nitrogen atoms, and the heat preservation time is around 60h.

Figure 1. 38CrMoA1A steel single-stage nitriding process

To reduce the brittleness of the nitrided layer, denitrification is performed 2-4 hours before the end of nitriding, increasing the ammonia decomposition rate to over 70% and the denitrification temperature to 560-570℃. Isothermal nitriding is simple, with lower nitriding temperatures, shallower nitrided layers, less part deformation, and higher surface hardness, but it is slow and has a long production cycle. It is suitable for parts with shallow nitriding depth and high dimensional accuracy and hardness requirements.

(2) Two-stage nitriding: The two-stage nitriding process curve is shown in Figure 2. The process parameters for the first stage (except for the holding time) are the same as for isothermal nitriding. In the second stage, the nitriding temperature is increased to 550-560℃ to accelerate nitrogen atom diffusion, shorten the nitriding cycle, and increase the helium decomposition rate to 40%-60%. Based on the brittleness requirements of the nitrided layer, denitrification should also be performed 2 hours in advance to increase the ammonia decomposition rate and temperature.

Figure 2: Two-stage nitriding process for 38CrMoAlA steel

Two-stage nitriding takes less time than isothermal nitriding, has slightly lower surface hardness, and slightly increased deformation. It is suitable for parts with deep nitrided layers and large batches.

(3) Three-stage nitriding: The three-stage nitriding process curve is shown in Figure 3. It is developed based on two-stage nitriding. This process appropriately increases the temperature of the second stage to accelerate nitriding, while adding a lower-temperature third stage to compensate for the low surface ammonia concentration resulting from rapid ammonia diffusion in the second stage, thereby maintaining surface nitrogen content and improving surface hardness.

Figure 3: Three-stage nitriding process for 38CrMoAlA steel

Three-stage nitriding can further increase the nitriding speed. Still, the hardness is lower than that of general nitriding processes, and the brittleness and deformation are slightly greater than those of general nitriding processes.

3.2 Ion nitriding

Ion nitriding, also known as glow discharge nitriding, is carried out using the principle of glow discharge. A metal workpiece is placed as the cathode in a negative-pressure container filled with a nitrogen-containing medium. When an electric current is applied, nitrogen and hydrogen atoms in the medium are ionized, forming a plasma region between the cathode and anode. Under the strong electric field of the plasma region, nitrogen and hydrogen ions bombard the workpiece surface at high speed. The high kinetic energy of the ions is converted into heat, raising the workpiece surface to the desired temperature. Due to the ion bombardment, atomic sputtering occurs on the workpiece surface, thus purifying it. Simultaneously, nitrogen penetrates the workpiece surface through adsorption and diffusion.

Compared to conventional gas nitriding, ion nitriding has the following characteristics: ① It can appropriately shorten the nitriding cycle; ② The nitrided layer is less brittle; ③ It can save energy by reducing nitrogen and hydrogen consumption; ④ Areas that do not require nitriding can be shielded, achieving localized nitriding; ⑤ Ion bombardment has a surface-purifying effect, removing the passivation film on the workpiece surface, allowing direct nitriding of stainless steel and heat-resistant steel workpieces; ⑥ The thickness and microstructure of the nitrided layer can be controlled.

Advantages and disadvantages of ion nitriding:

Advantages: Short nitriding time, easy quality control, fatigue-resistant, and a high-strength nitrided layer. Due to the nitriding temperature of 520~540℃, workpiece deformation is small, and the surface has high antimagnetic properties.

Disadvantages: Complex equipment control, poor furnace temperature uniformity.

3.3 Nitrogen-Carbonization

Low-temperature nitriding, also known as soft nitriding, involves infiltrating carbon into the workpiece surface simultaneously with nitrogen at temperatures below the iron-nitrogen eutectoid transition temperature. The fine carbides formed after carbon infiltration promote nitrogen diffusion and accelerate the formation of high-nitrogen compounds. These high-nitrogen compounds, in turn, increase the solubility of carbon. The mutual promotion of carbon and nitrogen atoms accelerates the infiltration rate. Furthermore, carbon in the nitrides reduces brittleness. The resulting compound layer after nitriding has good toughness, high hardness, wear resistance, corrosion resistance, and anti-galling properties.

Commonly used nitriding methods include liquid and gas methods. The treatment temperature is 530~570℃, and the holding time is 1~3 hours. Early liquid salt baths used cyanide salts, and later various salt bath formulations emerged. Two commonly used methods are neutral salts with ammonia gas and salts primarily composed of urea and carbonates; however, these reaction products remain toxic. Gaseous media mainly include: endothermic or exothermic gases (see Controlled Atmosphere), ammonia, urea thermal decomposition gas, and dripping of carbon- and nitrogen-containing organic solvents, such as formamide and triethanolamine.

Nitrocarburizing not only improves the fatigue life, wear resistance, corrosion resistance, and anti-galling ability of workpieces but also requires simple equipment, low investment, easy operation, short processing time, and minimal workpiece distortion. Sometimes it can even give the workpiece an aesthetically pleasing appearance.

4. Nitriding Process

In the entire manufacturing process of nitrided parts, nitriding is often the final step, followed by fine grinding or lapping. The general process flow for nitrided parts is: forging → normalizing (annealing) → rough machining → tempering → fine machining → stress relief → rough grinding → nitriding → fine grinding → assembly. Preheating treatment before nitriding includes normalizing (annealing), tempering, and stress relief.

(1) Normalizing (annealing): Its purpose is to refine grains, reduce hardness, and eliminate forging stress.

(2) Tempering: This improves the machinability of steel, obtains a uniform tempered sorbite structure to ensure sufficient strength and toughness in the core of the part, and also ensures a strong bond between the nitrided layer and the base material.

(3) Stress relief: For precision parts with complex shapes, stress relief should be performed 1-2 times before nitriding to reduce deformation during the nitriding process.

5. Preparatory work before nitriding production

(1) Degreasing treatment: Before loading the parts into the furnace, they should be degreased with gasoline or alcohol. Rust and dirt are not allowed on the parts’ surfaces.

(2) Anti-nitriding treatment: For non-nitrided parts, anti-nitriding treatment can be performed by electroplating or coating.

(3) The surface quality of nitrided parts should be good, and no decarburized layer is allowed. Therefore, a sufficient machining allowance should be left for parts before preheat treatment so that the decarburized layer can be completely removed during machining before nitriding, thereby ensuring the quality of the nitrided layer.

(4) Before loading the furnace, check that the equipment, nitriding fixtures, electrical system, pipelines, ammonia decomposition analyzer, etc., are in normal working order; nitriding fixtures must not have dirt or oxide scale, and if present, it should be removed.

(5) Furnace-borne test samples: The furnace-borne test samples should be made of the same material as the nitrided parts and undergo the same pre-treatment.