Application Research on Nitriding Process

Key words: Nitriding, workpiece distortion, surface treatment

Abstract: Compared with traditional metal surface heat treatment processes such as quenching, surface nitriding treatment has unparalleled advantages. This paper explores and studies the nitriding technology that is increasingly widely used at present, with the aim of achieving the goal of mutual learning and improvement with readers.

Nitriding, also known as nitriding, refers to a chemical heat treatment process in which nitrogen atoms penetrate the surface layer of steel parts. Its purpose is to enhance the surface hardness and wear resistance of the parts, as well as to improve fatigue strength and corrosion resistance. It utilizes the decomposition of ammonia gas when heated to release active nitrogen atoms, which are absorbed by the parts to form a nitrided layer on their surface and simultaneously diffuse towards the center. Nitriding is usually carried out using specialized equipment or well-type nitriding furnaces. Gas nitriding was first studied, developed and industrialized by the German Fry around 1923. Currently, nitriding has rapidly developed and become increasingly perfect in both theory and technology, and the applicable materials and workpieces are also expanding day by day. Due to the fact that the products treated with nitriding possess excellent wear resistance, fatigue resistance, corrosion resistance, high-temperature resistance, anti-seizing property, resistance to atmospheric and superheated steam corrosion, and anti-tempering softening ability, as well as reduced notch sensitivity, compared with carburizing process, the nitriding temperature is relatively low, resulting in less workpiece distortion. Therefore, it has become one of the important chemical heat treatment processes. It is widely applied to the surface treatment of parts and products such as gears, cams, crankshafts, tools, cold working dies and hot working dies in industries like machinery, metallurgy and mining.

I. Common Materials for Nitriding

The aluminum, chromium, vanadium and molybdenum elements in traditional alloy steel materials can form stable nitrides when they come into contact with the nascent nitrogen atoms during the nitriding process. Especially the molybdenum element, it not only generates nitride elements but also reduces the brittleness produced during nitriding. Other elements in alloy steel, such as nickel, copper, silicon, manganese, etc., do not contribute much to the nitriding properties. Generally speaking, if the steel material contains one or more nitride-generating elements, the effect after nitriding is relatively good. Among them, aluminum is the strongest nitride element. The nitriding result is the best when it contains 0.85% to 1.5% aluminum. If there is sufficient chromium content, a good effect can also be achieved. Carbon steel without alloys is not suitable as nitrided steel because the nitrided layer it generates is very brittle and prone to peeling off.

Ii. Nitriding Process Control

1. Surface cleaning of parts before nitriding

Usually, nitriding is carried out immediately after oil removal by the gas degreasing method

2 Remove the air from the nitriding furnace

Place the parts to be treated in the nitriding furnace, seal the furnace cover, and then heating can be carried out. However, before heating to 150℃, the air inside the furnace must be removed. The main purpose of removing the air from the furnace is to ensure that only ammonia and nitrogen are involved in the nitriding treatment, preventing the generation of explosive gases when ammonia decomposes and comes into contact with air, as well as preventing the surface oxidation of the treated parts and supports.

3 The decomposition rate of ammonia

Nitriding occurs when other alloying elements come into contact with nascent nitrogen (the generation of nascent nitrogen is achieved when ammonia gas comes into contact with heated parts, and the parts themselves act as catalysts to promote the decomposition of ammonia). Although nitriding can be carried out under various decomposition rates of ammonia gas, generally, a decomposition rate of 15% to 30% is adopted, and the required thickness for nitriding is maintained for 4 to 10 hours. The processing temperature is maintained at around 520℃.

4 Cooling

Most industrial nitriding furnaces are equipped with heat exchangers to cool the heating furnace and the treated parts after the nitriding work is completed. After nitriding is completed, turn off the heating power supply to lower the furnace temperature by approximately 50℃. Then, double the ammonia flow rate and turn on the heat exchanger. At this point, it is necessary to ensure that the pressure inside the furnace is positive. After the ammonia gas introduced into the furnace stabilizes, the flow rate of ammonia can be reduced until positive pressure is maintained inside the furnace. Only when the furnace temperature drops below 150℃ can the furnace cover be opened.

Iii. Nitriding Process Route

Forging? "Cool down?" Rough machining? (Quenching and tempering treatment)? "Finishing?" Grind or polish? Nitriding

Note: Due to the thin nitrided layer, when a core structure with higher strength is required, quenching and tempering heat treatment should be carried out first to obtain tempered sorbite, thereby improving the mechanical properties of the core and the quality of the nitrided layer

Iv. Common Nitriding Methods

1. Gas nitriding

Gas nitriding is generally aimed at enhancing the wear resistance of metals, and thus a high surface hardness needs to be achieved. The surface hardness of the workpiece after nitriding can reach HV850 to 1200. Gas nitriding can be carried out by general nitriding method (i.e., isothermal nitriding) or multi-stage (two-stage, three-stage) nitriding method. The former ensures that the nitriding temperature and ammonia decomposition rate remain constant throughout the nitriding process. The temperature is generally between 480 and 520℃, the ammonia decomposition rate is 15% to 30%, and the holding time is nearly 80 hours. This process is suitable for parts with shallow diffusion layers, strict distortion requirements and high hardness requirements, but the processing time is too long. Multi-stage nitriding refers to the process of nitriding and diffusion carried out at different stages throughout the nitriding process, using different temperatures, different ammonia decomposition rates, and different holding times respectively. The entire nitriding time can be shortened to nearly 50 hours, allowing for the acquisition of a relatively deep nitriding layer. However, this results in a higher nitriding temperature and greater distortion.

There is also gas nitriding for the purpose of corrosion resistance. The nitriding temperature is between 550 and 700 ° C, and it is held for 0.5 to 3 hours. The ammonia decomposition rate is 35% to 70%, and a compound layer with high chemical stability can be obtained on the surface of the workpiece, preventing the workpiece from being corroded by moist air, superheated steam, gas combustion products, etc.

2 Ion nitriding

Also known as glow nitriding, it is carried out based on the principle of glow discharge. The metal workpiece is used as the cathode and placed in a negative pressure container containing nitrogen medium. After being electrified, the nitrogen hydrogen atoms in the medium are ionized, forming a plasma zone between the anode and cathode. Under the strong electric field of the plasma zone, the positive ions of nitrogen and hydrogen bombshell the surface of the workpiece at high speed. The high kinetic energy of the ions is converted into thermal energy, heating the surface of the workpiece to the required temperature. Due to the bombardment of ions, atomic sputtering occurs on the surface of the workpiece, thus achieving purification. At the same time, due to adsorption and diffusion effects, nitrogen seeps into the surface of the workpiece.

One of the most important features of ion nitriding is that the structure of the surface compound layer (commonly known as the white bright layer) and the organization of the diffusion layer can be controlled by regulating factors such as the composition of the nitriding atmosphere, air pressure, electrical parameters, and temperature, thereby meeting the service conditions and performance requirements of the parts.

3 Nitrocarburizing

Also known as soft nitriding or low-temperature carbonitriding, it refers to the process where, below the iron-nitrogen eutectoid transition temperature, both nitrogen and carbon are mainly infiltrated into the surface of the workpiece. The fine carbides formed by the infiltration of carbon can promote the diffusion of nitrogen and accelerate the formation of high-nitrogen compounds. These high-nitrogen compounds, in turn, can increase the solubility of carbon. The mutual promotion of carbon and nitrogen atoms thus speeds up the infiltration rate. In addition, carbon in nitrides can also reduce brittleness.

The commonly used nitrocarburizing methods include liquid method and gas method. The treatment temperature is 530 to 570 ° C, and the holding time is 1 to 3 hours. Early liquid salt baths used cyanide salts, and later various salt bath formulas emerged. There are two commonly used types: neutral salts with ammonia gas and salts mainly composed of urea and carbonates, but these reaction products are still toxic. The main gas media include: endothermic or exothermic gas with ammonia, urea thermal decomposition gas, and the injection of organic solvents containing carbon and nitrogen, such as formamide and triethanolamine.

The processability comparison of various nitriding methods is detailed in the following table

V. Common Nitriding Defects and Their Causes

1. The hardness is relatively low

In production practice, the surface hardness of workpieces after nitriding sometimes fails to meet the requirements specified by the process. In mild cases, they can be reworked; in severe cases, they may be scrapped. The common causes of low hardness are:

In terms of equipment: For instance, air leakage in the system can cause oxidation.

In terms of materials: For instance, the selection of materials is unreasonable.

Pre-heat treatment: such as the hardness of the base material being too low, severe surface decarburization, etc.

Incomplete pretreatment of the workpiece: such as the cleaning method and cleanliness before entering the furnace.

In terms of process: For instance, the nitriding temperature is too high or too low, the time is too short, or the nitrogen concentration is insufficient, etc.

2 The hardness and diffusion layer are uneven

The main reasons are:

The way of loading the furnace is unreasonable.

Improper adjustment of air pressure.

The temperature was uneven during the nitriding treatment.

The airflow inside the furnace is unreasonable.

3 The deformation of the parts is too large after nitriding

Deformation is inevitable. For easily deformed parts, taking the following measures is conducive to reducing deformation:

Stabilization treatment should be carried out before nitriding.

The heating and cooling rates during the nitriding process should be slow.

During the holding stage, efforts should be made to ensure that the temperature at all parts of the workpiece is uniform. For workpieces with strict deformation requirements, within the process range, a lower nitriding temperature should be adopted as much as possible.

4 There are differences in the appearance colors

The surface color of parts made of different materials after nitriding treatment varies slightly. After the parts are taken out of the furnace, the appearance quality is first inspected with the naked eye. The surface of steel parts after nitriding treatment is usually silver-gray (blue-black) or dark gray (blue-black), while the surface of titanium and titanium alloy parts should be golden yellow.

5 Vein nitride

Nitriding (especially ionic nitriding) is prone to the formation of vein-like nitrides, which are nitrides with the diffusion layer running parallel to the surface in a white wavy pattern. The formation mechanism remains undetermined. Generally, it is believed to be related to the segregation of alloying elements at grain boundaries and the diffusion of nitrogen atoms. Therefore, measures to control the segregation of alloying elements are all conducive to reducing the formation of vein nitrides. In terms of process parameters, the higher the nitriding temperature and the longer the holding time, the more likely it is to promote the formation of vein structure. For instance, at the corners of the workpiece, due to the relatively high nitriding temperature, the vein structure is much more severe than in other parts.

References

1. Liu Yongquan: Heat Treatment of Steel, Metallurgical Industry Press, 1987

2 Hu Haitian, Chen Qiang, et al. : Metal Heat Treatment by microwave plasma Nitriding device, 1995

3 Zhou Xiaozhong, Chen Dakai, et al. Industrial Technology of Ionic Thermal Treatment. Machinery Industry Press, 1990

4 Qian Miaogen et al. : Modern Surface Technology, Machinery Industry Press, 1999