Nitridization
Encyclopedia
Nitriding is a heat treating process that diffuses
nitrogen
into the surface of a metal
to create a case hardened
surface. It is predominantly used on steel
, but also titanium
, aluminum and molybdenum
.
Typical applications include gear
s, crankshaft
s, camshaft
s, cam follower
s, valve
parts, extruder screws, die-casting tools, forging
dies, extrusion
dies, injector
s and plastic-mold
tools.
(NH3), which is why it is sometimes known as ammonia nitriding. When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses from the surface into the core of the material. This process has been around for nearly a century though only in the last few decades has there been a concentrated effort to investigate the thermodynamics and kinetics involved. Recent developments have led to a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. The advantages of gas nitriding over the other variants are:
The disadvantages of gas nitriding are:
The disadvantages are:
In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma
, naming the technique. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of gas nitriding with ammonia). There are hot plasmas typified by plasma jets used for metal cutting, welding
, cladding
or spraying. There are also cold plasmas, usually generated inside vacuum
chambers, at low pressure
regimes.
Usually steels are very beneficially treated with plasma nitriding. Plasma nitriding advantage is related to the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only the performance of metal parts gets enhanced but working lifespan gets boosted. So does the strain limit, and the fatigue strength of the metals being treated. For instance, mechanical properties of austenitic stainless steel like wear can be significantly reduced and the hardness of tool steels can be double on the surface.
A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any other post-nitriding operations. Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion.
This process was invented by Dr. Bernhardt Berghaus of Germany
who later settled in Zurich
to escape persecution of his community by the Nazis in 1939. It was only after his death in late 1960s that the process was acquired by Klockner group and popularized world over.
Plasma nitriding is often coupled with physical vapor deposition
(PVD) process and labeled Duplex Treatment, to avail of immensely enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to generate a smooth jetblack layer of oxides which is very resistant to not only wear but corrosion.
Since nitrogen ions are made available by ionization, differently from gas or salt bath, plasma nitriding efficiency does not depend on the temperature. Plasma nitriding can thus be performed in a broad temperature range, from 260°C to more than 600°C. For instance, at moderate temperatures (like 420°C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties.
In plasma nitriding processes nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like hydrogen or Argon are also used. Indeed, Argon and H2 can be used before the nitriding process during the heating up of the parts in order to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process H2 gas is also added in order to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding (see for instance ).
, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring nitriding take place at a lower temperature than the last tempering temperature. A fine-turned or ground surface finish is best.
Minimal amounts of material should be removed post nitriding to preserve the surface hardness.
Nitriding alloys are alloy steels with nitride forming elements such as aluminum, chromium
, molybdenum and titanium.
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
into the surface of a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
to create a case hardened
Case hardening
Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy...
surface. It is predominantly used on steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, but also titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, aluminum and molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
.
Typical applications include gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
s, crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
s, camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
s, cam follower
Cam follower
A cam follower, also known as a track follower, is a specialized type of roller or needle bearing designed to follow cams. Cam followers come in a vast array of different configurations, however the most defining characteristic is how the cam follower mounts to its mating part; stud style cam...
s, valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
parts, extruder screws, die-casting tools, forging
Forging
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...
dies, extrusion
Extrusion
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
dies, injector
Injector
ʎ̩An injector, ejector, steam ejector, steam injector, eductor-jet pump or thermocompressor is a pump-like device that uses the Venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid to velocity energy which creates a low pressure zone that dɯaws in and...
s and plastic-mold
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
tools.
Processes
The processes are named after the medium used to donate . The three main methods used are: gas nitriding, salt bath nitriding, and plasma nitriding.Gas nitriding
In gas nitriding the donor is a nitrogen rich gas, usually ammoniaAmmonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
(NH3), which is why it is sometimes known as ammonia nitriding. When ammonia comes into contact with the heated work piece it disassociates into nitrogen and hydrogen. The nitrogen then diffuses from the surface into the core of the material. This process has been around for nearly a century though only in the last few decades has there been a concentrated effort to investigate the thermodynamics and kinetics involved. Recent developments have led to a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. The advantages of gas nitriding over the other variants are:
- All round nitriding effect (can be a disadvantage in some cases, compared with plasma nitriding)
- Large batch sizes possible - the limiting factor being furnace size and gas flow
- With modern computer control of the atmosphere the nitriding results can be tightly controlled
- Relatively cheap equipment cost - especially compared with plasma
The disadvantages of gas nitriding are:
- Reaction kinetics heavily influenced by surface condition - an oily surface or one contaminated with cutting fluids for example will deliver poor results
- Surface activation is sometimes required to successfully treat steels with a high chromium content - compare sputtering during plasma nitriding
- Ammonia as nitriding medium - though not especially toxic it can be harmful when inhaled in large quantities. Also, care must be taken when heating in the presence of oxygen to reduce the risk of explosion
Salt bath nitriding
In salt bath nitriding the nitrogen donating medium is a nitrogen containing salt such as cyanide salt. The salts used also donate carbon to the workpiece surface making salt bath a nitrocarburizing process. The temperature used is typical of all nitrocarburizing processes: 550–590 °C (1,022–1,094 F). The advantages of salt nitriding are:- Quick processing time - usually in the order of 4 hours or so to achieve
- Simple operation - heat the salt and workpieces to temperature and submerge until the duration has transpired
The disadvantages are:
- The salts used are highly toxic - Disposal of salts are controlled by stringent environmental laws in western countries and has increased the costs involved in using salt baths. This is one of the most significant reasons the process has fallen out of favor in the last decade or so.
- Only one process possible with a particular salt type - since the nitrogen potential is set by the salt, only one type of process is possible
Plasma nitriding
Plasma nitriding, also known as ion nitriding, plasma ion nitriding or glow-discharge nitriding, is an industrial surface hardening treatment for metallic materials.In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, naming the technique. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of gas nitriding with ammonia). There are hot plasmas typified by plasma jets used for metal cutting, welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
, cladding
Cladding (metalworking)
Cladding is the bonding together of dissimilar metals. It is distinct from welding or gluing as a method to fasten the metals together. Cladding is often achieved by extruding two metals through a die as well as pressing or rolling sheets together under high pressure.The United States Mint uses...
or spraying. There are also cold plasmas, usually generated inside vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
chambers, at low pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
regimes.
Usually steels are very beneficially treated with plasma nitriding. Plasma nitriding advantage is related to the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only the performance of metal parts gets enhanced but working lifespan gets boosted. So does the strain limit, and the fatigue strength of the metals being treated. For instance, mechanical properties of austenitic stainless steel like wear can be significantly reduced and the hardness of tool steels can be double on the surface.
A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any other post-nitriding operations. Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion.
This process was invented by Dr. Bernhardt Berghaus of Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
who later settled in Zurich
Zürich
Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...
to escape persecution of his community by the Nazis in 1939. It was only after his death in late 1960s that the process was acquired by Klockner group and popularized world over.
Plasma nitriding is often coupled with physical vapor deposition
Physical vapor deposition
Physical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...
(PVD) process and labeled Duplex Treatment, to avail of immensely enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to generate a smooth jetblack layer of oxides which is very resistant to not only wear but corrosion.
Since nitrogen ions are made available by ionization, differently from gas or salt bath, plasma nitriding efficiency does not depend on the temperature. Plasma nitriding can thus be performed in a broad temperature range, from 260°C to more than 600°C. For instance, at moderate temperatures (like 420°C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties.
In plasma nitriding processes nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like hydrogen or Argon are also used. Indeed, Argon and H2 can be used before the nitriding process during the heating up of the parts in order to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process H2 gas is also added in order to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding (see for instance ).
Materials for nitriding
Examples of easily nitridable steels include the SAE 410041xx steel
41xx steel is a family of SAE steel grades, as specified by the Society of Automotive Engineers . Alloying elements include chromium and molybdenum, and as a result these materials are often referred to as chromoly steel or CRMO...
, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring nitriding take place at a lower temperature than the last tempering temperature. A fine-turned or ground surface finish is best.
Minimal amounts of material should be removed post nitriding to preserve the surface hardness.
Nitriding alloys are alloy steels with nitride forming elements such as aluminum, chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, molybdenum and titanium.
History
Systematic investigation into the effect of nitrogen on the surface properties of steel only started in the 1920s. Investigation into gas nitriding began independently in both Germany and America. The process was greeted with enthusiasm in Germany and several steel grades were developed with nitriding especially in mind, these are the so called nitriding steels. The reception in America, on the other hand, was less impressive. With so little demand the process was more or less forgotten in the US. It was only after World War II that the process was reintroduced from Europe. A great deal of research has taken place in the last few decades to understand the thermodynamics and kinetics of the reactions taking place.See also
- CarburizationCarburizationCarburizing, spelled carburising in the UK, is a heat treatment process in which iron or steel is heated in the presence of another material which liberates carbon as it decomposes. Depending on the amount of time and temperature, the affected area can vary in carbon content...
- CarbonitridingCarbonitridingCarbonitriding is a metallurgical surface modification technique that is used to increase the surface hardness of a metal, thereby reducing wear. During the process, atoms of carbon and nitrogen diffuse interstitially into the metal, creating barriers to slip, increasing the hardness and modulus...
- Ferritic nitrocarburizingFerritic nitrocarburizingFerritic nitrocarburizing is a range of case hardening processes that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures. The processing temperature ranges from to , but usually occurs at...
- Surface finishingSurface finishingSurface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, solderability, corrosion resistance, tarnish resistance, chemical resistance,...
Further reading
- Ruth Chatterjee-Fischer - Wärmebehandlung von Eisenwerkstoffen: Nitrieren und Nitrocarburieren [Heat treatment of ferrous materials: nitriding and nitrocarburising] 1995 2nd Edition Expert Verlag ISBN 3-8159-1092