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A review of the process of Nitriding including Gaseous and Plasma Nitriding

Introduction

It is without any doubt that the process of nitriding (and its derivative processes) is a growth and continuing development process for both the present day and for the future.

Of all of the thermo chemical diffusion processes, the process of nitriding and its derivative diffusion processes) offers the engineer a suitable process tool that will conserve on:

  • Reduction of distortion
  • A wide variety of steels that can be processed from low carbon steel, cast irons, alloy steels, high alloy tool steels, stainless steels and exotic materials
  • Because off the advancement of process control technology, it offers almost repeatable and consistent metallurgical results
  • The process consumables (process gas, process temperature, process time, choice of process methods) can be almost infinitely variable
  • The process chemistry is perhaps the simplest of all of the thermochemical diffusion processes, and based on two or three elemental gases
  • Final machining procedure times can be significantly reduced. And in some application’s eliminated
  • Improvement in corrosion resistance of most steel alloys
  • Improvement on most of the wear characteristics such as: Abrasion, Impact résistance (dependent of course on the core strength of the steel in question), frictional wear(erosion)
  • Improvement in ductility resistance (once again, dependent on the core treatment and characteristics of the steel in question)
  • A high resistance to hardness  reduction at  operating temperatures up to approximately 570⁰C (dependent on the steel chemistry)
  • An improvement in surface characteristics such as surface smoothness as can be experienced by the action of sputter cleaning when using the process method of Plasma Nitriding
  • Excellent dimensional stability
  • Simple furnace design. (Although the plasma nitriding electrical process control can be  complex)
  • With the  Plasma Nitriding procedure, there is no atmospheric pollution as the process gases are environmentally friendly due to low volumes of process gas consumption (hydrogen and nitrogen)

The Process

Nitriding is a diffusion process just as the process of carburizing and carbo-nitriding is. There are significant differences in the two surface hardening techniques which are:

  • Lower process temperature selection than with carburizing diffusion techniques
  • No quenching involved during the procedure
  • Surface metallurgy and mechanical properties (different surface phases and diffusion zone metallurgy)

Machlet living in Elizabeth, New Jersey first developed and patented the process in 1908. This was followed by the further development of the process in Germany in 1917 (patented in 1923) which ultimately lead to the manufacture of Nitriding steel by Dr. Adolph Fry at the Krupp Steelmaking company  and the marketing (and subsequent licensed manufacturing) of the Nitralloy group of nitriding steels.

The principle of the process is based on the simple premise of the limits of solubility of nitrogen in iron at low temperatures by reacting with alloying elements in the steel chemistry. In other words, nitrogen will diffuse in the iron or steel very easily with the application of heat and will interact with the some of the alloying elements.

The chemistry of the process is extremely simple and uncomplicated. The source of nitrogen is ammonia for gaseous nitriding. The surface reaction and the decomposition are as follows:

  • 2NH3 + heat ⇄ 2N + 3H2

The initial decomposition is to separate the nitrogen from the hydrogen of the ammonia (nitrogen source). For a fraction of a second, the nitrogen is atomic, which will react with the steel being treated which then diffuses into the steel surface. The hydrogen acts as a dilutant gas and also a reducing gas to assist in reducing surface oxide contaminants. The steel surface acts as the catalyst to assist in the gas decomposition.
The ratio of nitrogen to hydrogen in the ammonia decomposes to a ratio of 1 part nitrogen to 3 parts of hydrogen. This is a very important observation to note when defining what type of formed immediate surface metallurgy is required for the application of the part being nitride.
However from a fixed gas chemistry will be a fixed surface metallurgy.

As a result of the thermal decomposition of the process gas, atomic nitrogen will react at the steel surface and begin to diffuse into the steel as the process temperature increases. As the process gaseous decomposition temperature increase, the limit of solubility of nitrogen in iron will increase, to the point of saturation.

At the normal nitriding temperature operating range up to say 570⁰C, use is made of the Hansen Equilibrium diagram demonstrating the solubility of nitrogen in iron, but with a limited solubility concentration up an approximate value of 0.10%. Should the value of 0.10% be exceeded, then the surface metallurgy will begin to form what is known as ý nitride (Gamma Prime) (Fe4N).

Should the nitrogen concentration of the process atmosphere (ammonia) increase to approximately 6.0%,  then the  gamma prime will begin to undergo a reactionary change to what will now be known as ἑ nitride (Epsilon nitride)

The two phases will as described above will be observed in a combined phase which when etched in nital will be seen as a white layer at the surface metallurgy. This is known commonly as ‘The White Layer’. The two phases will not be seen as separate phases, but only as a combined white layer.

There are conditions that will determine the thickness of the surface white layer, such as:

  • Process gaseous decomposition temperature
  • Time at that temperature
  • Composition of the steel being processed and in particular the nitride forming elements in the steel composition
  • Depending on the steels carbon content and the available nitrogen at the steel surface, will determine the formation of what are known as carbo nitrides.
  • It can therefore be said that ALL STEELS CAN BE NITRIDED!! The resulting physical metallurgy and final process values will be determined by steel chemistry, process temperature, and available nitrogen.
  • High surface hardness values are generally determined by the formation of finely dispersed nitride (with nitride forming alloys) and resulting carbo-nitrides within the now distorted ferrite lattice structure.

Nitride forming elements

The nitride forming elements that will react with the atomic nitrogen and form stable nitrides are as follows:

  • Iron
  • Chromium
  • Aluminum
  • Molybdenum
  • Vanadium
  • Tungsten
  • Titanium
  • Silicon

A can be seen from above, that iron is also a nitride former. (Very often overlooked) However the resulting nitrides do not exhibit high hardness values, but will assist in the improvement of the steels resistance to corrosion.

As can be seen, there are many advantages to using the process of nitriding. One now has to choose the methodology of Nitriding or Ferritic Nitrocarburizing.

The Choice of Nitriding Methods

The choice of nitriding methods are as follows:

  • Liquid Nitriding (molten salts) (Effluent and part cleanliness are now of concern) as is depth diffusion depth limitations.
  • Simple Gaseous Nitriding using ammonia as the source gas for nitrogen and hydrogen being obtained by the thermal decomposition of ammonia. This is perhaps the simplest method of gaseous nitriding. The process control can be very simple as can the process conditions. The first gaseous nitriding systems were based on the solubility of Nitrogen in Iron and the Iron Nitrogen graphical phase diagram of Nitrogen in Iron
  • From the above procedure came the development of the Controlled Nitride process which was based on the final composition of the surface phase metallurgy as defined by The Lehrer diagram, which defined the Nitride Potential by controlling the relationship between the surface metallurgy of the Epsilon ( Ε ἑ) and Gamma Prime ( γ ý) phases of the steel being treated.
  • Active screen nitriding. This process was developed by Mr Jan George in Luxemburg with a very strong input and influence by Professor Tom Bell of Birmingham UK. The process makes use of an active metal screen between the anode and the work piece at cathode potential, from which it is claimed to produce a more active and controllable plasma glow seam at the work surface, thus producing a more uniform and controllable surface metallurgy.
  • The Plasma Nitriding process techniques can also be known also as Glow discharge nitriding, or Plasma nitriding Continuous DC nitriding, Pulsed Plasma nitriding. The process can be likened to natural plasma such as is often seen in the Northern Lights in the earths Northern hemisphere. Or such as is seen during a thunderstorm with lightning strikes. The principle of plasma generation is exactly the same as the two natural methods of glow discharge.

Nitriding, its Methods, Process and Metallurgy

At least 4 different, yet basic different methods of applying the nitride process to steel and are:

  • Gaseous nitriding (using ammonia as the nitrogen source)
  • Pack nitriding
  • Salt bath nitriding (using a cyanide based salt such as KCN and NaCN (Potassium Cyanide and Sodium cyanide))
  • Plasma assisted nitriding using molecular nitrogen and hydrogen

The decomposition of the ammonia to release both nitrogen and hydrogen diffusion is very similar with each of the above methods except with the Plasma nitriding. Time, temperature and material chemistry will also influence -

  • Surface metallurgy
  • Surface hardness
  • Core hardness
  • Diffusion zone enrichment
  • Nitride networks

Metallurgy

The final metallurgy will comprise of the following results in the order of observation (microscopically)

  • Compound layer ( also known as the white layer), because the immediate surface  etches out white with a cross sectional examination of the formed case) The compound layer comprises of two phases known as Gamma Prime (ỳ) and Epsilon nitride (έ) The compound zone formation can be  controlled  by adjustment of the process gas flows(or salt bath chemistry)
  • Diffusion zone. This is where the nitrogen has reacted with the alloying elements to form stable nitrides with the appropriate alloying elements.
  • The transition zone. (this is the area from the diffusion zone to the core material)
  • Core. This is the original core metallurgy of the steel being nitrided.

The exception to the above will be nitriding by plasma techniques and nitriding by dilution techniques.

Plasma nitriding

Plasma nitriding has the distinct advantage of being able to control the ratio of nitrogen to hydrogen in order to control the resulting surface metallurgy of the nitriding process. The formed compound zone can be constructed of:

  • Dual phase (ỳ and έ)
  • Single (Mono) phase (ỳ or έ)
  • No compound layer.

This is accomplished, simply by control and the ratios of nitrogen to hydrogen.

Dilution Nitriding (Precision Nitriding)

This method of gaseous nitriding can control the phase formation in the surface compound zone. In addition to this, carbon that is present in the steel analysis will also influence the compound layer formation.

The basis of this nitriding method is the measurement and analysis of insoluble exhaust gases from the process. The method analyzes the insoluble hydrogen and the insoluble nitrogen and adjusts the gas flow appropriately to construct the compound layer formation.

The basis of the control is therefore by gas analysis and not by volumetric methods as is with the conventional method of gas nitriding.

Steels for Nitriding

Any steel will nitride, simply because of the presence of iron. However they will not produce the same hardness values because of the steel chemistry's. The iron will assist in the surface corrosion resistance by the formation of Iron Nitrides.

Below is a general group of steels that will nitride. The list is by no means complete:

  • Cast Irons Cast iron grades will nitride without any significant difficulty. The problem then arises of the cast iron porosity and density. The ability of the nitriding to nitride cast iron has been known for many years and is not new. A new use has been developed for cast irons, which is the nitriding of cast  iron forming dies for the surface hardening of large auto bodies (such as the tractor in tractor/trailer. This has been pioneered by European and USA die manufacturers with commercial heattreaters.
  • Alloy steels Most alloy steels will nitride. However care needs to be taken when considering the choice of steel for nitriding and in particular with the carbon content. It is not generally necessary to have a high carbon percentage in the steel to give high core hardness in order to support the formed case. A carbon content of approximately 0.45% maximum is considered acceptable. Once again, please be aware that the carbon content of the steel will affect the ratio of gamma prime to epsilon phases in the compound layer.
  • Tool steels The typical tool steels for nitriding will be the Hot Work series of tool steels. The High Speed Steels will nitride very satisfactorily, as will the Air Hardening tool steels. There are some applications where the D series are nitrided, but care should be exercised when selecting the D series for nitriding.
  • Stainless steels All of the stainless steels will nitride. This is because of the ability of chromium to form high surface hardness values. However some will nitride easier than others. The martensitic stainless steels are perhaps the easiest to nitride. All of the other stainless steels require some form of surface de-passivation to remove the chrome oxide layer on the immediate surface of the stainless steels. Once the chrome oxide surface layer, then the stainless steel has lost its corrosion resistance.