Plasma Nitriding advantages and disadvantages to other surface treatments/surface hardening

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  1. Introduction

The process of Nitriding is not a new process. It is simply a process that is not well understood in general engineering terms and practice. The process is a mature process and has been used since its first applications back in 1908 after it was initially patented by Adolph Machlet of Elizabeth New Jersey. Machlet initially applied the process to cast irons and plain carbon steels with some good success.

 Surface treatments

Fig. 1 - Surface treatments

 

Thermal surface treatment categorization

Fig. 2 - Thermal Surface treatment categorization

 

Comparison of nitride process procedures

Fig. 3 - Comparison of Nitride Process procedures

 

There was also knowledge available for the development of plasma. It was also known that there are at least two natural plasma phenomena’s: 

That knowledge was known in the mid 1800’s. However it was two German physicists that developed the plasma generation as was applied to the process of nitriding named Wenheldt and Berghaus. They successfully applied the technology of plasma generation to decompose elemental gases such as nitrogen from its molecular form to its atomic form. The electrical current application was known as CONTINUOUS DC CURRENT. This meant that the electrical power had to be on all of the process cycle time until the completion of the plasma process.

They found that the generated plasma was difficult to control and keep the plasma stable. As a result, burning of sharp corners occurred and a great deal of overheating and burning.

 

Gas nitriding seemed to dominate the process scene until the nitriding by molten salts was discovered using cyanide based salt.

Because one was (or is) using a fixed gas/fixed salt chemistry, the resulting surface metallurgy was always constant. Therefore it can be said ‘fixed gas chemistry = fixed surface metallurgy. Variable gas chemistry = variable surface metallurgy. This is the great advantage of plasma nitriding (be it continuous DC Power or variable DC power.

Formation of the surface compound zone

Fig. 4 - Formation of the surface compound zone

 

This simply means that the metallurgical technician can create the surface metallurgy that best suites the part application.

The surface metallurgy can be created as;

Another common misunderstanding regarding the process of nitriding is that it requires special alloying elements in the steel to form nitrides. While that statement is true and the alloying elements are;

 

Nitride forming elements in steel

Fig. 5 - Nitride forming elements in steel

 

The one element that is present in steel and cast iron is iron itself. Iron is a strong nitride former. The iron will react strongly with nitrogen to form iron nitride. The iron nitrides in themselves are not very hard however; the formed iron nitrides will   afford the steel excellent corrosion resistance.

The formation of the surface nitrided case

Fig. 6 - The formation of the surface nitrided case

 

  2.  Nitriding versus other surface treatment process

 

  2.1  Flame hardening

The flame hardening process technique is a thermal process which means the steel being treated will be ‘thermally shocked’ by a direct flame onto the steel surface in order to raise the surface temperature up to an elevated temperature so that the surface of the steel will transform to the austenite phase and then be rapidly cooled down to transform into surface martensite. There is the risk of the steel cracking on the surface due to the;

Flame hardening using controlled oxy-acetylene flame

Fig. 7 - Flame hardening using controlled oxy-acetylene flame

 

  2.2  Induction heat treatment

The formed case will be determined by the selection of electrical frequency. The steel surface will be more subjective to thermal shock than with the process of flame hardening due to electrical power being almost instantaneous. In order to form a hard surface with induction heat treatment one will require;

 Once again the steel surface will be at risk for cracking due to:

      

Categorization of induction heating methods and their applications

Fig. 8 - Categorization of induction heating methods and their applications

 

   Induction heating of a solid shaft and quenching with water

Fig. 9 - Induction heating of a solid shaft and quenching with water

 

 

  2.3 Carburizing

Carburizing is perhaps the oldest known method of surface treatment and has been available to us for many, many years. Carburizing has been conducted by the following methods;

 Each of the above methods of carburizing has one process requirement in order to effect the diffusion of carbon into the steel surface which is that of an elevated temperature.

 

 Comparison of process time, at process temperature, between carburizing and gaseos ntiriding

Fig. 10 - Comparison of process time, at process temperature, between carburizing and gaseos ntiriding

 

Carburizing is time/temperature process. The higher the selected process temperature, the faster the diffusion of carbon into the steel surface. Therefore the following formula applies (as it does with the process of nitriding);

This is known as a simplified formula developed by F.E.Harris in the late 1930’s /early 1940’s. (The original formula is a great deal more complex than the simplified version)

Because of the use of high temperatures for the carburizing procedure (chosen simply for process economics) it is necessary for the treated steel to be at an elevated temperature for an extended period of time (depending on the total case depth requirement). This extended time at temperature will cause;

Pit carburizing furnace. Picture by elterma

Fig. 11 - Pit carburizing furnace. Picture by Elterma

 

 Disadvantages of carburizing

Fig. 12 - Disadvantages of carburizing

 

2.4 Carbo-nitriding

Engineers tend to consider the process of carbo-nitriding for low alloy steels because of the ability of the resulting metallurgy to produce hard case formations.

This is not the real reason for the use of ammonia to form some nitrides in the surface of the steel. The real reason is to reduce the A3 (upper critical line), which in turn will reduce the case austenitizing temperature and thus reduce the risk of distortion by reducing the ∆? of the steel temperature down to the quench medium temperature.

2.5 Salt bath nitriding

Salt bath nitridng is old technology and it also fraught with;

2.6 Gaseous nitriding

Gaseous nitriding was the first nitriding procedure to be utilized by engineering in 1908. It is a tried and tested process and has been used successfully for many years. The resulting metallurgy is and well known by academics and practitioners alike. The simplicity of the gas decomposition can become its own enemy.

 

Single stage gas nitriding process for the diffusion of 1 mm case depth on plain carbon steel

Fig. 13 - Single Stage gas nitriding process for the diffusion of 1mm case depth on plain carbon steel. (0.040”)

 

The decomposition of the gaseous anhydrous ammonia (source for nitrogen) is as follows;

               2NH₃ ⇄ 2N + 3H₂

It can be seen that the procedure is reliant on fixed gas chemistry and will produce a fixed surface metallurgy each time, all the time. In addition to this, the surface phase metallurgy will be approximately 50% Gamma Prime phase and 50% Epsilon phase. However the carbon content of the steel will contribute to the percentage phase formation.  The more carbon that is present in the steel, the greater the Epsilon phase formation will be. The Epsilon phase is a very hard brittle layer that will withstand wear resistance but does not perform well under impact conditions.

In addition, the surface metallurgy has a strong tendency to be porous. This will of course be dependent also on the chemistry of the steel being treated.

As a direct result of the process, the surface metallurgy will form into the following layer. This is known as the formed case. The formed case comprises of three group areas;

The carbon content of the steel will determine how the surface compound zone will form. High carbon steels will produce the very hard brittle phase of Epsilon which has excellent wear characteristics, but it has no impact strength. On the other hand, low carbon steels will tend to produce more of the Gamma Prime phase, which has excellent impact strength, but not good wear characteristics. The hardness of the surface is generally lower then if the Epsilon phase was the dominant phase.

The compound zone will form approximately 10% of the total formed case which is from the surface through to the core hardness (through the transition zone). In addition, there will be a growth factor of approximately 10% of the total formed case.

Contrary to popular belief, there will be growth occurring of the component steel during the process. In addition, if the steel has not been appropriately pre-thermally stress relieved, any residual stress present in the steel as a result of machining, will be relieved during the nitriding process, thus creating distortion.

The occurrence and amount of distortion that will occur will be dependent on;

It is a very simple fact that distortion will occur, however it can be controlled to some degree, by;

There is a ‘not so apparent’ advantage of the nitriding process (across the board of the various methods of nitriding), and the advantages are;

A significant disadvantage of the gas nitriding process is the large consumption of ammonia process gas. The reason for this is that the process gas needs to fill the process chamber and to continue to replenish the exhausted process gas.

Another disadvantage of the gaseous nitride procedure is the long process time. It should be remembered that the process is a ’time and temperature’ diffusion process. The process diffusion is still governed by the simple Harris formula;

For example, for a case depth of say 0.040” (1 mm) (total case) the estimated cycle time would be approximately 90 hours.

The question then needs to be asked, do I really need 0.040” (1 mm) of case depth. The obvious answer should be no!! Therefore if one can reduce the case depth requirement, then the following benefits occur;

The secret of any nitriding process is of course the pre-heat treatment for the core hardness to support the formed nitride case.

However, there are two ways to control the thickness of the compound layer (white layer);

2.7 Plasma Nitriding

The plasma nitriding process is not a new process, contrary to popular belief. It is a process that has been practiced since 1932 by Drs. Wenheldt and Berghaus in Germany. Plasma occurs naturally when we see the lightening in a thunderstorm, when we see the northern lights. It also can be seen in the form of a fluorescent light which is pure plasma. The color of the light is determined by the choice of gas in the fluorescent light tube.

Early plasma nitride systems were difficult to control precisely and to sustain a uniform plasma condition within the process retort. This was simply due to the fact that the plasma was generated by a continuous DC voltage. This situation persisted until approximately 1978 when three German physicists and metallurgists came together. They developed the system which we now know as Pulsed Plasma Nitriding. This was simply turning the DC power on and off at pre-determined intervals. The time on and off was principally determined by the part geometry, process pressure, pulse voltage and power and process gas composition.

 

Schematic of the steel surface reactions during the process cycle - plasma nitriding

Fig. 14 - Schematic of the steel surface reactions during the process cycle - plasma nitriding

 

The advantages of the Pulsed Plasma Nitride system are;

It should be noted that nitriding is a growth procedure industry that offers many metallurgical benefits to both the user and the application. It is:

 


 Prepared by: Mr. David Pye, Pye Metallurgical International Consulting

Newport News, Virginia 23602, USA

Website: www.heat-treatment-metallurgy.com

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