Microsoft Word WendiThesis docx

 

17

martensite.  

 

Nitriding is a ferritic thermochemical method. The diffusion process is based on the 

solubility of nitrogen in iron, as shown in the iron-nitrogen equilibrium diagram. 

(Figure 2) 

 

 

Fig2; Iron-nitrogen equilibrium diagram. [23] 

 

The solubility limit of nitrogen in iron is temperature - dependent, and at 450

 o

C the 

iron-base alloy will absorb up to 5.7% to 6.1% of nitrogen. Beyond this, the surface 

phase formation on alloy steels tends to be predominantly epsilon phase. The greater 

the nitrogen content, the more potential for the epsilon phase to form. When the 

temperature is increased to the gamma prime phase at 490

 o

C, the limit of solubility 

begins to decrease at 680

 o

C. [23] The equilibrium diagram shows that control of the 

nitrogen diffusion is critical to process success. 

 

Several operating process parameters must be adhered to and controlled in order to 

carry out the nitriding process. Most of them can be controlled with simple 

instrumentation and methods. Examples of process parameters for gas nitriding 

include: 

---

 

18



 

Furnace temperature 



 

Process control 



 

Time  



 

Gas flow 



 

Gas activity control 



 

Process chamber maintenance [24] 

The factors help to reduce the distortion during the process. Another benefit of 

nitriding is that it acts as a stabilizing process by providing an additional temper to the 

processed steel. Control of the process parameters is necessary to ensure formation of 

an acceptable metallurgical case. [25] Without control, repeatability of the 

metallurgical requirements cannot be guaranteed. 

 

The process control factors are the elements that will ensure a controlled process and 

acceptable results: 



 

Total surface area to be nitrided 



 

Process pressure inside the sealed process chamber 



 

Pressurization system and gas delivery into the sealed process chamber 



 

Exhaust gas system from the sealed process chamber 



 

Control the preheat treatment procedure prior to nitriding, including stress relief 

and prehardening and tempering 



 

Quality and integrity of the steel surface precleaning prior to nitriding [26] 

 

Of the alloying elements, aluminum, chromium, vanadium, tungsten, and 

molybdenum are beneficial in nitriding because they are strong nitride-forming 

elements which form stable nitrides at nitriding temperature. Aluminum especially is a 

very important nitride former and is used in amounts between 0.85 and 1.5% in 

nitriding steel. [27]

 

Aluminum- containing steels produce a nitrided case of very high 

hardness and excellent wear resistance. All hardenable steels must be hardened and 

tempered before being nitrided. Tempering is performed at temperatures between 540 

and 750

 o

C, usually at least 30

 o

C higher than the maximum temperature to be used in 

---

 

19

nitriding. Tempering above the nitriding temperature provides a core structure that 

will be stable during nitriding.   

 

Gas nitriding is accomplished with ammonia gas which dissociates on the surface of 

the steel according to the following reaction: [28] 

H

N

NH

3

3





 

The resulting atomic nitrogen is absorbed at the surface of the steel. The schematic of 

the process is shown in Figure 3. 

 

Fig 3: Process steps during nitriding [29] 

 

 

Gas nitriding is based on a heterogeneous reaction between an ammonia gas 

atmosphere and a steel surface at temperature between 500 and 580

 o

C, with several 

chemical reactions involved : 



 

Ammonia is transported to the metal surface by molecular diffusion 



 

Ammonia molecules absorb at convenient surface locations and dissociate by 

steps into atomic nitrogen and hydrogen 



 

Nitrogen diffuses into the surface forming an interstitial solution and iron nitrides 

Fe

x

N 



 

Hydrogen forms molecular H

2

 or reacts with oxygen to form H

2

O, both of which 

desorbed from the surface into the furnace atmosphere [29] 

---

 

20

 

The results are a compound layer at the surface consisting of iron nitrides, as well as a 

diffusion layer beneath the compound layer consisting of an interstitial solution of 

nitrogen in the 



-Fe lattice which is shown in the Figure 4. The compound layer is 

responsible for excellent wear and corrosion resistance and the diffusion layer for the 

increased surface hardness. [30] In the compound layer, nitrogen concentration can 

reach values of 20 at%. In the diffusion layer, nitrogen concentration is below 8 at%. 

The boundary region between these layers is characterized by dissolved atomic 

nitrogen as well as nitride precipitation, usually at the grain boundaries. [31] 

 

Figure 4: White layer and diffusion zone in nitrided steel. Steel is Nitralloy 135 

Modified. Etched in 1.5% nital. Magnification, 500X. [27] 

 

The overall reaction is slow and demands long furnace residence times up of up to 

20h. The performance of this process depends on the material. Each step of the 

process can be hindered by surface contamination from prior manufacturing steps. On 

the other hand, improvements in nitriding have been reported owing to a preoxidation 

process taking place in the nitriding furnace at temperatures up to 300

 o

C, resulting in 

the formation of oxide layers. [32] 

---