Microsoft Word WendiThesis docx

 

29

Chapter 3   

The Effects of surface contaminations on Gas nitriding 

process 

3.1 Introduction 

Surface cleaning should be employed prior to the thermochemical heat treatments. 

Manufacturing residue on parts and components soil the furnace and may lead to 

corrosion and harmful exhaust components. Furthermore, they affect the result of the 

thermochemical treatment. Manufacturing residues, residual coolants, lubricants and 

anti-corrosives from cold/hot work, cutting or machining operations can act as passive 

layers and prevent or hinder the diffusion process. Nonvolatile contaminants do not 

vaporize completely before the heat treatment temperature is reached in the furnace. 

Cleaning/degreasing agents will also leave some inorganic salt residues which are 

hard to volatilize, so they must be removed completely from the steel surfaces prior to 

heat treating. The rinsing stage becomes an important part of a cleaning process, 

especially if aqueous cleaning agents are used. [1] 

 

During cold or hot work, as well as during machining, the cooling lubricants, 

hydraulic oils, and machine grease may form stable reaction layer at the elevated 

temperatures and pressures which occur during the manufacturing process. Reaction 

layer can work as barrier layers and inhibit surface modification reactions. Reaction 

film from lubricant additives can affect the white layer formation and surface 

hardness, which makes white layer too thin and non-uniform. [2] 

 

Not all types of surface contaminants interfere with the nitriding. Generally, the more 

volatile components are desorbed or vaporized, whereas others can react with the 

metal to form stable surface films, preventing diffusion. Furthermore, Haas et al [3] 

---

 

30

presents that steels containing chromium as alloying element tend to form passive 

oxide surface films even if free from contaminants. In common cleaning procedures 

liquid cleaners are applied, either water- or solvent based. However, these cleaners are 

limited in their capability with respect to the removal of passive chemisorption or 

reaction layers. Electrochemical methods allow distinguishing types of surface 

contamination which do not interfere with the thermochemical process from passive 

layers which do, and thus must be removed - or prevented from formation during 

manufacturing.[3] 

   

In the present work, Nitralloy-135 steel samples have been contaminated with the 

cutting fluid, rust preventive oil and rust. The objective of this study is to understand 

the effect of surface contaminants on the nitriding behavior of steel. Microhardness 

and nitride flux after nitriding are used as the parameters to evaluate the heat 

treatment performance. To determine the effect of contamination on gas nitriding, 

weight gained by the parts and the surface hardness were also measured. 

 

Nitriding Process 

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

atmosphere and a steel surface at temperatures between 500 and 580°C. Residence 

times for the steel components to be treated are between 2 to 20 hours. Ammonia 

content, furnace temperature and residence time control the process result with respect 

to the hard layer morphology. 

 

Usually the hard layer can be divided into a "white layer" and a compound layer 

consisting of metal nitrides of thickness 20 um or less, and a diffusion layer below, 

containing nitride precipitation at grain boundaries and dissolved atomic nitrogen in 

the ά-iron lattice of some mm thickness.[4] 

 

As seen in Figure 1, the reaction is the typical heterogeneous reaction between a gas 

---

 

31

and the steel's surface. As ammonia is not thermodynamically stable at nitriding 

conditions with respect to the formation of nitrogen and hydrogen, the reaction is 

kinetically controlled. Several chemical reactions that control the overall reaction rate 

and thus, the growth rate of the hard layer: 



 

The molecular transport of ammonia 



 

The adsorption of ammonia molecules 



 

The stepwise dissociation into atomic N and H 



 

The combination of H and N atoms to form H

2

 and N

2

, and/or, in competition 

with the combination reactions 



 

The removal of atomic nitrogen from the surface owing to the formation of the 

interstitial solution representing the diffusion layer and at higher nitrogen content. 



 

The removal of H

2

 (and N

2

 if present) by desorption and molecular transport into 

the bulk gas phase. [5] 

 

Figure 1 The sketch for the liberation of nascent N [5] 

 

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

diffusion layer beneath the compound layer. The compound layer is responsible for 

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

Atmosphere 

Adsorption layer 

Steel 

---

 

32

surface hardness. In the compound layer, nitrogen concentration can reach values of 

20 at% and more. In the diffusion layer, nitrogen concentration is below 8 at%. [6] 

 

The overall reaction is slow and demands long furnace residence times of up to 20 

hours. The performance of this process depends on the material, on surface shape and 

condition, and on the pretreatment of the work-piece, as well as the nitriding process 

parameters. Each step of the process can be hindered by surface contamination from 

prior manufacturing steps. [7] 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

---