Microsoft Word WendiThesis docx

 

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processes. Thus, cleaning to provide a residue free surface before a surface treatment 

such as gas nitriding is important. Since surface contamination during the heat 

treatment process can greatly affect the quality of the end products, one process to 

obtain better control is the surface cleaning, to remove the residues from the prior 

manufacturing processes. Manufacturing residues, like residual coolants, lubricants 

and anti-corrosives from cold/hot work, cutting or machining operations and 

mill-scale or rust formed during storage or transit can act as surface contaminant that 

prevent or hinder the diffusion process. [4] 

 

1.2 Objective 

The objective of this study is to determine the effect of surface contaminants on the 

nitriding behavior of the Nitralloy-135 steel. Microhardness and nitrogen flux after 

nitriding are used as the parameter to evaluate the heat treatment performance. To 

determine the effect of contamination on gas nitriding, the weight gained by the parts 

and the surface hardness were measured. 

 

1.3 Organization of the thesis 

The thesis is divided into four chapters. Chapter 1 is an introduction that provides an 

overview of the research within and why it is important. Chapter 2 is a review of 

relevant literature and previous work completed by others in the field of heat treating 

process. The literature review focuses on the classification of surface contaminants 

and cleaning methods as well as the key aspects of the effect of contamination on the 

heat treating process. Chapter 3 presents the experimental set-up and testing 

procedures along with a test matrix that described the experiments to be conducted. 

Chapter 3 also describes the effect of contaminations on the gas nitriding. In particular, 

the cutting fluid, the rust preventive oil and the rust were selected to contaminate the 

samples. The rust samples were cleaned by the acid cleaner, and the oily samples were 

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cleaned by alkaline cleaners. Weight gain was calculated before and after 

contamination, cleaning and gas nitriding. Surface hardness and hardness profiles 

were measured after gas nitriding. Nitrogen flux was also calculated after the heat 

treating process. The results are discussed in terms of the weight gain and 

microstructures. The final chapter, Chapter 4, presents the summary and future work. 

In Appendix, the Questionnaire Results from the CHTE Cleaning Project are shown. 

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Chapter 2   

Literature review 

2.1 Surface contaminants 

Prior to heat treatment, the parts can be handled and treated in different ways, and the 

surface can be contaminated and then changed. There are two distinct changes: 

mechanical changes and chemical change of the part’s surface. The mechanical 

changes include deforming, compressing and abrasion. [5] During these processes, 

auxiliary agents are necessary for cooling, greasing, and corrosion resistance. They 

assist for a longer life time of the tools, allow faster processing speed and transport 

splinters away. But they are also responsible for the surface contamination. The 

chemical change of the part’s surface consisted of phosphate layers and adsorption or 

reaction layers caused by corrosion protection compounds and auxiliaries as cooling 

compounds, soaps, oils.[6]  The schematic of the layers that may form on a 

work-piece are presented in Figure 1. 

 

As seen in Figure 1, the bulk material may be covered by a deformed boundary layer 

that is formed during the manufacturing processing of the part. On top of the 

deformed layer, the reaction layer forms normally due to metal oxides. But it can also 

contain reaction products of auxiliary compounds used during manufacturing and 

metal, e.g. metal sulfides or phosphorous compounds. They are thin, but have 

excellent adhesion and they can be very dense. [7] 

 

On top of the reaction layer is a sorption layer. Compounds of the grease are bound by 

chemisorption or physisorption. This layer passes into a thicker contamination layer, 

containing residue of the previous working steps, e.g. oil, grease, cleaner residues or 

water ingredients. 

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Figure 1. Different layers on the surface of a work piece before the heat treatment. [3] 

 

Reaction layers may form during machining processes also. Reaction layers, however 

thin, may hinder chemical surface modification processes such as gas nitriding. Haas 

et al [7] reported that the samples of 42CrMo4 steel milled using mineral oil based 

cooling agents had reaction layers of oxides and the surfactant cleaner used in the 

study was unable to remove it.   

 

The top layer is the contamination layer, as shown in Figure.1, which will be 

investigated in this study.    The contamination layer may be loosely deposited, much 

like ordinary dust, or it may be adhered quite firmly by some chemical or physical 

means such as chemical bonding, adsorption, adhesion, and so on. This distinction is 

important, and the surface contamination is classified on the basis of how easily it can 

be removed.  A survey has been conducted among the members of CHTE [8] to 

identify the surface contaminants that may influence various heat treatment processes.   

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