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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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