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The Effects of Contaminants on the Gas
Nitriding of Nitralloy-135
A Thesis
Submitted to the Faculty of the
WORCESTER POLYTECHNIC INSTITUTE
In partial fulfillment of the requirements for the
Degree of Master of Science
In Material Science and Engineering
By
Wendi Liu
November 2008
Approved: ___________________
Richard D. Sisson, Jr.
Director of Manufacturing and Materials Engineering
George F. Fuller Professor
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Abstract
Surface contamination during the heat treatment process can greatly affect the quality
of the heat treated parts. Although cleaning the post-heat treated parts is considered a
value added process in heat treatment, cleaning parts prior to heat treatment is also
important and can influence the outcome of subsequent processes.
A series of experiments has been designed to determine the effects of the surface
contaminants on nitriding. Nitralloy-135 steel was selected to evaluate the cleaning
methods and the effects on the result of subsequent nitriding process. The samples
were contaminated with a variety of contaminants, including rust preventive oil,
cutting fluid and rust. The contaminated samples were cleaned using either acid or
alkaline cleaner. Both cleaned and contaminated samples were nitride in the furnace.
To determine the effect of contaminants on gas nitriding, weight gain and the surface
hardness were measured. Nitrogen flux was also calculated. The results show that the
acid cleaning effectively removes the rust layer. Rusted parts had lower nitrogen
absorption than that of the cleaned samples. The parts contaminated by the oil did not
reveal any effect on the nitrogen uptake. The surface hardness (Rc) did not show a
significant difference between the heavily rusted sample and the clean sample. It has
been observed that the nitrogen flux is smaller due to the rust layer on the heavily
rusted samples.
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Acknowledgements
This project was sponsored by WPI’s Center for Heat Treating Excellence.
I would like to thank my advisor Professor Richard D. Sisson, Jr. for allowing me the
opportunity to work on this project and for his help, encouragement, and advice
throughout this project as well as others to allow me to be where I am today.
I would like to thank Dr. Mohammed Maniruzzaman for all of his valuable assistance
and suggestions throughout this study. I also thank Dr. Boquan Li for the helping and
guiding the lab work. I would also like to thank Rita Shilansky for all of her time and
support throughout this project.
I also would like to express my appreciation for to Imad Jaber and Christopher Maki
of Bodycote Thermal Processing, Worcester plant to for thesis assistance in nitriding
the samples.
Lastly, I thank my family and friends for all of their help to get me where I am.
Wendi Liu
Worcester, MA
November 2008
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Table of Contents
Chapter 1 Definition of the problem ........................................................... 5
1.1 Introduction ..................................................................................... 5
1.2 Objective ......................................................................................... 6
1.3 Organization of the thesis ............................................................... 6
Chapter 2 Literature review ........................................................................ 8
2.1 Surface contaminants ...................................................................... 8
2.2 Cleaning methods ......................................................................... 11
2.3 Surface hardening methods ........................................................... 14
2.4 Effects of contaminants on the heat treating process ................... 21
Chapter 3 Effects of surface contaminations on Gas nitriding process .... 29
3.1 Introduction ................................................................................... 29
3.2 Experimental procedure ................................................................ 33
3.3 Results and Discussion ................................................................. 39
Chapter 4 Summary .................................................................................. 59
Appendix ................................................................................................... 60
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Chapter 1
Definition of the problem
1.1 Introduction
Surface hardening is the process of hardening the surface of a metal by diffusing
carbon or nitrogen atoms into the metal to form a thin layer of a harder alloy. It is also
a process to be used to improve the wear resistance of parts without affecting the
interior of the part. [1] The surface hardening of steel has an advantage over through
hardening or quenching because less expensive low-carbon and medium-carbon steels
can be surface hardened without the problems of distortion and cracking. There are
two distinctly different ways to perform the surface hardening:
Methods that involve an intentional buildup or addition of a new layer
Methods that involve surface and subsurface modification without any intentional
buildup or increase in part dimensions [2]
Surface hardening process investigated in the current research focuses on the second
set of methods, which is divided into diffusion methods and selective hardening
methods. Further, this study investigated the gas nitriding process which is one of the
diffusion methods. Diffusion methods modify the chemical composition of the surface
with hardening species such as carbon, nitrogen, or boron. [3] Diffusion methods
allow effective hardening of the entire surface of a part and are generally used when a
large number of parts are to be surface hardened.
The heat treatments are facing tougher specifications for process quality, safety and
results in term of reproducibility. That can be achieved only if the metals’ surface
condition is controlled during the process and, especially, prior to the heat treatment
process. It is well known that surface contamination may hinder surface modification
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