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- Bu səhifədəki naviqasiya:
- Acknowledgements
- Table of Contents
- Chapter 1 Definition of the problem 1.1 Introduction
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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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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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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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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 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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