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Title Partial Case Hardening of Stainless Steel
Author Nielsen, Kristian Holm
Supervisor Horsewell, Andy (Materials Science and Engineering, Department of Mechanical Engineering, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Institution Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark
Thesis level Master's thesis
Year 2006
Abstract Stainless steel is widely used all over the world due to its excellent corrosion resistance. It is well known that stainless steel achieves this resistance by the formation of a thin layer of chromium oxide on the surface forming a barrier so that the iron avoids contact with oxygen. Austenitic stainless steel, typically AISI 316 and 304, is ductile and easily processed into tubes, sheets, etcetera. The ductility is a consequence of the face centered cubic unit cell. However the hardness of stainless steel is relatively low and because of the chromium oxide layer a simple hardening by infusion of either carbon or nitrogen could until 1985 only be done at temperatures above 500◦C with precipitation of CrN and with a significant loss of corrosion resistance [1]. The chromium is withdrawn from the solid solution and when it reaches below 11 wt. %, the steel loses its self repairing ability of rebuilding the passive film at the surface [2]. In 1985 the possibility of low temperatures surface engineering of stainless steel by use of plasma or implantation techniques was recognized. It was discovered that a hard and wear resistant surface layer could be obtained without experiencing a loss of corrosion resistance when nitriding at temperature below 450-500◦C [3, 4]. The new phase responsible for the favorable properties associated with low temperature nitriding was by Ichii et al. [4] designated the S-phase. The term S-phase relates to the unidentified peaks in the X-ray diffraction patterns arbitrarily named S1-S5. The S phase is also called expanded austenite because of the interstitial diffusion of nitrogen in the austenite. It is a metastable phase and its stability is highly dependent on the applied process conditions. The S phase decomposes when thermally exposed to critical temperatures. The decomposing of stainless steel like AISI 316 used in this projects happens within minutes at 600◦C and within years at 300◦C [5]. Plasma and implantation technologies circumvent the problem of passive layer impenetrability either by sputtering the surface in a vacuum chamber or with a chemical removal of the passive layer. The problem with a chemical removal of the passive layer is that the surface starts rebuilding the moment it gets in contact with oxygen and thereby recreates the problem of passive layer impenetrability. However Somers et al. [6] have developed a patterned process where nickel is applied to the surface directly after the oxide removal, making the surface thermodynamically stable. The depassivation of the surface is carried out in a solution of 80 ml 15% w/w hydrochloric acid + 1 ml 35% hydrogen peroxide for 20-30 seconds thereby removing the chromium oxid layer. Then the specimen, while still wet, is directly placed in a Wood’s nickel bath, which is an acidic halogenide containing electrolyte, where a nickel layer ís grown in 90-120 seconds. Afterwards several articles have been published by the same group [7, 8, 9, 10, 11, 12, 13] and more are being prepared for publication [14, 15, 16]. The topic of this master thesis is low temperature surface hardening of stainless steel in micro scale patterns using photoresist to create the desired patterns. Thereby creating functional surfaces for different uses. The method of surface hardening is developed by Somers et al. [6] and the idea of creating a functional surface is suggested by A. Horsewell. The basic idea is that before case hardening of the surface a micro scale pattern of nickel is applied to the surface instead of applying it to the whole surface. This can either be done by protecting a pattern of the surface from getting in contact with the nickel bath or by protecting a pattern of nickel already applied to the whole surface. The technological advantages of only case hardening parts of a surface could be: a) The part to be welded can be left non case-hardened. b) The hardened contact surface area can be reduced, thereby allowing non-hardened surface area to be removed during sliding wear. c) Patterned surfaces of hardened and non-hardened areas respectively can be tailored to allow local reservoirs of oil or other lubricants. d) Stresses which could lead to surface cracking might be relaxed in untreated areas. Furthermore, basic understanding and fundamental research studies on interfaces hardened and non-hardened regions - stressed and non-stressed regions - are needed. Each of the chapters contains its own introduction, experimental, discussion and conclusion. The Chapters 2 and 4 deal with the two different types of micro scale pattern creation. Chapter 2 deals with the process of protecting a pattern of the surface from getting into contact with the nickel bath i.e. applying nickel in patterns and chapter 4 deals with the process of creating a pattern of nickel by protecting parts of nickel already applied to the whole surface i.e. removing nickel in patterns. The discussions and conclusions in Chapter 4 are based on preliminary observations as the sample preparation used is not final. Chapter 3 deals with a thickness determination of the nickel layer by use of energy dispersive X-ray spectrometry (EDS) and Monte Carlo electron simulations. Conclusions are presented in chapter5 along with an outlook.
Pages 48
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Admin Creation date: 2007-07-25    Update date: 2011-09-27    Source: dtu    ID: 201790    Original MXD