Surface Hardening of Stainless Steels Materials and Applications Series, Volume 20 SURFACE HARDENING OF STAINLESS STEELS Euro Inox Euro Inox is the European market development asso- Full members ciation for stainless steel. Members of Euro Inox Acerinox include: www.acerinox.com • European stainless steel producers; Aperam • national stainless steel development associations; www.aperam.com • development associations of the alloying element Outokumpu industries. www.outokumpu.com The prime objectives of Euro Inox are to create aware- ness of the unique properties of stainless steel and Associated members to further its use in existing applications and in Acroni new markets. To achieve these objectives, Euro Inox www.acroni.si organises conferences and seminars and issues guid- British Stainless Steel Association (BSSA) ance in printed and electronic form, to enable archi- www.bssa.org.uk tects, designers, specifiers, fabricators and end users Cedinox to become more familiar with the material. Euro Inox www.cedinox.es also supports technical and market research. Centro Inox www.centroinox.it ConstruirAcier www.construiracier.fr Industeel www.industeel.info Informationsstelle Edelstahl Rostfrei www.edelstahl-rostfrei.de International Chromium Development Association (ICDA) www.icdacr.com International Molybdenum Association (IMOA) www.imoa.info Nickel Institute www.nickelinstitute.org Paslanmaz Çelik Derneği (PASDER) www.turkpasder.com Stowarzyszenie Stal Nierdzewna www.stalenierdzewne.pl SWISS INOX www.swissinox.ch ISBN 978-2-87997-395-1 SURFACE HARDENING OF STAINLESS STEELS Contents Surface Hardening of Stainless Steels 1 Introduction 2 Second edition 2015 2 Principle 3 (Materials and Applications Series, Volume 20) 3 Thermochemical diffusion methods 5 © Euro Inox 2013-2015 3.1 Carburising 6 3.2 Gas nitriding 7 Publisher 3.3 Plasma (ion) nitriding and liquid nitriding 10 Euro Inox 3.4 Nitrocarburising 11 Avenue des Cerisiers 15, bte. 1 3.5 Boriding or boronising 13 1030 Brussels 4 Applied energy methods 14 Phone +32 486 50 37 53 4.1 Induction hardening 14 E-mail: [email protected] 5 Costs 15 Internet: www.euro-inox.org 6 Summary 16 7 References 17 Author Alenka Kosmač, Brussels (B) Cover photos Expanite, Hillerød (DK) (left) iStockphoto (top right) Bodycote Hardiff, Düsseldorf (D) (bottom right) Copyright notice This work is subject to copyright. Euro Inox reserves all rights of translation in any language, reprinting, re-use of illustrations, recitation and broadcasting. No part of this publication may be reproduced, stored Disclaimer in a retrieval system or transmitted in any form or by Euro Inox has made every effort to ensure that the any means, electronic, mechanical, photocopying, information presented in this document is techni- recording or otherwise, without the prior written per- cally correct. However, the reader is advised that the mission of the copyright owner, Euro Inox. Violations material contained herein is for general information may be subject to legal proceedings, involving mon- purposes only. Euro Inox and its members, specifically etary damages as well as compensation for costs disclaim any liability or responsibility for loss, dam- and legal fees, under Luxembourg copyright law and age or injury, resulting from the use of the information regulations within the European Union. contained in this publication. 1 SURFACE HARDENING OF STAINLESS STEELS 1 Introduction Scratch resistance of Surface hardening provides surfaces for the surface is often different consumer products and ensures requested for high- that stainless home appliances, household visibility applications. objects, cutlery and highly exposed objects maintain their pleasing appearance and durability. It is also necessary in industrial applications where exceptional wear resist- ance combined with corrosion resistance are needed. A wide range of such applica- Stainless steel is widely used in applications tions can be found in [1, 2, 3, 4, 5]: in which corrosion resistance is of high im- • consumer goods portance. In many end-uses, the material is • household appliances also expected to have a hard, scratch-resist- • food and beverage industry ant surface. When improved wear resistance • mobile devices is required, surface engineering provides • industrial fluid handling solutions. Industrially proven processes are • industrial and consumer fasteners available that improve surface hardness, • valve and pump parts scratch and wear resistance. • medical applications • marine applications Properties most often expected from sur- • automotive components face hardening [1, 2]: • axis and rotating parts • scratch resistance • surface hardness above 900 HV0.05 • unchanged corrosion resistance Surface hardening can • reduced friction coefficient be used in many differ- • no dimensional change ent areas, from building • minimal or no change in visual appearance and architecture to the food and beverage • no cracking or flaking of the hardened layer industry. Photo: Heat • minimal or no change in surface roughness & Surface Treatment, • no pre-treatment Eindhoven (NL) • no post-treatment • enhanced lifetime and reduced downtime and costs • weldability • suppression of fretting/galling (adhesion) 2 SURFACE HARDENING OF STAINLESS STEELS 2 Principle Surface hardening includes a wide variety There are three distinctly different ap- of techniques. Most often it is used to im- proaches to the various surface-hardening prove the wear resistance of parts without methods [7]: affecting the softer, tough base material 1. Thermochemical diffusion methods, necessary to resist impact occurring during which modify the chemical composition operation. of the surface with hardening species such as carbon, nitrogen and boron. Dif- Wear involves the physical removal of fusion methods allow effective hardening material from a solid object. It can be of the entire surface. They can be used for divided into three categories: abrasive, both single parts and batches. adhesive and fatigue wear. 2. Applied energy or thermal methods, Abrasive wear is when two surfaces rub which do not modify the chemical com- together and the harder surface grinds position of the surface but rather improve away the softer. It can be characterized properties by altering the surface struc- by a rough appearance. Often, work ture – that is they produce a quench-hard- hardening of the surface can occur. ened surface, without additional alloying Adhesive wear, like abrasive wear, is species. They can be used to harden the caused by loaded surfaces rubbing to- entire surface or only part of it (selective gether. With adhesive wear, high local- surface-hardening). ized temperatures are created by friction 3. Surface coating or surface-modification at the tips of opposing asperities on rub- methods, which involve the intentional bing surfaces. These tips can deform and build-up of a new layer on the steel sub- “weld” together, due to localized temper- strate. atures. They either break and fall away as debris or are cold-welded together. Various process methods for the surface Fatigue wear occurs whenever a surface hardening of steels are shown in Table 1. is subjected to repeated high-stress These long-established techniques are load. Wear rates are less affected by tem- continually improved and remain among perature than is corrosion [6]. the most widely applied ones. This publica- tion discusses the most important surface- hardening methods used on stainless steels (marked in italics in the following table). 3 SURFACE HARDENING OF STAINLESS STEELS Table 1. Process methods for the surface hardening of steels [7] Diffusion methods Coating and surface modification Carburising Hard chromium plating Nitriding Electroless nickel plating Nitrocarburising Thermal spraying Carbonitriding Weld hardfacing Boriding or boronising Chemical vapour deposition Thermal diffusion process Physical vapour deposition1 Applied energy methods Ion implantation Flame hardening Laser surface processing Induction hardening Laser-beam hardening Electron-beam hardening Vickers test scheme The pyramidal diamond An indentation left in case- indenter of a Vickers hardness hardened steel after a Vickers tester. hardness test. The difference in length of both diagonals and the illumination gradient, are both classic indications of an out-of-level sample. This is not Source: Wikipedia; Vickers hardness test a good indentation. Photos: R. Tanaka (middle); Dennis M. Clarke, (right) 1 Physical vapour deposition is discussed in the Euro Inox publication Colouring Stainless Steel, Materials and Applications Series, Volume 16, http://www.euro-inox.org/pdf/map/ColouringStainlessSteel_EN.pdf 4 SURFACE HARDENING OF STAINLESS STEELS 3 Thermochemical diffusion methods Traditional thermochemical treatment on ties and tribological properties [8]. Some stainless steel is associated with a loss of recently developed processes, in contrast, corrosion resistance as nitrogen and car- do not diminish the corrosion resistance of bon react with chromium to form carbides/ stainless steels. nitrides, thus withdrawing chromium from solid solution. In the case of stainless steels, hardening by thermochemical treat- ment has been considered bad practice or a compromise between corrosion proper- Table 2. Typical characteristics of thermochemical diffusion treatments for stainless steels [7, 8] Process Typical case Case Process Name of case temperature depth hardness Process characteristics °C µm HV Nitriding/Carburising/Nitrocarburising Hardest cases from