The Forming Potential of Stainless Steel Materials and Applications Series, Volume 8 THE FORMING POTENTIAL OF STAINLESS STEEL Euro Inox Euro Inox is the European market development associ- Full members ation for stainless steel. Acerinox www.acerinox.es Members of Euro Inox include: Outokumpu • European stainless steel producers www.outokumpu.com • national stainless steel development associations ThyssenKrupp Acciai Speciali Terni • development associations of the alloying element www.acciaiterni.it industries ThyssenKrupp Nirosta www.nirosta.de The prime objectives of Euro Inox are to create aware- ness of the unique properties of stainless steel and to UGINE & ALZ Belgium UGINE & ALZ France further its use in existing applications and in new mar- Groupe Arcelor kets. To achieve these objectives, Euro Inox organises www.ugine-alz.com conferences and seminars, issues guidance in printed and electronic form to enable architects, designers, Associate members specifiers, fabricators and end users to become more Acroni familiar with the material. Euro Inox also supports www.acroni.si technical and market research. British Stainless Steel Association (BSSA) www.bssa.org.uk Copyright notice Cedinox This work is subject to copyright. Euro Inox reserves all rights of www.cedinox.es translation in any language, reprinting, re-use of illustrations, recitation and broadcasting. No part of this publication may be Centro Inox www.centroinox.it reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or Informationsstelle Edelstahl Rostfrei otherwise, without the prior written permission of the copyright www.edelstahl-rostfrei.de owner, Euro-Inox, Luxembourg. Violations may be subject to legal Institut de Développement de l’Inox proceedings, involving monetary damages as well as compensation (I.D.-Inox) www.idinox.com for costs and legal fees, under Luxemburg copyright law and regula- tions within the European Union. International Chromium Development Association (ICDA) www.icdachromium.com ISBN 978-2-87997-211-4 International Molybdenum Association (IMOA) www.imoa.info Nickel Institute www.nickelinstitute.org Polska Unia Dystrybutorów Stali (PUDS) www.puds.com.pl SWISS INOX www.swissinox.ch THE FORMING POTENTIAL OF STAINLESS STEEL Contents The Forming Potential of Stainless Steel 1. Introduction 3 First Edition 2006 2. Mechanical properties 4 (Materials and Applications Series, Volume 8) 3. Forming potential 5 © Euro Inox 2006 4. Surface finish 5 5. Hydroformed nodes for automotive frames 6 Publisher 6. Hygienic design through seamless surfaces 8 Euro Inox 7. Pump efficiency through hydroformed casings 10 Registered office: 8. Metal spinning for exclusive designs 12 241 route d’Arlon 9. Decorative wheelrims made by spinning 14 1150 Luxembourg, Grand Duchy of Luxembourg 10. Cold rolled sections for superior strength 16 Tel.: +352 26 10 30 50 / Fax: + 352 26 10 30 51 11. Explosion formed heat exchanger plates 18 Executive office: 12. Deep drawn locknuts for wheel decoration 20 Diamant Building, Bd. A. Reyers 80 13. Corrugated sheet for higher cargo capacity 22 1030 Brussels, Belgium 14. References 24 Tel.: +32 2 706 82 67 / Fax: +32 2 706 82 69 E-mail: [email protected] Internet: www.euro-inox.org Disclaimer Author Euro Inox has made every effort to ensure that the information pre- Benoît Van Hecke, Brussels (B) sented in this document is technically correct. However, the reader is advised that the material contained herein is for general informa- Acknowledgements tion purposes only. Euro Inox, its members, specifically disclaim Cover photographs: any liability or responsibility for loss, damage, or injury, resulting • HDE Solutions, Menden (D) from the use of the information contained in this publication. • ThyssenKrupp Nirosta, Krefeld (D) • Alessi, Crusinallo (I) 1 THE FORMING POTENTIAL OF STAINLESS STEEL About stainless steels Stainless steels are iron alloys with a mini- These families also include grades contain- mum chromium content of 10.5% (by ing other elements, such as molybdenum, weight) and a maximum of 1.2% carbon, titanium, niobium and nitrogen. Austenitic necessary to ensure the build-up of a self- stainless steels account for approximately healing oxide layer – known as the passive two thirds of the world’s stainless steel use. layer – which provides the alloy’s corrosion Austenitic grades EN 1.4301/1.4307 (AISI resistance. This is the definition of stainless 304/304L) and EN 1.4401/1.4404 (AISI steels given in EN 10088-1. 316/316L), ferritic grade EN 1.4016 (AISI The composition of the alloying elements 430) and their variants are the best-known greatly influences the metallurgical struc- stainless steels and are widely commercial- ture of stainless steel and defines four main ly available. families of stainless steel, each with its typ- The main properties of stainless steel can ical mechanical, physical and chemical be summarised as follows: properties1: • corrosion resistance • Austenitic stainless steels: Fe-Cr-Ni, • aesthetic appeal C < 0.1 % (non-magnetic) • heat resistance • Ferritic stainless steels: Fe-Cr (> 10.5 %), • low life-cycle cost C < 0.1% (magnetic) • full recyclability • Duplex stainless steels: Fe-Cr-Ni, • biological neutrality combined austenitic-ferritic structure • ease of fabrication (magnetic) • high strength-to-weight ratio • Martensitic stainless steels: Fe-Cr, C > 0.1% (magnetic and hardenable) If the stainless steel surface is machined or accidentally damaged, the passive layer instantaneously reforms, in the presence of oxygen from air or water. 1 Detailed information about the chemical, mechanical and physical properties of stainless steels is available from www.euro- inox.org/technical_tables (an interactive database) or from the printed brochure Tables of Technical Properties (Materials and Applications Series, Volume 5), Luxembourg: Euro Inox, 2005. 2 THE FORMING POTENTIAL OF STAINLESS STEEL 1 Introduction Stainless steel has considerable potential Production cost involves: in forming applications, due to its interest- • material cost ing range of mechanical properties. The • transformation cost material’s high strength-to-weight ratio and While stainless steel may not always be the considerable elongation and work harden- cheapest material, production-process sim- ing properties mean it can often meet the plifications that its use may lead to can challenges of complex, three-dimensional, largely offset the higher material cost – for seamless designs. example, by reducing the number of deep Since its use in such designs does not drawing steps or heat treatments. impair any of its well-known corrosion resistant, heat resistant and decorative qualities, stainless steel is often the right material choice for both industrial and con- sumer products. Beer and beverage kegs (typically 20-70 l) can be manufactured in different ways, thanks to stainless steel’s versatile mechanical properties. Three-piece designs (left-hand example) are one option, using two dished ends and cold worked stainless steel sheet for the middle section. Cold working cold rolled stainless steel enhances its mechanical properties. The use of such sheet for the middle section will improve the keg’s strength or make thinner walls possible at equal strength. This design may be preferred if weight reduction is a key criterion. Alternatively, stainless steel’s forming capacity allows two-piece designs (right-hand example), composed of two identical deep drawn halves. This design is preferable when reduction of welds is the driving parameter. Apart from its forming potential, stainless steel is often the most appropriate material for con- tact with food, as it easily complies with European Food Safety Regulations. Three-piece versus two-piece design. Photos: AEB, Vimercate (I) 3 THE FORMING POTENTIAL OF STAINLESS STEEL 2 Mechanical properties Assessing the forming potential of any Stress-strain curves for different types of stainless steel. material requires understanding its Stress (N/mm2 ) mechanical properties. The most commonly 1250 used mechanical evaluation criteria are: Martensitic 1.4028 quenched and tempered 1000 Strength: the degree of resistance of a 750 Ferritic-austenitic 1.4462 material to deformation. Depending on 500 structural considerations, deformation Ferritic 1.4521 Austenitic 1.4401 can be defined as either: 250 • “yielding” or permanent plastic defor- 0 mation (hence “yield strength” R ), or 0 10 20 30 40 50 60 70 p Strain (%) • “breaking” or rupture (hence “tensile strength” Rm) Martensitic steels have high strength and Hardness: the degree of resistance to rather low ductility (or formability) whereas permanent indentation by an applied austenitic steels have lower strength and load. high ductility. Ferritic-austenitic (or duplex) Toughness: the capacity to absorb defor- and ferritic steels occupy an intermediate mation energy before fracture. position. The yield strength of ferritic steels Ductility: the ability to deform plastically is generally higher than that of austenitic without fracturing. steels, while the yield strength of duplex is considerably greater than that of both ferrit- The concepts “strong” and “weak”, “hard” ic and austenitic grades. Ferritic and duplex and “soft”, “tough” and “brittle” define dif- steels have similar ductility2. ferent aspects of a material’s mechanical With the exception of martensitic stain- properties and should not be confused. less steels, the typical relationships Some of these