Engineering Materials

Henry Tindell

• Publisher: Crowood
• Year: 2014
• Print ISBN: 9781847976796
• eBook ISBN: 9781847976802
• Topic: Technology & Engineering
• Subtopic: Engineering General
• Index: Technology & Engineering

A comprehensive guide to engineering materials used in the workshop, for processes such as milling, welding, and lathe and bench-work. Designed for the general enthusiast or amateur engineer, Engineering Materials provides in-depth information on the functions and limitations of commonly used metals, and valuable advice on material selection. With detailed diagrams and photographs throughout, the book covers: a history of engineering materials, and the forming and behaviour of a range of ferrous and non-ferrous metals; the practical application of materials in engineering and case studies on steam locomotive boilers, model aero engines and classic two-stroke motorcycle engines; authoritative advice on material selection for practical heat treatments, joining and other processes in the workshop; a review of the micro-structures and performance of familiar metals in critical applications, including fast fracture and fatigue, illustrated by a re-evaluation of some well-known dramatic engineering failures. Superbly illustrated with 144 colour photographs and 82 diagrams.

1 Case studies: materials in action

This chapter introduces materials used in three practical applications: model locomotive or steam engines (Case Study 1), a small model aero engine (Case Study 2), and a two-stroke motorcycle engine (Case Study 3). It is to be hoped that these items will be of interest to the amateur engineer: often the steam engine from earlier and later life; the aero engine from sore fingers flicking propellers when growing up; even the small two-stroke motorcycle might resonate with those Baby Boomers who were sufficiently fortunate (or otherwise) to encounter them as early personal transport, even if this particular model’s failings and missed potential reflect on the demise of this sector of UK manufacturing.

Photo 13 Dave Waggett of the Stockport Model Engineering Society powers around their track.

CASE STUDY 1: MODEL STEAM LOCO BOILER

Materials

The copper loco boiler is the heart of the model steam train. A similar construction can be found in model traction engines, and simplified examples are employed widely in many and various other steam plant, examples of which are illustrated in the introduction to this book.

The fabrication of the loco boiler is quite a demanding task, not to be undertaken before sufficient experience has been gained in the fields of rolling and bending plates, and the requisite joining methods. Design is also paramount; many established and proven designs are readily available. The importance of structural integrity must not be underestimated as mandatory requirements exist for the pressure testing and operational safety, as high-pressure gas (steam) provides a potentially dangerous energy source. Indeed, the catastrophic record of full-size boiler failures throughout the Industrial Revolution provided the drive to understand the design and manufacture of pressure vessels, which we explore in Chapter 3.

Therefore, boiler fabrication should be treated with due respect, but with proper experience it does provide a perfectly practical activity for the home worker, and there is no shortage of information with which to tackle the subject (see Bibliography).

Photo 14 Boiler build nearing completion in Ron Perry’s workshop.

Table 2 Copper for boilers.

We are primarily concerned with the materials of construction here, and it is worth noting the approach taken by makers of model loco boilers in the almost universal choice of copper (Cu) for the main construction. This contrasts with the full-size machines, which almost universally employ plain carbon steels for boilers, generally relying on riveting for joints, as arc welding was not available for practical application until early in the twentieth century.

Whilst there exist alternatives for model boilers, such as plain carbon steels and stainless steels, the consensus of makers indicates that copper remains the optimum choice. This is primarily driven by the exemplary corrosion resistance, important with the thicknesses involved, typically 3mm (⅛in), whereas the full-sized version uses relatively thick plate upon which a corrosion allowance may be applied without ­significant economic or practical impact.

Stainless steel might be thought a practical alternative, and indeed it has been tried on occasion, but it presents a more complex choice. For one-off fabrication the selection of a particular grade of stainless pres­­­ents significant practical difficulties which would not be the case for high-volume production with optimum material selection. As will be explored later, stainless steels are not necessarily free of corrosion problems, such as pitting, crevice and stress corrosion cracking in particular environments, which could be the case with certain boiler operations.

The grade of copper most commonly employed is generally recognized as ‘C106’, which relates to a superseded British Standard but, as explored later, is still common currency in practice. The equivalents include UNS C12200 and, in the USA, C12200, such as ASTM B75 for seamless tubing. This is a ‘phosphorus deoxidized non-arsenical copper’, thus avoiding potential problems with oxygen embrittlement, with properties shown in Table 2.

Photo 15 Firebox view of Photo 14, showing reinforcement universally employed.

Photo 16 LMS Duchess, showing varieties of silver soldered joints.

Joining

The method employed for fabrication of these Cu bodies is normally silver soldering. At one time it was thought that an extensive range of silver solders, with a wide range of discrete melting points (T_m), were required as the sequence of joints was made, with highest T_m followed by progressively lower, to finish at the lowest without re-melting previous joints. With experience, however, two alloys are sufficient to effect a full and complex boiler construction, as shown in the photos.

For these examples the grades SB40 and SB55 provided the melting points as shown in Table 3. These alloys are now cadmium (Cd) free, comprising essentially Cu-40 per cent silver (Ag), and Cu-55 per cent Ag. This provides around 50°C difference in T_m. Careful planning of the job is required to ensure that subsequent joints are far enough away to avoid disturbance of those previously made, and also with sufficient access.

Heat sources usually employ gas-air, such as when using specialist propane torches. It is possible to use oxyacetylene but the concentrated heat source requires experience to avoid overheating, although it can solve some problems of difficult access, and flame extinction in constricted areas.

Careful preparation, with regard to joint fit-up and especially cleanliness, is vital in the making of successful joints by this method, as is sufficient practice. Flux is required, an example being the general purpose ‘Easy-flo’ powder or paste flux marketed by Johnson ...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• Preface
• Introduction
• 1 Case studies: materials in action
• 2 Materials
• 3 Behaviour of metals
• 4 Material selection and limitations
• 5 Back to basics
• Appendix I: Hardness measurement
• Appendix II: Resources
• Appendix III: Elements, Abbreviations, Symbols, Conversions
• Glossary
• Bibliography
• Acknowledgments
• Index