Soldering and Brazing Handbook for Home Machinists E-Book

Tubal Cain

• Herausgeber: Fox Chapel Publishing
• Kategorie: Lebensstil
• Sprache: Englisch
• Veröffentlichungsjahr: 2022

Seitenzahl: 275

Beschreibung

Joining metal by soft or hard soldering, or brazing with alloys, is a common practice in welding and engineering workshops. But have you ever given thought to whether there could be quicker, more efficient, and less expensive methods? An extremely comprehensive book for model engineers, Soldering and Brazing Handbook for Home Machinists thoroughly explains the processes, equipment, and materials, as well as what happens in the joints as they're being made for an even deeper understanding. Featuring detailed sections on the characteristics of filler metals, brazing techniques, soft soldering techniques, capillary joint design, safety, data on fuel gases, and more, practical examples, test pieces, and organized data are also included throughout, making this must-have resource extremely useful for anyone in the metalworking industry. Author Tubal Cain was a skilled engineer and craftsman who wrote several best-selling home workshop and model engineering guides throughout his career.

Leseprobe

SOLDERING AND BRAZING HANDBOOK

FOR HOME MACHINISTS

Practical Information and Useful Exercises for the Small Shop

TUBAL CAIN

Copyright © 2022 by Tubal Cain and Fox Chapel Publishing Company, Inc., Mount Joy, PA.

Copyright © Special Interest Model Books Ltd 2003

First published in North America in 2022 by Fox Chapel Publishing, 903 Square Street, Mount Joy, PA 17552.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior permission of the copyright holder.

Print ISBN: 978-1-4971-0194-4eISBN: 978-1-6374-1147-6

Library of Congress Catalog Number: 2021945624

To learn more about the other great books from Fox Chapel Publishing, or to find a retailer near you, call toll-free 800-457-9112 or visit us at www.FoxChapelPublishing.com.

We are always looking for talented authors. To submit an idea, please send a brief inquiry to [email protected].

Contents

FOREWORD

Chapter 1 INTRODUCTION

Chapter 2 THE CHARACTERISTICS OF FILLER METALS

Chapter 3 SOFT SOLDERS

Chapter 4 BRAZING ALLOYS

Chapter 5 FLUXES

Chapter 6 SOFT SOLDERING TECHNIQUES

Chapter 7 TOOLS FOR SOFT SOLDERING

Chapter 8 BRAZING TECHNIQUES

Chapter 9 BRAZING EQUIPMENT

Chapter 10 CAPILLARY JOINT DESIGN

Chapter 11 SAFETY PRECAUTIONS

APPENDIX I Properties of some commercial brazing alloys

APPENDIX II Data on fuel gases

APPENDIX III An assessment of the hazards arising from cadmium-bearing alloys

Foreword

Both soldering and brazing are very old crafts – many thousands of years old – and unlike many such they have persisted, using very similar basic techniques over the centuries. This may be the reason why the model engineer – and, for that matter, the ‘jobbing professional’ – is so ill-served in the literature. Such books as do exist were either written so long ago as to be out-of-date, or they deal with the subject from the point of view of the production engineer, concerned with the joining of perhaps tens of thousands of identical parts per week. Equally unfortunate is the fact that many of the developments in recent years – and there have been many – are related to and described in terms of automatic brazing machines, the robot, or soldering machines costing more than a Prime Minister’s salary. Industrial soldering today is the province of the printed circuit board, and the car radiator or tin can manufacturer, and much of the brazing done in industry uses sophisticated heating processes and computer control. In very few books and papers does the ‘manual’ operator receive any consideration, and even when otherwise never more than a paragraph or two.

This book is written specifically for the manual operator, whether professional or amateur. I have tried to sort out the basic essentials from the mass of research literature so that you can see WHY the capillary joint gap is so important, HOW the joint action of flux and alloy conspires to make the bond, and – perhaps more important than is generally realized – how the joint design can influence both. It always helps to know the reasons behind the manipulation of the tools you are using. I have had great help from the manufacturers of both soft solder and brazing alloys and as a result have been able to offer some guidance through the proliferation of types, grades and specifications. In this connection I have, wherever possible, used the current British Standard nomenclature and I hope that you, too, will try to do the same when talking about the material you use. Specify BS 1845 AG21, and you will get the same material all over the world, but ‘Jointfabwerkegesellshaft Schlippenflow 15’ is likely to be obtainable only in Ruritania, and often only with some difficulty even there. Further, I would urge you to avoid the use of the term ‘silver solder’, for this CAN be confusing. It can describe alloys with melting points (‘solidus’) from 180°C up to 1200°C, and embrace materials containing metals as diverse as lead and palladium. The recommended terminology is– ‘Soldering’ or ‘Soft Soldering’ when the working temperature lies below about 400°C, perhaps with the addition of ‘silver bearing’ to describe the solder when appropriate, and ‘Brazing’ or ‘Hard Soldering’ when the working temperature is higher than that – again with the appropriate prefix when describing the alloy. Being old-fashioned I find that I myself use the term ‘Braze’ when I am using spelter or plain brass as the filler alloy and ‘Silverbraze’ when using silver-bearing alloys.

I have done my best to cover all the general types of soldering and brazing you are likely to meet, and almost all the illustrations are from fabrications I have made or from ‘exercise pieces’ specially devised to illustrate the point. However, I have, throughout, concentrated on the making of joints (which is the important part) rather than on the finished article. Only where the nature of the fabrication may affect the making of the joint have I felt it necessary to go further. For this reason I have not dealt in detail with locomotive boiler-making. The loco boiler is large (and heavy) and it does require a fairly high capacity burner to deal with it, but compared with many smaller fabrications all the joints are simple and the procedures I have outlined will cover them. I have, in my time, made a number of boilers, locomotive and traction engine types included, but the really difficult brazing jobs have been quite small ones by contrast.

I have, throughout, had the ‘beginner’ or the relatively inexperienced reader in mind, and if the more expert reader finds at times that I am ‘stating the obvious’ I would simply ask him (or her) to remember the first time they handled a torch or soldering iron! However, I believe that there is a considerable body of information in the following pages which will be of interest, if not of value, to all. I can, perhaps, emphasize this point by saying that I know far more now than I did when I started to write, and am now making far better joints!

The chapter on ‘Joint Design’ gave me far more difficulty in the writing than any of the others. I cannot tell you HOW to design a joint, for they serve so many different functions, and any design is of necessity constrained by the nature of the component being made. In the end it seemed best to start by outlining the principles, then to cover the most common types of joints, and to complete the picture with a number of ‘case histories’, each chosen to illustrate one or more design factors. On the important question of workshop safety I have taken a different approach, by outlining in general the hazards and remedies in the chapter devoted to that subject, but providing a more specialized analysis as an appendix. This is a matter to be taken seriously, but we must retain a sense of proportion; most people die in bed, but this is a poor reason for sleeping on the floor!

Also in the appendix you will find data on the three most common fuel gases, and tables giving comprehensive details of brazing alloys under their commercial names. I am deeply indebted to the Calor Gas Company, and to Messrs Fry’s Metals, Ltd., Johnson Matthey & Co. Ltd., and the Sheffield Smelting Co. Ltd (now Thessco Ltd.) whose staff have been generous in their help. I am also grateful to the British Standards Institution, for permission to reproduce data from their publications.*

I will conclude this foreword by making a point which is reiterated several times in the text. Soldering and brazing have established themselves as sound and reliable methods of joining metals over a period of several thousand years. Properly made joints can be expected to last for centuries – indeed, in many cases it is the parent metal which proves to be the more vulnerable to the ravages of time. Both ‘soft’ and ‘hard’ solders each have their proper place, each has its own virtues, and both have the right to be considered as ‘sound engineering practice’. I hope that this book will help you to make joints which the archaeologists of the future will find worthy of their approbation.

Tubal Cain

September 1984

*Copies of all British Standards can be had from the British Standards Institution, Linford Wood, Milton Keynes MK14 6LE.

Chapter 1

Introduction

The accepted definition of both soldering and brazing is ‘. . . the joining of metals using a filler metal of lower melting point than that of the parent metals to be joined. . .’ This is true enough, but leaves much unsaid. ‘Bronze-welding’ for example, uses just such a filler-rod, but is NOT ‘Brazing’. To get to the bottom of the matter, let us compare the three processes of ‘Welding’, ‘Gluing’ and ‘Soldering’, noting incidentally that the only real difference between soldering and brazing is the melting-point of the filler material used. (The distinction was much clearer in the old days when the two processes were known as ‘soft’ and ‘hard’ soldering).

In Fig. 1 at ‘A’ we have a fusion welded joint. The two parts are united by means of the fillets ‘w’, which are made up of the same material as the parent plates. The latter have been melted locally, so the plates and fillets are literally one piece of the same material (though it should always be remembered that these fillets will be in the cast condition, whereas the plates themselves may be rolled or forged). The strength of the joint depends on the area of contact at the fillets, and in a butt joint as at 1B the parent plates have been ‘prepared’ by beveling the edges, both to ensure complete penetration of the weld metal (absent in 1A) and to increase the effective contact area.

In Fig. 2 we have a glued joint. There is no fillet, the glue being disposed between the mating parts beforehand. Again, the strength will depend on the area in contact and, of course, on the strength of the glue and its bond to the parent material. With glues and cements this bond is usually due to the filler material – the glue – engaging with the surface irregularities of the joint faces; hence the need to roughen the surfaces. However, the important point to note for our comparison is that the glue is set in place on the surfaces before they are brought together and that as a rule these parts must be clamped together until it sets. Some glues set by chemical action (the ‘epoxy’ type for example), some by evaporation of a solvent, and others are applied hot and set as they cool – resembling a ‘solder’ in that respect.

Fig. 1

Fig. 2

Fig. 3 shows a soldered joint, and at first sight it differs not at all from that of the glued joint in Fig. 2A. The difference, and it is a crucial one, is that unlike the glue the solder has been applied to the joint from the edge, and has penetrated the joint line by CAPILLARY ACTION. This is the crux of the matter. In ‘welding’ the filler material is deposited drop by drop onto the molten

surface of the joint. In a ‘glued’ joint the glue is applied before the two parts are united. In both ‘soldering’ and ‘brazing’ the filler penetrates the joint area from outside the mating surfaces. This is not to exclude altogether the setting of filler material between the parts before heating (a process known as ‘sweating’) but even when this is done the flow of the molten filler is the result of capillary forces.

These forces are considerable, and the filler (whether soft solder or brazing alloy) can literally climb uphill. In Fig. 4 I have bent a piece of tinplate to a VEE shape, which was then heated up to the melting point of soft (tin-lead) solder. The solder was applied to the base of the crevice formed by the bend and then, when it had set, the bend has been torn open to reveal the solder filling. You will see that there is a marked increase in height as the gap between the two sides of the vee diminished – in this case the solder climbed up 11/2 inches. In Fig. 5 I have made a lap joint. The soft solder was laid against the upright at the right-hand end and the joint gently heated from below until it melted. The joint is shown torn open at 5B, and you can see that the solder has flowed right through the joint. There is a small patch which was not properly wetted (we shall come to this point later) but this was due to the fact that the mating part was not thoroughly cleaned at that point. (The ‘parent metal’ in this case was cut from the can of one of Mr. Heinz’ 57 varieties) Fig. 6 shows an attempt to make a similar demonstration using a brazing alloy melting at around 630°C. A groove of tapered depth ranging from 0.002 to 0.012 inch deep was cut in a piece of mild steel, and a small hole drilled at the blind end. A second piece of flat steel was clamped to it, with the groove filled with flux, and the whole heated up to brazing temperature. Brazing rod was then applied through the hole with the test-piece set vertically. The rod was fed in until it would take no more. After all had cooled the cover-plate was milled away, and the brazing alloy revealed. You will see that in this case the alloy climbed the full height of the test piece – about 2.8 inches. It did not climb up at all, however, on the wide side of the groove on the right, another point of importance we shall return to later. There are a few ‘inclusions’ in the joint, as you can see; these are my fault; the surface was left ‘as milled’ and I did not take enough care over cleaning before applying a water-based paste flux. However, the sample does show that the capillary attraction is very strong and can be relied upon to carry the alloy well into any joint provided that the joint gap is reasonable.

Fig. 4Showing that soft solder can ‘climb’ up a narrow gap.

Fig. 5Flow through a capillary gap by soft solder. Top, (a) The lap joint, ‘fed’ from the right. (b) The same joint, torn open.

Fig. 6Climbing action of a silver-brazing alloy.

Fig. 7Wetting action of solder on brass. Left, no flux. Center, using resin. Right, a semi-active flux.

Wetting Capillary action can occur only when the fluid, whatever it is, wets the surface. This means that the nature of both the parent metal and the filler alloy will have an effect on the performance. Fortunately the manufacturers of solder and brazing alloys have tackled this problem for us, and it is rare to find a base metal which cannot be soldered or brazed, though some may be very difficult. However, there is a great deal of difference between their laboratories and your workshop, and the pristine surface which they may have used will seldom be found in practice. It is imperative that the surfaces be clean if proper wetting is to be achieved, and even more so if a proper bond is to be made. (Bonding is dealt with later in the chapter.) The most common obstacle is the oxide film which forms (surprisingly quickly) on almost all metal surfaces. To overcome this a ‘flux’ is used which, at the temperatures used in the process, will attack and remove any reasonable oxide film. These are dealt with later in detail, and it will suffice to say now that the flux should not be used to clean up a dirty joint surface – it has enough on its hands in preventing the filler metal and joint surface from oxidizing at the jointing temperature. The surfaces should be as clean as can be managed before starting work, and as we shall see later the cleaning method used can have quite an effect on the integrity of the joint.

You can check this point very easily for yourself – and, incidentally, compare the effectiveness of fluxes, too. Cut a strip of clean brass about 1 inch wide x 3 inch long and thoroughly clean the surface, preferably not with emery; use pumice powder and water, finish by washing with detergent and hot water, and then air drying it. Coat the center third of the length with a paste flux, and one end with whatever other flux you have available. Take care to get no flux at all on the other third of the length. Cut off three small pieces of soft solder (not the resin cored stuff) and set one in the middle of each third of the length. Now heat the strip gently and evenly from underneath until the solder melts. You will see for yourself that even though the one end was really clean the solder has not spread at all – it does not ‘wet’ the surface. The effectiveness of the other fluxes used can be compared by the amount of ‘spread’ of the solder lump. I have done this in Fig. 7, with the unfluxed section on the left, pure resin used in the center, and a proprietary paste flux on the right.

Bonding We have seen that ‘glue’ acts by getting a grip on the surface irregularities of the parent materials. This can happen with soldering and brazing too; the little lump of solder on the left of Fig. 7 was quite firmly stuck on, but it parted from the brass quite simply when given a sideways tap. Examination of a properly soldered joint when torn apart shows quite a different state of affairs. Indeed, most of you will have had the experience of trying to remove solder when it has ‘got where it shouldn’t have’! Somehow it almost seems to be necessary to go right below the surface of the original metal. The bonding effected in both hard and soft soldering is a metallurgical process. It is not necessary to go into this in any detail – indeed, it would be difficult to do so in a book this size; but a few words may help you to understand what is going on and, more important, to give you some idea of what may have happened when things don’t go quite as they should.

The basis of all ‘solders’, hard or soft, is one or more ‘active metals’, which can form a type of alloy with the parent metals to be joined. Tin is one of the most active and is the basis for almost all soft solder. Copper, silver, zinc and many others show this form of activity, some quite general, others with greater affinity for one parent or base metal than others. The manufacturer of the solder or brazing alloy will have carried out extensive research to ensure that the composition of ‘general purpose’ fillers have a reasonably wide application, and at the same time will have developed special alloys to deal with the more difficult joints. There are few base metals (using the term to indicate ‘the metal to be jointed’) which cannot be brazed or soldered.

Fig. 7AErosion of a screw-in bit.

The active element in the filler forms an ALLOY with the base metal during the heating period. This may be surprising to those who have been told that alloys are formed by melting two (or more) metals together. However, again a little observation will show that it is possible for an alloy to form between a hot liquid and a solid. I suppose that everyone who has used a soldering iron will at some time or another have found it necessary to dress up the business end with a file, and will have found that once the solder itself has been removed there seems to be a hard coating on the bit. This is a tin-copper alloy, which has formed during previous use of the tool. It is quite marked. You may have found, too, that the end of a soldering iron bit seems to ‘dissolve away’ (see Fig. 7A). This again is due to the action of the tin in the solder on the copper, and some commercial soldering irons are ‘iron plated’ over the copper to reduce this. So, it can work both ways: copper being dissolved by the tin, and tin being alloyed with the solid copper of the bit.

Fig. 8Alloy formation between tin/lead solder and copper. (After Thwaites).

The ‘bond’ between the base or parent metal and the filler is, therefore, an alloy layer as shown in Fig. 8. The bond is not mechanical, as in a glue, but metallurgical, and is very strong indeed. If the joint is close enough the joint may be almost entirely ‘alloy’, but it is seldom that a gap as close as this will permit the necessary capillary movement of the filler. It can happen, however, in a sweated joint. Incidentally, this alloy formation is one explanation of the fact that it seems to need a higher temperature to ‘unbraze’ a joint than it does to make it! Note that this alloy must not be confused with ‘Intermetallic Compounds’ sometimes referred to in articles on soft soldering. These do occur, but need an electron microscope to see them, and they are usually regarded as deleterious rather than forming part of the bond.

Conclusions From what I have said it will be seen that the basic principles of a soldered or brazed joint are (1) that the bond is the result of the formation of an alloy between one or more of the constituents of the filler material and the base or parent metal. (2) That the filler penetrates the joint by capillary action. These are the important points – the fact that the filler has a lower melting point than the base is only a matter of convenience; it would be very difficult to carry out the process if the reverse held true!

These principles lead to certain consequences, the understanding of which is imperative if good joints are to be made. (Or any joint at all, for that matter.) First, as the joint is a ‘capillary’ THERE MUST BE A GAP. It is only possible to make a brazed or soldered joint without an initial gap in certain specialized applications (e.g. ‘sweating’) which I shall deal with when the time comes. Second, the gap must lie between fairly well-defined limits. Too narrow, and the alloy will not flow; too wide, and the surface tension, which causes the capillary flow, will be insufficient. These limits are fairly wide for soft (tin-lead, or tin-lead-silver) solders, but smaller for the ‘hard’ or brazing solders. Third, capillary flow cannot occur unless the alloy can ‘wet’ the surface. The filler alloys are compounded by the makers so that this will occur, but the preparation of the base surfaces is entirely in the hands of the user; cleanliness is important. Fourth, the bond results from the formation of an alloy between filler and base, and this cannot form if any oxide layer intervenes. To prevent this we must use a flux which either destroys the oxide or prevents it from forming. (You may hear of the ‘fluxless brazing’ of copper from time to time. This is a misnomer; true, no flux is ADDED to the joint, but in fact the filler alloy contains a substance, usually phosphorus, which acts as a flux once the filler is molten.) Other important matters, such as the temperature at which the joint is made, the formation of fillets and so on, all spring from these four requirements, and will be dealt with later on.

However, I think I must deal with one other point now – nothing to do with what has gone before. Both soft soldering and brazing are jointing processes which have their own special merits. Welding is not ‘better’ than either, unless the job in question is such that welding is ‘appropriate’. Despite the relatively high temperatures used in brazing (600-f700°C) thermal distortion can be much less, and the process can be applied to workpieces so delicate that welding would be impossible. Similarly, soft soldering (at temperatures around 200°C) is not necessarily inferior to brazing, and has several advantages; the joint is easily undone if need be, thermal distortion is almost entirely absent, and the cost is very low. Which process is ‘best’ depends entirely on the application – and, perhaps, convenience.

Readers should beware of being influenced too much by ‘the professionals’. Their advice is always to be welcomed, but it is only prudent to remember that what is appropriate in a commercial undertaking may not always be relevant to model making, and procedures which can pay their way in flow production plants may well prove to be hopelessly uneconomic in a jobbing workshop. Horses for courses, always!

Chapter 2

The Characteristics of Filler Metals

I use the term ‘Filler Metal’ to indicate the brazing alloy, spelter, or solder and in contrast to the ‘base’ or ‘parent’ metal which is being jointed. This is a general term and can be applied either to ‘solder’ (low melting-point material) or to brazing alloys of any sort. Where my remarks apply only to one or other of these I shall use the term ‘solder’, ‘spelter’ or ‘braze metal’ as is appropriate. I hope that this will avoid confusion.

It is possible to use pure metals as fillers, and for special reasons in the past I have often ‘soldered’ with pure tin, melting point 232°C. Pure copper (M.P. 1083°C) is used increasingly in industry for the fabrication of steel components. In fact, almost any of the ‘active’ metals can be used as jointing fillers. The almost universal use of alloys arises from several considerations. (1) Cost. Tin is very expensive, and except in a few situations the addition of a ‘padder’ material will effect an economy with no serious reduction either in strength or capillary action. (2) Application temperature. The addition of alloying elements reduces the melting temperature. We shall be dealing with this in detail shortly, but it is worth noting now that the addition of a more expensive alloying element can and often does reduce the overall cost of the joint, due to the reduced heating costs. (3) Improved flow. Quite small additions of alloying elements can effect a marked improvement in ‘capillarity’ – the cost may be somewhat increased but the joint is considerably improved. (4) Mechanical properties. Strength is but one of these. Resistance to corrosion, ability to resist higher temperatures, shock, or fatigue are also of importance. It must always be remembered that the filler metal is, within the joint, in the ‘as cast’ condition and the physical properties specified by the manufacturers apply to this condition.

This almost universal use of alloys as filler metal has very important consequences both for the designer of the joint and for the chap who is making it, and it is, unfortunately, true that many users do not fully understand the implications. At first sight it might appear reasonable to suppose that if we made an alloy from equal proportions of two metals the melting point would lie more or less midway between that of each component. This is far from being the case – indeed, for almost all alloys (certainly all used in metal joining) a proportion of the two alloy metals will be found which has a melting point below that of either. At other proportions of the two constituents you will find that there is no well defined melting point at all; the metal starts to melt, but remains ‘pasty’ until a higher temperature is reached when the whole becomes molten. In some cases this can be a nuisance, but in others (‘wiped’ joints for lead pipes, for example) is a positive advantage. The effect can best be seen from a melting-point diagram.

Eutectics and Melting Ranges. Fig. 9 shows the effective melting points of an alloy of tin and lead – soft solder, in fact. I am using this as an example because it is not untypical of most two-metal (‘binary’) alloys, and I have simplified the diagram to show only the essential points. The bottom scale shows the percent of tin in the alloy, and that at the top of the diagram – running the other way – the percentage of lead. The left-hand scale is temperature in degrees C. It will be seen that pure lead (left hand end) melts at 327°C and pure tin at the right, at 232°C. Above the line ABC the alloy is fully molten whatever the composition. Below the line (almost straight) ADBEC it is always fully solid. Between the two lines the alloy will be more or less ‘pasty’ – a mixture of solid crystals swimming in molten material.

You will notice that the two lines coincide at B. This is the only proportion of a tin-lead alloy which has a definite melting temperature, at 183°C; it melts or solidifies almost instantaneously at this temperature. This particular composition is called the EUTECTIC alloy, and consists of 61.9% tin, 38.1% lead. At all other compositions there is a melting range during which the metal is pasty. The line ABC is called the ‘LIQUIDUS’, being the temperature above which the alloy is wholly liquid, and ADBEC is the ‘SOLIDUS’, below which the metal is wholly solid. You will see that this solidus coincides with the eutectic temperature and this is characteristic of all simple alloys; the melting point of the ‘eutectic’ composition is also the ‘solidus’ for all compositions of the system.

Fig. 9Simplified melting-range diagram for tin-/lead alloys.

Fig. 10Sketch of a microscope view of 80% tin-20% lead alloy. The tin-rich crystals are surrounded by ‘eutectic’ material containing 61.9% tin.

Fig. 11A similar micrograph to Fig. 10 but for a 30% tin alloy. The dark patches are lead-rich alloy, again surrounded by the eutectic material.

Now look at the line RQ, which represents the heating of an alloy containing 80% tin. In the solid state the metal looks something like Fig. 10 under the microscope. The Christmas-tree shapes are crystals of tin-rich alloy and are surrounded by a matrix of eutectic material, 61.9% tin. There is an excess of tin in the alloy and this has separated out. As we reach the solidus temperature (DBE) the eutectic mass melts, but the tin-rich crystals stay solid. These begin to melt as the temperature rises still further until finally, when the liquidus temperature is achieved, they are all liquid. The reverse situation holds on cooling; the tin-rich parts solidify first, the mass gets less fluid as the temperature falls, until we reach the solidus line when the main body of eutectic alloy solidifies.

The line PO indicates a solder containing 30% tin. We now have an excess of lead over the eutectic composition. The solid metal would look like Fig. 11, with conglomerations of lead surrounded by the eutectic – not much of the latter, as there is so little tin present. Again, on heating this eutectic melts first and holds the lead-rich crystals in suspension as a pasty mass. Not until the liquidus line is reached will these melt.

So, in these two alloys we have a well-defined MELTING RANGE. From 183°C up to just over 200°C for the 80% tin alloy and from 183 up to 260°C for the 30% tin. This applies to all ‘noneutectic’ alloys though you will appreciate that the shape of the melting curve (it is called an ‘Equilibrium Diagram’, for what that is worth) will be different for different materials. Many eutectics occur at very small proportions of one of the constituents, down at 5% or so. When you start looking at more elaborate alloys the diagram gets very complicated indeed. Some ‘silver solders’ or brazing alloys may have three or four constituents and one, Ag18 of BS 1984, has six. The eutectic temperature of these silver-bearing brazing alloys can be as low as 600°C, or as high as 960°C, and the melting range – the width of the pasty stage – may be quite short at 10°C for Ag2 or as much as 90°C in other cases. We shall deal with these differences later, but there are some general consequences of considerable importance which we had better look at first.

Liquation This is the name given to the situation where part of the alloy is fluid and the rest solid – the pasty stage. This need