Brazing and Soldering E-Book

Richard Lofting

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 237

Veröffentlichungsjahr: 2014

Beschreibung

Brazing and soldering are essential metal joining techniques for a range of jobs in the workshop. This new practical guide will introduce you to the methods and show you how to enhance these skills safely and effectively, with step-by-step photography throughout. Included in this new book is advice on equipment and building a brazing hearth; an overview of alloys and fluxes; lead loading, body and electrical soldering and pickling and cleaning. The main brazing and soldering types are also covered including soft soldering, silver soldering and brazing. There is also a range of case studies to show practical technique applications. Fully illustrated with 298 colour photographs and 2 tables.

Leseprobe

  C R O W O O D   M E T A L W O R K I N G   G U I D E S  

BRAZING ANDSOLDERING

RICHARD LOFTING

First published in 2014 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2014

© The Crowood Press Ltd 2014

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN 978 1 84797 837 0

Disclaimer

Safety is of the utmost importance in every aspect of the workshop. The practical procedures and the tools and equipment used in engineering workshops are potentially dangerous. Tools should be used in strict accordance with the manufacturer’s recommended procedures and current health and safety regulations. The author and publisher cannot accept responsibility for any accident or injury caused by following the advice given in this book.

Acknowledgements

I would like to thank my family for their help in writing this book, in particular my son William Lofting and my niece Bethany Old, who posed for some of the photographs, my wife’s aunt, Audrey Peters, for reading through the text, and of course Pam, my wife, for the endless cups of tea. Thank you.

All photographs by Richard Lofting.

Contents

Introduction

1  Alloys and Fluxes

2  Soldering Irons and Blowlamps

3  Designing and Building a Brazing Hearth

4  Brazing

5  Silver Soldering

6  Preparation, Cleaning and Pickling

7  Soft Soldering

8  Lead Loading, or Body Solder

9  Electrical Soldering

Useful Addresses

Index

All forms of soldering or brazing require the use of heat; as long as the correct precautions are taken, these tasks can be done safely.

Introduction

Brazing and soldering are essentially the same joining technique, the difference between the two being the temperature at which each method is performed. In essence, a joint is made in metal using an alloy of two or more metals to, in effect, hot-glue the parts together. The word ‘glue’ here does not refer to just sticking something together with a sticky substance, as the items to be joined will be bonded at a molecular level to the alloy, which imparts considerable strength to the joint. The weakness will be in the strength of the brazing or soldering alloy, although the strength of some alloys that are used for filler rods are very near to the strength of the materials being joined.

In general terms, the hotter the melting temperature of the alloy, the stronger the joint. The weakest joints are made with soft solder, where the soldering alloys melt at temperatures below 450°C; in fact, some alloys will melt at extremely low temperatures and even in hot water, but these are beyond the remit of this book as they are not used for soldering. Historically, most soft solders were predominantly made from lead. However, the lead content has lately been substituted for other alloying metals, such as tin due to health concerns regarding the use of lead and its poisonous effect on the human body and of course the environment.

The middle ground is held by hard soldering, or silver soldering, where traditionally the soldering alloy contains a significant proportion of silver; details of the actual alloy contents will be given in a later chapter. Hard/silver soldering covers a temperature range of approximately 450°C through to around 850°C. As can be seen, this is already significantly higher than soft soldering, with joints effectively being made with a blow-lamp as these temperatures are beyond a soldering iron’s capability.

At the top of the temperature range is brazing, with a heat range of approximately 800°C through to 1,000°C and sometimes higher. In this process, the alloying components in the filler rod are mainly copper, tin and zinc; copper and tin form the alloy bronze, while copper and zinc form the alloy of brass, both very common alloys in everyday use. While these temperatures may seem hot, they are still a fair way below welding temperatures, which are required to melt and then fuse the component parts together; typically for mild steel this is around 1,300°C.

In reality, there is no division between silver soldering and brazing – you cannot say that on one side of a temperature line it is silver soldering and the other side of the line it is brazing. All the preparation is the same, from thorough cleaning through to heating; the only difference is in the alloys being used to complete the joint. The technique of silver soldering and brazing covers an enormous field in modern engineering, as it enables the joining of dissimilar metals. For example, it is possible to braze hard tungsten carbide tips into a steel circular saw disc or lathe tool, thus giving the best of both worlds – the tips are hard and so retain their cutting edge for longer, but the centre disc or tool holder is made from steel, giving the flexibility to stand the forces produced while cutting. With the relatively new important materials like ceramics entering the field of engineering, new ways to join these have had to be devised. Originally, they were given a metallic coating to which brazing alloys would adhere, but by accident it was discovered that with an active element in the alloy, usually titanium, used in the right atmosphere, ceramic substances could be brazed directly.

With the increasing temperature range that has just been shown, so the relative strength of the joint correspondingly increases with it. This is due, mainly, to the alloying metals used in the brazing rods or sticks of solder, as stronger materials generally have a higher melting temperature than the weaker, or softer, ones. This is the derivation of the terms ‘hard’ and ‘soft’ solder. The soft solders contain lead and tin, while the hard solders contain harder metals in the alloys such as silver. Once past the hard soldering metals and into brazing, the metals used in the alloys contain brass, which in itself contains copper and zinc. At the top of the temperature and strength range alloys using nickel are used. An in-depth overview of soldering and brazing alloys will be looked at in Chapter 1, along with the fluxes that are available and their attributes.

A soft-soldered lap joint. Soft soldering is performed at the lowest of the temperature range, but will give a moderately strong and air-tight joint.

Brazing is the strongest joining technique, without melting the parent metal; this allows dissimilar metals to be joined.

Particularly with hard soldering and brazing, where a brazing torch/blowlamp is to be used, this must be carried out in a safe environment, away from inflammables. The hotter the temperature used for the process, the more oxides are produced from not only the parent metal but also the alloy bearing rods. Chapter 3 is dedicated to building a brazing hearth. Although this sounds rather grandiose, it is nothing more than a frame made from angle iron or sheet steel with the working area lined with commercially available firebricks.

When heating metals, or anything else for that matter, the increase in temperature has the effect of increasing the production of surface oxidation, so in all forms of soldering and brazing a flux is required. In general terms, a flux keeps the metal from oxidizing during heating and removes any that has already formed, from both the item being joined and the soldering/brazing rod. The exception is specialist brazing rods that are available for joining copper to copper, which are in effect self-fluxing due to their phosphorous content.

The technique of lead loading automotive body panels with lead solder has now been, mainly, superseded by the use of plastic fillers, although there is a niche in the vintage and classic car world where originality is important. Plastic fillers are very good, being easy to mix and apply and then contour to the desired shape, but they are not as permanent as lead loading. A higher skill level is required with lead loading, as the vagaries of the alloy are exploited during the application process, spreading the lead alloy like butter with wooden paddles, unlike polyester fillers, which are mixed with a hardener; the only skill here is to get the mix on to the panel before it sets. Once again, the poisoning effect of the lead content rears its ugly head. The finish cannot be done with power tools, as this will fill the air with lead particles, but is done by hand with body files and abrasives.

At the end of the book, Chapter 9 is dedicated to electrical soldering. Although it is basically just soft soldering, there are several criteria that need to be considered to produce a satisfactory electrical joint.

Alloy rods and fluxes required for soldering and brazing operations.

1  Alloys and Fluxes

The three categories of soldering and brazing are:

◆  soft soldering, which is mainly carried out with a soldering iron and low temperature alloys

◆  silver soldering, also known as hard soldering, which is performed in a brazing hearth using alloys containing silver

◆  brazing, which is also performed in a brazing hearth, but using higher temperatures and higher temperature melting alloys, producing the strongest joints.

SOLDER AND BRAZING RODS

To help understand the process of soldering it will be advantageous to look at the composition of the alloys used in the rods for each process. First, the basic question needs to be answered: what is solder? Traditionally, soft solder was mainly based on alloys predominantly made of lead, but with the increasing awareness of the poisonous effects of lead on persons and the environment, lead solders are no longer used in plumbing. In addition, lead-based electrical solders are now virtually banned worldwide due to the possibility of the lead content leaching into water supplies from landfill rubbish sites, as a result of the ever increasing amount of discarded electronic equipment. European Union directives are now in place to prohibit the use of lead in various industrial processes and the construction industry, with it being replaced by other non- or less poisonous metals such as tin, copper and silver as the main alloy. Apart from the attributes that lead gave to solder alloys, such as good wettability, it was a cheap and readily available commodity, whereas the alternatives are relatively more expensive to source and extract from the ore containing the metal.

Another question is why use alloys for the filler material instead of a pure metal? There are several reasons. A pure metal melts at a higher temperature than an alloy containing the same metal and by combining two or more metals the attributes from each can be taken advantage of in the alloy. Also, varying the mixture will vary the melting temperature and other factors such as wettability or the tensile strength of the resultant solder alloy.

Soft solder is available as sticks or wire on a reel; most soft solders are predominantly non-lead based today, due to environmental concerns.

As already seen in the introduction, soldering has been neatly divided up into three sections, namely soft soldering, silver (or hard) soldering and brazing. These are arbitrary boundaries that have been historically developed over the years. As far as the alloys used for all three categories are concerned, there are no delineating boundaries, as the only difference between any of the alloys used for soldering or brazing is the temperature at which they melt and consequently can be used at, and their service life. It will be explained later on in this chapter that most alloys have two temperatures given in their specification, unless they are a eutectic alloy and therefore have a single melting point.

Silver solder comes as wire or rods depending on the diameter being used and for what purpose; thin foils are also available for specific purposes.

Brazing rods are very much like silver soldering rods, the difference being the melting temperature of the alloy they contain and that they are usually brassy coloured.

NAME THAT ALLOY

There are various ways that manufacturers of soldering and brazing rods describe their particular soldering and brazing alloys, sometimes using the main alloying metal’s percentage with a range description, for example Silver-flo 40. This alloy, which is from the Johnson Matthey range, has a silver content of 40 per cent, copper 30 per cent, zinc 28 per cent with 2 per cent tin, a melting range of 650–710°C and a tensile/shear strength of 450/155 N/mm2. This particular alloy mix meets the widely recognized international standards and is variously known as AG20, L-Ag 40 Sn and AG105, depending upon whether you are looking at the BS1845, DIN 8513 or the EN 1044 standards. Or the alloy’s name may contain the temperature as part of its description, for example Indalloy 281, this being a eutectic alloy having a single melting temperature of 281°F or 138°C. The use of the Fahrenheit temperature scale in the description usually means that the manufacturer is from America, where the Fahrenheit scale is standard. Where a manufacturer produces a range of alloys in a series it makes it relatively easy to pick various alloys from the range so that soldering in a sequence or step soldering can be performed in confidence, without disturbing the previously made joints. This very useful technique will be covered in detail in a later chapter.

THE DEFINITION OF AN ALLOY

The term ‘alloy’ describes a mixture of two or more pure metals. The most common feature of an alloy is that its melting temperature is always lower than any of its constituent parts. Solders use the differing alloying contents mainly to determine the melting characteristics of the solder. Alloys are not confined to soldering and brazing, but have many uses, such as brass made from copper and zinc, bronze made from copper and tin, and of course the world would be a different place without the steel alloys that are commonly used. Alloys have many uses in producing properties in materials, such as hardness and wear-resistance, which the parent materials do not possess.

ALLOY MELTING TEMPERATURE

A feature of an alloy is that it does not usually go directly from a solid to a liquid, as does a pure element such as copper, although there are exceptions to this. Two terms are used in defining the melting and solidifying of an alloy – these are the liquidus and solidus points.

Liquidus

The liquidus point in an alloy describes where the whole alloy is deemed to be a homogenous liquid without any remaining solid crystals from any of the constituencies of the alloy. This is the upper temperature quoted for a particular alloy. For example, a 5 per cent silver (Ag), 95 per cent tin (Sn) soft solder is quoted in data charts as having a melting temperature range of 221–235°C; in this case, the liquidus point is 235°C. As the liquid alloy cools down, the liquidus point marks where crystals once again start to form within the liquid alloy, with the amount of crystals increasing as the temperature falls. Once cooled to the solidus point, the alloy is once again a solid.

Solidus

The solidus point is the start of a range at which an alloy will melt, as different crystals within the alloy will melt at differing temperatures. As in the example above, the lower figure of 221°C given in the range for that particular soldering alloy is the solidus point. The alloy will not be completely melted until the liquidus point is reached; this is when the whole alloy is a homogenous liquid.

The difference between the liquidus and solidus points can be small or relatively large, depending on the alloying materials. Within the range of the two points, the alloy will become a mush, not unlike melting snow, and is neither completely solid nor a liquid. Normally during soldering and brazing operations it is nothing more than an inconvenience, but movement of items being soldered during this phase will possibly result in a dry or porous joint. However, as will be seen later in Chapter 8, this mushiness can be used to our advantage. For example, when lead loading an automotive body panel with body solder, if the alloy is kept between the solidus and liquidus points, it can be shaped and contoured, not unlike plastic. But of course should a little bit more heat be applied, so that the liquidus point is reached or superseded, the body solder will be all over the floor!

Liquation

As stated above, generally the solidus and liquidus range is nothing more than an inconvenience while soldering. However, with some alloys that have a large temperature range and particular alloying elements a problem called liquation can occur. This is where on reaching the solidus temperature, as the alloy begins to melt, the prolonged heating cycle can cause the alloying elements to separate. The ones with the lower melting temperature melt first, leaving crystals of the remaining alloys behind. Once this has occurred, the only effective remedy is to allow everything to cool and then clean off the separated constituents of the alloy, and the inevitable surface oxides, and start all over again. In order to prevent liquation, the best solution is to complete the joint as quickly as possible by rapid heating of the items being soldered or brazed through the solidus and liquidus points. Apart from the unsightly appearance of a joint where liquation has occurred, the resultant joint will possibly be brittle and porous and thus unreliable in service.

Eutectic Alloys

The exception to an alloy with both a solidus and liquidus point is a eutectic alloy. This type of alloy has a single melting point, with the solidus and liquidus points being effectively one and the same, as in a pure element such as iron or copper.

Although not connected to soldering, it is of interest to note that low-temperature eutectic alloys are used in the fusible plug within automatic fire-prevention sprinklers, whereby an alloy with a melting temperature of below 100°C melts in the heat of an emerging fire and so releases water from the sprinkler. One such alloy, named Wood’s Alloy after the American physicist Robert W. Wood, melts at 70°C. It contains 50 per cent bismuth (Bi), 26.7 per cent lead (Pb), 13.3 per cent tin (Sn) and 10 per cent cadmium (Cd), although it is detrimental to health as it contains lead and cadmium. Another safer eutectic alloy is Field’s Alloy, named after scientist Simon Quellen Field, which has a melting temperature of just 62°C and is an alloy of 32.5 per cent bismuth (Bi), 51 per cent indium (In) and 16.5 per cent tin (Sn).

ALLOY SELECTION

All sorts of base metals are used in solder and brazing alloys, with the main ones being shown in the table. Some relatively rare or exotic metals are used in small amounts; these give the alloy certain advantageous qualities, such as wettability of the solder, which means that the solder will flow on to the item being soldered with ease.

The Internet will reveal a huge range of silver soldering and brazing alloys, with some interesting proprietary names, such as Silvaloy 5, Matti-sil, Sil-Fos 56, Argo-braze 40N and so on. Often a clue to the main alloy in the rod is in the name, for example Silvaloy; here, the main alloying component is silver. However, this is not always the case; therefore, before undertaking any serious silver soldering or brazing it is a good idea to have a look at the manufacturers’ data sheets, which are freely available online and elsewhere. This will allow an informed decision to be made on the suitability of the rods to be used for a specific job where exacting standards are required.

Most manufacturers have a range, or several ranges, of rods with specific alloy content, but, by varying the amounts, small changes in the alloy mix can have profound effects on the alloy’s abilities, such as wettability, flow characteristics and gap filling. The other main effect is that of changing the solidus and liquidus temperatures, which may be critical for the metal being joined, or to the service to which the soldered or brazed joint is to be put.

Alloying Metals

When selecting a suitable alloy filler rod, there is more to it than just finding one that melts in the correct temperature range. Other factors come in to play, such as the suitability of the alloying elements within the rod. For example, it is no good using a solder containing lead when soldering is undertaken connecting the gold wires on an integrated circuit chip to the external connections on the case, because the lead component will leach away the gold from the delicate wires. While this may be a somewhat rarefied example, a more common case would be the need for correct rod selection when joining stainless steel fittings where these are going to be in contact with, in particular, salt water. If the right rods are not used, what is known as crevice or interfacial corrosion will occur between the stainless steel and alloy interface, causing possible porosity or weakness due to one of the alloying metals within the alloy being attacked by the salt.

When deciding what alloy composition of the filler rod to choose for the task in hand, be it soft soldering, silver soldering or brazing, it must be noted that all alloys weaken before signs of melting occur. This has implications in the serviceability of a joint, especially if the joint is subject to heat under working conditions. Most alloy data sheets will give the service temperature limits of individual alloys.

Most silver solders are now produced to the ISO 17672 standard, with fresh virgin materials being used rather than reclaimed materials so as to meet this standard. It is particularly important when building a pressure vessel, such as a steam boiler by silver soldering, for example, that solder made to this standard is used along with the correct grade of copper. This will avoid any possibility of intermetallic compounds forming that will cause embrittlement of the silver soldered joint and the surrounding area, by alloying with any metals such as aluminium and titanium contained within the silver solder. Failure of the boiler joints and seams in this example would have huge safety implications to not only you, but to any bystanders in the vicinity, rather than be just a disappointment. The usual course of events during boiler construction is to employ a hydraulic test; this entails filling the boiler completely with cold water, then pressurizing it to above the working pressure at which the boiler is designed to work. This will indicate any leaks within the boiler, but, more importantly, will not cause a catastrophe, should one of the seams fail. Water, being virtually incompressible, will lose pressure immediately upon a seam failure. If air or steam were to be used, being compressible a lot more expanding energy would lie behind the failing seam and therefore be a lot more dangerous.

For anything other than general work, data sheets are available from the manufacturers of soldering and brazing alloys. These detail the constituents and characteristics of the alloys and, importantly, their melting temperatures.

Size Matters

One factor that needs consideration, especially with silver soldering and brazing work, is selection of the correct size of filler rod diameter. As already stated, the rods come in all shapes and sizes and indeed some have specific specialist uses. As most of the filler is drawn into the joint, from this perspective the rod size will have little effect on the finished joint, which will take so much filling alloy and that will be that. As a general rule, when working with thin materials such as sheet work, the smaller sized rods or even wire are ideal; if a thicker rod were to be used it would take more heat from the items being heated to melt the alloy, causing a pause in the alloy flow and resulting in a patchy joint. If larger sections are being joined, which in themselves will have more stored heat in them, this is the time for using thicker rods. If thinner rods or wire were used on the larger job, while the joint would be successfully completed, it would take a lot of rods or wire and, with the thinner section rods being more expensive pro rata, the cost of the job will escalate. In an ideal world, we could stock up on the common grades used and in all diameters, but this would also be somewhat expensive.

Different sized jobs require different rod sizes.

If buying rods for a specific job and there are a few rods left over, label them so that at a later date you will know what the alloy content of the rods is, as one rod looks much the same as the next. The only discernible feature between various rods might be a slight difference in colour; for general work this may not matter, but for use in the construction of pressure vessels it may be critical to a safe job.

Solder and Brazing Rod Storage

The above statement leads nicely into rod storage. The best storage for solder and brazing rods is a dry, warm place away from the dirt in the workshop; plain rods will not be affected by moisture as the flux-coated ones will be, but any moisture on the rods will allow the general detritus, grinding dust and so on floating around the work-shop to stick to the rods. This will not hurt the rods themselves, but could lead to contamination of the joints made using them. The flux coating on the flux-coated rods will deteriorate if allowed to remain in a damp atmosphere for any length of time. If buying in bulk, the rods will usually come in a protective plastic tube and if the lid is kept on this will keep most contaminants at bay during storage. If purchasing smaller amounts, storage in a plastic container would be ideal.

At a minimum, store your rods in a bag to keep off contaminants and label them so that you know the composition of the rods for future use. Once the data is lost and the rods muddled up, it will just be guesswork.

An ideal way to store your brazing/silver soldering rods is in a stout tube to keep them from contaminants, also, keep the lid on when not in use.

HEALTH CONCERNS

As has already been noted, lead is poisonous to the human body and the environment, a fact that is now well known. Cadmium has traditionally been used in silver solders as it imparts very good flow characteristics to a silver solder alloy, making it easy to use. The downside is that as an alloy containing even small quantities of cadmium will give off dangerous fumes as it is heated. There is a vast range of alloys now available that do not contain cadmium and it would be sensible to use an alloy from these ranges so as to avoid the risks from cadmium fumes. If it is found that an alloy has to be used that contains cadmium, then all the correct fume extraction should be in place before attempting to use these rods. If using rods of an unknown source and the exact alloy content cannot be verified, it would be prudent to assume that they contain cadmium and take the correct precautions.

SILVER SOLDERING

One of the problems associated with silver soldering and indeed brazing, is applying the filler rod before the parts to be joined are hot enough. The joint may look to be at the right temperature, that is, glowing red hot, but it may have not quite reached the melting temperature of the alloy being used. As the rod is advanced into the joint it melts in the heat of the flame from the blow-lamp, not from the heat of the parts being joined. Providing more heat is applied until the silver solder is seen to be drawn into the joint by capillary action, there will be no problem. However, if the alloy is left sitting on the top surface, this will achieve nothing other than an ugly mess.

Do not use the heat from the flame to melt the rod, as this will inevitably lead to failed joints.

Use the heat in the items being joined to melt the rod, as this will ensure penetration by capillary action, as the alloy is drawn into the joint rather than being melted on top.