Heat Treatment E-Book

Richard Lofting

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

Veröffentlichungsjahr: 2018

Beschreibung

The ability to perform heat treatments in the home workshop can be a very useful asset, enabling you to make, repair and maintain tools, to anneal and normalize work-hardened metals, and even to create decorative finishes. Heat Treatment is a practical guide to this valuable range of workshop techniques and how to employ them safely and effectively. Featuring step-by-step photography throughout, this book covers metals and their properties; building a heat treatment oven for the home workshop; case hardening, flame hardening and tempering and finally, decorative finishes with colour case hardening, oil blacking and enamelling. Metals and their properties Will be of great interest to model engineers, tool makers, car restorers and anyone with an interest in metalworking. Features step-by-step photography throughout with 291 colour photographs. Richard Lofting has over forty years' experience of performing heat treatments in the workshop and is a regular contributor to Farming Heritage magazine.Another title in the highly successful Crowood Metalworking Guides series.

Leseprobe

CROWOOD METALWORKING GUIDES

HEAT TREATMENT

RICHARD LOFTING

THE CROWOOD PRESS

First published in 2018 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2018

© The Crowood Press 2018

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of thistext may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication Data

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

ISBN 978 1 78500 442 1

Disclaimer

Safety is of the utmost importance in every aspect of metalworking. The practical workshop procedures and the tools and equipment used in metalworking are potentially dangerous. Tools should 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.

Contents

Introduction

1 METAL PROPERTIES

2 BUILDING A SMALL HEAT TREATMENT OVEN

3 HARDENING AND QUENCHING

4 TEMPERING

5 ANNEALING, NORMALIZING AND STRESS RELIEF

6 CASE HARDENING

7 WORK HARDENING

8 DECORATIVE FINISHES

Appendix

Index

Introduction

The term ‘heat treatment’ can conjure up all sorts of connotations, but in this book we are dealing with the heat treatment and its consequences to metal that we use in the home or hobby workshop. Ostensibly one piece of steel looks very much like another, but steel is an alloy of two or more elements and depending on what these elements are, and in what proportions they are added, will make all sorts of differences to the properties of that particular alloy. As most people know, steel is widely used in buildings and the world’s infrastructure, such as bridges, electricity pylons, etc. If the wrong grade of alloy was used there would be all sorts of disasters, such as bridges bending like rubber or pylons snapping off in high winds – I digress.

Heat treatment requires elevated temperatures and soaking periods, the ideal equipment is a gas-powered oven/kiln.

Obviously we are talking about materials in the home workshop, so not on so large a scale. In the world of industry there have evolved many techniques and alloys that, although very interesting, are beyond the realms of our world. For example, some hardenable steels require soaking for many hours at high temperature to fully change the crystal structure within, and then special tempering sequences including cryogenic treatments to attain the full hardness and toughness required.

In this book we will be looking at materials and techniques that can be used in our home workshops to produce useful tools, etc. by the use of simple heating sources, i.e. a propane blowtorch and the use of water, brine and oil as quenching mediums.

For those who are interested, Chapter 1 goes some way to describe the technical details as to how the steel composition affects the hardening characteristics. In simple terms, to harden steel it needs to heated to a temperature where the crystal structure changes. If allowed to cool slowly it will revert to its original soft condition, but if cooled fast enough, i.e. quenched, then the altered structure will remain, in effect freezing the crystal structure and leaving the steel as hard as glass. The only problem is that with hardness comes brittleness. The process of tempering loses some of the hardness obtained during hardening in exchange for toughness, making the tool or whatever we have made useable without breaking.

A washer being reheated after being placed in case hardening compound, before quenching in cold water.

A typical steel specification on the wrapping sheet, in this case O1 gauge plate.

Steels such as mild steel that have a low percentage of carbon as an alloying element are not hardenable as they are, but by allowing carbon to come into contact with the steel at red hot temperatures, a shallow surface region with a high enough carbon content can be produced. This thin layer can then be hardened, giving a hard-wearing surface, while the main body remains relatively soft. The process using case hardening compounds and flame hardening will be demonstrated. Another use for case hardening is to produce a decorative surface often used on guns and knives called colour case hardening and this will also be demonstrated.

Most metal when worked will work harden to a certain degree. If you continue to work the metal once it has hardened, cracks will inevitably form, rendering whatever you are making into scrap. The process of annealing and normalizing materials will be discussed and how to avoid melting items made from aluminium, which goes from solid to liquid when heated without the change in colour associated with other materials. Once annealed, further work can then be resumed on soft material.

1 Metal Properties

Although this book is about heat treatment in the home workshop in general, the vast majority of treatments will concern steel alloys and in this chapter I will try to unravel some of the mysteries of this wonderful, but very common, alloy used in myriad places and all walks of life. Since the beginning of the Iron Age when man first discovered that by heating certain rocks to a high enough temperature, probably by accident in the first place, a metal was produced that was in many ways better than what had been found before. Copper, along with its alloys of brass and bronze, as usable materials have been around a lot longer than iron and its alloys. Tools had been made from these alloys and were very successful, the biggest drawback being that they did not keep a keen edge for long. Once iron was discovered most cutting tools were made from iron and more latterly steel alloys. The only exception to this is the use of tools in an environment where sparks could cause a safety issue. Here bronze is used instead as it does not produce sparks if hit, similar to hardened steel under the right circumstances.

CRYSTAL STRUCTURE

Any metal whether ferrous, iron or steel, or non-ferrous aluminium, copper, brass, etc. has a base characteristic that is more or less predetermined by the way the atoms and molecules in the metal structure are arranged and how they are bonded together. If you alter one, say hardness, as this increases by whatever means, usually by heat or mechanical working, the ductility will decrease as a result. Ductility, along with malleability, is the property that allows the metal to be worked without breaking. Avoiding getting too bogged down in structures at atomic level, it is the metallic bond that allows the free valence electrons to be shared between atoms; this allows the metal particles to slide over one another, rather than to snap apart as in other bond structures.

A typical iron–carbon phase diagram, showing at what percentage of carbon the critical temperature is for the maximum amount of austenite.

Any piece of steel looks much the same as another, even if cut in two, although there may be a variation in the perceived colour of the metal due mostly to the alloying elements that it may contain. There is no visual indication of how the metals microstructure is formed; even with a conventional microscope there is little to see, but with an electron microscope the full internal microstructure can be seen in detail.

Whether by accident or design, it was discovered that by mixing carbon with the iron a much better material was produced. There would have been a certain amount of carbon in the early smelting furnace introduced with the coke or coal used to provide the necessary heat to melt the iron ore. The hardening characteristics were probably discovered by chance, as carbon steel in its annealed state is very much like any other kind of steel. The hardenable attributes were probably found when quenching a red hot piece of steel when making a small tool of some description.

Steel making is now a state of the art process and with modern chemical analysis on hand in the foundry, steel of almost infinite variety or quality can be produced with many alloying elements being added, either to suppress undesirable characteristics or enhance the more desirable ones.

Austenite

A metallic non-magnetic solid solution of iron and carbon above the critical temperature.

Pearlite

A layered structure of alternate layers of cementite and ferrite.

Bainite

A microstructure similar to pearlite, but forms between 250 and 550°C.

Martensite

A hard crystalline structure produced from rapidly cooling austenite.

Ferrite

A form of iron that gives it its magnetic qualities, also known as alpha iron.

Cementite

A carbide formed from iron and carbon.

Some of the common crystal formations found in steel.

THE FE–C PHASE DIAGRAM

The iron–carbon phase diagram gives a graphic illustration as to what, in simple terms, is happening to the crystal structure within the steel at certain temperatures and varying amounts of carbon. It shows that steel with 0.76 per cent carbon has the lowest critical temperature at which all the ferrite and cementite has converted to austenite; this is known as eutectoid steel, the critical temperature is 724°C, and at this point the steel will have become non-magnetic. It also illustrates that as the carbon percentage increases, and to some extent as the carbon percentage decreases, a higher temperature will be needed to reach the critical temperature at which the maximum amount of all the various crystal structures convert to austenite. Of course, if the heated steel with its austenite was left to cool slowly, as the critical temperature was passed, the austenite would transform into ferrite and cementite once again, leaving the steel in an annealed condition. Quench the steel above the critical temperature and the austenite will transform into its hard form of martensite, leaving the steel glass hard.

Quenching a red hot piece of steel in water: note the red hot steel is plunged vertically into the water to avoid distortion due to uneven cooling.

Many steel alloys require accurate temperature control during the hardening and tempering process, and long soaking times at elevated temperatures. These are impossible to reproduce in the home workshop with any degree of accuracy. We will be looking in depth at three or four types of steel suitable to harden and temper with our limited equipment to produce many items of use, either as tools or as an end product.

ALLOYING ELEMENTS

The hardenability is primarily down to alloying element carbon, but there are many other alloying elements that are added to steels and each or a combination of these various elements is what alters some of the other characteristics of the steel. For example, silver steel, which is a water-hardening steel normally designated as W1 steel, contains 1 per cent carbon, 0.4 per cent chromium, 0.35 per cent manganese and 0.3 per cent silicon. The chromium and manganese impart extra toughness and wear resistance than would otherwise be obtained with a plain carbon alloy. The silicon, although less effective than the manganese, helps the hardness. High percentages of silicon will have a detrimental effect on the surface finish, although in steels destined for the manufacture of transformers and motor armatures the inclusion of silicon helps the metal electrically.

Alloying Element

Chemical Abrv

Effects on Steel

Aluminium

Al

Controls grain size, deoxidiser.

Boron

B

Hardening agent and helps machinability

Carbon

C

Percentages above 0.6% produces a hardenable steel.

Chromium

Cr

Above 12% increased corrosion resistance hardenability strength, toughness.

Cobalt

Co

Improves strength at high temps and magnetic permeability.

Copper

Cn

Helps improve corrosion resistance.

Manganese

Mn

Increases strength at high temps improves hardenability and ductility.

Molybdenum

Mo

Increases hardenability and strength at high temps.

Nickel

Ni

Impact strength, toughness.

Silicon

Si

Increases strength and elasticity improves magnetic qualities.

Titanium

Ti

Improves strength and corrosion resistance, limits austenite grain size.

Tungsten

W

Refines grain size, increases elevated temp hardness.

Vanadium

V

Limits grain size, increases strength and hardness.

Zirconium

Zr

Increases strength and limits grain size.

Some of the common alloying elements added to steel.

A commonly used oil hardening steel such as O1 tool steel, often referred to as gauge plate, has 0.95 per cent carbon, 1.25 per cent manganese, 0.5 per cent chromium, 0.5 per cent tungsten, and 0.2 per cent vanadium. Here the tungsten and vanadium give the tool steel extra toughness throughout, but it requires quenching in oil as water would give this type of alloy a much higher thermal shock than it could stand, with the real possibility of causing it to crack due to the more rapid cooling. Most metalworkers will have a tool in their tool box stamped with the words chrome vanadium or CrMo, especially decent quality socket spanners. Do not just assume that sockets with a shiny chrome finish must be made from good quality chrome steel as the shine is achieved by the chrome plating on the surface, not from the metal beneath! As with a lot of things in life, quality almost always invariably comes at a price, and if you require tools of a certain standard then you must expect to pay for them. Having said that, socket and spanner sets are now available from several of the discount supermarket chains at reasonable prices. The tools are made to the German TUV standards and will take a fair bit of abuse, however the decorative finish may be slightly lacking compared with more professional brands.

Wood turning chisels are often made out of air-hardening steel alloy, as they can work at a fairly high temperature without loosing their temper.

The air-hardening tool steels A2 and D2 have even more alloying elements in their composition and it is these, in particular the large percentage of chromium, that imparts the air-hardening quality to the steel. It has a typical composition of 12 per cent chromium, 1.55 per cent carbon, 0.85 per cent molybdenum, 0.8 per cent vanadium, and 0.45 per cent manganese. The molybdenum and vanadium have the effect of increasing hardness and reducing the grain size. The benefit of this type of steel alloy is the fact that, along with the air-hardening qualities imparted by the chromium, it is also very good at resisting abrasion. Also, the fact that it does not de-temper easily makes it good for tools that are used dry without coolant, such as wood-engaging tools. Wood turning chisels, for example, have to contend with being used dry against fast revolving wood, some of which can be very abrasive indeed. This produces heat as well as interrupted cuts if the wood is not round, so the tool needs to be made of the right grade of steel to do its job.

The downside of such an alloy in the home workshop is that if you need to anneal it to a soft workable state, so you can work the steel or machine it to shape, then long soaking periods at high temperatures are required with equally long cooling periods with specific drops in temperature such as 20°C per hour until cool enough to handle. Unfortunately most of us will not have a temperature-controlled oven capable of this kind of accuracy in our workshops. The best we could achieve with an alloy such as this would be to purchase the steel in its annealed state so it could then be machined to the profile required, and then only when any machining and shaping has been completed harden it to a usable condition and leave it at that except for final sharpening once in a hard condition.

From our home workshop viewpoint, the more exotic the alloy the more difficult it is to produce the correct hardening and tempering regime to get the best out of the material. Some of the more exotic alloys have been produced for specialist industries where the tools made are used under arduous conditions, such as in CNC production facilities for example, where downtime costs a lot of money through lost production while a tool is changed. In our world, out in the shed (workshop), does it really matter that once in a while we will have to stop what we are doing and re-sharpen a tool? It may be that we have produced a tool that is required for a one-off job and never used again, or maybe perhaps for two or three jobs a year. In the home workshop environment, stopping to sharpen the tool occasionally is not too much of a chore, as most if not all will stop anyway for the occasional cup of tea or coffee on a regular basis. From the machine’s point of view, be it lathe or milling machine, etc., an occasional stop will allow the relatively small motor to cool down. Of course, if you have a three-phase supply to the workshop this will not be such a problem.

Sharpening a lathe tool on an off-hand grinder.

CARBIDES

Although carbides cannot be hardened and tempered in the same way as steel alloys, and are not machinable in the normal sense, it is worth describing them here so that we can form a complete picture of materials that we can use in the workshop. A carbide is a compound of carbon and usually a metal such as tungsten, with approximately a 50:50 mix of each element. The bonding structure of the carbide molecules at an atomic level assures a very hard material, and in the case of tungsten carbide (WC), the only thing that is harder that can be used to cut or sharpen is a diamond. The fine carbide powder can be shaped under pressure and heat to form a cutting tip. The benefits of this is that a tool holder can be made in a less expensive grade of steel with a machined pocket that will fit one of the predefined carbide tips available on the market. Although these tips may seem a bit expensive at £2 to £3 each, some of the more common ones used in industry may be cheaper, especially if purchased in quantity. Most have two or more cutting faces so that when one is worn out the tip simply needs to be turned around and you are ready to work again, so in effect you get two or three for the price of one. The benefit of using the replaceable tips is that they are very hard and wear resistant and can withstand heat during use without detriment to the cutting ability as they do not soften. This allows use without cooling and faster turning speeds, but the use of a cutting fluid might well extend the tips’ useful life and improve the finish, especially in a production environment. Another benefit is the fact that the tips are what are described as ‘indexed’, this allows the new tip or the original one to be turned around, to be placed in exactly the same position on the tool shank as the previous one. This avoids having to reset the tool’s position, as you would if using a conventional tool that had to be removed completely from the machine every time it was sharpened.

A selection of different types of tungsten carbide tool tips for various machining operations on the lathe.

Various tipped tools, left: boring tool, top: thread cutting tool, centre: internal thread cutting tool, bottom: a box of CCMT tool tips.

To sharpen tungsten carbide, a green grit wheel is required to cut the hard material.

For a superior cutting edge, a fine diamond sharpening card will give the ultimate finish on carbide tooling.

One of the downsides of using carbide is the fact that it is relatively brittle and under impact loads such as interrupted cuts when turning in a lathe, for example, the tip is easily chipped. In this situation the best material for a cutting tool would be HSS or carbon alloy steel. For the thrifty and inventive carbides can and are easily brazed, as described in the book Brazing and Soldering by Richard Lofting, so the worn or chipped carbide tips can be brazed on to a steel shank to make your own specialized cutting tools, as long as you have the correct grade of grinding wheel to reshape and sharpen the carbide.

STEEL CLASSIFICATION

There are several differing sets of standards that apply to the classification of steel and any other type of metal. Many of the older standards have been superceded over the years, but they still seem to be used along with the newer systems. An example is the old, now redundant, EN system where EN stands for Emergency Number. This system was set up as a stopgap classification in 1941, during the Second World War; it was a simple system with numbers starting at EN1 representing mild steel through to EN58 for a high alloy stainless steel. It was soon discovered that, with many new alloys being produced, this system was inadequate and suffix letters were added, e.g. EN24T, where the T suffix indicates that the steel is in one of its hardened forms. Following on from this, it was realized that steels produced with similar alloying elements and properties could not be listed within the group of closely related steels, as there were not any spare numbers left in the system to use.

Worldwide it seems that every continent has its own classification system. In the USA there is the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE). In Europe, apart from the UK with its BS (British Standard) numbers, there are the German Deutsches Institut Normag (DIN/ Werkstoff) numbers, the French AFNOR and the modern EN numbering system. While Japan has its own JIS system of classification. China has also its own GB classification system. As already mentioned, the old EN classification numbers are often quoted in Britain, as it is probably a lot easier to remember ‘EN19’ rather than its close equivalent of DIN:42CrMo4 or W-Nr:1.7225. Just to add a bit more confusion to the mix of standards, here in the UK we have our own British Standards (BS) systems as already mentioned above, but there are the old and new standards to contend with, and currently as members of the European Union we are obliged to use its standards mentioned above as well.

Cold-rolled and bright steel is usually identified by the lack of surface scale.

Hot rolled steel is usually identified by the black scale produced during the surface oxidising while being at red hot temperature.

The same grade of steel can be purchased in different forms; for example mild steel can be bought as hot rolled, cold rolled or bright, which is ostensibly the same steel alloy, but having gone through a differing process in its production; depending what you subsequently want to do with the material will have a bearing on what condition you purchase it in. If the finish is important then you may well buy the steel in bright condition, but on the other hand you may well be bending up brackets, in which case it may be better to purchase the steel as hot rolled as it will be easier to work with. If you were to do the same with bright or cold rolled steel it would be worth annealing it before use. The same would be true if you wanted to use hot rolled steel in a bright condition. The surface scale would need to be removed before use, although the result would be somewhat softer than steel purchased in a bright condition as this would be to some extent in a work-hardened condition as it would have been cold rolled.

STAINLESS STEEL

Stainless steel has a minimum of 10.5 per cent chromium and it is the chromium that gives it its stainless qualities. The chromium constituent in the alloy creates a chromium oxide layer at the surface when exposed to the air, preventing further oxidizing (rust). This is very similar to the way aluminium produces an oxide layer at the metal’s surface, and it is this that stops further oxidization occurring as the thin oxide layer forms a barrier to further corrosion. In fact, on the Continent stainless steel is known as Inox, from the French word inoxydable. In environments with low oxygen levels stainless steel will indeed tend to corrode somewhat, as the lack of oxygen will inhibit the development of the chromium oxide layer, thus losing or preventing the development of its protective properties.

Drills containing cobalt will withstand drilling into work-hardening materials such as stainless steel much better than standard high-speed steel drill bits.

Cobalt drill bits will be marked with HSCO on the shank rather than the HSS of standard drill bits.

Stainless steel, although having many of the same constituents as normal steel, is a totally different animal when it comes to machining and treatments. It tends to work harden easily and galls readily; in fact it is known that in a workshop environment it is a waste of time trying to drill holes in the material with anything less than cobalt steel drill bits. This is because even before the first hole has been drilled right through, the localized work hardening produced by the drilling action will have blunted an ordinary carbon or HSS drill bit; the cobalt makes the drill much harder and tougher. The normal procedure when drilling through a piece of steel is to occasionally lift the drill bit from the work to clear the swarf being produced. In a deep hole this helps accuracy, but do not do this when drilling stainless steel as each time you lift the drill from cutting you will produce a work-hardened zone that has to be cut through when the drill bit is re-engaged with the work. When drilling stainless steel use a slower speed than you would normally and keep a steady pressure on the drill bit all the way through the drilling operation. And when using stainless steel fastenings, such as nuts and bolts, it is a good idea to use something such as copper grease on assembly as often when undoing them at a later date the galling will destroy the thread as the metal picks up on the thread or the nut. The copper grease acts in this case as a lubricant rather than a corrosion inhibitor, as the stainless steel will inhibit the corrosion itself.

There are three main classes of stainless steel and they are classified by the way the alloying elements affect the crystal structure within the metal itself. Each type lends itself to particular applications, whether it is corrosion resistance, mechanical properties and machinability or appearance, or a combination of all.

Austenitic

Austenitic stainless steel is classed in the 200 and 300 series of alloys, the most common being 304 (A2) and 316 (A4) alloys, and they are used in decorative applications where a resistance to corrosion is required. This type of stainless steel is most often used for fastenings where its corrosion resistance is an advantage, such as nuts and bolts and, of course, motor vehicle exhaust systems where it not only resists corrosion from within, but also from the salt-laden roads in winter. Austenitic stainless steel cannot be hardened in the same way that carbon steels can, but it can be work hardened.

A2 stainless steel fasteners are commonplace and cheap. Used in non-critical places they will not corrode as ordinary steel ones will.

All the information you need is generally stamped into the head of the bolt, in this case a socket cap screw.

Martensitic

Martensitic stainless steel alloys contain lower amounts of chromium and therefore are less corrosion-resistant than austenitic alloy stainless steels. However, the advantages of these alloys are that they are hardenable and can be tempered just like any other carbon steel. They are used extensively in the medical profession, making excellent surgical instruments and scalpel blades as their resistance to corrosion is a distinct advantage. The slightest corrosion on a blade could harbour germs and bacteria, which would be detrimental to the outcome of an otherwise successful medical operation.

Ferritic

Ferritic stainless steel is cheaper than either austenitic or martensitic types, and has the lowest resistance of all three forms to corrosion. However, it is an excellent engineering material, has high strength and toughness and is resistant to work hardening, but it still has a good surface finish and is to a degree corrosion-resistant, far better than ordinary steels. There are in fact five sub classes within the ferritic range; the second group, the 403 types, are the most used, producing many household items such as kitchen sinks and food preparation surfaces. They are also used in the automotive industry where high corrosion resistance is not strictly necessary.

I turned this sleeve on the left believing it to be W1 steel as I wanted to harden and then temper it. The piece on the right is silver steel.

I later found that a magnet was not attracted to it cold, so it must have been stainless steel.

The martensitic stainless steels are the only ones that have any interest to us as they hardenable, usually by a precipitation process, and can be tempered to a degree. These attributes are ideal for making surgical instruments; the last thing you would need during an operation would be a rusty scalpel.