Heat Treatment

Richard Lofting

• Editore: Crowood
• Anno: 2018
• ISBN:
  9781785004421
• Argomento: Tecnologia e ingegneria
• Categoria: Commercio tecnico e manifatturiero

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.

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.
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