Heat Treatment of Metals: History and Objectives | Metallurgy

In this article we will discuss about:- 1. Introduction to Heat Treatment of Metals 2. History and Art of Heat Treatment of Metals 3. Process 4. Objectives.

Introduction to Heat Treatment of Metals:

Heat treatment is an endeavor to obtain the maximum efficiency of the material under the demanding conditions of service. Steel is an outstanding versatile engineering material, with which the reader will also agree because it is used in the widest variety of products. Much of the versatility of steel arises due to the fact that the properties of the steel can be controlled and changed at will (though within reasonable good limits) by heat treatment.

For example, if the steel is to be deformed into intricate shape, then, it can be made very soft and ductile by one heat treatment cycle; if on the other hand, it is supposed to resist wear, it can be heat treated to a very hard and wear resistant state by another cycle.

Metals Hand book defines heat treatment as, “A combination of heating and cooling operations, timed and applied to a metal or alloy in the solid state in a way that will produce desired properties, that is, it is an operation or combination of operations of heating and cooling of a solid metal or an alloy to endow it with certain predetermined physical and mechanical properties.”

These properties are dependent on the microstructure (or even substructure) of the alloy, i.e. the nature, shape, size, distribution and amount of these micro- constituents (it is different than the chemical composition), which are controlled by the changes in alloy composition and the heat treatment. Different heat treatment cycles bring about a very wide range of micro-constitutional variation, which result in very wide range of properties, and that is why the steel is so versatile an engineering material.

This can be explained in a way that particular steel after a particular heat treatment develops a particular microstructure which produces particular physical and mechanical properties in that steel. If the heat treatment cycle is changed, a different microstructure with ensuing different physical and mechanical shall be obtained in that steel.

If now the composition of the steel is also changed along with different heat treatments, a wide variety of microstructures could be obtained resulting to give a very wide range of properties in steels. If this had not been possible in steels, we would still be in the Bronze age.

The high purity of alloys and their correct and well-controlled heat treatment are the chief criterion in imparting properties within narrow limits to a definite specification. It is normally desired to preserve as nearly as possible the shape, dimensions and the surface of the part being heat treated. It may be remembered that all the heat treatment-operations are performed when the metal or the alloy remains in the solid state.

Though now a days, the term heat treatment is employed in a more generalised sense to any heating and cooling cycle of the metal or alloy, or a number of such operations, but a true heat treatment cycle involves a change of phase (in solid state) when heating or cooling takes place. It happens in steels very commonly. It is this aspect of steels and also in some non-ferrous alloys which makes more varied manipulations easier to control and change the properties.

For example a eutectoid steel (0.77 % carbon steel) when heated to 760°C, phase transformation takes place to obtain austenite and, when this austenite phase is cooled fast by quenching in water from this temperature, austenite transforms to martensite, phase transformation takes place again.

History and Art of Heat Treatment of Metals:

The classical alloy for heat treatment is, of course medium and high carbon steel. From the time of its discovery, steel has been regularly subjected to heat treatment of one form or another. The histories of the swordsmiths and cutlers trades make it very clear that precise method of hardening of steel, by plunging the solid red-hot steel into water (thus producing ‘martensite’), and its toughening, by tempering the quench-hardened steel at a moderate temperature, have been known empirically and used for thousands of years.

Even the antique literary book, “The Odyssey of Homer” finds description such as “Just as a smith plunges into cold water some great axe-head or edge and it hisses angrily-for that is the treatment, and the strength of iron lies in its temper…”.

The art of heat treatment of steel must have been known quite precisely to people in various parts of the world several hundreds of years ago, because the steel articles like daggers, knives and swords, etc., produced by them, could favorably be compared with the best that can be produced by using most modern methods and equipment.

There is every reason to believe that Chaucer’s ‘Scheffeld thwitel’ was hardened and tempered in a way similar to a modern carving knife by a cutler now. This spectacular early advancement of metallurgy of iron and steel and the methods of heat treatment of steels must have been due to the strategic military importance of steel in the manufacture of both offensive and defensive equipment of war through the ages.

The long experience in the heating of the steel to high temperatures, before plunging it into water must have been developed as an art of judging the correct hardening temperature from the colour of the radiations emitted by the hot solid steel. It has now been proved scientifically that a fairly accurate measurement of the temperature can be done from the colour of the radiations which appear very-dull-red just above 600°C; cherry-red at 760°C and then changes to orange, yellow, lemon yellow, yellowish white to almost white at 1500°C.

This property is shown not only by iron and steel but also, by other metals and substances. The steel smiths must have learnt the art of judging correct tempering temperature from the type of temper colours produced on a polished surface of plain carbon steel. It is well known now, that the temper colours starting from light-straw, through dark-straw, purple, dark blue, light-blue, and blackish-gray cover a range of temperatures approximately from 200°C to 375°C and are due to the interference of light with varying (increasing) thickness of transparent iron oxide layer formed on the surface.

The knowledge of temper colours and the related tempering temperature must have been a great boon to the craftsmen then (even now) because this range of tempering temperatures is ideal for tempering of many high carbon steel tools, and weapons like knives, daggers and swords giving them the requisite combination of hardness and toughness. These and other secretly developed arts of heat treatment of daggers, swords, etc. earned world-wide reputation for the craftsmen of Damascus (in Syria), India and Spain.

The development of some of the closely kept secretive art of hardening of steel have been quite often cruelly barbarous and many times unhealthy superstitious. In Middle East countries, in the middle ages, the sword makers used to harden the swords by plunging the red-hot swords in the bodies of live goats and lambs resulting in excellent combination of hardness and toughness.

This inhumanly cruel process of hardening the swords, etc., resembles the modern process of mar tempering. It is quite surprising to know that famous steel treaters of Sheffield could be unhealthily superstitious until almost the beginning of this century.

It was a practice in vogue there that a blonde was preferred for a job of an apprentice in steel-treating mills over others, as it was believed that his urine when mixed with water for quenching baths, conferred beneficial properties. Such a bath as is well known now resembles in process with a brine solution, or caustic solution. A hard-headed Yorkshireman once exported the Sheffield water, which was credited with unique properties, in bags to Japan at a fair price.

The secrets of heat treatment of steels which produced desired superior properties, were credited to have been conferred to steels by supernatural powers and were kept closed secrets, passed on from one generation to another till the mid of 19th century, when art started to develop as science. A Sauveur of USA and Portevin (France) started the work. The drawing and use of phase diagrams had started at the end of last century and in the beginning of this century.

Until the beginning of this century, it was not realised that alloys (other) than the classical alloys of steels could be substantially hardened by heat treatment. It was left to Mr. A. William of Germany, who discovered in 1906 that aluminium alloys containing copper could be hardened by quenching, and aging and this was the starting point for the development of duralumin and in-numerable other heat treatable alloys, particularly non-ferrous alloys. In 1919-20, R. Merica, Waltenberg and Scott postulated the reasons for age-hardening or tempering treatment to the formation of disperse precipitates in a supersaturated solid solution.

This work opened the way to the large scale development of age-hardenable and temper-hardening alloys, because the essential requirement of the decrease in solid solubility with the decrease in temperature is a simple and common feature of phase diagrams and thus, there exist a large number of potential heat treatable alloys. This is because once the scientific principle was known; it became a simple matter to choose from the phase diagrams, the likely alloy composition and heat treatment temperatures.

The theory of heat treatment of metals progressed well, as the different physical methods were used to ‘ investigate the nature, mechanism and kinetics of solid state transformations, correlating the structure with the properties of metals and alloys. X-rays were used as a tool to do structural analysis.

The use of transmission electron microscope since late 1950’s had resulted in a more precise understanding of sub-structural changes taking place due to heat treatment cycles, etc. The theory of defects in crystals has become the base of modern theory of heat treatment of metals and alloys.

Even pure metals can be in fact hardened by quenching and aging. Quenching a-pure metal from temperature close to its melting point leads to supersaturated solution of vacancies, which on ageing produce dislocation rings, or other defects which harden the metal by acting as barriers to the gliding dislocations. This type of hardening however is small and rather difficult to produce because of highly mobile nature of vacancies, and these vacancies exist in low concentrations.

Thus alloying of a metal becomes essential to manage easily and get large hardening effects. Alloying increases the number of phases possible in the solid state and provides sloping solid solubility lines, the eutectoid transformations (base of heat treatment of steels) in the phase diagram, and the kinetics of the solid state transformation are easily manageable. Alloys of noble metals can be hardened by the heat treatment called order-hardening in which domain structure on fine scale is produced.

In most of the methods of heat treatment of metals and alloys, the intermediate non-equilibrium precipitates, or zones of clustered solutes, or vacancies are the transitory structures. These transitory structures when formed on an extremely fine scale produce great hardening effects.

The practice in the heat treatment of metals also incorporated the advances in the theory of heat treatment. The thermo-mechanical treatments using simultaneous application of heat treatment and the deformation process to an alloy resulted in not only new hardenable alloys to obtain very high strength levels combined with toughness, but also resulted in the improvement of older practices, such as the controlled rolling, now of greatest importance to get reliable mechanical properties in steels for pipe lines, bridges etc.

The micro alloyed steels are increasingly being used for heavy duty truck frames, tractor parts, crane booms, etc. The good metallurgical control has led to the development of steels called transformation induced plasticity (TRIP) steels.

Lot of work has been done lately to improve and understand the process of heat treatment of metals and alloys, Auger electron spectroscopy, Laser hardening, Plasma carburising etc. to name a few.

Process of Heat Treatment of Metals:

Normally a heat treatment cycle, for example for steel, can be divided into three steps as:

1. Heating Steel to a Predetermined Temperature:

The first step in most of the heat treatments is the heating of steel to a predetermined temperature, generally the austenitising temperature which depends on (a) the composition of the steel, (b) the heat treatment to be given to the steel like annealing, normalising or hardening, etc. The type of heat treatment chosen for the steel depends on the properties to be developed in steel.

For plain carbon steel, the austenitising temperature can be read from the iron-iron carbide diagram. To obtain generally total transformation to austenite and homogeneity in austenite (uniform composition of austenite), the steel is to be heated to a high enough temperature for sufficient time but not too high a temperature and not too long a time, otherwise austenite becomes coarse grained to induce inferior properties in the steel.

2. Soaking:

It is holding the steel for a definite minimum time at the austenitising temperature without grain growth. Soaking time is the minimum time in which the whole section (including centre) of the part attains the desired microstructure, that is, the pre- existing phases change to fine grained and homogeneous austenite without grain growth. It is during soaking, that the phase transformation of ferrite-carbide takes place to austenite of smallest grain size and also the diffusion of carbon and other alloying element (if present) takes place to get austenite of uniform composition.

3. Cooling at a Predetermined Rate:

The cooling rate fixed for a part depends on:

(a) Heat treatment fixed for the alloy depending on the properties to be developed in the steel,

(b) Composition of the steel and

(c) The thickness of the part.

Basically, the aim is to obtain a definite microstructure throughout the section of the part which gives certain desired uniform physical and mechanical properties in it. For example, a homogeneous austenite in a 0.77% carbon steel at 760°C, when quenched In water produces martensite in micro structure with a hardness of Rc 65 [Fig. 1.1 (a)] but the same steel if cooled in furnace results in micro structure of coarse pearlite (process called annealing) having a hardness of around Rc 10 [Fig. 1.1 (b).] Spheroidizing annealing produces still lesser hardness, but more ductility.

Objectives of Heat Treatment of Metals:

Heat treatment may be done to derive one, or more of the following objectives:

1. To increase ductility and softness.

2. To increase hardness and wear resistance.

3. To increase toughness, i.e. to obtain both high tensile strength and good ductility to withstand high impact.

4. To obtain fine and proper grain size.

5. To remove internal stresses induced by cold working, or by non-uniform cooling of heated parts, or during casting or welding.

6. To improve machinability of steels.

7. To improve cutting properties of tool steels.

8. To improve specific properties such as high surface hardness with a tough core, or high temperature properties, or corrosion resistance, etc.

9. To improve electrical properties.

10. To change or, modify the magnetic properties of the steels.