INTRODUCTION

DEFINITION:

         Heat treatment refers to an operation or combination of operations, involving heating and cooling to alter the properties of metals, alloys plastic and ceramic materials.

          Changes in material's properties result from changes made in micro-structure of the material. During heat-treatment of a suitable rate, there occur changes in the micro-constituents of the metal. These changes in the micro-constituents of the metal may be in their nature, form, size and distribution in the metal piece. Obviously, temperature of heating and rate of cooling are the main controlling factors of changes in micro-constituents. These changes in micro-constituents then control the changes in physical and mechanical properties of heat treated metal specimen. Heat treatment can be applied to ingots, castings, semi-finished products, welded joints and various elements of machines and instruments. For various fabrication and manufacturing operations, heat treatment is a very important process.

OBJECTIVES:

The purpose of heat treatment is to achieve any one or more objectives as follows:

1. To remove gases from castings, to soften a metal to improve its machinability, and to increase the resistance to wear, heat and corrosion.
2. To remove strain hardening of a cold worked metal and to improve its ductility.
3. To relieve internal stresses set up during cold-working, casting, welding and hot-working treatments.
4. To improve electric properties.
5. To improve the cutting ability.
6. To improve magnetisation property, especially of steels, for producing permanent magnets.
7. To change or refine grain structure after hot working a metal.
8. To soften and toughen a high carbon steel piece.
9. To produce a single phase alloy in stainless steel and to produce a hard, wear resistant case on a tough core of a steel part.
10. To harden non-ferrous metals and alloys, especially aluminium alloys and to produce a single phase alloy in stainless steel.
11. To produce a hard, wear resistant case on a tough core of a steel plant and to toughen a hardened steel piece at the cost of its hardness.

Object of Heat Treatment

Object of Heat Treatment:

        The Metals and alloys are heat treated to improve their mechanical properties, to relieve stresses (or) to improve their machinability. The properties of carbon steels can also be altered significantly by subjecting them to heat treatment processes. Heat treatment consists of three basic steps:

(i) Heat the metal/alloy to a predetermined temperature. This temperature will, ideally depends upon the actual composition of carbon steel (i.e., carbon percentage),

(ii) Soaking or holding the metal/alloy at that temperature for some time, so that the temperature across the entire cross-section becomes uniform, and

(iii) Cooling the metal/alloy at a predetermined rate in a suitable medium like water, oil or air. The rate of cooling is the most important factor.

MICRO-Constituents of iron & steel

MICRO-Constituents of Iron & Steel:

          We can observe the various constituents of Fe and Steel produced due to decomposition of austenite under a microscope. The following are the important micro-constituents of Iron and Steel:

     1. Ferrite

     2. Cementite

     3. Pearlite

     4. Bianite

     5. Martensite

     6. Austenite

     7. Troosite, and

     8. Sorbite

Ferrite

i. Ferrite:

          Iron which contains little or no carbon is called Ferrite. This is a soft and ductile phase and is known as alpha iron generally by metallurgists. Ferrite is the name given to pure iron crystals. Ferrite is present to some extent in a great range of steels, particularly those low in carbon content, and it is also present, in soft cast iron. Ferrite has the following mechanical properties: Its KCU = 2.5 MJ/m2 , and HB = 80-100. Below the critical temperature, the slow cooling of low carbon steel produces ferrite structure. Ferrite is very soft and highly magnetic and does not harden when cooled rapidly. It forms smaller crystals when cooled from a bright red heat at a rapid rate.

Cementite

ii. Cementite:

          This is essentially an iron carbide Fe3C (of almost constant composition). It is extremely hard in nature and brittle, being harder than ordinary hardened steel or glass. Cementite increases with the proportion of carbon present, and the hardness and also the brittleness of cast iron is believed to be due to the presence of cementite in it.

          It contains 6.69% C and has a complex rhombic lattice. It is weakly ferromagnetic below 25°C, but loses this property on heating to 210°C. The melting temperature of cementite is difficult to determine, since cementite decomposes on heating. In experiments with laser-beam heating, its melting point has been measured to be 1260°C.

Pearlite

iii. Pearlite:

          Pearlite is the name given to a mechanical mixture of about 87% ferrite and 13% cementite having a two phase micro-structure and found in some steels and cast irons. Pearlite results from the transformation of austenite of eutectoid composition and consists of alternating layers (or lamellae) of ferrite and cementite. When e seen in the microscope the surface of appears like mother of pearl, hence it is give name as pearlite. The thickness of alternate plates and the distance between them is governed by the rate of cooling. Slow cooling produces a coarse structure than rapid cooling.

          A steel with 0.8% carbon is wholly pearlite, with less than 0.8% carbon is hypoeutectoid and with more than 0.8% carbon is hypereutectoid steel. The former contains ferrite and pearlite and is soft while the latter contains pearlite and cementite which are hard and brittle.

Bainite

iv. Bainite:

          This is a ferrite cementite aggregate, i.e., an austenitic transformation product found in some steels and cast irons. This is formed by the growth of a ferrite nucleus. It forms at temperatures between those at which pearlite and martensite transformations occur. It is the product of isothermal decomposition of austenite. The micro-structure consists of ferrite and a fine dispersion of cementite. Bainite is present in two forms; the feathery bainite obtained in the upper part of the temperature range and needle-like or accicular bainite produced by lower reaction temperature.

Martensite

v. Martensite:

          Martensite is magnetic and hard brittle mass of fibrous or needle like structure, it contains carbon upto 2% and it is main constituent of hardened steel. This is a body-centered, tetragonal meta-stable iron phase produced by entraping carbon on decomposition of austenite when the rapid quenching of high carbon steel from a slightly higher temperature than the maximum temperature of critical interval.

          We may note that the decomposition of austenite below 320°C starts the formation of martensite. It is not as tough as austenite. It is differs from austenite in being magnetic.

Austenite

vi. Austenite:

          This is the solid interstitial solution of carbon in gamma iron (FeC), which is stable only within a particular range of composition and temperature. and is non-magnetic. It has a FCC lattice in which the interstices are larger than in the BCC lattice, because of which the solubility of carbon in FeC is much higher and attains 2.14%. Austenite is ductile and has a higher strength than ferrite (HB 160-200) at a temperature 20-25°C. On cooling below 723°C it starts transforming into ferrite and which is magnetic and cementite to form the eutectoid pearlite, together with free ferrite or free cementite, depending on whether the carbon content is less or greater than 0.87% respectively. We may note that the austenite in a eutectoid steel is unstable at all temperatures.

Troosite

vii. Troosite:

          This is obtained either by tempering a martensite steel at between 250 and 450°C or by quenching steel at a speed insufficient to suppress the thermal change point fully. In comparison to martensite, this is less hard and brittle and also weaker than martensite. At the transformation temperature, the inter lamellar spacing decreases to 1X10-7 -- 2X10-7 m. It has dark appearance on etching.

Sorbite

viii. Sorbite:

          This is also produced by the transformation of tempered martensite at a temperature of 590-640°C. It is produced when steel is heated at a fairly rapid rate from the temperature of the solid solution to normal room temperature. Its properties are intermediate between those of pearlite and troosite and has a good strength. It is practically pearlite. We may note that troosite and sorbite division of pearlite structure is quite conventional, since the mixture dispersity increases monotonously with decreasing temperature of transformation.

IRON-CARBON EQUILIBRIUM OR PHASE DIAGRAM

          Figure shows the iron-carbon equilibrium diagram representing the entire range of iron-carbon alloys. Diagram indicates transformations that take place in an alloy of iron-carbon from pure iron to cementite (carbon content 6.67%). The carbon percentage is represented on the horizantal axis and the temperature on the vertical axis. We have also shown the names of the phases existing at temperatures and concentrations determined by the lines (boundaries) of these areas on the equilibrium diagram.

          Any Iron-carbon alloy in the moltan state may be considered to be solution of Fe3C in iron. When this solution cools and solidifies, it depends upon the amount of carbon present whether the alloy will solidify as a solid solution or form a eutectic.

Different kinds of Heat Treatments Given to Carbon Steels

Kinds of Heat Treatments Given to Carbon Steels

Carbon steels are subjected to the following basic heat-treatment processes:

1. Annealing,
2. Normalising,
3. Hardening,
4. Tempering,
5. Surface hardening,
6. Case hardening, and 
7. Ageing. 

Each of them has a number of varieties. We shall now describe these processes in the next posts.

Annealing

          This is a kind of heat treatment after which a metal or alloy acquires a structure close to the equilibrium one. A material is exposed to an elevated temperature for an extended time period and then slowly cooled. Normally, annealing is carried out to:

1. To improve machinability,
2. To relative internal stresses,
3. To increase softness, ductility and toughness, 
4. To produce a specific micro-structure, 
5. To remove gases,
6. To refine grain size due to phase recrystallisation,
7. To modify electrical and magnetic properties.

          The approximate temperatures to which the steel-sample should be heated will depends upon its carbon content. The recommended temperatures are shown in the following table:

          There are a variety of annealing heat treatments are possible. These are characterized by the changes that are induced, which many times are microstructural and are responsible for the alteration of the mechanical properties.

An annealing process consists of three stages: 

      (i) heating to the desired temperature, 

      (ii) holding or 'soaking' at that temperature, and 

      (iii) cooling, usually to room temperature. 

             In these annealing procedures, time is an important parameter. There exist temperature gradients between the outside and interior portions of the piece during heating and cooling. The magnitudes of temperature gradients depend on the size and geometry of the piece. Soaking time may be given at the rate of 3-4 minutes for every one mm thickness of the cross-section of material. If the rate of temperature, change is too great, temperature gradients and internal stresses may be induced that may lead to warping or even cracking. Moreover, the actual annealing time must be long enough to allow for any necessary transformation reactions. An annealing temperature is also an important consideration. Since diffusional processes are normally involved and therefore annealing may be accelerated by increasing temperature. The various types of annealing operations are: 

1. Full annealing,
2. Isothermal annealing,
3. Process (or) sub critical annealing,
4. Spheroidise annealing,
5. Diffusion annealing, and 
6. Recrystallisation annealing.

Full Annealing

          If it is desired to reline the grain structure and produce a lamellar pearlite, a full annealing cycle should be used. This consists of heating the steel to a temperature above the transformation range, holding for one or two hours, and then cooling at a predetermined rate to obtain the desired microstructure. Grain refinement is accomplished in this instance by the re-crystallisation of the steel in passing through the critical range both in heating and in cooling. The microstructure obtained in cooling any steel from above the critical temperature range is dependent both upon the temperature range in that ranges. Thus, it is obvious that the rate at which any steel is cooled determines the final microstructure, since the degree of transformation will depend on the amount of time allowed for it to occur. Therefore, the slower the rate of cooling and the higher the temperature at which complete transformation occurs during full annealing, the coarse the pearlite will be with correspondingly lower hardness.

          Such treatment is performed usually on steel of 0.3 to 0.6 % carbon content which is to be machined.

Isothermal annealing

          It is a type of full annealing in which the steel first is cooled to the temperature at which it is desired to have transformation occurs, at a rate sufficiently rapid to prevent any structural change above that temperature. The steel then is held at the selected temperature for the time necessary to complete such transformation. Thus it possible, with this process to obtain a more uniform micro-structure that could be expected by continuous cooling. However, since it is necessary to drop the temperature rapidly to prevent any transformation above the desired temperature, there are definite limitations as to the mass that can be co-treated. It is applicable, therefore, only to small sections and would be suitable for large bars or large load in batch type furnace since in would be impossible to cool them at a rate sufficiently rapid to prevent some transformation.

          Isothermal annealing process not only improves machinability in general, but also results in a better finish by machining. However, it has the following important limitations:

          It is suitable only for small components. Heavy components cannot be subjected to this treatment because it is not possible to cool them rapidly and uniformly to the holding temperature at which transformation occurs. 

Process or Sub critical annealing

          Another type annealing called Process or Sub critical annealing consists of heating the steel to a temperature first under lower critical point and holding at this temperature for the proper time (2-4 hrs) followed by air cooling. This type of annealing results in softening the steel due to particle coagulation of the carbide to form the spheroids or small globules of carbide. It is not suitable when a close control of hardness or structure is desired, because the prior structure of steel determined to a marked degree the extent of spheroidisation which will occur.

          This process is quite satisfactory for rendering bars more suitable for cold sawing or shearing and is used to great extent for these purposes. Since the temperature to which the bars are heated is somewhat lower than in full annealing there is less scaling and warping can be controlled.   

Spheroidisation annealing

          It is a type of annealing which causes practically all carbides in the steel to agglomerate in the form of small globules or spheroids. There may be wide range of hardness with such a structure for any grade of steel since the size of the globules has a direct relation to hardness. Spheroidising may be accomplished by heating to a temperature just below the lower critical and holding for sufficient period of time. A more desirable and commonly used method for spheroidising is to heat to temperature just above the critical and cool very slowly (6°C per hour) through the critical range or to heat to a temperature within the critical range but not above the upper critical and cool slowly.

          This treatment is used for practically all steels containing over 0.6 percent carbon that are to be machined or cold formed.  

Diffusion annealing

          This process, also known as homogenizing annealing, is employed to remove any structural non-uniformity, like dendrites, columnar grains and chemical inhomogeneities are generally observed in the case of ingots, heavy plain carbon steels castings. These defects promote brittleness, and reduce ductility and toughness of steel.

Process:

• Steel is sufficiently above the upper critical temperature (say 1000-2000°C), and held at this temperature for prolonged time, usually 10-20 hours, followed by slow cooling.
• Segregated zones are eliminated, and a chemically homogeneous steel is obtained by this treatment as a result of diffusion.
• Heating to such a high temperature results in considerable coarsening of austenitic grains & heavy scale formation. The coarse austenite thus obtained further transforms to coarse pearlite on cooling, which is not a desirable structure as mechanical properties are impaired.
• The main aim of homogenising annealing is to make the composition uniform, i.e., to remove chemical heterogeneity.
• The impact energy and ductility of the steel increases as the homogenizing temperature increases and the hardness yield strength and tensile strength decrease with an increase in the homogenizing temperature.
• Homogenizing annealing has a few shortcomings as well. It results in:
• Grain coarsening of austenite, there by impairing the properties.
• Thick scales on the surface of steels.
• It is an expensive process.

Recrystallisation Annealing

• Practically all steels, which have been heavily cold worked, are subjected to this treatment.
• The process consists of heating steel above the recrystallisation temperature, holding at this temperature and cooling thereafter. It results in decrease in hardness or strength and increase in ductility. The process is used both as an intermediate operation and as a final treatment. The treatment is very important and is frequently employed in industries manufacturing steel wires, sheets and strips.
• As little scaling and decarburisation occurs in recrystallisation annealing, it is preferred over full annealing.
• High carbon steels and alloy steels require higher recrystallisation temperatures.