The properties of metals and alloys can be changed by heating followed by cooling under definite conditions to make them suitable for specific applications. Accordingly steel can be hardened to resist cutting action and prevent abrasion. The rate of cooling and the manner of cooling are the controlling factors in heat treatment processes.

Heat treatment not only increases the hardness but also increases the tensile strength and toughness. Different heat treatment processes are carried-out in temperature controlled furnaces and ovens.

Purpose of Heat Treatment Processes

The heat treatment processes serve the following purposes. They

• produce hard surfaces and tough interior portions.
• increase resistance to wear, abrasion and corrosion.
• modify magnetic and electrical properties.
• improve mechanical properties such as tensile strength, ductility, shock resistance etc.
• improve machinability.
• refine the grains.
• relieve the internal stresses produced during cold working and other operations.
• change the chemical composition.

Classification of Heat Treatment Processes

Various heat treatment processes may be classified as follows.

1. Annealing

• Process annealing
• Full annealing
• Spheroidise annealing
• Diffusion annealing.

2. Hardening or Quench hardening

3. Tempering

• Austempering,
• Martempering, and
• Temperature based tempering — (i) low temperature tempering, (ii) medium temperature tempering, and (iii) high temperature tempering.

4. Normalising

5. Chemical heat treatment processes  

Annealing Heat Treatment Process

Annealing of steel is done to impart softness to it. The process involves heating of steel above the critical temperatures (Figure – Heating temperature range), and holding it there for about 1 hour. It is then allowed to cool slowly in the furnace at a rate of about 30° to 15°C/hour.

On cooling, the steel changes to ferrite and pearlite in case of hypoeutectoid steels, and into pearlite and cementite for hypereutectoid steels.

Objectives: Annealing serves the following objectives.

• It softens the steel.
• It enhances machinability.
• Internal stresses, if any, are also relieved.
• It improves ductility of steel.
• It refines the grain structure.

Different types of annealing processes are employed to serve different requirements of steel and alloys.

Types of Annealing Processes: Various types of annealing processes are 

1. Process annealing
2. Full annealing
3. Spheroidise annealing
4. Diffusion annealing,
5. Recrystallization annealing
6. Stress relief annealing

Process annealing: In this process, the steel is heated to little below the critical temperatures, and held there for prolonged duration. It is then cooled slowly which results in formation of pearlite. New crystals of the same structure are formed after the heating-cooling operation. Process annealing is carried-out on low carbon steel, cold rolled products used in wire drawing and deep drawing operations. The process is shown in Figure.

Full annealing: This process consists of heating steel above the critical temperatures, holding it there for a considerable time, and then allowing it to cool slowly in the furnace. The phases recrystallize completely in this process and the structure is refined. After this process the steel structure becomes chemically homogeneous and gets soft. Full annealing is applied on steel castings and ingots. Precaution should be taken to avoid overheating of steel.

Spheroidise annealing: The knot form of cementite is converted into granular form, generally spheroidal, by this process. The process is normally applied to high carbon steel by heating between 680°C and lower critical temperature, holding it at this temperature and then cooling it slowly to about 600°C. This process is costly and time consuming, and is shown in above Figure.

Diffusion Annealing: This annealing process is carried-out in heavy castings to make austenitic grains homogeneous. This process employs heating of the job above upper critical temperature, about 820°C, followed by full annealing process to obtain fine grained structure. The approximate annealing temperatures are listed in the following Table.

Type of Steel	Percentage of Carbon	Annealing Temperature (oC)
Dead mild steel	0.10 or less	900 ± 25
Mild steel	0.10 to 0.30	875 ± 25
Medium carbon steel	0.30 to 0.60	825 ± 20
Medium carbon steel	0.60 to 0.75	800 ± 20
High carbon steel	0.75 to 1.60	770 ± 10

Hardening Heat Treatment Process

To perform hardening process, steel is heated to a temperature above its critical range. It is held at this temperature for a considerable time and then allowed to cool by quenching in water, oil or brine solution.

If the carbon content of the steel is known, the proper temperature to which the steel should be heated may be obtained from the iron-carbon equilibrium diagram.

On heating above the critical temperature, the basic structure changes into austenite which contains considerable part of cementite. On rapid cooling this austenite change into martensite that imparts hardness in steel.

Purpose of hardening: The objectives of hardening are

• to improve the strength of steel,
• to develop hardness in the metal to resist wear, abrasion and to enable it to cut other metals.

Factors affecting the hardness: Various factors responsible for hardness in steel are the following.

• carbon content,
• quenching rate,
• quenching medium, and
• work size.

Quench hardening: In this process, the heated steel is suddenly dipped into a cooling medium bath. Cooling media such as oil, cold water, blend of water and soluble oil, brine (cold water + 5 to 10% salt), air and fused salts are used. The salts may be sodium chloride, calcium chloride, or sodium hydro-oxide etc.

Effect of quenching rate: The hardness in steel depends essentially on its quenching rate. A very rapid quenching is necessary to harden low and medium plain carbon steels. Certain alloys can be hardened at a too low rate of cooling. Quenching in water bath is commonly employed.

For high carbon steel and alloy steels, oil is generally used as quenching medium. Various commercial oils are used for this purpose. They have different cooling effects and impart different hardness on quenching. For extreme cooling, brine or water spray is most effective.

The structures obtained with different rates of cooling differ in appearance and properties like tensile strength, hardness, yield point etc. Faster the cooling rate, greater will be the hardness of steel.

The rate of cooling of high carbon steel from a temperature above the critical range determines the amount of martensite, troostite, sorbite or pearlite. If the steel is slowly cooled in a furnace, the structure obtained will be pearlite. If it is cooled in still air, the structure will be sorbite. Troostite is obtained when the steel is quenched in oil. Martensite is obtained when the quenching done in water is drastic.

Effect of quenching media: The effect of quenching media on properties of eutectoid steel with different structures is given in the Table below.

Quenching medium	Type of microconstituent	Elongation (%)	Hardness number (HRC)	Ultimate tensile strength (MPa)
Water	Martensite	1 – 3	60 – 70	1700
Oil	Pearlite (very fine)	4 – 6	30 – 40	1100
Air	Pearlite (fine)	7 – 9	20 – 30	800
Cooling within furnace	Pearlite (coarse)	10 – 12	10 – 20	500

Tempering Heat Treatment

Steel after hardening becomes brittle, develops non-visible micro-cracks and is strained due to residual (internal) stresses. These undesired symptoms are reduced by tempering the steel.

Infact, tempering is an essential operation that has to be performed after hardening. This process involves reheating of the hardened steel to a certain temperature below lower critical temperature followed by a slow cooling rate.

Reheating permits partial transformation of martensite, and relieving of the internal stresses. With the increasing tempering temperature, breakdown of martensite occurs at a faster rate.

Purpose of tempering: Tempering process serves the following objectives.

• It reduces brittleness of hardened steel.
• It increases ductility.
• It relieves internal stresses.
• It improves toughness of steel.

Besides the above desired results, the tensile strength and hardness of steel decreases a little which is undesired.

Low, medium and high temperature tempering: Based on the reheating temperature range of hardened steel, the tempering process may be sub-classified as follows.

1. Low temperature tempering,
2. Medium temperature tempering, and
3. High temperature tempering.

In low temperature tempering, the steel is heated to about 200°C (Figure – Heating temperature range). The process retains hard martensite. Case hardened components are tempered by this process.

In medium temperature tempering, the steel is heated between 175 to 275°C. The process yields troosite which provides high elastic limit to the steel. This process is applied on the laminated springs and coils etc. where toughness is desirable.

In high temperature tempering, the steel is heated between 275 to 375°C. The process produces sorbite that eliminates the internal stresses completely. This process is applied on the structural steel.

Some special tempering processes are also employed to serve specific purposes. These processes are:

• Austempering or Isothermal quenching, and
• Martempering.

Austempering: In this process, the steel is heated to about 700°C, and held there for some time. It is then quenched in a molten salt bath down to about 500°C, and held there for prolonged duration. This causes formation of bainite. The steel is then quenched in water to room temperature.

Important features: Important features of austempering are as follows.

• The hardness of austempered steel does not decrease.
• Steel becomes more tough and ductile.
• Cracks developed during quenching are eliminated.
• Warping and distortions are minimized.
• Almost uniform microstructure is obtained.
• The process is time taking and costly.

Austempering is suitably performed on the components of aircraft engines.

Martempering: In this process, the steel is heated to about 600°C, held there for some time and then quenched in a molten salt bath down to 300°C. After holding it at this temperature for a certain time, the steel is allowed to cool gradually in air. During the process, austenite transforms into martensite.

Martempering minimizes cracks, distortions and internal stresses in steel while the toughness increases. This process is also referred to as stepped quenching process.

Normalizing Heat Treatment Process

It is a process of heating steel 40^o to 50^oC above the upper critical temperature, holding it there for a certain duration, and then allowing it to cool in the surrounding air. 

Objectives of normalizing: It is done to get the following results:

• To refine the grain structure completely.
• To improve the machinability.
• To eliminate any leftover internal stresses.
•  To increase the strength of medium carbon steel.
• To maintain granular homogeneity.
• To enhance the toughness.

Normalizing is usually performed on rolled and cast steels, on components subjected to high stresses, and high carbon steel. During normalizing, the cementite present at the grain boundaries dissolves in austenite. Comparison of normalizing and annealing processes are illustrated in the above figure.

Microstructures of Steel and Iron

Carbon is found in different forms in iron and steel, therefore various microstructures are found in them. These are:

Ferrite: It is soft and ductile. It cannot be hardened by rapid cooling. Low carbon steel and wrought iron consist of ferrite. It is in the form of flakes.

Cementite: It is carbide of iron (Fe₃C), and is extremely hard. The hardness increases with the increase in carbon content. Presence of carbon in iron and steel increases the hardness but decreases the ductility. The carbon is in the form of knots in cementite.

Pearlite: It is a mixture of ferrite and cementite, and is found in low and medium carbon steels. Hardness of steel increases with the increasing proportion of pearlite. The coarse crystals of pearlite are formed during slow rate of cooling between 723°C and 625°C.

Ludeburite: Its microstructure is of layered formation, and is found in cast iron. It possesses lubricating property that imparts good machinability to cast iron. It is brittle, cannot be forged or rolled, but can be easily casted.

Austenite: The solid solution of iron carbide (Fe₃C) in y-iron is known as austenite. Complete structural changes occur in steel when it is heated above higher critical temperature shown in Figure (Heating temperature range). The non-magnetic austenite absorbs excess ferrite and cementite.

Bainite: It is a mixture of ferrite and cementite having finer crystals  than pearlite. Its formation starts below 625°C when formation of pearlite finishes.

Martensite: It is the hardest constituent of steel, and has a needle like structure. It is a solid solution of iron carbide in ex-iron. It is produced when steel heated above 723°C is rapidly cooled by sudden quenching. Martensite is least ductile and negligibly tough.

Troostite: It is obtained by sudden quenching of heated steel, or by cooling the steel rapidly. Its structure is slightly granular. Troostite may be of primary or secondary types. Primary troostite is formed directly from austenite. Secondary troostite results from tempering of martensite. Troostite is intermediate in hardness between martensite and sorbite.

Sorbite: When the reheating temperature is increased, decomposition of steel begins that causes change of troostite into sorbite. The formation of sorbite does not take place spontaneously, rather it occurs gradually.

Although sorbite is less ductile than pearlite but its tensile strength and yield point is higher. Depending upon the chemical composition, size of the job and degree of hardening, the sorbite begins to form at about 400°C and ends at about 680°C.

Allotropic Forms of Steel

Steel possesses allotropy, and its allotropic forms are

• α-iron,
• β-iron (accepted initially but discarded later-on),
• y-iron, and
• δ-iron.

These allotropic forms are observed at different temperatures during heating and cooling.

An allotropic change is a reversible change in the structure of steel during heating and the cooling. Figure shows such a curve for pure iron during heating and cooling.

When molten iron solidifies below 1539^oC, the liquid to solid state is accompanied by evolution of heat. This is represented by a horizontal line at 1539^oC

For a considerable period this temperature remains unchanged. When solidification is complete, iron attains delta form (δ-iron) and the structure becomes body centered cubic (BCC).

After sometime, the temperature falls to 1410^oC and allotropic change of iron takes place from delta to gamma iron (y-iron). This iron has a face centered cubic structure (FCC), and is non-magnetic.

On further cooling to 910^oC gamma iron changes to alpha iron (α-iron) with body centered cubic structure. The evolution of heat is represented by a horizontal line at 910^oC. Alpha iron is non-magnetic till the temperature of 768^oC.  At 768^oC, no allotropic change takes place, and the α-iron remains as such but becomes magnetic. On further cooling, no further change takes place.

Critical Points

The structural changes occur at the critical points A_c1, A_c2, A_c3, A_r1, A_r2, A_r3 shown in above Figure. The critical points A_r, obtained on cooling are slightly lower than those obtained on heating A_c. This change in temperature is due to thermal hysteresis.

Critical points A_c1, A_c2, A_c3 during heating and A_r1, A_r2, A_r3 during cooling are known as recalescence points. The critical points during heating are also known as chauffage and during cooling as refroidissement.

Due to change in the structure of steel from BCC to FCC, and again to BCC on heating and vice-versa, its lattice dimension changes. This results in change of volume of steel.

Iron-Carbon Phase Diagram

Iron-carbon phase diagram depicts the relation among percentage carbon, temperature and the constituent microstructures. The effects of heat treatments on alloying elements and on the induced specific properties in steel may be well understood by an iron-carbon phase diagram. The iron-carbon phase diagram is explained in preceding article.

α-iron is called ferrite and iron carbide (Fe₃C) is known as cementite. Iron containing carbon up to 2.0% is called steel and between 2.0% to 4.5% is termed as cast iron. Pure iron has melting temperature of 1539°C. The carbon content in cast iron does not exceed 4.5%. Iron containing 4.5% to 6.67% carbon is called Pig iron.

The iron-carbon equilibrium diagram terminates at 6.67%) carbon since all commercial alloys are made by carbon composition well below 6.67%.

Hypoeutectoid and hypereutectoid steels: As the carbon content increases above 0.1%, the ferrite is rejected from austenite y, and hence y-phase transforms to (α + y) phase before 0.83% C. At about 0.83% carbon, no free ferrite is rejected from austenite. This steel is called eutectoid steel and contains almost 100% pearlite.

Steels containing less carbon than that at eutectoid point are called hypoeutectoid steel, and those with more carbon percentage are called hypereutectoid steels.

Eutectoid, Eutectic and peridotic temperatures, their compositions and phases have already been described in preceding article.

The complete solidification of eutectic phases takes place at eutectic temperature during transformation from liquid to solid phase. Ledeburite is an example of eutectic constituent in the iron-carbon equilibrium system.

In liquid condition, 0.83% carbon is homogeneously mixed with the molten iron. 0.83% carbon steel does not solidify at some specific temperature, rather it has a solidification range. The structure of 0.4% carbon steel consists of 50% pearlite and 50% ferrite. Similarly the structureof 0.2% carbon steel consists of 25% pearlite and 75% ferrite.

The 1.2% carbon steel solidifies at temperatures much lower than that of 0.4% or 0.8% carbon steel. After solidification, the 1.2% carbon steel in austenitic solid solution is in the form of y-iron. On further cooling, more cementite separates as the temperature further decreases. In hypoeutectoid steels, ferrite separates and austenite gets richer in carbon; but in hypereutectoid steels, the cementite separates and austenite gets poorer in carbon.

Transformation in Steel and Critical Cooling Curve

The transformations in steel can be conveniently understood by time-temperature-transformation (T-T-T) curve as shown in Figure. Due to its shape from the start to the end of transformation, it is also called C-curve.

The nose indicates the least time taken for certain transformation. Line AB passing through the nose indicates fall in temperature in OB time. This line is known as critical cooling curve, and its rate is termed as critical cooling rate. Its value is about 300°C/s.

Effect of rate of cooling: The rate of cooling, slow or fast, decides the transformation processes. These processes may be summarized as follows, and are shown in Figure (T-T-T curve).

1. Annealing, when rate of cooling is very slow, as slow as 10°C/s or less.
2. Normalizing, when rate of air cooling is slow, as slow as 50°C/s or less.
3. Hardening, (by sudden quenching), when rate of cooling is faster than the critical cooling rate. This rate may be 350°C/s or more.

In Figure (T-T-T curves) various phases and microstructures are shown for different temperatures as a function log (time). The time is given in seconds. The formation of martensite starts at M_s and finishes at M_f.

In ferrous alloys, martensitic structure distorts to Body Centred Tetragon (BCT) due to interstitial carbon atoms. The height and base ratio (c/a) of a unit cell determines the tetragonally of martensitic lattice. It is expressed by

c/a = (1 ± 0.005) + 0.045(W_c)

Where W_e, is the weight percent of carbon. When W_e tends to 0, (c/a) tends to 1. In this case, the structure will be BCC ferrite.

Effect of Carbon Content on Hardness

The hardness of different phases and microstructures as a function of carbon content of steel is shown in Figure.

Temperature Ranges in Heat Treatment Processes

To understand the heat treatment processes, it is essential to their heating temperature range. Figure shows heating ranges for different processes and corresponding phases. The two important temperature ranges are:

1. Lower critical temperature (lct) for hypereutectoid steel, and
2. Higher (or upper) critical temperature (hct) for hypoeutectoid steel.

The tempering process requires heating much below the lct; while full annealing, hardening, carburizing and normalizing processes require heating above hct.

Spheroidizing annealing, recrystallization annealing and stress relief annealing are performed below lct. The temperature range of tempering may be further sub-classified into following three categories.

• low temperature tempering, normally below 200°C,
• medium temperature tempering, normally between 175-275°C,
• high temperature tempering, normally between 275-375°C but below 400°C.

For hypoeutectoid and hypereutectoid steels, normalising requires heating at a higher temperature range than that for annealing and hardening.