Sometimes, it is necessary to mold a metal into a desirable shape. To obtain this shape, a certain amount of heat must be given to the metal. This heat converts a solid metal into a plastic one.
The process of converting the solid metal into the desired shape by heating is known as the heat treatment process.
Heat treatment is defined as a controlled operation that involves the heating and cooling of a metal or alloy in the solid state for the purpose of obtaining a certain desirable shape of metal. The properties of a metal depend upon its structure.
Objectives of Heat Treatment
The objectives of heat treatment are described as follows.
(i) To increase the hardness of metals.
(ii) To improve machinability.
(iii) To change the grain size.
(iv) To modify the internal structure of the material ie., on the basis of electrical and magnetic properties.
(v) To improve the mechanical properties of, tensile, ductility, and shock resistance.
(vi) To increase the qualities of metal to provide better resistance to heat, corrosion, and wear.
(vii) To soften the metal. In this chapter, we will study the heat treatment processes at different kinds of temperature points.
Steel
As we know, steel can be heat treated to produce a great variety of microstructures and properties hence, it is the most popular solid metal. It is an alloy of iron and carbon, which may contribute up to 2.1% of its weight. Steels are a large family of metals. All of them are alloys in which, iron is mixed with carbon and other element.
Structure of Steel
The shape of steel depends upon the structure of iron or different types of molecules and the quantity of carbon in it. Many types of steel structures are as follows.
(i) Ferrite
It is a chemical compound of ceramic materials with iron i.e., Fe2O3. It is almost the pure state of iron due to the presence of 0% carbon in it. It is soft, pliable, and comparatively weak. It cannot be hardened by rapid cooling. It has more magnetic properties.
(ii) Cementite
When carbon exists in steel as a chemical compound it is called iron carbide (Fe, C) or cementite. This alloy is tough and brittle but it is not strong. It increases with more percentage of carbon. It shows magnetic properties below 250°C.
(iii) Eutectoid/Pearlite Steel
It is the mechanical aggregate of ferrite and cementite in equilibrium containing 0.88% 3 carbon. It is made up of alternate white and dark bonds. The size of the pearlite depends upon the rate of cooling. The rapid cooling will lead to the formation of the pearlite. The strength of the iron and steel is due to pearlite. It is much stronger than ferrite or cementite.
(iv) Hypoeutectoid Steel
It contains carbon of more than 0.8%. Hypoeutectoid steel consists of pearlite and cementité at room temperature. When slowly cooling a hypoeutectoid steel, the cementite will begin to crystallize first. When the steel remaining becomes eutectoid in composition, it will crystallize into pearlite. Since cementite is much harder than pearlite.
To change the internal structure of steel, apply the heat treatment in a furnace. The internal changes depend upon the heating temperature conditions. Now, we will discuss the effects of heating on the internal structure of iron.
Effect of Heat on the Internal Structure of Steel
If we heat a specimen of steel, then its temperature increases continuously up to 723°C but after reaching the temperature. 723 C, it remains the same for some time. Thereafter, it increases slowly and reaches a certain temperature, then it again increases at the same rate as before. After reaching the temperature of 723°C, the heat energy consumption by the specimen of steel becomes maximum which is further used in changing the internal structure of the steel, and the remaining part of the heat energy is used to increase the temperature of the steel.
In this process, the Lower Critical Point (LCP) of all carbon quantities has the same temperature i.e., at 723°C but its Upper Critical Point (UCP) always varies. We will further discuss the critical temperature point.
Critical Temperature
The critical temperature of a substance is the temperature at and above which the vapor of the substance cannot be liquified.
There are mainly two states of the critical temperature, which are as follows
(i) Lower critical point temperature
(ii) Upper critical point temperature
(i) Lower Critical Point Temperature
The temperature at which the structure of steel starts to be austenite (ie, at 723°C), is called the lower critical temperature for all plain carbon steels. Normally, the value of the lower critical temperature for all types of steel is 723°C but the upper critical temperature is changeable.
(ii) Upper Critical Point Temperature
The temperature at which the structure of steel completely changes to austenite is called the upper critical temperature. This varies depending on the percentage of carbon in steel.
Critical Range
The temperature difference between lower critical temperature and upper critical temperature is called critical range.
Methods of Heat Treatment
In the process of heat treatment, steel is heated through various methods.
These methods of heat treatment are as follows.
- Hardening
- Tempering
- Annealing
- Normalizing
- Case Hardening
1. Hardening
Hardening is a hardness indexing kind of heat treatment process in which the hardness and strength of a metal are increased.
In this process, the steel is heated to the required temperature held at that temperature for the required time period, and then quenched in water. Due to this internal distortion takes place which increases the hardness of steel with a corresponding reduction in in strength and ductility.
The mechanical properties produced as a result of this treatment will depend upon the following factors.
(i) The carbon content of the steel.
(ii) The temperature to which it is heated.
(iii) The duration of heating.
(iv) The temperature of the steel at the start of quenching.
(iv) The cooling rate produced by quenching.
The effect of carbon content on the hardness produced by the process is illustrated.
The increase in carbon content will result in an increase in the hardness produced by the treatment.
Steel with less than about 0.15% carbon will not respond to this treatment.
Process of Hardening
It is done for medium and high-carbon steels. The steel heated at its critical temperature Le, 750 to 850°C called the soaking period of temperature. After this, it cooled in coolant. The cooling in some coolants is known as quenching.
In order to produce the desired effect, sufficient carbon must be put into the solid solution to cause internal distortion, when it is trapped in the iron by quenching
When the carbon content is less than 0.83% the steel is heated only to just above its Lower Critical Point (LCP).
The figure illustrates the temperatures at which steels are heated before quenching.
Soaking Period
The heating of a metal at a constant temperature for a suitable duration of time is known as the soaking period of a metal. Normally, 5 min is allowed as the soaking period for 10 mm thickness of steel.
Cooling
After the soaking of steel, it is cooled in a suitable quenching medium at a certain minimum rate called the critical cooling rate. The critical cooling rate depends upon the composition of the steel. This cooling transforms all the austenite into a fine needle structure called martensite.
The structure of steel treated in this way is very is very hard and strong, but very brittle.
2. Tempering
It is usually re-heated to a suitable temperature below the lower critical point (heating) to improve, its toughness and ductility but it is done at the expense of hardness and strength. It is done in order to make the steel more suitable for service requirements.
Purposes of Tempering the Steel
Steel, where it is in hardened condition, is generally brittle and too severely strained. In this condition, steel cannot be used and hence it has to be tempered.
The purpose of tempering is as follows.
(i) To relieve the steel from internal stresses and strains.
(ii) To regulate the hardness and toughness.
(iii) To reduce the brittleness.
(iv) To restore some ductility.
(v) To induce shock resistance.
Process of Tempering
The tempering temperature depends upon the properties required, but it is between 180 and 650°C. The duration of heating depends upon the thickness of the material. Tools are usually tempered at a low temperature. The temperature itself is judged by the color of the oxide film produced upon heating.
This method is not, however, suitable for accurate temperature assessment.
Quenching Medium
The quenching medium controls the rate of cooling. For a rapid quenching, a solution of salt or caustic soda in water is used. For very slow quenching, a blast of air is sufficient. Oil gives an intermediate quenching. Water and oil are the most common quenching media used. Air quenching is suitable only for certain special alloy steels. When steel is heated, its color changes and the variation in colors indicates the various tempering temperatures.
Tempering Temperatures for Different Colours
| SL NO | Color | Temperatures (in °C) |
| 01 | Faint straw | 220 |
| 02 | Dark straw | 240 |
| 03 | Brown | 250 |
| 04 | Brownish purple | 260 |
| 05 | Purple | 270 |
| 06 | Light blue | 320 |
| 07 | Blue | 300 |
In a manufacturing plant, when heat treatment is done on a production basis, modern methods are used. Tempering is done in controlled atmosphere furnaces with the temperature controlled by modern instruments. Under such conditions, it is possible to obtain accurate and uniform results in any number of pieces.
Generally, tempering in the lower temperature range for an increased time provides greater control in securing the desirable mechanical properties.
3. Annealing
In this process, steel is heated to a suitable temperature depending upon its carbon content and is held at that temperature for sufficient time and then slowly cooled to room temperature.
The heating, soaking (holding the temperature), and slow cooling cause the grains to become large and so, produce softness and ductility.
For annealing, hypoeutectoid steel is heated at 30 to 50°C above the upper critical temperature and 50°C above the lower critical temperature for hypoeutectoid steel.
The soaking period at temperature is 5 min/10 mm of thickness for carbon steel.
The cooling rate for carbon steel is 100 to 150°C/h.
The cooling is done in the furnace itself by switching the furnace or the steel is covered either in sand or dry lime and dry ash.
Annealing Temperatures
| SL NO | Carbon content (in %) | Temperature (in °C) |
| 01 | <0.12 | 875 to 925 |
| 02 | 0.12 to 0.25 | 840 to 970 |
| 03 | 0.25 to 0.50 | 815 to 840 |
| 04 | 0.50 to 0.90 | 780 to 820 |
| 05 | 0.90 to 0.1.3 | 760 to 780 |
Purposes of Annealing
The purpose of annealing is as follows.
(I) To soften the steel.
(ii) To relieve internal stresses.
(iii) To reduce or eliminate structural in-homogeneity.
(iv) To refine grain size.
(v) To improve machinability.
(vi) To increase or restore ductility and toughness.
4. Normalising
Due to continuous hammering or uneven cooling, strains and stresses are formed in the internal structure of steel. These should be removed from forgings or castings, otherwise, they may fail at any time while in use.
Normalizing is done to produce a fine grain for uniformity of structure and for improved mechanical properties.
Normalising Process
In this process, steel is heated to the required temperature depending on its carbon content held at that temperature and then, cooled freely in air.
Normalizing is usually done before machining and hardening, to put the steel in the best condition for these operations.
The steel is heated to a temperature (30 to 40°C above the upper critical temperature) at which all the austenite is present even in the case of high-carbon steel.
Purposes of Normalising
The purposes of normalizing are as follows:
(i) To soften the metal.
(ii) To refine grain structure.
(iii) To improve machinability after forging and rolling.
(iv) To improve grain size.
(v) To improve the structure of the weld.
(vi) To prepare steel for sub-heat treatment.
Comparison between Annealing and Normalising Processes
| SL NO | Annealing | Normalizing |
| 01 | In this hypoeutectoid steel is heated to a temperature approximately 2 to 30°C above the temperature of the higher critical temperature and hypereutectoid steel is heated 20 to 30°C above the lower critical temperature. | In this process, metal is heated 30 to 50°C above higher critical temperature. |
| 02 | It gives good results for low and medium-carbon steel. | It also gives very good results for low and medium-carbon steel. |
| 03 | It gives high ductility. | It induces higher ultimate strength, yield point, and impact strength in ferrous material. |
| 04 | It is basically required to soften the metal, to improve machinability, to increase ductility, and to improve grain size. | It is basically required to refine grain size, improve the structure of the weld, to relieve internal stresses. |
5. Case Hardening
We know about the many engineering components that have the internal toughness to protect them from any impact. e.g., Gear, camshaft, etc. Case hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft.
Thus, forming a thin layer of harder metal is called a case at the surface. The internal part of the job remains soft and the upper part becomes hard, which protects the metal from breakdown. Case hardening is usually done after the part has been formed into its final shape.
This technique is used for steel with a low carbon content. The carbon is added to the outer surface of the steel to a depth of approximately 0.03 mm.
The following types of processes are used for case hardening.
(i) Carburising
(ii) Nitriding
(iii) Cyaniding
(iv) Flame hardening
(v) Induction hardening
(i) Carburising
Carburising is a heat treatment process used to case harden the steel with a carbon content between 0.1 and 0.3%. In this process, steel is introduced to a carbon-rich environment at elevated temperatures for a certain amount of time period and then quenched.
Mild steel or low carbon steel is heated at 925°C for 5 h approximately, due to which, carbon diffuses in the upper surface of steel up to 1-2 mm depth. In this way, case hardening is done by carburizing. By this method, the hardness of the case can be increased up to R 65 grade (i.e., a grading system for measuring the case hardness of a material).
To obtain fine grain of core, firstly the metal is heated above its critical temperature (870-925°C) and then cooled in oil at 760°C. This results in the development of softcore and the changing of the whole structure in pearlite and martensite. The carburizing is a diffusion-controlled process and is used for case hardening of gear, cam and camshaft, bearing, etc.
Carburising can be classified as follows.
(a) Pack carburising
(b) Liquid carburising
(c) Gas carburising
(a) Pack Carburising
The carburizing in a solid medium is called pack carburizing. In this process, the job which is to be case hardened is placed in a cast iron box, with the power of carburizing substances such as powder of wood, charcoal, bone charcoal, etc. An energizer in the form of barium carbonate (BaCO3) is also used in the box.
The joints are sealed with fire clay and the box is gradually heated at a temperature of 900-950°C in a furnace for a specific period of time, which depends on the case depth requirement. Then, the box is removed from the furnace and keep the box tight for slow cooling. This surface of the job becomes hard on cooling.
(b) Liquid Carburising
It is performed by immersing the workpiece in a bath of molten salts, containing 20 to 50% of sodium cyanide,
40% of sodium carbonate and a small amount of sodium or barium chloride. This process takes 0.5 to 2 hours and requires a temperature of 850 to 1000° C. This process gives a thin harden layer up to 0.8 mm thickness. This process is mostly suitable for small parts.
(c) Gas Carburising
In this method, the parts to be gas-carburized are surrounded by the atmosphere of carburizing gas. Generally, the carburizing atmosphere consists mixture of 20% of CO, 40% H₂, and 40% nitrogen. This mixture is also known as carrier gas. Besides carrier gas, some hydrocarbon gases are mixed in a furnace. The common carburizing gases are methane, ethane, propane, butane, etc. The furnace is either gas-fired or electrically heated. The temperature usually varies from 950 to 1030° C and it will take very little time for case hardening of job, therefore used in mass production.
(ii) Nitriding
Nitriding is a special process of case hardening process, in which nitrogen instead of carbon is used for case hardening. In the nitriding process, the steel part is heated to 482-621°C in an atmosphere of ammonia gas and dissociated ammonia.
This process is used for those alloys, which are susceptible to the formation of a chemical nitride. The alloy steel containing chromium, nickel, aluminum, manganese, etc., or nitre-alloy are widely used for this process. There are some important applications of this process.
Applications of Nitriding
Many automobile, and diesel engine parts, pumps, shafts, gears, clutches, etc. are treated with the nitriding process. This process is used for the parts which require high wear resistance at elevated temperatures such as automobile and valve parts, piston pins, crankshafts, cylinder liners, etc. It is also used in ball and roller bearing parts, die casting dies, wire drawing dies, etc.
(iii) Cyaniding
Cyaniding is a case hardening process, which is used on low-carbon steel and mild steel. In this process, a cyanide bath is prepared by heating a mixture of 30% sodium cyanide, 40% sodium carbonate, and 30% sodium chloride at 800 to 900°C in a crucible. At this temperature, sodium cyanide is in molten form. The part is then bathed in the solution and thereafter quenching is done under oil or water. In this process, the following reactions take place.
2 NaCN + O2 2 Na CNO
2 Na CNO +02 →Na₂ CO3 + CO + N₂
2CO +CO2 + C
The hardness of the material increased in two steps. Firstly, the formation of nitride compound increases the hardness, and secondly hardness increases after heat treatment due to an increase in the percentage of carbon. It is typically used in small parts such as bolts, nuts, screws, and small gears. The major drawback of cyaniding is cyanide salt i.e., poisonous.
(iv) Flame Hardening
Flame hardening is a surface hardening method that involves hardening of steel object that contains 0.3 to 0.6% of carbon. In the flame hardening process, hardening is done with the help of a high-temperature flame followed by a quenching process. The aim of this process is to harden the metal while making it malleable for subsequent work. It is used for medium carbon alloy steels and cast iron to produce a hard, wear-resistant surface.
These are using an oxyacetylene flame on the surface of the steel being hardened and heating the surface above the upper critical temperature before quenching the steel with a spray of water. This process is widely used for hardening beds of lathes and other machines, teeth of a gear, spindle, wrench, pulley, and rolls of the rolling machines.
(v) Induction Hardening
Induction hardening is an extremely versatile heating method that can perform uniform surface hardening, localized surface hardening, and tempering of hardened pieces. It is accomplished by placing a steel ferrous part in the magnetic field generated by high-frequency alternating current passing through an inductor, usually a water-cooled copper coil.
The depth of heating produced by induction is related to the frequency of the alternating current, power input time, part coupling, and quench delay. The electrical consideration involves the phenomena of hysteresis and eddy current. Because secondary and radiant heat is eliminated, the process is suited for inline production In this process, the chemical composition of steel is not affected. Induction hardening includes plain carbon steel (0.30%), medium carbon steel, and alloy steel.
It is cheaper takes less time and has less possibility of job bending so, due to the many advantages of this system is very useful and does not require any furnace.