Physical behaviour of material during reduction at High temperatures

The behaviour of material from the time are charged on the stock line until their descent in to the bottom of the stock is of direct interest in assesiass the blast furnace operation. During this period the material may disintegrate by any one or more of the of the below reason :

1. Decrepitation
2. Low temperature breakdown under reducing condition.
3. Failure under overlying load at high temperature.
4. Swelling at high temperature under reducing condition.
5. Premature softening of materials.

● The permeability of the stack may in consequence decrease and affect the furnace operation adversely. Tests have, therefore been developed under as closely simulated conditions as is otherwise feasible, to measure the tendency of burden material to degradation.

Decrepitation :

When iron bearing materials are suddenly exposed to the exhaust gas temperature at the stock level on charging, breakdown may occur due to thermal shock. This is known as decrepitation.

● It is measured by dropping a known weight of material in a previously heated to a temperature level of 400 to 600℃ under normal atmosphere, inert atmosphere or under mildly reducing conditions. After the charge attains the temperature it is removed cooled and sieved to measure the breakdown.

Low Temperature Breakdown Test :

● It has been observed in the experimental blast furnace that the iron bearing materials do disintegrate at low temperature under mildly reducing conditions, that is in the upper part in the stack affecting the furnace permeability and consequently the output adversely. It is believed that deposition of carbon in the region of the stack is also a contributory factor with sinters the breakdown has been associated with the presence of micro cracks.

Reduction Degradation Indeix Test :

● All the methods are followed the temperature of sample is raised to 900℃ and mixed gas of N2:CO :: 70:30 at the rate of 15 lit/min is passes for 3hrs. The cooled sample is weighed to find out the percentage of weight loss against 500gms.

Reducibility index = {% wt. Loss / (Fe(t) - FeO) × 48/112 + FeO × 16/56 } × 100

Coke Reactivity Test (CRI) | Coke Strength after Reaction (CSR)

● The dry coke is placed in the reacting vessel made of inconnel-600 grade steel. The vessel is pushed in an electric muffle furnace kept at 1100℃ temperature. Pure N2 is passed @ 5lit/min till the sample temperature stabilise at 1100℃. Then pure CO gas is passed for 2hrs. @ 5lit/min. Subsequently sample temperature is brought down to room temperature by passing N2 gas.

● Cold reacted coke is weighted and the % wt loss against initial wt. is reported as CRI (coke reactivity index). Then the reacted coke is rotated in an I/drum at 20rpm for 30min. Tumbled coke is screened on 10mm round screen. The percentage of +10mm is reported as CSR ( coke strength reaction)

Hot Compression Strength :

● The compression strength of the burden materials falls rapidly with increasing temperature. A cold compression strength strength of 150 - 300 kg for certain pellets fall to as low as 25kg at 1000℃. It means that practically half way down the furnace stack the pallets loose practically much of their strength.

Swelling :

● Some materials or peticupart the pellets show a maxima in their volume change with increasing degree of reduction. This is due to swelling of the pellets. If it is excessive it can give rise to serious trouble in the furnace operation. Swelling can be measured for individual particles but bulk swelling data are practical use. In the CNRM or the BISRA tests for the swelling four samples of 58 -62gms each are used.

Temperature : 1000℃
Reducing gas : 4% CO and 60% N2
Gas flow         : 1000 lit/min
Pre heating.   : Under nitrogen atmosphere.

Softening of Materials in the furnace :

● During its descent in the furnace stack the burden comes to a temperature level where it soften. The charge becomes sticky and bed permiabilper is consequently decreased resulting in a considerable pressure drop in this zone. The wider is the sticky zone greater is the pressure drop in the furnace and more the furnace operation gets affected adversely.

● The earlier the softening commence wider is the softening zone and consequently wider is the sticky zone in the furnace. The softening behaviour also varies with the degree of reduction. Therefore it necessary to measure the softening temperature of the burden material under reducing condition.

● Softening temperature vary with the nature of the iron bearing material its content, initial state of oxidation, basicity ratio etc. In particular alkali content could have a marked effect on the softening temperatures.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

4. Iron making by : Dr. R. H Tupkary & V. R Tupkary

Author :
Subir Kumar Sahu.



Some laboratory tests have been devised to investigate, identify and quantify on a relative basis, an increasing number of burden properties to assess the extent of suitability of burden materials. These tests are empirical in nature and hence the results of two different investigation do not readily agree unless extensive standardisation is sought.

● The properties of burden material of interest are those properties which have bearing on its performance during handlings, until it is charge I'm the furnace, and subsequently on its behaviour inside the furnace. These properties are :

1. Room temperature physical properties
2. Reducibility
3. Physical behaviour during reduction at high temperature.

Room Temperature Physical Properties

It includes the physical properties such as the cold strength, resistance to impact and abrasion and porosity of the burden materials. The shatter and tumbler tests have mainly been devised for this purpose beside direct compression strength measurements.

Shatter Tests :

It essentially consists of dropping a certain amount of material from a standard height for certain number of drops. The amount of the material, retained on or passed through certain sieves expressed as percentage of the original weight which indicated as the shatter index.

Tumbling and Abrasion Test :

The tumbling test essentially consists of tumbling a standard weight of material of certain size in a standard drum. Tumbling carried out at a standard speed for a fixed number of revolutions. The % material passing through or retained on a certain seive is the index.

● This tests have shown that the pellets are a high quality product and sinter a poor quality with natural ore, in general falling intermediate between the two in respected of physics strength.

Compression Test :

The direct mesurment of compression strength of iron bearing material is difficult because of the uncertain geometry of the specimens. The compression strength of pellets, being more regular in shape, can be measured with better accuracy than those of ores and sinters. A large number of tests should be carried out and the maximum minimum and mean strength along with the standard deviation should be indicated.

Porosity :

It is the most difficult property to measure and expressed as the volume of the material tested. Two types of porosities, open and closed are recognized. Open porosity is accessible to fluids where the closed porosity is not. It is usually measured by simple method based on archimeds principle.


● The reducibility test essentially aims at measuring the rate of reduction of iron bearing material under blast furnace condition. Since the condition in a blast furnace vary from top to the bottom of the stack, a standardized condition.

● The use of reduction temperature in the range of 900 to 1000℃ and a reducing gas of CO or CO + N2 of constant composition is increasingly being used. Several tests have been devised that are.

1. Gakushin test
2. Verein Deutscher Eisenhuttenleute method
3. Centre National de recherches Metallurgists method
4. Non isothermal test.
5. Chiba test.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

4. Iron making by : Dr. R. H Tupkary & V. R Tupkary

Author :
Subir Kumar Sahu.


Firing machine

There are three types of pellet firing machines that in commercial use which have discuss below :

1. The shaft kiln
2. The grate machine
3. The grate kiln.

The Shaft Kiln :

● It was developed in the 1950's. The green or dried pellets are fed vertically downwards in a central shaft of usually a rectangular cross section. Fuel is burned in two fire chambers, one on each long side of the shaft, and the hot gasses are allowed to enter the main shaft through multiple flues.

● It is in a way counter-current gas solid heater. The fired pellets are cooled in the lower portion and if any chunks are formed these are broken by the chunk breakers and cooled pellets are discharged from the bottom.

● Cooling air, introduced from below, gets pre heated and is either taken out to burn the fuel in the chamber or to make available preheated gas for completing the combustion in the firing zone of the shaft.
The shaft kiln - induration of pellets
The shaft kiln

● Here a typical furnace has a shaft of around 4×2m cross section and is nearly 20m height from the discharge point to the stock level. The feed and the discharge rates are adjusted to maintain a nearly constant stock line level.

● Nearly a third of the total air required for combustion is admitted in the combustion chamber for partial burning of the fuel. The remainder being introduce in the shaft for cooling from below and which is then available as preheated secondary air for completing the fuel combustion inside shaft.

● The temperature in the combustion chamber is around 1300℃. The furnace contains nearly 200t of pellets and production rate of hardened pellets is around 1000 to 1200 ton/day. Now some recent designs have Incorporated external coolers or internal coolers and heat exchanger to improve the thermal efficiency of the process.

The Grate Machine :

This types of machine are modified version of Dwight-Lloyed sintering machine in which the green balls are fed at open end on the continues traveling grate and the hardened end cooled pellets are discharge from the other end. The length of the grate is divided in to four different zones
1. Drying
2. Pre-heating
3. Firing
4. Cooling

● The hot air from the cooling zones is circulated in a complex manner to carry out drying, dehydration, pre-heating etc.
The grate kiln for induration of pellets
The grate kiln for induration of pellets

● Down draught, up-draught or a combination of the two is suitably employed in the design to carry out the preliminary operations before firing. The combustion varies with the mineralogical Constitution of the base feed and as a result the design differ considerably from place to place.

● A peculiar feature is the necessity of the protective layer of previously fired pellets on the bottom as well as on the sides of the bed. The temperature therefore need not be raised to the firing temperature level near bottom or the walls and the danger of overheating the metallic pellet is there by eliminated.
The straight grate kiln for induration of pellets
The straight grate kiln

● If such a precaution is not taken the pellets near the bottom or the walls will remain underscored in trying to protect the metallic grate and the walls.

● The latest modified version of grate machine is the use of a circular grate in place of straight grate. It eliminate the operational difficulties that are encountered in the straight grate. Only one such unit has been recently commissioned in Mexico and it's performance is being watched with interest.

The Grate Kiln :

● It is a combination of a grate and kiln system. The drying and pre-heating is carried out in this machine on the straight traveling grate, as in the grate system and firing is completed in a long rotating kiln, wherein the fuel is burned to generate the necessary temperature.

● Hot gases from the kiln are carried over the grate, where drying and pre-heating are carried out under down draught condition or in combination with up draught movement as well.

● The traveling grate in this case is not subject to very high temperature and hence the life of the grate is very high. It is one single factor that makes the machine a costly operation.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

4. Iron making by : Dr. R. H Tupkary & V. R Tupkary

Author :
Subir Kumar Sahu.


Fatigue Test :

Failure of a metal can take place at much lower stress as compared to tensile strength when it is subjected to repetitive or fluctuating load. Such failure is referred to as fatigue failure, which always result in brittle fracture.

● It takes place instantaneously without any warning. Many components such as crank shafts, gears, connecting rods, springs and blades of power driven machines, which are subjected to cyclic loading are prone to failure by fatigue.

● Such failure has been observed to be promoted mainly by application of sufficiently high tensile stresses, large fluctuations or variation in the applied stress or large number of cycles of applied stress.

Fatigue Testing Machine :

Fatigue testing machine
Block dia of fatigue testing machine
And stress development in specimen

● The principle of operation of a fatigue testing machine is very simple. The test specimen is mounted on the machine and subjected to rotation. Due to rotation, the upper surface of the test specimen is subjected to tension, where as the lower surface experience compression.

The rotation of specimen continue till failure occurs. The test is performed for given metal at varing cyclic stresses. Thus a graph is obtained between cyclic stress and number of cycles. This grape is known as a S-N curve.
S-N curve
S-N curve for ferrous and non ferrous metal

● Fatigue failure is generally starts at the surface. Hence in addition of stress, surface finish should be given due to attention for components subjected to cycling loading.

Remedies of Fatigue Failure :

It has been experimentally observed that resistance to fatigue failure improved significantly by improving the surface finish, removal of decarburized layer, hardening of surface and electro polishing. The compressive residual stresses in the surface considerably enhances resistance towards failure of fatigue. Shot peening and surface rolling are two commercial method available for the introduction of the compressive residual stresses in the metal surface.

Factor Affecting Fatigue Strength :

● Here certain metallurgical factors such as heat treatment, grain size, presence of alloying element and non-metallic inclusions also affect fatigue strength. The presence of equilibrium products in steel greatly reduces the fatigue strength.
● In general a hardened and tempered steel has optimum fatigue properties. Further improvement in fatigue properties can be achieved by bainitic structure.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.


Impact Test

Impact test measure the strength of a material under dynamic loading, where most of the structural components are subjected to dynamic loading. So tensile strength along will not be of sufficient use as a seeing parameter.

● In impact test the material is subjected to sudden load, a hammer is made to swing from a fixed height and strike the standard impact specimen. Impact strength of a material is defined as the capability of the material to absorb energy without failure under impact loading.
Impact testing machine
Impact testing machine

● Depending on the nature of the standard impact specimen, there are two most common method for the measurement of impact strength. They are Izod and Charpy impact tests.

Izod Impact Test :

The Izod specimen may have either square or round cross section. These are usually V-shape notch. The depth of notch is 2mm and included at an angle of 45°. In this test a hammer strike the specimen which is fixed in vertical position. The notch faces the hammer.

Impact test specimen
(a) single notch square Izod
(b) single notch round Izod
(c) beam V- notch
(d) Charpy U- notch

Charpy Impact Test :

The Charpy impact specimen are usually square in cross section and V-shape or key hole shaped notch. The specimen is fixed in horizontal position. The hammer strike the impact specimen on the unnotched face.

Impact Strength :

Impact strength of a material is governed by many factor such as temperature, heat treatment, chemical composition and grain size. Temperature has marked influence on impact strength. As temperature comes down impact strength also decrease. There is sudden drop in impact strength when material is cooled below a particular temperature.

The temperature at which ductile materials changes  to brittle is known as transition temperature. The temperature at which fracture is 50% brittle is called fracture appearance transition temperature (FATT). Carbon raises transition temperature significantly in steels. The deleterious effect of carbon is counteracted by manganese.

● The manganese to carbon ratio should be at least 3 for satisfactory impact strength. Other elements which raises the transition temperature are phosphorous, silicon and molybdenum. Heat treatment also affects impact strength. A tempered martensitic structure provides the best combination of tensile and impact strength.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.


Tensile Test

Tensile test is one of the most widely performed tests. In this process we determined yield stress, upper and lower yield points, tensile strength, elongation and reduction in area.
● Universal tensile testing machine is frequently used for performing tensile test. The specimen is subjected to tensile load till fracture occurs. These results of tensile tests can be represented in the from of engineering or true stress-strain curve.

Yield Stress :

Stress strain curve
Stress strain curve

The yield stress is defined as the stress at which plastic deformation of the tensile specimen takes place at a constant load. For example : carbon and low carbon steels in which elongation and extension have occurred.

Tensile Strength :

● The tensile strength also known as ultimate strength which is denfind as the maximum stress withstand capacity of a material. It is obtained by dividing maximum load by original cross sectional area. Tensile strength value provides idea about hardness and fatigue strength of the material.

● When two samples of the same material with identical microstructure but with different composition are taken in to consideration, it observed that their fatigue strength and hardness bear directly relation to tensile strength.

Let's consider steels : The tensile strength in Newton per square (N/mm2) is related to its hardness in BHN like

Tensile strength : 3.242 × BHN for heat treated alloy steels.
Tensile strength : 3.396 × BHN for heat treated medium carbon steels.
Tensile strength : 3.551 × BHN for heat treated low carbon steels.

● Fatigue strength of most of the steel is of the 50% of tensile strength.

Elongation :

Elongation is generally given by change in length per unit length of the tensile specimen multiplied by 100. The change in length is always measured relative to some length marked on the tensile specimen prior to testing which is referred to as gauge length. The gauge length is always reported along with the reported value of percentage elongation.

Percentage elongation :     (Lf - Lo) × 100 / Lo
Where Lf : final length
             Lo : initial length (gauge length)

Percentage reduction in area =   (Ao - Af) ×100 / Ao
Where Ao : original cross sectional area of the tensile specimen.
             Af : final cross sectional area of the tensile specimen.

Hounsfield tensometer
Line diagram of hounsfield tensometer

For research work and in laboratories, Hounsfield tensometer is most commonly used. Load on tensile specimen in this tester a applied with the help of spring beams. The maximum load which can be applied on tensile specimen in a tensometer is 2000kg. Other loads available with tensometer are 1000kg, 500kg, 250kg, 125kg, 62.50kg, and 31.25kg. So a large number of materials can be tested on this machine. For reproducibility of results, samples of standard size should be tested.
Specimen of hounsfield tensometer
Standard tensile test specimens of hounsfield tensometer

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.



Testing of material is one of the important and essential steps for judging suitability for engineering application after heat treatment. Most of the properties of interest are mechanical properties since heat treatment essentially alters mechanical properties. Many testes and testing materials have also be included now a days.

Hardness has been defined in several ways, based on the principle and the manner in which test is conducted. There are 30 types of methods used for measuring different types of hardness. There are generally three types -

1. Scratch hardness :

Scratch hardness can be defined as that property of the material by virtue of which it resists wear or abrasion.

Moh's Scale of Hardness :

● Moh's scale of hardness consists of 10 standard minerals. It is most commonly used for scratch hardness. Each one of the minerals has been assigned as a hardness number. Though Moh's scale finds wide application in the field of minerology, it is hardly used for metal and alloys which may have hardness value lying between two consecutive scratch hardness according to Moh's scale. Thus comparative hardness value of metal and alloys, which are of great significance can not be estimated.

Standard moh's scale of hardness
Standard Moh's scale of Hardness

2. Indentation hardness :

● Indentation hardness is a measure of resistance offered by a material to plastic deformation. It is measure by estimating the size of indentation. Indentation hardness measurement involve pressing an indenter of known material and well defined geometry in to the surface of the sample or work piece. The size or depth of indentation is used as hardness measuring parameter.

● Indentation hardness test is very common and finds varied applications in the field of metallurgy. It is most widely used for metals and alloys. Three most commonly used are :
1. Rockwell hardness test
2. Brinell hardness test
3. Vickers hardness test

3. Rebound or dynamic hardness :

Rebound hardness is a measure of resistance of the materials to strike and rebound. For determining rebound hardness, an indentor is dropped on the surface of the material under specific set of conditions. The energy of the impact or height of rebound of the indentor forms the basic of measurement of rebound or dynamic hardness. Shore seleroscope is the most commonly used rebound hardness tester.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

NORMALIZING | Heat treatment for steels


Normalizing is a process of heating steel to about 40 to 50℃ above the critical temperature A3 or Acm, then holding for proper time and then cooling in still air or slightly agitated air to room temperature.

● In special cases cooling rate can be controlled either by changing air temperature or air volume. After normalizing the resultant microstructure should be pearlitic. This is particularly important for some alloy steels which are air hardening for martensite forms on air cooling.

● Slower cooling rates are required for this process. Since the temperature involved in this process is more than that for annealing, the homogeneity of austenite increases and it results in better dispersion of ferrite and cementite in the final structure, which enhanced mechanical properties.

Microstructure during normalizing :

The grain size is finer in normalized structure than in annealing structure. Grain size of normalized steel is governed by section thickness. As cooling rate deffer considerably from case to core, there is variation in grain size of normalized steel over it's cross section. This process is used for rolled and forged steels, possessing coarse grains due to high temperature involved in this, which is subjected to normalizing treatment for grain refinement.

Application of Normalizing Process :

The important application of this process is it's adoption as post treatment after achieving a homogeneous structure. Generally heavy castings including ingots and rolled and forged steels.

Normalizing - Hardening - heat treatment for steels
Normalizing and Hardening

Process of Normalizing :

● Here steels is heated to a temperature which is higher than that recommend for normalizing. The casting is held for sufficient period of time so that chemical homogeneity is achieved by diffusion.

● Then followed by air cooling. In fact cooling rate are not of much important because it followed by second cycle of treatment. The aim of the second heat treatment cycle is to refine the coarse grained structure developed to high temperature heat treatment.

● Refining is done by normalizing steel at lower temperature. Some internal stresses are developed in theses heavy casting or ingots because of large section thickness which results in variation in cooling rates from case to core.

Properties after Normalizing :

Normalized steels are generally stronger and harder than fully annealed steels. Correspondingly the machinability of steels shows an improvement on normalizing. By normalizing an optimum combination of strength and soften is achieved, which is results in satisfactory level of machinability in steels. This methods of improving machinability is specially applicable to hypoeutectoid steel.

● It has been observed that carbide gets precipitated at grain boundaries and forms continuous network, particularly in hypeeeutectoid steel by annealing. This process is very effective to eliminate carbide network. Such a network is quite stable and not eliminated by annealing treatment since there is repreciation during cooling.

● Normalizing treatment is frequently applied to sterling order to achieve any one or more of the objectives like grain refinement, improvement in machinability and enhance mechanical properties such as hardness, strength and toughness.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

SPHEROIDIZING | Heat treatment for steels


Spheroidizing is a heat treatment process which result in a structure consisting of globules or spheroids of carbide in a matrix of ferrite. Here cementite of lamellar pearlite for hypoeutectoid and eutectoid steels, both lamellar and free cementite in the case of hypereuctoid steels, coalesce in to tiny spheroids.

● The degree of spheroidization depends on heat treatment temperature and holding time. The process can be completed in short time by increasing the treatment temperature. At high temperature dissolved carbide carbide particle can reapper as lamellae during cooling.
Spheroidizing - heat treatment for steels

Methods of Spheroidizing :

● In spheroidization heating of steel is just below the lower critical temperature, then holding at this temperature for prolonged periods after that cooled slowly.
• Another method involves heating and cooling steel alternately just above and below the lower critical temperature. It can consists of heating of steel to a temperature above the lower critical temperature, followed by slow cooling to a temperature below the lower critical temperature and holding at this temperature for a period till the shape of all carbide particles changes in to spheroids.

● The spheroidizing treatment is also carried out by heating steel above the lower critical temperature. This results in a completely spheroidized structure. The steel is to be heated above the lower critical temperature depends on the chemical composition of steel. Eutectoid steels are heated to about 20 to 30℃ above the lower critical temperature, where as hypereutectoid steels are heated 30 to 50℃ above the lower critical temperature. Medium carbon steels can be spheroidized either by heating just above or below the lower critical temperature.

Purpose of Spheroidizing : 

● High carbon and alloy steels are frequently spheroidized in order to improve machinability and ductility. Low carbon steels are not generally spheroidized. The aim is to make these steels suitable for serve deformation in case they are spheroidized. Low carbon steels, on spheroidization, become very soft and gummy.

● It has been observed that the fine lammellar pearlite coalescess more easily than coarse pearlite. Very fine pearlite spheroidizes still more readily. Quenched structures consisting of fine and well dispersed carbide phase shows greatest spheroidization rates. Cold working of steel prior to the treatment also helps in accelerating the rate of spheroidization.

Q. What is Spheroidizing ?

Ans : Spheroidizing refers to a heat treatment material modification process that is used to convert granular structures of the material into a spheroidal form. The process is performed to improve a metal's cold forming capability.

Short note on Spheroidizing :

Spheroidizing is performed by annealing steels with more than 0.8% carbon. The metal is heated to a temperature of about 1200°F (650°C) and maintained at this temperature for a predetermined amount of time to convert its microstructure. This allows for a cementite steel structure to change from a lamella formation to an alpha ferrite matrix. The alpha ferrite matrix is made up of particles of spheroidal cementite formations.
Spherodizing is industrially performed in an endothermic atmosphere to prevent oxidation and decarburization.
Spheroidizing is used primarily to treat various types of steels, and is used to improve the machinability of hypereutectoid and tool steels. This is accomplished by lowering the metal's steel flow stress.
Spherodizing is typically done on parts that have been work hardened for improved ductility, toughness, strength and reduced hardness.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

ANNEALING | Heat treatment for steels


Annealing involves heating to a predetermined temperature holding at this temperature, and finally cooling at a very slow rate. Annealing can from either the final treatment or a preparatory step for further treatment. The various purpose of the treatment are to

1. Relieve internal stresses developed during solidification, machining, forging, rolling or welding
2. Improve or restore ductility and toughness.
3. Enhance machinability
4. Eliminate chemical non uniformity.
5. Refine grain size.
6. Reduce the gaseous contents in steel.

Depending on heat treatment temperature annealing treatment can be subdivided into three classes, namely
1. Full annealing : here steel is heated above the upper critical temperature A3 and then cooled very slowly.
2. Partial annealing : It also known as incomplete annealing or intercritical annealing. It involve heating of steel to a temperature lying between lower critical temperature A1 and upper critical temperature (A3 or Acm).

3. Subcritical annealing :

Subcritical annealing is a process in which the Maximum temperature to which steel is heated always, less than the lower critical temperature A1. In this process no phase transformation takes place, only thermally activated phenomenon such as recovery, recrystallization, grain growth, agglomeration of carbide and softening occurs. The rate of cooling from a subcritical temperature is of little significant since practically there is no variation as far as microstructure and final properties are concerned.
Annealing - heat treatment for steel

The various annealing processes are discussed below :

Full Annealing :

It consist of heating steel to austenite region, followed by slow cooling. Steel is heated to about 30 to 50℃ above the upper critical temperature A3 for hypoeutectoid steels. Then steel is held at this temperature for a predetermine period and then slowly cooled inside the furnace or a heated insulated container by which, equilibrium structure as predicted by equilibrium diagram are obtained in the steel. Due to this full annealing the steel get a homogeneous austenite structure.

Isothermal Annealing :

In this process hypoeutectoid steel is heated above the upper critical temperature A3 and held for some time at this temperature and then cooled rapidly to a temperature less than the lower critical temperature A1 (usually 600 to 700℃). Fast cooling can be achieved by rapidly transferring steel to another furnace and then maintained at the desired temperature. At which super cooled austenite has minimum stability within the pearlitic region, then held it until the all austenite gets transformed to pearlite. After all the austenite is transformed in to lamellar pearlite, steel is cooled in air. By which the magnitude of internal stresses developed within the steel will vary with cooling rate. By which this process not only improve the machinability but also results in a better surface finish by machining. So that it used more used for making alloys steel.

Diffusion Annealing :

Diffusion Annealing also known as homogenized annealing which is employed to removed any structural non uniformity, like dendrites, columnar grains and chemical inhomogeneous. Theses are generally observed in the case of ingots, heavy plain carbon steel casting and high alloy steel casting. These defect promote brittleness and reduce ductility and toughness of steel. In this type of annealing steel is heated sufficiently above the upper critical temperature (1000 to 1200℃) and is held at this temperature for usually 10 to 20 hours, then cool slowly. Heating to such a high temperature results in considerable coarsening of austenitic grains and heavy scale formation. The coarse austenite obtained further transforms to coarse pearlite on cooling. The coarse grain structure can be refined either by plastic working for invitations or by employing a second heat treatment for casting.

Partial Annealing :

Partial annealing is also referred to as intercritial annealing or incomplete annealing. In this process steel is heated between the lower critical temperature A1 and the upper critical temperature A3 or Acm, which is followed by slow cooling. Generally hyperutectoid steels are subjected to this treatment. The resultant microstructure consists of fine pearlite and cementite instead of coarse pearlite and a network of cementite at grain boundaries, as observed in the case of full annealing. As low temperature involve in this process, it is less expensive than full annealing. Hypoeutectoid steels are also subjected to this treatment in order to improve their machinability.

Recrystallization Annealing :

All steels, which have been heavily cold worked are subjected to this treatment. The process consists of heating steel above the recrystallization temperature, holding at this temperature and then cooling. It results in decrease in hardness or strength and increase in ductility. The desired extent of reduction in cross sectional area is possible with adoption of cold work recrystallization anneal cycle. The process is used both as an intermediate operation and as a final treatment. The treatment is frequently employed in manufacturing steel industries for making wires, sheets and strips. The final structure after the treatment consists of strain free grains produced at the expense of deformed original grains.

Process Annealing :

In process annealing steel is heated to a temperature below the lower critical temperature, and is held at this temperature for sufficient time and then cooled. Since it is a subcritical annealing, cooling rate is of little importance. The purpose of this treatment is to reduce hardness and to increase ductility of cold-worked steel so that further  working may be carried out easily. It is an intermediate operation and is sometimes referred to as in-process annealing. The process is less expensive than recrystallization annealing. It differs from recrystallization annealing in the sense that complete recrystallization of cold-worked steel may or may not take place in this treatment. Parts which are fabricated by cold forming such as stamping, extrusion, upsetting and drawing are frequently given this treatment as an intermediate step.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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