TYPES OF HEAT TREATMENT OF STEELS

. purpose of heat treatment

. purpose of annealing

. difference between the processess of annealing and normalising

The properties of steel depend upon its composition and its structure. The properties of steel and its structure can be changed by heating it to a particular temparature and then, allowing it to cool at a definite rate. The process of heating and cooling for changing the structure of steel,and thus obtaining the required properties is called 'heat treatment of steel'.

If steel is heated to a suitable temaprature, and then, slowly cooled, the steel will be soft, weak and ductile.

If steel is heated to a suitable temaprature, and then, rapidly cooled (quenched) the steel will be hard and brittle.

Classification of heat treatments

The treatments that produce equilibrium conditions are annealing and normalising.

Treatments that produce non-equilibrium conditions are hardening and tempering (usually done in conjunction with each other)

Annealing

In this process, steel is heated to a suitable temparature depending upon its carbon content, and is held at that temparature for sufficient time, and then slowly cooled to room temparature.

The heating, soaking (holding the room temparature )and slow cooling cause the grains to become large, and so produce softness and ductility.

For annealing, hypoeutectiod steel is heated to 30° to  50°c above the upper critical temparature,and it is 50°c above the lower critical temparature for hypereutectoid steel.

Soaking time at temparature is 5mts/10 mm of thickness for carbon steel.

The cooling rate for carbon steel is 100 to 150°C/hour.

The cooling is done in the furnace itself by switching off the furnace or the steel is covered either in sand or dry lime  and dry ash.

ANNEALING TEMAPRATURE

carbon content %         temaprature °c

< 0.12                                 875 to 925

0.12 to 0.25                       840 to 970

0.25 to 0.50                       815 to 840

0.50 to 0.90                       780 to 810

0.90 to 1.3                         760 to 780

Purpose of annealing

Annealing is done

- to obtain softness

-to improve machinability

-to increase ductility

-to relieve internal stresses

-to reduce or eliminate structural inhomogenity

-to refine the grain size

-to prepare the steel for subsequent heat treament process.

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 castins. Otherwise, they may fail at any time while in use.

Normalising is done to produce a fine grain for uniformly of structure and for improved machanical properties.

The normalising process

In this process, steel is heated to a suitable temaprature depending upon its carbon content, and held at that temparature, and then cooled freely in air.

Normalising is usually done, before machining and before hardening, to put the steel in the best condition for these operations.

The steel is heated to a temaprature (30 to 40° c above the upper critical temparature )at which all the austenite is present even in  the case of high carbon steel. This is because this process is the first step towards producing the final properties, and it is necessary to start with austenite to ensure uniformly.

PRECAUTION

The heated piece for normalising should not be kept at any wet place, in wet air or kept in forced air as they will induce some hardness 

Heating and cooling process of steel

. structural changes of plain carbon steel while heating

. the structural changes of plain carbon steel in normal cooling

Distinguish between the lower and upper critical temparature during heating and cooling.

Heating

While heating uniformly a piece of plain carbon steel in a furnace, the temparature will increase uniformly and it will be halted or will fall slightly for a short time although the heat is supplied to the steel.

At this point, the heat, is being used to  cause a rearrangement of the iron atoms which, in turn,, causes the formation of a solid solution called austenite.

The temparature at which austenite is formed is called the lower critical point (heating ) and is at 723°c.

If the steel contains less than 0.83% carbon there will be some ferrite present after the above change also. And if it contains more than 0.83% carbon, there will be some cementite present after this change.

If the heating is further continued, the ferrite or cementite present will be gradually tranformed into austenite. Less heat is required for this page.

The  temparature at which the formation of austenite is total called the upper critical point(heating ).

The temparature of upper critical point(heating )  depends upon the percentage of carbon in steel.

Cooling

While cooling, the temparature of the piece will fall uniformly untill a certain temparature is reached at which stage it starts to cool less rapidly.

The temparature at which the above is happening is called the upper critical point (cooling ) and is 30° c below upper critical point (heating ).

At this stage the austenite starts to break down to produce either ferrite or cementite according to the percentage of carbon in the steel.

The total breaking of the austenite to ferrite or cementite will continue untill the lower critical point (cooling )is reached at 693° c.

Then there will be a halt in the decrease of temparature. This halt in temparature willl be utilised for the breakdown of austenite caused by the rearrangement of the iorn atoms.

This rearrangement reduces heat which causes the changes in the rate of fall of the temparature of the steel.

The changes observed during this heating and cooling of steel are important in the study of its heat treatment. The changes indicated are those that occur when the steel is COOLED SLOWLY and the diagram is accordingly called a thermal equilibrium diagram. It is usual to indicate the temparature at which the changes occur during cooling, the changes also occur during heating, but the temparature at which they  occure are all 30° c higher than those associated with cooling.

METHODS OF IDENTIFICATION OF METALSS

.The different methods of identifying ferrous metals and alloys

. state how non ferrous metals and alloys are identified

A fitter has to handle different types of metals in his  work. A knowledge about how to recognise and differentiate the commonly used metals will help him in many ways.

Ferrous metals and alloys can be identified by

Their appearance (colour, texture etc,)

Their weight (light or heavy )

The sound

Cold hammering

The spark test (grinding )

Apart from the above tests steel bars are also identified by the code colours painted on them.

Different colours are marked, based on the different composition of materials and grade. Colours charts are available to determine the different metals.

Most non ferrous metals and alloys can be identified by their colour.

copper colour is distinctive red colour

Aluminum colour is  dull  white

Lead colour is bluish grey colour

Tin colour is silvery white, with a slightly yellowish tinge

Brass colour is distinctive yellow colour

Bronze colour is colour between copper and brgiven

Characteristics of different ferrous metals and alloys given

Low carbon steel

The appearance is smooth scale with blue/black sheen/silver grey. The density and weight is 7.85 medium weight. The sound (drop  a 15 to 25 cm long, on to the ground ) is medium metalic sound. The cold hammering is flattens easily. The spark test is stream of  yellow white sparks varying in length slightly fiery.

Medium carbon steel

The appearance is smooth scale black sheen. The density is 7.85 and weight is medium weight. The sound is higher note than that of low carbon steel. Cold of hammering is fairly difficult to flatten. The spark test is yellow  sparks shorter than those of low carbon steel, finer and more feathery.

High carbon steel

The appearance is rougher scale black. The density of metal is 7.85 and weight is medium weight. The soundof metal is good ringing sound. The cold hammering is difficult to flatten. The spark test is sparkless bright, starting near grinding wheel and more feathery with secondary branching.

High speed steel

The appearance of metal is roughness scale black with reddish tint. The density of metal is 9 and the weight is comparatively heavy. The sound of a metal is lower ringing more like low carbon steel.. The cold hammering is very difficult to flatten tends to crack easily. The spark test is faint red streak ending in fork.

Most use manufacturing process of steel?

The principle of the L. D converter

Name the raw materials used for the charge in L.D process👍

L. D. Process

The name L.D. is derived from the initials of two towns,LINZ & DONAWITZ in Austria where this process was first used.

The L. D. is converter or the basic oxygen converter is designed to produce high quality steel in a relatively shorter time. The basic raw material used in the process are the molten pig iron from the blast furnace and the steel strap. Fluxing agents are used for reacting with impurities and form a slag on the top of the molten metal.

In this process of 👍producing steel, a jet of pure oxygen is blown through a lance which is water cooled.The lance is brought near the molten metal and oxygen is blown at high pressure on the molten metal bath. This leads to the oxidation of carbon and other impurities causing violent agitation which allows all the molten metal to come in contact with the oxygen stream. When the blowing is over,the converter is tipped to remove the slag and again turned the other way to collect the metal into a ladle.

Controlled quantity of carbon and other elements are added to obtain the desired quality of steel.

Electric process

Two types of electric furnace are in use

-electric arc furnace

-high frequency furnace

High grade steels and alloy steels are made by the electric process

No fuel or air is required for burning. Because of this there is no contamination of the charge.

Electric arc  furnace

In this type of furnace, the heat required is generated by electric arcs struck between carbon electrodes and the metal bath. This type of furnace is used for making alloy steels, such as stainless steel, high speed steel etc.

The composition of the steel can be accurately controlled in this process. This process is not influenced by the external conditions and the temparature of the furnace can be maintained to the desired level.

The furnace uses pig iron and selected scrap, and produces high quality steel.

When the metal is molten and the slag is formed on the top surface, the furnace is tilted and the slag is removed. Then the furnace is again tilted in the other way to collect the molten metal. Silicon, manganese and other necessary elements are added in the ladle, and the composition of steel regulated.

The high frequency furnace

This furnace consists of a crucible made of refractory materials, surrounded by an inductor coil which carries the alternating current, and is insulated and water cooled. The outer ring is made of special magnetic iron laminations,, and shields the casting from getting too hot. It also serves as a magnetic sheild to increase the magnetic flux in the coil.

The charge used consists of steel scrap and iron oxide.

The steel is further refined by re melting and addition of the required alloying elements. Steel, produced by these processes, is used for making press tools, cutting tools and precision measuring instruments.

OPEN HEARTH PROCESS

This process eliminates the  disadvantage of the Bessemer process. Most of the steel in our country is produced by SIEMENS MARTIN process using an open hearth furnace.

Open hearth furnace

An open hearth furnace is a reverberatory furnace, having a melting chamber built with refractory materials, bounded at the top the roof, and at the bottom by the hearth. The charging doors are provided at the side walls. The melting area is connected with regenerators through ports.

Fuel for this process is the producer gas generated by a plant which can be fed  to the hearth through two generating Chambers. The air, which forms a combustible mixture, may be fed the same way through the other two chambers

Heating through producer gas and air is the characteristic of the open hearth furnace. For the chemical reaction of the charge, intense heat is required. This can be achieved through regenerating chambers.

The process

The hearth is first prepared and heated  well up to 1500°c. The charge is a mixture of pig iron and steel scrap in different proportions as required according to the class of steel to be manufactured.

The ratio of pig iron to scrap varies from 1:4 to 4:1 A reasonable ratio is 3:2. After the charging is over, the gas is allowed in to the melting champer. During melting, part of  the carbon, most of the silicon and  manganese are removed by oxidation. More carbon will be oxidised away at the  boil stage by adding iron ore to the charge. When the bath reaches the requisite percentage of carbon, the furnace is allowed for tapping. While the metal is flowing into the ladle, a de-oxidiser is added to the steel to remove air, and ensure soundness in the metal.

Manufacturing process of steel by Bessemer process ok on on

.The different manufacturing processes of steel

. manufacturing process of steel by the bessemer process

. distinguish between acid process and basic process

Manufacturing processes

The different manufacturing processes of steel

-Bessemer process

-L. D process

-Electric process

-Open hearth process

Bessemer process

Sir Henry Bessemer, a British engineer invented this process of converting molten pig-iron to steel  using a large pear shaped container. This is called the Bessemer converter.

It is a method of producing steel by blasting compressed air through molten iron and burning out the excess carbon and other impurities. This does not use any source of heat other than obtained from the oxidising process used to remove carbon and other impurities.

The three stages of this process are

-the preliminary stage

(The slag formation )

-the boil stage

(The brilliant flame blowing period)

-the finishing stage

(The reddish smoke period )

The molten pig iare from the blast furnace is poured into the converter. Then the blast of air at 150 to 250 KN/m2(killo Newton /m2) is directed into the molten metal, and the converter is rotated into an upright position. The conversion stage  commence now.

At the preliminary stage, the oxygen of the blast oxidises the iron to ferrous oxide. As a slag, the ferrous oxide mixes with the metal. The silicon and manganese, due to their affinity for oxygen, are seperated as oxides and go into the slag. During this reaction, a large amount of heat is generated which increases the  temparature from  1250°c to 1525°c. This stage lasts for 3 to 4 minutes.

The 2nd stage helps the burning of carbon from the molten bath. The  dissolved carbon is oxidised by the ferrous oxide of the slag. At the same time, carbon monoxide will be burning at the nose of the converter with a dazzling white flame. This elimination of carbon takes place for about 8 to 12 minutes. The 3rd stage starts with the subsiding of the flame. It means that the carbon  has been practically removed from the charge. This stage lasts  for one or two minutes. Then the converter is brought to a horizontal position. To eliminate oxygen and to bring silicon, manganese and carbon content of the steel to a specific limit, de oxiders like ferro manganese, ferro silicon or aluminum are added to the metal bath.

The Bessemer process has  the following disadvantages.

The steel produced by the Bessemer process will have an increased oxygen content which will impair its mechanical properties. Due to this reason, it cannot be used for producing cutting tools, spring and parts subject to impact, load etc..

When the Bessemer converter is line with silicous refractory material,this process is called acid process.

When the converter is  lined with a basic refractory, it is called a basic process or thomas  process.