Hardening and Temaparing of steel

. the principle of hardening

. the effect of the carbon content in hardening

. the process of hardening

. the tempering of steel

. the purpose of temaparing

Introduction

If a piece of steel is heated to a sufficiently high temparature, all the carbon will be dissolved in the solid iron to form the solid solution, austenite of the steel. When it is slowly cooled, the change in the arrangement of the iron atoms will cause a solid solution can only contain up to 0.006% carbon, and so the excess carbon will be forced to leave the solid solution, and produce cementite. This will, with ferrite, form a laminated structure called pearlite.

The principle of hardening

If steel is cooled rapidly (quenched ) the excess carbon will not have sufficient time to leave the solid solution with the  result that it will be trapped in the iron, and so cause an internal distortion. This internal distortion is the cause for the increase in the hardness of steel with a corresponding reduction in its strength and ductility. This is the basis of the hardening process.

The mechanical properties produced as a result of this treatment will depend upon:

-the carbon content of the steel

-the temparature to which it is heated

-the duration of heating

-the temparature of tbe steel at the start of quenching

-the cooling rate produced by quenching

The effect of carbon content upon 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

In order to produce the desired effect, sufficient carbon must be put into the soild 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  to only just above its upper critical point (heating ).

When its  carbon content is more than 0?83% the steel is heated only to just above its lower critical point (heating ).

The temparature to which steel are heated before quenching.

Soaking time

After heating the steel is held at that temaprature for some time. Normally 5 mts  are allowed as soaking time for 10 mm thickness of steel.

Cooling

Then the steel 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 like structure called martensite

The structure of steel treated this way is very hard and strong but very brittle.

The quenching medium

The quenching medium controls the rate of cooling. For a rapid quenching a solution of salt or casuistic coda in water is used.

For very slow quenching a blast of air is sufficient.

Oil gives intermediate quenching.

Water and oil are the most common quenching media used.

Air quenching is suitable only for certain special alloy steel.

Temparing

After hardening, steel is usually reheated to a suitable temparature below the lower critical point 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.

Purpose of temaparing of steel

Steel in its hardened condition is generally too brittile and too severely strained. In this condition steel cannot be used and hence it has to be tempered.

The aims of temaparing are

-to relive the steel from internal stresses and strains

- to regulate the hardness and toughness

-to reduce the brittleness

-to restore some ductility

-to reduce shock resistance

Process of temaparing

The temparing temaprature depends upon the properties required, but it is between 180°c and 650° c. The duration of heating depends upon the thickeness of the material. Tools are usually tempered at a low temparature. The temparature itself is judged by the colour of the oxide film produced upon heating.

This method is not however, suitable for accurate temaprature assessment.

   Temaparing Temparature

Temper colour              temaprature °c

Pale straw                       230

Dark straw                      240

Brown                              250

Brownish purple           260

Purple                              270

Dark purple                    280

Blue                                  300

In a manufacturing plant, when heat-treating is done on a production basis, modern methods are used. Temparing is done in controlled atmosphere furnaces with the temparature controlled by modern instruments. Under such conditions, it is possible to obtain accurate and  uniform results in any number of pieces.

Illustrated the appearance of the microstructure of hardened and tempered steel.

Generally, temaparing in the lower temaprature range for an increased time provides greater control in securing the desirable mechanical properties. Such heat treatment may no be feasible under all conditions. For precision work, where results justify the method, and for certain combination of mechanical properties, temaparing for long periods of time in a lower temparature range provides a reliable method of getting the desired results.