Changes of Carbon Steel Properties During Hardening, Tempering, Annealing, Normalizing and Refining

  Hardening

1. When carbon steel is heated to a temperature, a little above its upper critical point  and then cooled by drastic quenching, the normal structural change does not occur. The  rapid cooling arrests the formation of pearlite, and the austenite is changed into another micro-structure. "Martensite ” which is extremely hard and brittle.

If the rate of ‘cooling  is not quite so drastic, then the austenite transforms into another structure called “ Troostite ”  which is less hard but very tough.

An even lower rate of cooling will produce “ Sorbite,”  a finely grained structure which is strong and ductile. This hardening effect on carbon steel is  only slightly apparent upto 0.25% carbon content, appreciable above 0.6% carbon and almost  attains its full value in a 0.87% carbon steel. Thus the degree of hardness of steel is dependent upon its carbon content and the rate of cooling. 

2. This hardening treatment gives the steel:
(a) Small grain size.
(b) Maximum hardness
(c) Maximum tensile strength.
(d) Minimum ductility   

Tempering

3. Martensite is stable only at temperatures below 200°C. With increasing temperature  martensite is slowly modified first into troostite and then into sorbite. Producing these  changes in the hardened steel  low temperature heat treatment is known as “Tempering”.

The higher the tempering temperature (which is always below the lower critical point)  the lesser the hardness and the greater the toughness of the steel.     

4. Hardened steel is brittle. So to relieve this brittleness rand induce toughness the  steel is tempered. Tempering consists of reheating the hardened steel in a fairly low temperature and then quenching it. The temperature desired may be judged from the “Temper colors" which appear on the bright surface of slowly heated steel. Each color corresponds to a particular temperature. 

 Annealing 

Steel in a hardened or tempered condition can be restored to its softest state by heating it to its upper critical point, thereby bringing it to its austenitic condition, and then allowing it to cool very slowly by leaving the steel in the hot ashes or in the furnace after  the source of heat has been removed.

The slow cooling allows the normal structural changes  to take place and the steel reverts to its pearlite and ferrite, or pearlite  cementite condition.

6. This heat treatment makes the steel very soft in its cold state, thus enabling the  steel to be easily bent without cracking, or soft enough to be cut  stamped or machined  without unnecessarily blunting the cutting tool. Annealing also relieves the metal of any  internal stresses set up by the previous heat treatment.          

Normalizing 

When steel is cold worked (hammered, bent, rolled etc.) its crystal structure is  disturbed, stresses and strains set up and the metal becomes brittle and weakened. Normalizing restores the crystalline structure to a  normal condition and relieves the stresses.

The  process is carried out by heating the steel slowly to its annealing temperature and then allowing  it to cool freely in the air. This more rapid rate of cooling as compared with the cooling  rate in the annealing process gives the steel a  finer grain than that obtained by annealing.   

Refining 

8. Prolonged heating at temperatures well above the upper critical point followed by slow cooling causes the grain structure of steel to coarsen, reducing toughness and strength. 

The refining process which has the effect of reducing the size of the crystalline structure, consists of reheating the material to a specific temperature, usually between 840°C to 900°C  followed by rapid cooling either in water or oil.

This process may be carried out two or  three times if desired, each heating being at a lower temperature than the previous one and  finished with the final operation of hardening the case or skin of the steel by heating to  about 760°C and quenching in oil or water.