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What Is Tempering In Heat Treatment of metals?

 When a piece of steel is taken out of the quenching medium, as already stated, it is hard, brittle and will have severe unequally distributed internal stresses besides other unfavorable characteristics. In general, tempering restores ductility and reduces hardness and results in some decrease in hardness. The primary objects of tempering are , therefore as follows :

1. To stabilize the structure of the metal.

2. To reduce the internal stresses produced during previous heating.

3. To reduce some of the hardness produced during hardening and to increase the ductility of the metal.

4. To give the metal right structural condition combined with toughness and shock - resistance. 






The tempering treatment requires :

1. Reheating the steel after hardening to temperatures below low critical temperature point.

2. Holding it for considerable time.

3. Slow cooling. It is desirable that the temperature of  the steel shall be maintained for not less than 4 to 5 minutes for each millimeter of the section. 

Exact Temperatures at which tempering should be carried out depends upon the purpose for which the article or tool is to be used. According to the heating temperatures, Tempering is classified into 3 types , they are : 

i). Low Temperature Tempering which is done in the temperature from 150*C to 250*C. The purpose of this procedure is to relieve internal stresses and to increase the ductility without appreciably reducing its hardness. Low Temperature tempering is applied in the heat treatment of carbon and low alloy steel cutting tools, as well as measuring tools and parts that have been carburized and surface hardened.

ii). Medium - Temperature Tempering which involves heating of the work to 350*C to 450*C. The structure of steel is altered by this procedure. Martensite is transformed into secondary troostite. The results are a reduction in hardness and strength of the metal and an increase in elongation and ductility. Medium temperature tempering is mainly applied to the articles and parts which are subjected to impact loads : chisels, hammers, springs, spring plates etc.

iii) High - Temperature Tempering which is done in the range of 500*C to 650*C. At these temperatures sorbite is formed in the steel and the internal stresses are almost completely eliminated. High Temperature tempering Imparts high ductility to parts, yet permits them to retain adequate hardness. This is applied to machine parts which are subjected to high  stresses and impacts , gear wheels , shafts , connecting rods , etc. 

TEMPERING BATHS :

Baths using tempering oils may be employed for temperatures up to approximately 230*C. Tempering oils are usually mineral oils. For Temperatures above about 230*C, liquid salt baths are preferred. These salt baths usually consist of mixture of  nitrates, or nitrates and nitrites. For high temperatures, chlorides and fluorides are usually employed. Steel parts are often tempered in electrically heated or gas-fired furnaces within which the air is circulated to produce uniform temperature.




TEMPERING COLORS :

There is curious phenomenon associated with low-temperature tempering of carbon tool steels. This is the formation of temper colors on the surface of the steel being treated. These colors are caused by the surface oxidation of the steel with the formation of thin films of iron oxide. The temperature from 220*C to 300 *C above which the tempering colors vanish and surface films of oxide become opaque.




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