Heat treatment of plain-carbon steel

Heat treatment is a process in which a metal, in its solid state, is subjected to one or more cycles of heating and cooling in order to obtain certain desired mechanical properties. The mechanical properties listed in Section 13.2 can be altered by changing the size, shape and structure of the grains from which the material is made up. 

When a plain-carbon steel is heated through a sufficient temperature range, there is a particular temperature at which the internal grain structure begins to change. This temperature, known as the lower critical temperature, is about 700°C and is the same for aU plain-carbon steels. Fig. 13.3. 

When the steel is heated still further, the stmctural changes continue until a second temperature is reached where the change in the internal stmcture of the steel is complete. This temperature is known as the upper critical temperature and varies for plain-carbon steel according to the percentage carbon content. Fig. 13.3. 

The influence of the carbon content is so great tbat tbe heat treatment and subsequent employment of the steel is determined by this factor. 

The temperature range between the lower and upper critical temperatures is known as the critical range. 

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