Heat treatment of metal

Precision CNC machining parts The
generally accepted definition for heat treating metals and metal alloys is
“heating and cooling a solid metal or alloy in a way so as to obtain specific
conditions or properties.” Heating for the sole purpose of hot working (as in
forging operations) is excluded from this definition．Likewise，the types of
heat treatment that are sometimes used for products such as glass or plastics
are also excluded from coverage by this definition．

Transformation Curves

Precision CNC machining parts The basis for heat treatment is the time-temperature-transformation
curves or TTT curves where，in a single
diagram all the three parameters are plotted．Because of the shape of the curves，they are also sometimes called C-curves or S-curves．

To plot TTT curves，the particular steel is held at a given temperature and the
structure is examined at predetermined intervals to record the amount of
transformation taken place．It is known
that the eutectoid steel (T80) under equilibrium conditions contains，all austenite above 723℃，whereas below，it is the pearlite．To form pearlite，the carbon
atoms should diffuse to form cementite．The diffusion being a rate process，would require sufficient time for complete transformation of
austenite to pearlite．From different
samples，it is possible to note
the amount of the transformation taking place at any temperature．These points are then plotted on a graph with time and temperature
as the axes．Through these points，transformation curves can be plotted as shown in Fig.1 for eutectoid
steel． Precision CNC machining parts The curve at extreme left
represents the time required for the transformation of austenite to pearlite to
start at any given temperature．Similarly，the curve at extreme right represents the time required for
completing the transformation．Between the two
curves are the points representing partial transformation. The horizontal lines
Ms and Mf represent the start and finish of martensitic transformation.