Explain the Purposes of tempering

Tempering is a heat treatment approach for alloys and metals. In steels, tempering is done to "toughen" the metal by transforming brittle martensite into bainite or a combination of cementite and ferrite. Precipitation hardening alloys, such as numerous grades of aluminum and super alloys, are tempered to precipitate intermetallic particles that strengthen metal.

Brittle  martensite  becomes  strong  and  ductile  after  it  is  tempered.  Carbon  atoms  were trapped  in  the  austenite  when  it  was  rapidly  cooled,  typically  by  water  or  oil  quenching, forming  martensite.  Martensite  becomes  strong  after  being  tempered  as when reheated,  microstructure  can  rearrange  and  the  carbon  atoms  can  diffuse  out  of  distorted BCT structure. After carbon diffuses, result is nearly pure ferrite.

In  metallurgy,  there  is  always  a  trade-off  between  brittleness and  ductility. This delicate balance highlights many of the subtleties inherent to tempering process. Precise control of time and temperature during the tempering process are critical to achieve a metal with well-balanced mechanical properties.