CCT Diagrams

As  for  heating  diagrams,  it  is  important  to  clearly  state  what  type  of  cooling  curve  the  transformation  diagram  was derived from. Use of a constant cooling rate is very common in experimental practice. However, this regime rarely occurs in a  practical  situation. One can  also find curves  for  so-called  natural cooling  rates according to Newton's law  of  cooling.

These curves simulate the behavior in the interior of a large part such as the cooling rate of a Jominy bar at some distance from  the  quenched  end.  Close  to  the  surface  the  characteristics  of  the  cooling  rate  can  be  very  complex  as  will  be described  below.  In  the  lower  part  of  Fig.  9  is  shown  a  CCT  diagram  (fully  drawn  lines)  for  4130  steel.  Ferrite,  pearlite, and  bainite  regions  are  indicated  as  well  as  the Ms temperature.  Note  that  theMs temperature  is  not  constant  when martensite formation is preceded by bainite formation, but typically decreases with longer times.

The effect of different cooling curves is shown in Fig. 10. Each CCT diagram contains a family of curves representing the cooling  rates  at  different  depths  of  a  cylinder  with  a  300  mm  (12  in.)  diameter.  The  slowest  cooling  rate  represents  the center  of  the  cylinder. As  shown  in  Fig.  10,  the  rate  of cooling  and the  position  of  the  CCT  curves depend  on the  cooling medium  (water  produced  the  highest  cooling  rate  followed  by  oil  and  air,  respectively).  The  more  severe  the  cooling medium, the longer the times to which the C-shaped curves are shifted. The Ms temperature is unaffected.